Presbyopia: Therapies and Further Prospects Alain-Nicolas Gilg
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Accommodation and PresbyopiaCHAPTER 1

 
1.1 DEFINITIONS AND SYMPTOMATOLOGY
In my ophthalmological practice, I do not spend a single day without dealing with a problem of my patients’ presbyopia.
 
1.1.1 Etymologies
This definition of presbyopia does not prejudge the origin of the accommodative loss (traumatic, infectious, tumoral), the senile dysfunction (dystrophia) being the most frequent cause in its broadest acceptance: presbyopia.
Its Latin origin (accommodātĭo, ōnis, f.) translates the action of adapting, adjusting, sorting out. It is indeed evoking, here, the idea of a dynamic focus of the vision while adapting to different distances, going from distant vision to near one, or the other way round.
Accommodation is a symmetrical and consensual phenomenon: that is to say, the ocular couple should accommodate the same value at the same time.
This consideration has its importance given that some patients present a problem of refraction that can disturb both eyes’ simultaneous focus capacity, and then impair the quality of binocular fusion.2
zoom view
Fig. 1.1: Accommodating eye in a non-presbyopic patient
On a non-presbyopic eye, with straight gaze fixing, the focus works in central vision. Then, the image of a near object crosses the optical systems which focus it on the central foveal retina.
After these inescapable definitions, what are the symptoms of presbyopia?
 
1.1.2 Functional Signs
The functional signs are mainly the discomforts felt and mentioned by the patient, such as:
  • blurred near vision,
  • reading unusual fatigue,3
  • slow focus,
  • headaches,
  • tendency to hold reading at arm's length (the patients commonly say they need a “forearm graft”!),
  • search for better lighting (to improve contrasts), or lack of interest in reading small capitals.
Many presbyopes rightly complain about the little consideration given to their “handicap”:
  • the use of small capitals (insurance contracts, drugs precautions),
  • the poor quality of some printed documents,
  • the overuse of colored texts on colored backgrounds, considerably reducing contrasts,
  • and finally the bad quality of paper, whether recycled or not, spoiling the albedo of the page, as much as its legibility.
 
1.1.3 Physical Signs
Presbyopia also appears through physical signs, according to the patient, and confirmed by ocular redness after prolonged reading and lacrimations after near vision efforts.
Presbyopia does appear in some human activities. In pictorial art, the modification of the artists’ sight can have more or less obvious consequences on their work. Let's take Rembrandt (1606–1669) as an example. We do not know anything about his ophthalmological condition, but the maturity of his work coincides with the introduction of impasto, a technique used in painting, where paint is laid very thickly with brush or painting-knife, usually thickly enough that the wide and vigorous strokes are visible. On none of his self-portraits does the Dutch painter represent correcting lenses or pince-nez. Since impasto does not demand a vision of details from the artist, this process is more typical of presbyopic artists whose first works do not contain any (last Titian's paintings). The optical explanation thus seems more plausible than the stylistic justification, although, judging by the handrest, the working distance of oil paint must have been stretched-arm distance, for this technique.
The whole symptomatology shares aspects allowing some diagnosis: the disorders concern both eyes at the same time, start around forty, then get to expand with no possible regression, except at the beginning of the affection.4
 
1.2 PREVALENCE AND INCIDENCE OF PRESBYOPIA
Neither in the human race, nor amongst the superior primates, there is no one who escapes it; except for some of the patients who do not follow the usual development of presbyopia, for reasons that have still to be figured out.
 
1.2.1 Compared Ontogeny
As Professor Pouliquen[1] emphasizes it, it is interesting to notice that in the animal species adventure of eye's development, the mammals keep an ontological memory of their aquatic origin.
Consequently, during the accommodation, the human race uses the advantages of:
  • stenopeisation (Cephalopod's eye),
  • focal dynamic (Copilia's visual system),
  • biotopical ametropization (Aquatrols’ visual organ).
All the unavoidability, bilaterality and symmetry of the presbyopia phenomenon plead in favor of a still widely discussed dynamic process of tissular aging.
From a finalist viewpoint, it is interesting to notice how nature has endowed the human beings with accommodative power decreasing during life.
 
1.2.2 Various Enlarger Systems in Vertebrates
In vertebrates, many systems permit to enlarge the images.
We notice a power increase in corneal diopter during the air quest (reptiles, birds, mammals), crystalline movements when it is spherical and not deformable (abyssal fish) and finally possible deformations (birds and mammals).
Fish eye is big and made of a scleral shell, particularly developed in deep sea species, allowing a resistance to pressure. A flat cornea and a great crystalline lens not deformable and spherical are typical of this eye.
Fishes are going to accommodate to see in the distance by moving their crystalline lenses backwards towards the retina thanks to a muscle (Figs 1.2A and B).5
zoom view
Figs 1.2A and B: Accommodation in fish. Paradoxically, and because of the aquatic environment refractive index, the fish accommodates (B) to see in the distance and rests in near vision (A)
During the amphibians (Figs 1.3A and B) and reptiles (Figs 1.4A and B) air quest, lacrymal glands as well as eyelids allow cornea hydration modifying curvature radius to adapt to refraction index change between outdoor (air) and indoor (water) but accommodation occurs just as for fish, with crystalline lens moving.
In mammals, crystalline lens takes a lenticular shape and can deform itself thanks to a complex muscular system.
The most advanced system belongs to the diving/fishing birds such as the cormorant where the very large eye occupies the whole eye-socket.6
zoom view
Figs 1.3A and B: Accommodation in amphibian. From their aquatic origins, like fish the amphibians accommodate for distant vision (B) with the intervention of crystalline lens retraction muscle (A)
When the bird is diving, the Crompton muscle (Fig. 1.5A) allows the corneal flattening (like for fish), and under the action of Brüch's muscle, crystalline lens can deform until forming a lenticonus (Fig. 1.5B).
The cormorant can thus have optical power vary from around 50 diopters!
Great sea predators, sharks for instance, do not base their hunter capacities upon visual sense. Effectively, high myopes, sharks compensate deficient vision with hearing, sense of smell, of touch and above all electromagnetism (pores, ampullae of Lorenzini, located on the surface of the body) to detect their prey or spot the possible coming of enemies. Hammerhead sharks have their eyes at the end of the two cephalic extensions.7
zoom view
Figs 1.4A and B: Accommodation in reptiles. Like in fish (where they come from) and the amphibians, reptiles accommodate to see in the distance by contracting the crystalline lens retraction muscle (A). Crystalline lens lenticular shape seems to be an adaptation to life on earth. Lizards have a fovea and reptiles have a vitreous less liquefied than fish and amphibians with persisting papillary conus, a vitreous vestigial vascularization (B)
Sharks’ voluminous eyes are mobile thanks to the intervention of some muscles. They also have two thick and still eyelids to protect them. In some shark species, there is a winking membrane considered as a third transversal and mobile eyelid. It serves as a protection by covering the eye while the shark is about to bite. The transparent cornea is flattened and prolonged with an extremely resistant sclerotic, partly cartilaginous. The protruding crystalline lens is almost spherical: exaggerated convexity, added to the fact it does not deform itself much or easily accommodate, explains the shark myopia. According to the species, the pupil is often round or oval. Apart from this quasi-non-existent accommodation, sharks see colors, contrasts, regulate the sudden variations in surrounding light intensity (bright tapestry or tapetum lucidum), have a high sensitivity as opposed to a low visual acuity.8
zoom view
Figs 1.5A and B: Accommodation in birds. The bird's eye looks like the mammals’ one insofar as the accommodated state enables near vision thanks to muscles deforming crystalline lens and cornea leaning on a sclera's hard part (cartilage, ossicles) (A). The bird's accommodated eye then takes the shape of “a bell” (B)
Myopes, presbyopes, astigmats and animals also affected by natural cataract: it seems that nature has not been really kind to bovines.
Having a protruding crystalline lens, bovines own a near vision enabling them to see herb in front of them very clearly (Fig. 1.6). However, on the contrary, since ciliary muscles are not worked out, accommodation capacity remains quite low for distant vision.9
zoom view
Fig. 1.6: Bovine vision
 
