The combination of Ignacio Barraquer work and the introduction of the excimer lasers lead to a new surgical technique. Ioannis Pallikaris, MD coined the term LASIK (laser-assisted in situ keratomileusis), and was the first to create a “flap” of tissue with the microkeratome, rather than remove the entire top layer (Figure 1.1). He conducted the first animal trials of what is now modern LASIK in the late 1980s in his native Greece. Today LASIK is the dominant corneal refractive technique used to correct ammetropias around the world. Improvements in the creation of the corneal flap continue with improved 3keratome reliability. Today's microkeratomes have the ability to create 90 µm thick flaps with a high degree of repeatability and safety. Flap creation has also improved after the introduction of femtosecond laser technology.
Keratometry and corneal topography with placido disks systems were originally invented to measure anterior corneal curvature. Computer analysis of the more complete data acquired by the latter has in recent years has been increasingly more valuable in the practice of refractive surgery. The problem in the placido disk systems is that one cannot perform a slit scan topography of the cornea. This has been solved by an instrument called the Orbscan that combines both slit scan and placido images 5to give a very good composite picture for topographic analysis. Bausch and Lomb manufacture this.
Orbscan measure's elevation, which is not possible in other topographic machines. Elevation is especially important because it is the only complete scalar measure of surface shape. Both slope and curvature can be mathematically derived from a single elevation map, but the converse is not necessarily true. As both slope and curvature have different values in different directions, neither can be completely represented by a single map of the surface. Thus, when characterizing the surface of non-spherical test objects used to verify instrument accuracy, elevation is always the gold standard.
Curvature maps in corneal topography (usually misnamed as power or dioptric maps) only display curvature measured in radial directions from the map center. Such a presentation is not shift-invariant, which means its values and topography change as the center of the map is shifted. In contrast, elevation is shift-invariant. An object shifted with respect to the map center is just shifted in its elevation map. In a meridional curvature view it is also described. This makes elevation maps more intuitively understood, making diagnosis easier.6
To summarize:
- Curvature is not relevant in raytrace optics.
- Elevation is complete and can be used to derive surface curvature and slope.
- Elevation is the standard measure of surface shape.
- Elevation is easy to understand.
The problem we face is that there is a cost in converting elevation to curvature (or slope) and vice versa. To go from elevation to curvature requires mathematical differentiation, which accentuates the high spatial frequency components of the elevation function. As a result, random measurement error or noise in an elevation measurement is significantly multiplied in the curvature result. The inverse operation, mathematical integration used to convert curvature to elevation, accentuates low-frequency error. The Orbscan helps in good mathematical integration. This makes it easy for the ophthalmologist to understand as the machine does all the conversion.
The general quad map in the Orbscan of a normal eye (Figure 1.2) shows four pictures. The upper left is the anterior float, which is the topography of the anterior surface of the cornea. The upper right shows the posterior float, which is the topography of the posterior surface of the cornea. The lower left map shows the keratometric 7pattern and the lower right map shows the pachymetry (thickness of the cornea). The Orbscan is a three-dimensional slit scan topographic machine. If we were doing topography with a machine, which does not have slit scan imaging facility, we would not be able to see the topography of the posterior surface of the cornea. Now, if the patient had an abnormality in the posterior surface of the cornea, for example as in primary posterior corneal elevation this would not be diagnosed. Then if we perform LASIK on such a patient we would create an iatrogenic keratectasia. The Orbscan helps us to detect the abnormalities on the posterior surface of the cornea.
Another facility, which we can move onto once we have the general quad map, is to put on the normal band scale filter (Figure 1.3). If we are in suspicion of any abnormality in the general quad map then we put on the normal band scale filter. This highlights the abnormal areas in the cornea in orange to red colors. The normal areas are all shown in green. This is very helpful in generalized screening in pre-operative examination of a LASIK patient.
Figure 1.4: General quad map of a primary posterior corneal elevation. Notice the upper right map has an abnormality, whereas the upper left map is normal. This shows the anterior surface of the cornea is normal and the problem is in the posterior surface of the cornea.