1.2.3 Accommodative Involution
So the infant's mother, while holding him in her arms and instinctively coming closer, until standing only few centimeters from him, thus provides him with the capacity of a 20 diopters near accommodation, meaning clear vision until 5 cm for an emmetropic subject, just as the distance between him and the rattle in his hand.
As child grows up, accommodative power progressively decreases and seems to match visual needs. It happens at the time of walk, draw learning, and while developing fine motricity. So many parents tell me, “Doctor, you are saying my baby may well need spectacles, and yet, he is able to find a minute thread hidden in the fitted-carpet.”10
zoom view
Fig. 1.7: Distant, near vision and accommodation in a non-presbyopic not compensated emmetropeIn emmetropes, an image located to infinite (DV = Distant Vision: in practice, more than 5 meters) crosses the pupillary aperture and the eye's transparent media. It then converges on the fovea (no accommodation). As for a near object (NV = Near Vision), the emmetropic eye passively focuses at the back of the retina, but the accommodation allows an active focus on the fovea.
In the absence of other refractive problems, physiological accommodative loss progressively leads to keep only a residual accommodation diopter. It occurs without bothering the subject during his first 40 years, until 60 years old.
Table 1.1   Expected addition (expressed in diopters) according to the patient's age, for a working distance of 40 cm
Addition
+0.50
+0.75
+1.00
+1.25
+1.50
+1.75
+2.00
+2.25
+2.50
Age
40
42
44
46
48
50
52
56
60
The complexity of the system has a variable speed of deterioration, based upon biological non-unitarian factors laid out in series. It follows a biomechanical exhaustion, which we would rather qualify as viscoelastical, if we could establish a biophysical modelization.
Contrary to other affections in medicine, presbyopia does not present any gender or race's characteristic. Its prevalence and incidence are identical for men and women, for Caucasians, Asians, or melanoderms.
Environmental factors themselves do not seem to play a major role in this affection, whether they are biotope's latitude grade, sunlight, diet habits, life socioeconomical conditions, or ergonomy at workstation. Even in unilateral or bilateral amblyopia (defect in functional development of the vision with no anatomical impairment).
Statistically, none of these elements seems to influence the emergence of this affection within the population. So, as far as accommodation is concerned, would our organism, remarkable system, then be destined to a genetically programmed involution like a species’ characteristic?
It is far from being certain. At a time when molecular genetics is proudly cited in the media, when geneticians and biologists solve more enigma of human genome every day, nobody has yet discovered a series of genes commanding accommodation and presbyopia.
Even so, searchers identify genes that accelerate ocular aging in glaucoma, macular degeneration bound to the age, pigmentary degeneration of the retina and the other familial heredomacular degenerations.12
Nothing seems to hamper the inescapable way to presbyopia, but nothing can accelerate it either, except pharmacological actions. We will come back to this later (See paragraph 1.3.3, Convergence, page 14).
Sleep deprivation or jetlag, usually faced in critical situations, in emergency medicine, or aeronautical medicine, are ordeals intensely felt by the organism, which sets adaptations for the various organs of relation life. If the threshold of vigilance lessens, along with the fall of its attendant superior brain function, the other vital functions, pulmonary and cardiac ones, remain untouched, just as the other vegetative functions, especially digestive and urinary ones. It is as if the organism in difficulty centered its adaptation efforts on elements essential to survival. This is also the case of the situations of great malnutrition due to starvation or hunger strike.
In all these extreme situations, accommodation and the ongoing presbyopia seem preserved, what tends to link intrinsic processes to vegetative functions, normally unconscious and involuntary.
There does not seem to be any hormonal influence over accommodation or the course of presbyopia: pregnant woman and endocrine diseased people, apart from secondary cataracts they may have, do not suffer from either accommodation problems or premature presbyopia. Nonetheless, some medical books typically confirm a quicker development of presbyopia in the presence of certain affections: open angle glaucoma, diabetes, myasthenia, Graves’ disease, debilitating affections, overwork, and neurosis.[2]
 
1.3 ANATOMICAL PATHWAYS OF ACCOMMODATION
Accommodation and presbyopia thus represent quite a singular entity in the organism and its physiological processes. Neurological pathways of accommodation are in close relation with oculomotricity pathways and pupillary motility.
In usual situation, the frontal motor areas voluntarily activate the accommodation at the cerebral cortex level. They give this order to the brain stem's motor nucleus by a descending pathway.
 
1.3.1 Synkinesis
This is the phenomenon implemented to read this book in near vision. The ocular couple puts itself in convergence, meaning in oculomuscular adduction, under the influence of the medial rectus muscle, 13stimulated by the oculomotor nerves. The convergence phenomenon happens at the same time as the accommodative focusing on the text along with the pupillary miosis. Pupils are stimulated in miosis by stimulation of the parasympathetic nervous contingent on its way to the iris annular sphincter constrictor muscle.
The corollary of this physiological observation is that the effect of the eye's extrinsic musculature, or pupillary motility, can indirectly activate the accommodation whether positively or negatively.
There are pharmacological or optical situations in which such interferences between accommodation, oculomotricity and pupillary mobility, are possible. Let us first view the old drugs, known since antiquity for their action on pupillary size.
 
1.3.2 Miosis
It is above all pilocarpine, an alkaloid molecule obtained from the leaves of tropical American shrubs called jaborandi. Thanks to its hydrophilic nature, its chlorhydrate formula, instilled on the surface of the eye, gets into the tissues and stimulates the iris sphincter's muscarinic receptor M3, thus provoking both miosis and ciliary spasm responsible for transitory spastic accommodation.
Conversely, atropine or its derivatives, belladonna extracts, acts with sulphated formula laid on the eye in instillation, by provoking pupillary dilation (mydriasis), together with cycloplegia (pharmacologically-induced paralysis of ciliary muscle).
Senescence ordinarily comes with a physiological decrease of the pupillary diameter which can somewhat bind the synkinetic reflex analysis.
The pupillary diameter in the population varies from 8.5 mm to 3.0 mm with a predominance of subjects between 5.5 and 4.5 mm.
Pupil, playing the role of an optical diaphragm, allows reducing some visual aberrations, the way a stenopeic hole placed before the eye would do it.
From this hole, light beam faces interference on the edge of the whole (pupillary bank) with increase in the image clearness (sensitivity to contrasts) and in the field depth (clear area on both sides of the image), but 14decrease in its intensity (which needs an image enlightened enough). Stenopeisation during accommodation contributes to near vision focus, in as much as the read page remains well illuminated.
 
1.3.3 Convergence
Moreover, convergence and accommodation are tightly bound during the synkinetic reflex. Excessive accommodation for children can come together with convergent strabismus, completely reversible in its pure form, after adequate correction of the refraction.
Other accommodative disorders, such as accommodative spasm, can go with spasmodic esotropia (or convergent strabismus) added with convergence, sometimes replaced by insufficient convergence in decompensate cases.
The etiologies of bilateral spasms are linked to intoxications with:
  • opium,
  • morphine,
  • aconitine,
  • digitaline,
  • arsenobenzol, sulphonamides,
  • general parasympathomimetic drug (jaborandi, Calabar beans),
  • muscarine (amanita muscaria),
  • parathion,
  • curare, and
  • exceptionally brought up, methylene blue.
Some neurological diseases can be responsible in cases of:
  • diphtheria,
  • certain encephalitis,
  • meningitis,
  • tabetic crisis,
  • periodic spasm during oculomotor nerve cyclic phenomenon, and
  • chorea, exceptionally.
Other affections can trigger accommodative spasms such as helminthiasis (ascariasis, oxyuriasis), hypervitaminosis B1, electrocution or different types of tooth extraction.15
 
1.3.4 Accommodative Insufficiencies
However, accommodative insufficiencies from reduction or removal of accommodation, apart from crystalline lens affections or presbyopia, as mentioned earlier, are most of the time related to ciliary muscle palsies.
These palsies settle suddenly, or sometimes, progressively, and might be associated with signs like inconstant micropsia (see above for definition), as well as frequent paralytic mydriasis stemming from the relations, previously described, between accommodation and pupil (See paragraph 1.3.1, Synkinesis, page 12).
Then light can remove the pupillary reflex while convergence preserves it, what the anatomical distinction of those two reflex chains emphasizes. This is Argyll Robertson's sign that associates a removal of the pupillary contraction by light and preservation of synkinetic miosis, with accommodation-convergence.
Conversely, during synkinetic accommodation-convergence of near vision with a preserved pupil light reflex, we also notice an abnormality of the pupillary contraction: this is the reverse Argyll Robertson's sign.
Both of them cover various etiologies: neurological, traumatic ones, intoxications and miscellaneous affections.
Eyeball traumas add areflexive mydriasis to cycloplegia as in the related case of hymenoptera stings to sclerocorneal limbus.
We can also observe cycloparesis in cranial trauma with third nerve paralysis or thoracic trauma, or even in case of electrocution.
Cycloparesis can occur with ocular pathologies such as subacute glaucoma, uveitis or congenital aniridia.
A few intoxications and the absorption of some products may lead to bilateral cycloplegia. Especially the use of certain anticholinergical active drugs, well known for their contraindication if there is a risk of glaucoma by iridocorneal angle closure:
  • solanaceae species,
  • belladonna and its derivatives (homatropine, tropicamide, cyclopentolate),
  • scopolamine,
  • antiparkinsonians,
  • synthetic antihistaminics,16
  • ganglioplegics (hexamethonium, tetraethylammonium chloride),
  • central nervous system stimulants, tranquillizers (chlorpromazine, phenothiazines),
  • monkshood,
  • anticoagulants,
  • organic arsenicals,
  • barbiturates,
  • sodium and potassium bromides,
  • allyl dibromide,
  • carbon bisulfide,
  • cannabis,
  • carbon monoxide, carbon dioxide,
  • chloral,
  • chloramphenicol,
  • chloroquine,
  • cinchonine sulfate,
  • coniine and conhydrine,
  • dinitrophenol, disulphone,
  • iodoform,
  • Virginia jasmine,
  • henbane, mecamylamine,
  • mercury and its salts,
  • diphenylhydantoine methyl,
  • methyl bromide and chloride,
  • nutmeg,
  • hydrogen phosphide,
  • morphine,
  • estrogens, ponalide,
  • quinine,
  • sulfamides,
  • sodium and potassium thioglycolates,
  • valerian,17
  • valethamate bromide,
  • lead,
  • ergot,
  • antidiphteric vaccine,
  • food poisoning,
  • poisonous fungus, and
  • snake venom.
Some infections and parasitosis going with cycloplegia:
  • leprosy,
  • diphtheria,
  • botulism,
  • tetanus,
  • dengue,
  • epidemic hepatitis,
  • trichinosis,
  • ankylostomiasis, and
  • amebiasis.
Neurological affections come with ciliary muscle palsy during the clinical affection of an internal ophthalmoplegia with third nerve lesion:
  • Economo's disease (encephalitis lethargica),
  • Infectious encephalitis (measles, mumps, typhoid fever, scarlet fever, cow pox, influenza, undetermined viruses),
  • neuro-syphilis,
  • tuberculous meningitis,
  • Heine-Medin disease,
  • Guillain-Barré syndrome (acute idiopathic polyneuritis),
  • Little's disease,
  • posterior cranial fossa syndrome, and
  • Wilson's disease.18
The appearance of bilateral cycloparesis has implied several metabolic and endocrine disorders:
  • diabetes,
  • Graves’ disease,
  • lactation,
  • avitaminosis (B1, B2, C),
  • depression, and
  • anoxia.
Some lesions, close to the pathway of the oculomotor nerve, can breed a more or less important and more or less reversible palsy of the ciliary muscle. As it is the case in sphenoid sinusitis, dental affection, or other causes like Takayashu's disease, pithiatism or simulation.
That long and tedious list of what causes ciliary muscle palsy, which leads to some near-vision isolated affection, still shows the complexity of the ciliary effector accommodation pathway. It also proves that, in bilateral forms, the diversity of contexts can get us to diagnose a starting presbyopia, whereas it is a secondary loss of accommodation, which is often caused by:
  • certain affections,
  • unknown administration of anticholinergical drug,
  • clinical forms of diphtheria, botulism, tetanus, encephalitis, syphilis, diabetes, brainstem or hypophyseal fossa tumor.
In the absence of ciliary muscle, the congenital forms show familial bilateral palsy, hereditary and of predominant autosomal genetic transmission.
As we will see this further (See also paragraph 2.2.1, Hyperopic compensation, page 47), hyperopia in its acquired form, and which, according to its value, requires uncomfortable accommodation to focus, also expresses itself throughout a near-vision isolated affection.
Oculo-orbital affections leading to unilateral hyperopia represent local causes: orbital tumor lying in the retrobulbar cone, an intraocular tumor concerning the posterior pole, idiopathic retinal detachment, retinal or secondary chorioretinal detachment, exudative suffusion, in the macular region, some retinal detachment surgeries (with reduction of the scleral surface), ocular hypotonia which may bring about axial hyperopia (such after glaucoma filtering surgeries); some scars, corneal flattening, whether traumatically-induced or not (superficial or deep keratitis after corneal wound healing).
Astigmatisms, we will come back to it (See also paragraph 2.4, Presbyopia and astigmatisms, page 60), can give simultaneous affection of near and distal vision.19
Affections ones we are interested in are mainly transient and they appear as some visual disorder when accommodation changes, what is known as accommodotonia.
This neologism goes to show a slowing down in the accommodated states changes. As mentioned before, not only is accommodotonia the main reason why the new presbyopes should consult, but it is also present in unbalanced diabetes, of chronic alcoholism, Graves’ disease, cluster headache, some cases of cranial trauma, syphilis, measles, or in streptomycin historical treatment, obviously in connection with some neurological or selective muscular problem.
As for children, the accommodative fall or loss involving a difficulty or impossibility to read remains exceptional, no matter what the underlying refractive disorder may be, due to the high accommodative amplitude explained above. After a complete ophthalmic check-up along with pharmacological provocation test, it has to look after a brutal decompensation of unknown high ametropia, or a simulation (child's reactional depression to a divorce, after family loss, or other psychological trauma).
 