Let us now understand this better in a case of a primary posterior corneal elevation. If we see the General quad map of a primary posterior corneal elevation (Figure 1.4) we will see the upper left map is normal. The upper right map shows abnormality highlighted in red. This indicates the abnormality in the posterior surface of the cornea. 10The lower left keratometric map is normal and if we see the lower right map, which is the pachymetry map one will see slightly, thin cornea of 505 microns but still one cannot diagnose the primary posterior corneal elevation only from this reading. Thus, we can understand that if not for the upper right map, which denotes the posterior surface of the cornea, one would miss this condition. The Orbscan can only diagnose this.
Figure 1.5: Quad map of a primary posterior corneal elevation with the normal band scale filter on. This shows the abnormal areas in red and the normal areas are all green. Notice the abnormality in the upper right map
Now, we can put on the normal band scale filter on (Figure 1.5) and this will highlight the abnormal areas in red. Notice in Figure 1.5 the upper right map shows a lot of abnormality denoting the primary posterior corneal elevation. One can also take the three-dimensional map of the posterior surface of the cornea and notice the amount of elevation in respect to the normal reference 12sphere shown as a black grid. In a case of a keratoconus all four maps show an abnormality, which confirms the diagnosis. In the Orbscan, the calibrated slit, which falls on the cornea, gives a topographical information, which is captured and analyzed by the video camera. Both slit beam surfaces are determined in camera object space. Object space luminance is determined for each pixel value and framegrabber setting. Forty slit images are acquired in two 0.7-second periods. During acquisition, involuntary saccades typically move the eye by 50 microns. Eye movement is measured from anterior reflections of stationary slit beam and other light sources. Eye tracking data permits saccadic movements to be subtracted form the final topographic surface. Each of the 40 slit images triangulates one slice of ocular surface. Before an interpolating surface is constructed, each slice is registered in accordance with measured eye movement. Distance between data slices averages 250 microns in the coarse scan mode (40 slits limbus to limbus). So Orbscan exam consists of a set of mathematical topographic surfaces (x, y), for the anterior and posterior cornea, anterior iris and lens and backscattering coefficient of layers between the topographic surfaces (and over the pupil). Color contour maps have become a standard method for 13displaying 2-D data in corneal and anterior segment topography. Although there are no universally standardized colors, the spectral direction (from blue to red) is always organized in definite and intuitive way.
Blue = low, level, flat, deep, thick, or aberrated.
Red = high, steep, sharp, shallow, thin, or focused.
Keratoconus is characterized by non-inflammatory stromal thinning and anterior protrusion of the cornea. Keratoconus is a slowly progressive condition often presenting in the teen or early twenties with decreased vision or visual distortion. Family history of keratoconus is seen occasionally. Patients with this disorder are poor candidates for refractive surgery because of the possibility of exacerbating keratectasia. The development of corneal ectasia is a well recognized complication of LASIK and attributed to unrecognized preoperative forme fruste keratoconus.
All eyes to undergo LASIK are examined by Orbscan. Eyes are screened using quad maps (Figure 1.6A) with the normal band (NB) filter turned on. Figure 1.6B shows quad map with normal band scale filter on in the same eye as in Figure 1.6A
Four maps included (a) anterior corneal elevation: NB = ± 25 μ of best-fit sphere. (b) Posterior corneal elevation: NB = ± 25 μ of best-fit sphere. (c) Keratometric mean curvature: NB = 40 to 48 D, K. (d) Corneal thickness (pachymetry): NB = 500 to 600 μ. Map features within normal band are colored green. This effectively filters out variation falling within normal band. When abnormalities are seen on the normal band quad map screening, a standard scale quad map is examined.
For those cases with anterior keratoconus, we also generate three-dimensional views of anterior (Figure 1.7) and posterior corneal elevation. Figure 1.7A shows three-dimensional anterior float. Figure 1.7B shows three-dimensional posterior float. Figure 1.7C shows three-dimensional anterior corneal elevation measured in microns.18
The following parameters are considered to detect anterior keratoconus (a) Radii of anterior and posterior curvature of the cornea, (b) posterior best-fit sphere, (c) difference between the thickest corneal pachymetry value in 7 mm zone and thinnest pachymetry value of the cornea, (d) normal band (NB) scale map, (e) elevation on the anterior float of the cornea, (f) elevation on the posterior float of the cornea, (g) location of the cone on the cornea. On Orbscan analysis in patients with anterior keratoconus the average ratio of radius of the anterior curvature to the posterior curvature of cornea is 1.25 (range 1.21 to 1.38), average posterior best-fit sphere is −56.98 Dsph (range −52.1 Dsph to −64.5), average difference in pachymetry value between thinnest point on the cornea and thickest point in 7 mm zone on the cornea is 172.7 μm (range 117 to 282 μm), average elevation of anterior corneal float is 55.25 μm (range 25 to 103 μm), average elevation of posterior corneal float is 113.6 μm (range 41 to 167 μm).