1.4 ACCOMMODATIVE OPTOMETRY
As it is often the case in medicine, all affection deserves a qualification, which is precisely what we have been considering until now (See paragraph 1.3.4, Accommodative insufficiencies, page 15).
Moreover, accommodation and presbyopia require quantitative assessment to determine the status of the affection, understand its functional slowing down, predict its evolution, and to appreciate therapeutic efficiency. Then how can we get a method to measure presbyopia?
 
1.4.1 Measures
In practice, not to disturb the synkinetic reflex (See paragraph 1.3.1, Synkinesis, page 12), we determine the accommodative power of both eyes in biocular vision, and we can carry out the examination in various luminance conditions (scotopic or nocturnal environment, mesopic or twilight intermediate one, photopic or diurnal surrounding).
Lighting conditions play no small part in it, for it determines how easily, quickly, precisely, and how amply the vision is able to adjust the focus thanks to the contrasts.20
If we cannot have the reading light vary, we can also propose, in photopic conditions, to get optotypes varied in contrast (20%, 50%, 70%, 100% saturation), on a white background.
As we usually notice it in far visual acuity test type, for sociocultural reasons, numerical optotypes are an alternative to alphabetical optotypes (no Roman alphabet, pronunciation difficulties, schooling level).
However, all these optotypes of morphoscopic determination in visual acuity (alphanumerical optotypes, drawings, Snellen, “C” or Landolt's rings) have sensitivity different from the optotypes representing the visual acuity in network (sinusoidal stimuli) or the angular visual acuity (Landolt's ring), which we can use for near vision.
Some related periodicals propose a near visual acuity, expressed in percentage of central visual efficiency, where 100% is an excellent visual acuity.
Nevertheless, usual measuring systems did not obtain international recognition for the rating scale lacks linearity decimal visual acuity, textual content (revised standard Parinaud and Jaeger scale, American type point, notation “M”), possible interferences with the ametropias (dioptric scale).
Regarding its accuracy in high and low visions, we should only use the spatial resolution logMAR chart of the minimal angle, since the diversity of the normative pads excludes all possibility of getting used to it, or of interference with underlying ametropia, if any.
Immediate precisions to the reader about the fact that, in near vision, instead of using aim devices, we must favor the use of optotypes chart. Indeed, its stand is unfailing, its whiteness impeccable, the page albedo and reflectance are controlled.
 
1.4.2 Ergonomics
Effectively, the automatic refractor for the optometric consultations, as well as the screening visiotest used in occupational medicine, both have the faculty of leading, for a certain number of our patients, to an accommodation impairing the performance test whether in near, intermediate or distance vision.
The static measure of near vision, graded 40 cm (35 cm for others, 14 inches in the USA), only gives partial account of accommodative efforts necessary from far to near vision, through the intermediate one (an arm's length eye-to-computer screen distance).21
We measure dynamic near vision by evaluation of the reading speed. It is quantified by the number of per-minute words the subject can read in a running text, supposing he or she has acquired the principles of reading and disposes of a normal visuospatial strategy (normal population read at a pace between 140 and 250 words/minute).
The recent revival of interest in posturology among visuomotor disorders (dyslexia, dysgraphia, visuospatial dyspraxia) allows to screen and take charge of proprioceptive deficiency syndromes causing various problems (exaggerated headache while reading, feeling to read without understanding). These syndromes of postural deficiency are frequent within the population, since we have estimated that 5 to 10% show dysproprioceptive symptomatology.
Not being as conspicuous as the sense of hearing, eyesight or smell, proprioception is an occult sense often forgotten by doctors and patients, despite concerning several organs. It is still essential for body tonico-postural regulation in time and space (somesthesia) (See also paragraph 3.1.3, Multifocal lenses, page 84). This is why its dysfunction hampers normal reading flow, adequate concentration and normal regulation of muscular contractility. It explains the exacerbation of signs we observe when presbyopia starts, for an adult who was not posturally reprogrammed during childhood (See also bibliography, “SDP et l'ophtalmologie autrement”, page 204).
Accommodative course or amplitude gives the most elements about the accommodative capacities of the subject at a given time of the day, without considering fatigability factor.
 
1.4.3 Tests
 
1.4.3.1 Search for Maximal Accommodation
In practice, we advise the “push-up test” technique with a patient corrected in distance vision. This method consists of asking the patient to stare at the smallest visible text on reading board standing one meter from him, then to bring it closer and closer until the text gets illegible.
The distance covered (expressed in centimeter) gives the subject's accommodative course (expressed in accommodative diopter), since the focal power expressed in diopter is reversely proportional to the focal measured distance. The maximal accommodation will be the opposite of the measured distance.
The other way round works as well: “the accommodative blur technique”. The reading distance is stable; we have the subject's dioptric power vary, along the same lines as the evaluation of penalization's maximum tolerated.22
The text is 33 cm or 40 cm away, two possibilities:
  • The subject can read the text: we add negative lenses until blur, and we note down the value of the last additional sphere diopter with which the subject could read.
  • The subject cannot read the text and it is too blurred: we add positive power lenses until the subject can make the text out, and we note down the value of the additional sphere diopter enabling the reading.
It will give the maximal accommodation:
  • from 33 cm by: maximal accommodation = 3.00 diopters,
  • from 40 cm by: maximal accommodation = 2.50 diopters.
 
1.4.3.2 Duochrome Test (Fig. 1.8)
zoom view
Fig. 1.8: Duochrome test (red/green) in near vision
If, without optical compensation, the reader:
  • sees in the red part more clearly, it means the retina optical image of the test is focused forward,
  • if he sees in the green part more clearly, it means the optical image of the test is focused at the back of the retina.
With its optical compensation, suitable for near vision, the red area should not stand out more than the green one.
The addition, thus, determined should provide the patient with clear and comfortable vision at usual reading distance.23
zoom view
Fig. 1.9: Red-green test with equal contrast
Duochrome test lies in the optical principle of differential refraction or eye's longitudinal chromatic aberration. Chromatic differential refraction lays through a higher dioptric convergence for the shortest wavelength monochromatic lights (red) and a lower dioptric convergence for green lights (Fig. 1.9).
Consequently, a subject with balanced near-vision correction must see as clearly in the red part of the duochrome optotype, as in the green one. This test allows to screen and rebalance the optical sub or under corrections in near vision.
A typical answer about the green part translates an accommodative delay for a non-presbyope: about blacker optotypes, more contrasted, and clearer in the green. The accommodative delay for the young subject is of roughly 0.25 to 1.00 diopter.
By “accommodative delay”, we mean the fact that the focus is done at the background of the test. The “young” subject is supposed to put at stake the minimal accommodation that allows him to recognize the characters that he is reading. So the smaller these characters are, the lower the accommodative delay is.24
A contrary carry, an “accommodative excess” shows in a focus at the foreground of the test. The effort at stake is then inappropriate. When an accommodative excess is shown, we have to:
  • determine the cause,
  • know if it provokes any trouble,
  • and, if needed, to solve it.
 
1.4.3.3 Jackson's Cross-cylinder Test (Fig. 1.10)
zoom view
Fig. 1.10: Jackson's cross-cylinder test
Jackson's cross-cylinder is a visual test based upon the principle that, in the case of an eye with induced astigmatism, the vertical and horizontal lines are clear only at the level of the circle of least confusion. We will review this notion with the study of astigmatic presbyopia (See also paragraph 2.4.2, Circle of least confusion, page 64).
An accommodative delay for non-presbyope shows in an expected answer about the horizontals: said to be blacker, more contrasted, and clearer than the vertical ones. The accommodative expected delay for the young subject is of 0.25 to 1.00 diopter (Fig. 1.11).25
zoom view
Fig. 1.11: Jackson's cross-cylinder test with equal contrast
A fixed cross-cylinder of +0.50(–1.00)90° is located forward in the ocular couple. If, without optical compensation, the reader:
  • sees the verticals more clearly, it means the optical image of the test is focused forward in the retina,
  • if he sees the horizontals more clearly, the test image is then focused at the back of the retina.
With its optical compensation suitable for near vision, the subject should make out the horizontals as contrasted as the verticals.
The addition thus determined is the one that will enable the patient to benefit from a clear and comfortable vision at usual reading distance.
 
1.4.4 Other Considerations
Is there any relationship between the far best visual acuity and the near best visual acuity? We have to know that these two visual acuities do not appeal to exactly the same anatomical structures. Indeed, if ocular discrimination power (the smallest discontinuity the eye can see) is a given subject's constant, at a given 26time, the patient uses different visual strategies in distance vision (scan) and in near vision (accommodation and microfluctuations so the image grows, pursuits, saccades), which prevents from establishing any equivalence between far 20/20 and near 20/20.
The optical systems can increase the size of the pictures in near vision (possibility of foveolar excentration according to the preferential loci), whereas these systems become inoperative in distance (low visions in age-related macular degeneration).
More recently, other methods of accommodation measurement have been the subject of research, especially in the United States, with the introduction of the wavefront technology; we will come back to it (See also paragraph 2.5.6.1, Aberrometry, page 69).
However, the only reliable and objective method of accommodation measurement, defended by Adrian Glasser, is the refractometry compared before and after instilling pilocarpine 3%, possibly instilled with phenylephrine to limit the miosis, pharmacologically induced, which disturbs measures (See paragraph 1.3.2, Miosis, page 13).
Normal human accommodation naturally shows microfluctuations all along the day and maybe even during sleep and certain of its phases (Rapid Eye Movement). The dominant role of the ciliary muscle during the accommodation accounts for these variations. The ciliary muscle shows electrophysiological resting potentials responsible for a kind of unceasing dynamic balance, similar to the muscular microcontractions needed for the adaptation of well-balanced postural system during static tests.
 
1.4.5 Accommodative Microfluctuations
It is useless to remind human eye is able to stare objects with great accuracy at different distances.
Actually, it has been proven that the eye stares slightly too close at far objects and slightly too far at the near ones.
The tension grade of the ciliary muscle, applied to increase lens refractive power when the subject fixes his gaze on a near point, varies a lot from an individual to the other.
The muscular charge causing fluctuations varies in accordance with the strength applied as well as the subject's vitality or state of tiredness.27
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Fig. 1.12: Diagram of accommodative fluctuations in different clinical cases according to the target distance
If we analyze the tension degree of the ciliary muscle in emmetropic and non-presbyopic subject, eye refraction gradually increases as the visual targets gets closer (Fig. 1.12).
For presbyopic patients, at a distance of around 2 meters, there is some tension in the ciliary muscle but at a closer distance, it is not possible to focus anymore and the ciliary muscle does not work actively.
For the patients who have accommodotonia, focusing has an extent in accordance with the distance of the visual target; but for all distance, ciliary muscle exerts a strong tension.
In case of accommodation spasms (spastic accommodation in Figure 1.12), there is strong eye myopization and continuous tension in the ciliary muscle, without relation with the place of the visual target.
In case of technostress ophthalmopathy (Computer Vision Syndrome), the muscular tension of the ciliary muscle reacting to a distant visual target is not as important, but when the subject tries to look at a near visual target, he finds himself in pseudo-spasmodic state of accommodation.28
 
1.5 ACCOMMODATIVE ETIOPATHOGENESIS
What are the other anatomical structures implied in accommodation, then?
To answer this delicate question, it is necessary to enter the controversial debate of accommodation mechanism and its relationships with presbyopia.
 