Figure 1.8: Quad map with normal band scale filter of an eye with primary posterior corneal elevation
The diagnosis of frank keratoconus is a clinical one. Early diagnosis of forme fruste can be difficult on clinical examination alone. Orbscan has become a useful tool for evaluating the disease, and with its advent, abnormalities in posterior corneal surface topography have been identified in keratoconus. Posterior corneal surface data is problematic because it is not a direct measure and there is little published information on normal values for each age group. In the patient with increased posterior corneal elevation in the absence of other changes, it is unknown 20whether this finding represents a manifestation of early keratoconus. The decision to proceed with refractive surgery is therefore more difficult.
One should always use the Orbscan system (Figure 1.8) to evaluate potential LASIK candidates preoperatively to rule out primary posterior corneal elevations. Eyes are screened using quad maps with the normal band (NB) filter turned on. Four maps include (a) anterior corneal elevation: NB = ± 25 μ of best-fit sphere. (b) posterior corneal elvevation: NB = ± 25 μ of best fit sphere. (c) Keratometric mean curvature: NB = 40 to 48 D (d) Corneal thickness (pachymetry): NB = 500 to 600 μ. Map features within normal band are colored green. This effectively filters out variations falling within the normal band. When abnormalities are seen on normal band quad map screening, a standard scale quad map should be examined. For those cases with posterior corneal elevation, three-dimensional views of posterior corneal elevation can also be generated. In all eyes with posterior corneal elevation, the following parameters are generated (a) radii of anterior and posterior curvature of the cornea, (b) posterior best-fit sphere, (c) difference between the corneal pachymetry value in 7 mm zone and thinnest pachymetry value of the cornea.21
e Agarwal criteria to diagnose primary posterior corneal elevation.
- Ratio of the radii of anterior and posterior curvature of the cornea should be more than 1.2. In Figure 1.8 note the radii of the anterior curvature is 7.86 mm and the radii of the posterior curvature is 6.02 mm. The ratio is 1.3.
- Posterior best-fit sphere should be more than 52 D. In Figure 1.8 note the posterior best-fit sphere is 56.1 D.
- Difference between the thickest and thinnest corneal pachymetry value in the 7 mm zone should be more than 100 microns. The thickest pachymetry value as seen in Figure 1.2 is 651 microns and the thinnest value is 409 microns. The difference is 242 microns.
- The thinnest point on the cornea should correspond with the highest point of elevation of the posterior corneal surface. The thinnest point as seen in Figure 1.8 bottom right picture is seen as a cross. This point or cursor corresponds to the same cross or cursor in Figure 1.8 top right picture which indicates the highest point of elevation on the posterior cornea.
- Elevation of the posterior corneal surface should be more than 45 microns above the posterior best fit sphere. In Figure 1.2 you will notice it is 0.062 mm or 62 microns.
Figures 1.9A to D: Three-dimensional normal band scale map In the top right note the red areas which shows the elevation on the posterior cornea. The anterior cornea is normal
In the light of the fact that keratoconus may have posterior corneal elevation as the earliest manifestation (Figure 1.9A), preoperative analysis of posterior corneal curvature to detect a posterior corneal bulge is important to avoid post-LASIK keratectasia. The rate of progression of posterior corneal elevation to frank keratoconus is unknown. It is also difficult to specify that exact amount of posterior corneal elevation beyond which it may be unsafe to carry out LASIK. Atypical elevation in the posterior corneal map more than 45 μm should alert us against a post-LASIK surprise. Orbscan provides reliable, reproducible data of the posterior corneal surface and all LASIK candidates must be evaluated by this method preoperatively to detect an “early keratoconus”. Elevation is not measured directly by placido based topographers, but certain assumptions allow the construction of elevation maps. Elevation of a point on the corneal surface displays the height of the point on the corneal surface relative to a spherical reference surface. Reference surface is chosen to be a sphere. Best mathematical approximation of the actual corneal surface called best-fit sphere is calculated. 25One of the criteria for defining forme fruste keratoconus is a posterior best fit sphere of >55.0 D. Figure 1.9B shows three-dimensional anterior float. Notice it is normal. Figure 1.9C shows three-dimensional posterior float. Notice in this there is marked elevation as seen in the red areas. Figure 1.9D shows three-dimensional posterior corneal elevation measured in microns.