1.5.1 History
Historically speaking, and this is not always well known, it is Johannes Mueller who, in 1854 was the first to identify the ciliary body circular muscle and to theorize that its contraction produced an anterior movement of the vitreous and secondarily anterior crystalline lens displacement with increase of the refractive power.
While everyone had accepted this theory, Frans Donders himself engraved it as a dogma in his 1864 work on ophthalmology.
It was not before 1924 that Lindsay Johnson recalled Helmholtz’ theory into question. He argued about how unacceptable the theory was, as far as the muscular tension is concerned, since it is not usually occurring during relaxation.
As an alternative, Johnson said the change in lens curvature involved fluids compression in the circumlenticular space (See also paragraph 4.3.3, Ciliary exploration, page 161) during accommodation, with anterior lenticular movement and curving of the crystalline lens anterior side.
Coleman approved this theory inspired by fluids mechanism 50 years later.
Later, Coleman defended his catenary model to explain the accommodation mechanisms, by analogy with the catenary physical problem, in which a supple rope, uniformly dense and inextensible, follows the curve freely hanging by its extremities.
In 1986, he proposed to associate this curve with the anterior surface of the crystalline lens, and to liken the ciliary bodies to the pylons of a suspension bridge.
From this modelization of Coleman's catenary theory, the lenticulo-zonular complex and the anterior vitreous make up a diaphragm separating the eye anterior and posterior segments.
During the accommodation, the contraction of the ciliary muscle pushes the whole diaphragm forward in a piston moving, creating a pressure gradient between the eye two segments and leading to an anterior swelling of the crystalline lens and a flattening of its posterior face (Fig. 1.13).
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Fig. 1.13: Anterior swelling of crystalline lens and posterior flattening (leaders) in a piston movement (arrow) according to Coleman's catenary accommodation theory
30
 
1.5.2 Experimental Modelization
Experimentally, electric stimulation of a 10 volt bipolar current to a mammalian animal comes with a pressure rise of 20 cm of water (H2O) in the anterior chamber, of 2 cm H2O in the vitreous cavity.
This represents a pressure gradient of 5 cm H2O between both segments, taking into account the initial pressures corrected from the differential viscosities measured in each of the compartments.
The facts pleading for this catenary theory:
  • the relatively non-stretch lenticular capsule,
  • the presence of the vitreous to mold the crystalline lens,
  • the presence of collagenous and zonular structure in the peripheral anterior vitreous, not generating equatorial forces, but tangential tractive forces at the level of crystalline lens.
The construction of a mechanical model in accordance with the catenary theory demonstrates how, during the accommodation, the crystalline lens can undergo so quick, reproducible and accurate morphological changes.
Back in the nineties, Schachar reopened controversy about accommodation mechanisms with an adaptation of Tscherning's theory.
In Schachar's opinion, with aging, the progressive loss of tension within the zonule would be related to a lifelong increasing equatorial lens. Having an ectodermic origin, this increase is making the zonular fibers unable to stretch the crystalline lens in its adequate shape for accommodation.
Beyond the interest of this theory, which thwarts received ideas and calls the dogmatic theories about accommodation into question, Schachar's initiative has enabled experiments, launched with modern tools to confirm or refute each hypothesis.31
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Fig. 1.14: Lenticular swelling (leaders) during accommodation (arrows) according to Schachar's theory
According to Schachar's capsular theory, the stretching of an elastic capsule comes with a mobilization of the central lenticular mass that is swelling up (Fig. 1.14).
Coleman maintains the lens capsule does not own the required biomechanical characteristics allowing the crystalline lens, not only to swell in the middle, but also to make its anterior crystalloid draw a parabolic curve while the posterior crystalloid remains unchanged.
It is then hard to believe that human beings and primates can see clearly, and within a few milliseconds, with such an elastic system.
We have underlined other breaches of the capsular theory, especially the exclusion of the vitreous role as being part of the accommodative process, and the inability to explain the translational lens moving that develops a lateral vectorial force exerted through the zonule.
Coleman cannot believe that zonule may directly exert a lateral force vector upon lens, and that ciliary muscle may show insufficient anatomical stiffness, just as the zonular bounds solidity on lenticular equator, to stretch the crystalline lens efficiently through equator.32
 
1.5.3 Clinical Approach
Moreover, we will see this further (See also paragraph 4.4, Accommodative experimental surgery, page 172), should the lenticular equatorial diameter increase with aging, everything that can restore the zonular tension by increasing the working distance ought to also restore accommodation (supposing that, meanwhile, the crystalline lens has not become too sclerosed).
Conversely, as some defend it, if a hardening of the crystalline lens were the main cause of presbyopia, there would be no reason for the experiments relating to the increase in circumlenticular space to be conclusive.
If these experiments were relevant, then Helmholtz would have been mistaken about the causes of presbyopia, and by extension, about the accommodation mechanism. Besides, we will see it in the therapeutic chapter (See also chapter 4, Presbyopia and accommodative restoration, page 136), although in an inconstant and transient manner, all the ciliary or scleral expansion surgeries have still demonstrated a subjective accommodative gain.
We have then attempted to rationalize the anatomical changes during accommodation.
Medical echographic devices use ultrasonic technology applied on the organic tissues. Ultrasounds (US) easily the soft tissues but not the aerial cavities, diffuse through which represent real obstacles for US. Melanic pigmentation or body liquids do not stop them, and their tissular penetration depends on their initial energy.
Then, in a secondary step, after the anterior translation of the crystalline lens during accommodation, Mode A unidimensional echography shows a flattening of the anterior chamber, the ocular depth going from 4.09 mm in distance vision to 3.49 mm in near vision.
However, the difficulty to visualize the ciliary body and the zonular fibers, hidden behind the iris, causes many experimental biases.
Ocular albinism and aniridia clinical models, in which we can observe the entire crystalline lens by retroillumination, both contribute to Helmholtz’ theory. In 1937, Fincham led studies on aniridic patients, which report a decrease of the lens equatorial diameter of about 6 to 7%, between accommodated and non-accommodated states.
The ocular high-resolution magnetic resonance imaging (HR MRI) allows a total visualization of the living. We can visualize:
  • the iris,
  • the ciliary muscle and processes,
  • the crystalline lens, and
  • the geometrical relationships between these different structures with no interference in iris pigmentation.
Another advantage of this technology is that it offers unequalled contrast within soft tissues, a contrast we can use to improve the observation of the tissular modifications.34
With this technique, the human in vivo study on healthy volunteers aged between 22 and 91 enables, in binocular vision, to compare the accommodative performances in the physiological conditions (setting of a target at varying distance) with the pharmacologically-induced accommodation.
Some recent studies using ocular HR MRI on the rhesus monkey (Glasser) and on man (Strenk) show, unambiguously, that the crystalline lens becomes rounded during accommodation and that the equatorial diameter decreases of around 7%.
These data are incompatible with the model proposed by Schachar who states there is an increase in the equatorial diameter during the accommodation, under the effect of a greater tension that the zonule applies (See paragraph 1.5.2, Experimental modelization, page 30). Judging by the results of these investigations, the accommodation Helmholtz’ theory thus appears to be true.
 