Figure 1.10: A patient with iatrogenic keratectasia after LASIK. Note the upper right hand corner pictures showing the posterior float has thinning and this is also seen in the bottom right picture in which pachymetry reading is 329
Iatrogenic keratectasia may be seen in some patients following ablative refractive surgery (Figure 1.10). The anterior cornea is composed of alternating collagen fibrils and has a more complicated interwoven structure than the deeper stroma and it acts as the major stress-bearing layer. The flap used for LASIK is made in this layer and thus results in a weakening of that strongest layer of the cornea which contributes maximum to the biomechanical stability of the cornea.27
The residual bed thickness (RBT) of the cornea is the crucial factor contributing to the biomechanical stability of the cornea after LASIK. The flap as such does not contribute much after its repositioning to the stromal bed. This is easily seen by the fact that the flap can be easily lifted up even up to 1 year after treatment. The decreased RBT as well as the lamellar cut in the cornea both contribute to the decreased biomechanical stability of the cornea. A reduction in the RBT results in a long-term increase in the surface parallel stress on the cornea. The intraocular pressure (IOP) can cause further forward bowing and thinning of a structurally compromised cornea. Inadvertent excessive eye rubbing, prone position sleeping, and the normal wear and tear of the cornea may also play a role. The RBT should not be less than 250 µm to avoid subsequent iatrogenic keratectasias. Reoperations should be undertaken very carefully in corneae with RBT less than 300 µm. Increasing myopia after every operation is known as “dandelion keratectasia”.
The ablation diameter also plays a very important role in LASIK. Postoperative optical distortions are more common with diameters less than 5.5 mm. Use of larger ablation diameters implies a lesser RBT postoperatively. Considering the formula: Ablation depth [µm] = 1/3. 28(diameter [mm])2 × (intended correction diopters [D])), it becomes clear that to preserve a sufficient bed thickness, the range of myopic correction is limited and the upper limit of possible myopic correction may be around 12 D.
Detection of a mild keratectasia requires knowledge about the posterior curvature of the cornea. Posterior corneal surface topographic changes after LASIK are known. Increased negative keratometric diopters and oblate asphericity of the PCC, which correlate significantly with the intended correction are common after LASIK leading to mild keratectasia. This change in posterior power and the risk of keratectasia was more significant with a RBT of 250 μm or less. The difference in the refractive indices results in a 0.2 D difference at the back surface of the cornea becoming equivalent to a 2.0 D change in the front surface of the cornea. Increase in posterior power and asphericity also correlates with the difference between the intended and achieved correction 3 months after LASIK. This is because factors like drying of the stromal bed may result in an ablation depth more than that intended. Reinstein et al predict that the standard deviation of uncertainty in predicting the RBT preoperatively is around 30 μm. [Invest Ophthalmol Vis Sci 40 (Suppl):S403, 1999]. Age, attempted correction, the 29optical zone diameter and the flap thickness are other parameters that have to be considered to avoid post-LASIK ectasia.
The flap thickness may not be uniform throughout its length. In studies by Seitz et al, it has been shown that the Moris Model One microkeratome and the Supratome cut deeper towards the hinge, whereas the Automated Corneal Shaper and the Hansatome create flaps that are thinner towards the hinge. Thus, accordingly, the area of corneal ectasia may not be in the center but paracentral, especially if it is also associated with decentered ablation. Flap thickness has also been found to vary considerably, even up to 40 μm, under similar conditions and this may also result in a lesser RBT than intended. It is known that corneal ectasias and keratoconus have posterior corneal elevation as the earliest manifestation. The precise course of progression of posterior corneal elevation to frank keratoconus is not known. Hence, it is necessary to study the posterior corneal surface preoperatively in all LASIK candidates.
Figures 1.11A to I: Overview display from a patient with a history of conductive keratoplasty and cataract using the Pentacam(Courtesy-Tracy Swartz)
The Pentacam ocular scanner (Figure 1.11A) is a specialized camera which utilizes Scheimpflug imaging to accomplish with a variety of ophthalmic applications. 35Scheimpflug imaging was patented by Theodor Scheimpflug in 1904 after he diskovered that when the planes within a camera intersect rather than be placed in parallel, the depth of focus is extended. In a typical camera, three imaginary surfaces exist: the film plane, lens plane and sharp image plane. These are parallel to each other such that the image of the object placed in the plane of sharp focus will pass through the lens plane perpendicular to the lens axis, and fall on to the film plane. The depth of focus is limited in such a camera. Figure 1.11B shows a Scheimpflug image of a flap tear. Thinning is seen secondary to loss of tissue where the flap was rotated away from the bed.
In a Scheimpflug camera, the three planes are not parallel but intersect in a line, called the “Scheimpflug line”. When the lens is tilted such that it intersects the film plane, the plane of sharp focus also passes through the Scheimpflug line, extending the depth of focus. Note that this results in mild image distortion, which is then corrected by the Pentacam system. A two-dimensional cross-sectional image results. When performing a scan, two cameras are used to capture the image. One centrally located camera detects pupil size and orientation, and controls fixation. The second rotates 180 degrees to 36capture 25 or 50 images of the anterior segment to the level of the iris, and through the pupil to evaluate the lens. 500 true elevation data points are generated per image to yield up to 25,000 points for each surface. Data points are captured for the center of the cornea, an area that placido disk topographers and slit-scanning devices are unable to evaluate.
Elevation data measured using this technique has several advantages. Because it is independent of axis, orientation and position, it yields a more accurate representation of true corneal shape. Thus, the Pentacam's curvature map, because it is not sensitive to position, is theoretically more accurate. The elevation maps are created using one of three reference bodies: A best fit sphere, an ellipse of revolution, and toric. The best fit sphere calculation approximates the sphere as accurately as possible to the true nature of the cornea. This facilitates comparison between other topographers but is not the best fit for the aspheric cornea. The ellipsoid of revolution is calculated from the keratometry eccentricity and the mean central radius. This reference shape correlates well with the true shape of the normal cornea. The toric is based on the central radii and keratometry eccentricity as well. The flat and steep radii are automatically used. The toric is a good estimation for astigmatic corneas. The toric ellipsoid float display best facilitates pattern recognition of 37abnormalities on the front and back surfaces, such as found in keratoconus.
Figure 1.11C shows refractive display for the patient in Figure 1.11A. It is commonly used when evaluating patients for elective vision correction. Figure 1.11D shows topometric display for the patient in Figure 1.11A. It is most commonly used when fitting contact lenses. Figure 1.11E shows that when considering a patient for refractive surgical correction, look at the relationship between the four maps on the refractive display. This illustrates a suspicious “two point touch” where the posterior elevation corresponds to a mild anterior elevation. This patient had low pachymetry, but the pachymetry map was otherwise normal, symmetrical around the center. Figure 1.11F shows an example of a “three point touch” where the elevation on the posterior and anterior surface corresponds to a steep area on the curvature map. Figure 1.11G is an example of a classic ectasia following excimer ablation for high myopia, where all four maps show characteristic signs of ectasia. Figure 1.11G shows that astigmatism manifests as a “saddle” pattern on the posterior surface. Figure 1.11I shows a pachymetry map of a patient with keratoconus. Note the displacement of the thinnest point, and the overall reduction of corneal thickness.
Figure 1.12: The anterior chamber OCT “Visante™ OCT” developed by Carl Zeiss Meditec(Courtesy: Georges Baikoff)
The equipment (Figure 1.12) uses a 1310 nm wavelength but in its present form, the infrared light is blocked by pigments. However, the non-pigmented opaque structures are permeable and images can be obtained through a cloudy or white cornea, through the conjunctiva and the sclera. Axial resolution is 18 microns and transverse resolution 50 microns. Procedure is non-contact and very easy. Because of its simplicity, a technician can be rapidly 39trained to carry out the examinations. It is possible to chose the axis to be explored or carry out an automatic 360° exploration along the four meridians. There is an optical target that can be focused or defocused with positive or negative lenses. Natural accommodation can be stimulated and anterior segment modifications during accom-modation can be explored in vivo
Until recently, measuring the depth of the anterior chamber and checking the endothelium cell count with a specular microscope were considered sufficient when performing phakic implants. With the development of techniques such as the OCT, surgical indications can be streamlined and a regular check-up of the anterior chamber following such an intervention is mandatory. Figure 1.12, shows a posterior chamber ICL inserted in a patient over the age of 45 having developed cataract and severe optical problems. Although the ICL has been placed in the posterior chamber, on the endothelial safety scale we note that the edges of the optic are approximately 1mm from the endothelium. This distance is insufficient as it has been proved that a minimum safety distance of 1.5 mm is necessary between the edges of the lens’ optic and the endothelium.40
In Figure 1.12B, a pigment dispersion syndrome was observed following insertion of an Artisan hyperopic implant. Compared with a normal anterior segment, the iris is very thin and pigment cysts have developed on the pupil between the implant and the patient's anterior capsule. A convex iris, which is a contraindication for Artisan implants can be evaluated in a very precise way using the crystalline lens rise method (distance from the crystalline lens’ anterior pole to the internal diameter of the irido-corneal angle). When the crystalline lens rise is above 600 microns, the risk of developing pigment dispersion syndrome with a drop in visual acuity is probable in 70% of cases.