1.5.4 Accommodative Structures
The crystalline lens morphological changes, which lead to presbyopia, are less clear among the scientist community.
Another evidence of the continuous growth of the crystalline lens comes from the hydrated lens weight measures revealed by the human eye bank.
It is interesting to note that, at least for the human being, only the anterior part of the crystalline lens keeps growing, the equator remaining stable with aging, just as the posterior surface. However, putting the lens in the eye posterior part, this is not valid for the rhesus monkey whose lens grows in both the anterior and the posterior parts.
The growth of lens in its anterior portion results in more sphericity with aging. This leads for ciliary bodies to move forward, and then limits the possibility of a complementary anterior movement during accommodation.
Although in MRI the lens equatorial diameter does not change with aging, the ciliary muscular ring obviously decreases, what lowers the zonular tension down to zero in non-accommodated state.
The ciliary muscular ring moving inward under the effect of the zonule seems more plausible.
Nevertheless, MRI claims that this ciliary displacement does not come with any amyotrophy since the contractile activity persists whatever the age.
As the lens grows, the axial thickness increasing, the zonular fibers seem to slacken, without the crystalline lens becoming loose or unstable with aging.
Effectively, an unstable lens would imply accommodation microfluctuations for the older, which is not the case.
The data given by MRI show the uveal tract solidarily reacts as an answer to the growing lenticular thickness in presbyopia.
While we admit the lens hardness increases with aging, the data of a study led by Weeber show there is an exponential increase of the lenticular resistivity with aging that could accompany the accommodation linear decrease.
This model comes from the data of dynamic mechanical analysis made to measure the lens total resistivity, as well as its segmentary resistivity, on subjects recently deceased and aged between 18 and 90.
The results show a change in lens global resistivity and inside, the appearance of a gradient of resistivity with aging.
While putting that crystalline lens is a homogeneous structure and only by keeping the global resistivity, accommodation should linearly decrease with aging at around 1 diopter per decade.
However, if we modelize the lens as a heterogeneous structure with the appearance of a resistivity gradient with aging, we obtain an accommodation change curve according to the age in the shape of a sigmoid (Fig. 1.15).36
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Fig. 1.15: Accommodation change curve according to age
Firstly, this curve shows an initial slow decline of the accommodation amplitude between 30 and 40, secondly, a quicker drop during the next decade, and finally, a progressive decline between 50 and 60, until reaching another steady state.
We can use this new discovery about lens differential resistivity changes to explain why the aging crystalline lens loses its capacity to change its shape during accommodation.
The resistivity differentials can also permit to modelize a referential frame of the accommodative amplitude changes according to the age, as close as possible to clinical data that we have noted during the progressive settlement of presbyopia.
In order to determine the segmentary resistivity, the investigators have cut the lenses along the equatorial plane and have proceeded with localized microprobes. The measures they obtain have integrated the mathematical Shear Modulus (modulus of rigidity) and Young's Modulus (modulus of elasticity), then enabling to show the exponential increase of the resistivity with aging.37
The results of the segmentary resistivities study point that both the nucleus and the cortex become more resistant with aging.
Nonetheless, the magnitude of change differs for these two structures and is clearly greater for the nucleus than for the cortex.
In all the samples of crystalline lens measured, the nuclear resistivity grows by a factor of 10,000 where cortical resistivity only increases by 100.
The crystalline lens nucleus seems tenfold softer than the cortex in a young lens, with a resistivity relatively uniform around 50 years old, while the nucleus has a resistivity 200 times greater at the level of the cortex in older samples.
The mechanical model of finite elements used to describe the accommodation amplitude takes into account:
  • the crystalline lens,
  • the capsular bag and
  • a part of the zonular apparatus.
On different studies of the human myopia pathogenesis, some authors studied the ocular biometrical changes during the accommodation.
The ocular axial length increases by 60 μm with 3 diopters of accommodation, and by around 120 μm for 8 diopters.
An eyeball axial extension of 12.7 μm added with 5.1 accommodation diopters tallies with a change of the total power of the eye of −0.036 diopters.
 
1.5.5 Pseudo-accommodation
After this presentation of the actual debate about accommodation and the origin of presbyopia, it is essential to distinguish accommodation from pseudo-accommodation.
Effectively, understanding the difference between these two entities is paramount to deal with ametropic presbyopia, as well as assessing the success of therapeutic approaches of presbyopia.38
If we take them separately, each of these elements produce a partial accommodation, but not a pseudo-accommodation.
Optically speaking we usually consider cornea as neutral during accommodation.
Nevertheless, some authors, by the observation of a corneal accommodation in chicken, have called into question the role of the cornea during accommodation.
Since cornea has a major refractive role in the eye (See paragraph 1.2.2, Various enlarger systems in vertebrates, page 4), subtle changes in the power of the cornea center during the accommodation can be sufficient to play a part in modifying the ocular total optical power.
If we could demonstrate that the cornea played a part in the accommodative process, we should thus exclude this anatomical element from the pseudo-accommodation field.
According to Yasuda, there are many modifications of the corneal curvature, which videokeratotopo-graphy confirmed, during accommodation.
Is this about a passive training of the sclerocorneal limbus (anatomically linked with the ciliary muscle via the scleral spur and the trabecular filter)? (See also Fig. 4.25).
Is it about an indirect effect linked with the pressural modifications or with a cyclotorsion effect during accommodation?39
The corneal curvature increases with the ciliary muscle contraction, with, therefore, an increase of the refractive power between 0.60 and 0.72 diopter during accommodation.
The instilling of high-dose pilocarpine (4%) to induce accommodation pharmacologically comes with topographic curvature change, with no cyclotorsion, in the first 30 minutes while we have not even measured the pressural effect.
 