Figure 1.13: Concept of eye tracking for more accurate corneal ablations during movements of the eye new eye tracking technology can trace eye movements by detecting displacement of the pupil. In microseconds the eye tracking computer can move the treatment spot of an excimer laser beam appropriately to compensate for these eye movements. For example, laser beam (LA) is treating an area of the cornea when the eye is in position (A). Suddenly, during treatment, the eye moves slightly to the left to position (B). The eye tracking computer detects the movement of the pupil to the left (dotted circle) and commands the laser to track left (LB) the same amount, within microseconds. Thus the laser continues treating the same area of the cornea as desired before the eye movement took place. Such technology aims to increase the accuracy of the desired ablation and resulting correctionCourtesy: Benjamin F. Boyd, MD FACS, Editor-in-Chief “Atlas of Refractive Surgery”–Highlights of Ophthalmology, English Edition, 2000
The size of the entrance pupil (Figure 1.13) we currently see and measure does not correspond to the actual anatomical pupil size, because the optical properties of the cornea magnify and displace it anteriorly, but for clinical purposes we may consider and measure the entrance pupil. There are several methods to measure pupil size. Needless to say, the measurement of pupil necessary for refractive surgical purposes is the scotopic one, as pupil dilation enhances visual symptoms.
- Rulers and reference diameters. This method has been almost abandoned for refractive surgery because of its unreliability and unavailability of measuring pupil sizes at different established light conditions.
- Monocular infrared pupillometers associated with corneal topographers. They provide more reliable and consistent measurements than portable pupillometers, and some of them measure some pupillary dynamic changes with different light conditions.
- Binocular infrared pupillometers. Today these instruments are the most reliable ones to assess pupil size under different, set light conditions. They compensate for theoretical changes in pupil size due to accommodation thanks to a simultaneous measurement for both eyes. Some of them truly provide a dynamic measurement of changes in pupil size related to illumination.
Figure 1.14: Laser In Situ Keratomileusis should not be done in eyes in which the herpes has not been inactive for at least 1 year before(Courtesy Guillermo Simon Castellvi and Pablo Gili).
The correct approach to a patient in seeks for refractive surgery (Figure 1.14) begins with detailed medical history and careful physical and ophthalmologic examination. The medical interview collects information of patient's psychological (e.g. depression, future patient's compliance), emotional (e.g. reasons and motivation for refractive surgery) and medical state (ocular and general complaints, physiologic aspects, past and present diseases, laboratory findings, allergies, medications, etc.) of the patient. In medicine, preventing disease is more important than treating it (“primum non-nocere”), and this first 45interview is essential in screening potentially “dangerous” patients (e.g. Is the medical history significant for AIDS, diabetes or arterial hypertension?) and to improve future patient's compliance by means of building a good patient-doctor relationship.
To be a good candidate for vision correction surgery, patient must meet the physical, health and age criteria for the particular surgery (Laser In Situ Keratomileusis LASIK, Laser Epithelial Keratomileusis LASEK, Photorefractive Keratomileusis PRK, clear lens exchange, epikeratoplasty-epikeratophakia, laser thermal keratoplasty LTK, astigmatic keratotomies, implantable contact lenses-phakic intraocular lenses, conductive keratoplasty CK to treat presbyopia, …).
The refractive candidate must fully understand the procedure and be aware of the risks and possible side effects. Limitations for refractive surgery can be ophthalmologic and general. Medical history is important in estimating patient's suitability for surgery: All refractive procedures have ocular, physical, health and age criteria.
Figure 1.15: Dilated episcleral vessels in St\rge-Weber-Dimitri syndrome (encephalotrigeminal angiomatosis). Stürge-Weber syndrome is a rare neurological disorder present at birth, characterized by a birthmark (usually on the face) known as a port-wine stain caused by an overabundance of capillaries around the trigeminal nerve beneath the surface of the face, and neurologic problems due to loss of nerve cells and calcification of tissue in the cerebral cortex of the brain on the same side of the body as the birthmark (angiomatosis of the central nervous system). Note the large facial port-wine purple stain on the forehead and upper eyelid of one side of the face: When superior lid is affected, ocular complications are probable (e.g. angiomatous glaucoma). Note the angioma and hypertrophia of the ipsilateral lip. We do not perform refractive procedures in such patients: most neurological syndromes present at birth are contraindications for elective refractive procedures.(Courtesy: Guillermo Simon Castellvi)
Most surgical procedures cannot be safely performed if the patient has a history of autoimmune diseases (like 47collagenopathies, rheumatoid arthritis, systemic lupus erythematosous, dermatomyositis, psoriasis, Behet's disease, Crohn's disease, histocytosis or multiple sclerosis). Most autoimmune diseases are listed in the US Food and Drug Administration (FDA) as contraindications for LASIK due to concerns about potentially damaging effect of wound healing. The American Academy of Ophthalmology (AAO), lists in its guidelines relative and absolute contraindications to laser assisted in situ keratomileusis (LASIK) and considers connective tissue or autoimmune diseases and systemic immunosuppression as relative contraindications and only uncontrolled diseases and uncontrolled ocular allergy as absolute contraindications.
Some diseases, like Ehlers-Danlos syndrome (cutis laxa with laxity of joints) still remain absolute contraindications for corneal refractive procedures. While the molecular basis of this syndrome is heterogeneous, there are three fundamental mechanisms of disease known to produce Ehlers-Danlos syndrome: Deficiency of collagen processing enzymes, dominant-negative effects of mutant collagen α-chains, and haploinsufficiency. These mechanisms compromise the strength of the connective tissue complex, and often the collagen fibril itself. This 48abnormal collagen strength contraindicates laser refractive surgery, as the risk of post-surgical ectasia is presumed higher and the risk of devastating intra operative complication like globe rupture is possible. Apart from angioid streaks, strabismus or retinal detachment Ehlers-Danlos patients may present limbus to limbus thin corneas, keratoglobus, keratoconus, cornea plana, and corneal opacities: The cornea is very fragile (fragilitas oculi in Ehlers-Danlos syndrome type VI, or kyphoscoliosis) and the risk of keloid formation is extremely high. Indeed, abnormal bleeding may cause extreme difficulty with any surgical procedure.
Nevertheless, every case has to be considered and evaluated specifically: Careful preoperative evaluation holds the key to identifying appropriate candidates. When cornea is intact, LASIK is the safest refractive technique in risky patients. Avoid PRK and other superficial techniques that suppose a higher degree of inflammation.
There are special considerations for autoimmune disorders or collagen disease patients who undergo cataract, clear lens exchange or refractive surgery (Figure 1.15).
- Consider LASIK as your first refractive option: PRK creates a large epithelial defect that may predispose the cornea to ulceration.
- Profit from periods of calm of the disease, especially in treatment pauses or when the disease is stable.
- Make sure that biologic constants are stable when you perform surgery.
- Carefully check for infectious concomitant diseases.
- Give antiherpetic oral prophylaxis prior to surgery and a few days after surgery (ocular herpes can be devastating in such cases).
- Corneal melting tends to occur mainly in elderly autoimmune patients, and almost exclusively in those with extraarticular disease. LASIK is relatively safe in rheumatoid arthritis patients that only manifest in the joints. Modern immune response modulators such as etanercept (a class of medications called tumor necrosis factor —TNF-inhibitors) may help to stabilize rheumatoid arthritis, thus making viable the practice of a refractive procedure. Etanercept is used alone or in combination with other medications to reduce the pain and swelling associated with rheumatoid arthritis, juvenile rheumatoid arthritis, and psoriatic arthritis.