1.5.6 Compared Imaging of Accommodation
The compared physiology with modern imaging techniques permitted to better apprehend the accommodative phenomena peculiar to humans.
Because of the iris hampering all direct observation of the ciliary muscle in intact human's eye, many attempts to study the ciliary muscle accommodative behavior are based on in vivo and in vitro animal's data (especially in rhesus monkey) and upon in vitro human studies.
In primates (monkeys, humans) accommodative systems, happen to be similar on many points, but very dissimilar as for the accommodative loss with aging.
Inter-species differences do exist in crystalline lens development and its physiological growth, which leads to significant differences in geometrical interconnections between:
  • the ciliary muscle and
  • the crystalline lens and the corresponding vectorial forces implied in the accommodative mechanisms.
Even in the aging of the ciliary muscle remain inter species differences, from what the probable differences of development of presbyopia in rhesus monkey and human.
For example, the human ciliary muscle contraction does not decrease with aging (See paragraph 1.5.4, Accommodative structures, page 34), as opposed to the monkey's ciliary muscle that shows a reduction of the contractile answer with aging, certainly due to choroidal modifications (this contraction restores after it frees from its posterior choroid ties).
Moreover, while the human ciliary muscle apex makes an anterior displacement with aging, the rhesus’ ciliary muscle remains at an obvious posterior location in the eye.40
With accommodation, monkey's ciliary muscle simultaneously makes anterior and inward displacements, whereas the human's one only moves inward during accommodation.
Finally, the studies in microscopy indicate the aging human ciliary muscle develops more connective tissue—and in more muscle anatomical areas—than the monkey.
The fact that the human ciliary muscle contraction does not decrease with aging, remains true for the phakic or posterior chamber pseudophakic subject, at advanced stage of presbyopia.
Though the presbyope's ciliary muscle contracts as much as in young, we notice a significant decrease of the ciliary muscular ring diameter in phakic or pseudophakic presbyope.
To the most advanced stage of presbyopia, the zonular tension can reach zero in non-accommodated state, what shows through a muscular ciliary contraction unable to diminishing a ciliary tension already nil.
As we will see this (See also paragraph 3.3.9, Lenticular surgeries, page 109), it goes the same for the movements observed in the pseudophakic implanted patients with so-called accommodative artificial crystalline lenses where the ciliary muscle and choroid-dependent strengths are efficient, even in advanced presbyopia.
 
1.5.7 Lens Equator Growth and Accommodative Amplitude
Between 18 and 50 years old, the accommodation amplitude invariably decreases at a near linear speed of 0.3 diopters a year.41
However, during childhood, the accommodative amplitude decreases much quicker between 5 and 10 years old, at a speed twice greater than in teenagers.
This fast decline in accommodation amplitude during childhood is a track to understand the fall of accommodative amplitude with aging.
We cannot explain the rapid drop of accommodation amplitude during childhood by a structural modification of:
  • the cornea,
  • the axial length, even if the measures by OCT show a change of up to 12 μm during accommodation,
  • the ciliary musculature and the zonule, and
  • the neurosensory innervation.
We generally propose that a modification in hardness and resistivity of the lens nucleus might be responsible for presbyopia. Nonetheless, neither lens hardness nor resistivity undergoes any change during childhood (See paragraph 1.5.4, Accommodative structures, page 34).
Actually, with the use of approved parallel-plate rheometer to measure the accurate viscoelastical figures, we demonstrated that the hardness and/or resistivity of fresh human lens nuclei, obtained from volunteer and deceased donors aged less than 40, were not correlated with aging.
Moreover, the clinical observation corroborated these experimental data, as long as the lens optical density (See also Fig. 3.5) remained unchanged until 40 years old and the lens natural yellowing is not correlated with the accommodative amplitude.
Nevertheless, the lens capsule regularly thickens and hardens during the three first decades in life, whereas it modifies very little during childhood.
The increase in capsular hardness and resistivity happens to increase indeed the efficiency of the zonular tension, which does not constitute any element to explain the drop in accommodative amplitude.
If we now pay attention to the lens central thickness, it decreases during the first two decades of life and slowly increases shortly thereafter: in fact, regarding this biphasic evolution, the lenticular nucleus responsibility for the decline in accommodative amplitude with aging is unlikely.
As all the ectodermal tissues do, the whole lenticular stroma grows all along its life.
The lens equatorial diameter grows following a logarithmic model (Fig. 1.16):
  • quickly during the first two decades of life,
  • and then slowly after 30.
Zonule connects the lens equator with the ciliary muscle. There is no argument for a modification with aging. Therefore, as the lens equatorial diameter increases with aging, there is a decrease with aging in the basic length of ciliary muscle.
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Fig. 1.16: Increase in lens equatorial diameter and in decline of accommodative amplitude depending on age
43
Since all the muscles follow a rule between length and tension, and as the basic length decreases, the maximum amount of force the ciliary muscle displays tends to go down and cause an accommodative amplitude loss with aging.
Since the lens equatorial diameter increases rapidly during childhood, the basic length decreases quickly at that time, this period being responsible for the fast decline in accommodation all childhood long.
After the two first decades of life, the lens equatorial diameter increases slowly, so much that, between 18 and 50 years old, the basic length of the ciliary muscle decreases linearly, just as the decline in accommodation amplitude.
 
1.5.8 Decrease of Accommodative Amplitude on Intraocular Pressure (IOP)
Equatorial lenticular growth follows a logarithmic curve (See paragraph 1.5.7, Lens equator growth and accommodative amplitude, page 40) that also forecast the growth profile of intraocular pressure (IOP) with aging.
Scleral spur (See also Fig. 4.25) links ciliary muscle and trabecular network together. An increase in the ciliary muscular tonus extends the flow of aqueous humor leading to a fall in IOP.
Thereby, the lens equatorial growth brings about changes in the basic length of the muscle ciliary that are likely to affect the basic IOP.
For the lens equatorial diameter increases in accordance with a logarithmic progress, IOP should also increase logarithmically.
Indeed, IOP logarithmically increases with aging, rapidly during childhood, then slowly.
Actually, the outcome of IOP exactly matches the logarithmic increase of the lens equatorial growth.
Besides, with the strength of those considerations that permit to better understand the actual relationships between accommodation and presbyopia, it is about time to leave emmetropic accommodative physiology for the place of presbyopia in various ocular pathologies disorders and in refractive (spherocylindrical ametropias) (See also chapter 2, Presbyopia and ametropias, page 44).