ABSTRACT
Regional anaesthesia techniques are commonly used in ophthalmic surgery because most patients are elderly and suffer from comorbidities. Regional blocks are performed by injecting local anaesthetic agent in a very small area packed with important structures around the globe either through a needle (intraconal, extraconal) or cannula (sub-Tenon's). For regional blocks to be effective, local anaesthetic agent must find and reach the target motor and sensory nerves. Introduction of a needle is associated with serious sight and life threatening complications and very rarely so with a cannula block. It is important to have a thorough knowledge and understanding of anatomy of the orbit, globe and surrounding structures for safe outcome.
INTRODUCTION
The knowledge of anatomy of the orbit and its surrounding contents is essential to the safe practice of ophthalmic regional anaesthesia. Anatomy relevant to conducting ophthalmic regional anaesthesia and their clinical implications will be included in this chapter. There are several excellent textbooks1,2 and article3 on orbital anatomy and readers are advised to consult these material for further details.
OPHTHALMIC REGIONAL BLOCK TERMINOLOGY
The orbital fat is divided into central intraconal (retrobulbar) external or peripheral extraconal (peribulbar/periconal) compartments by the cone of the recti muscles.4 The cone refers to the “cone- and end-shaped” an arbitrary division formed and bounded by the four rectus muscles, which arise from the Annulus of Zinn at the orbital apex, to their penetration through Tenon's capsule before attaching to the globe. Major nerves and blood vessels enter the cone from behind.
Needle Based Blocks
Terminology used for needle blocks is controversial. Scientific names based on the placement of needle in the anatomical areas are accepted widely.4 If injection of local anaesthetic agent is deposited into the part of the orbital cavity behind the globe inside 4the muscle cone is described as intraconal (retrobulbar) block (Fig. 1). In extraconal block (peribulbar, Fig. 2) the needle tip remains outside the muscle cone and tangential to the globe These terms are used interchangeably. A combined intraconal and extraconal is also known as retro/peribulbar block. Computed tomography (CT) studies after injections of radio contrast material into intraconal (Fig. 3A) and extraconal (Fig. 3B) have demonstrated the existence of multiple communications between these two compartments.5 The injected material diffuses from one to the other compartment thus the extraocular muscles and the septal compartments function as a unit and there are no anatomical discrete divisions.
Classical Retrobulbar Block
Atkinson described the classical retrobulbar block in which patients looked upward and inward.6 A 38 mm long needle is inserted through the skin between the medial 2/3rd and lateral 1/3rd of the inferior orbital margin. The needle was directed towards the apex and 2–3 mL of local anaesthetic injected very close to the optic nerve. A facial nerve block was essential to block the orbicularis oculi muscle and thus prevent squeezing of eyelids.
Figs 3A and B: Injection of blue dye into the intraconal (retrobulbar) and extraconal (peribulpar) spaces. Spread of dye is similar after both injectionsSource: Ripart J in Ophthalmic Anaesthesia, Editors: Kumar CM, Dodds C & Gayer S, Oxford University Press, 2012.
Modern Retrobulbar Block
The globe is kept in a neutral gaze position and a shorter needle <31 mm is inserted as far as possible in the extreme inferonasal quadrant either through the conjunctiva (perconjunctival) or skin (percutaneous). The needle is directed upwards and inwards just to enter the cone but the needle remains tangential to the globe and 4–5 mL of local anaesthetic agent injected.7
Peribulbar Block
The globe is kept in a neutral gaze position and a shorter needle <31 mm is inserted as far as possible in the extreme inferonasal quadrant through the conjunctiva and the needle is not directed upwards and inwards along the inferior orbital floor but remains tangential to the globe.7 Five to six ml of local anaesthetic agent is injected outside the muscle cone.
Medial Peribulbar Block
This block is used mainly as a supplementary injection to inferotemporal intraconal or extraconal blocks and sometimes as a main injection in long eyes.8 The needle is inserted between the carcuncle and the medial canthus to a depth of 1–1.5 cm and 3–5 mL of local anaesthetic is injected.
Cannula-based Block
The local anaesthetic agent is injected through a blunt cannula under the Tenon capsule and this is widely known as sub-Tenon's block.9 Space under the Tenon capsule is usually accessed in the inferonasal but other quadrants such as inferotemporal, superotemporal and medial scleral areas can also be accessed. There are several names of this block such as parabulbar block,10 pinpoint anaesthesia11 and medial episcleral block12 Injection of local anaesthetic agent under the Tenon capsule blocks the sensations of the globe by action on the short ciliary nerves as they pass through the Tenon capsule to the globe and also blocks other sensory nerves. Motor nerves are blocked by diffusion of local anaesthetic agent into the intraconal space. Magnetic resonance imaging13 and ultrasound studies (Fig. 4)14,15 have demonstrated that the injected local anaesthetic diffuses into the intraconal space.
BONY ORBIT
The orbit is an irregular four-sided pyramidal cavity with its apex posteromedially and its base facing anteriorly corresponding to the anterior aperture. The orbit is 40–50 mm deep with a total volume of approximately 30 mL (7 mL of which is occupied by the globe and its muscular cone and the remainder volume is composed of loose connective tissue). There are several foramina and fissures (Fig. 5). The optic foramen lies at the apex which transmits the optic nerve and accompanying vessels. Superior and inferior orbital fissures transmit other nerves and vessels. The base is formed by the hind surface of the globe. The annulus of Zinn is a fibrous ring which arises from the superior orbital fissure.
The axial length of the globe is the distance from the corneal surface to the retina. Axial length in adults measures roughly 25 mm (ranges from 12–35 mm). Myopic globe becomes elongated and sclera becomes thin leading to outpouching (Fig. 6) of sclera (staphyloma).
Fig. 4: Ultrasound image shows the opening of the sub-Tenon's space and the characteristic T-signSource: Kumar CM, Dodds C. Clinic of North America, 2008.
Presence of staphyloma increases the risk of perforation of the globe during needle based blocks.8 If axial length is >26, the incidence of perforation with needle based blocks is increased hence alternative and safer block such as sub-Tenon's block should be considered.16
The orbital axis is not the same as axial length. It is not strictly anteroposterior but slightly oblique laterally and anteriorly with an angle of about 30° (Fig. 7). There are wide variations in orbital dimensions but average anteroposterior length of the orbit is about 40 mm. It is, therefore, possible to reach the apex of the orbit with longer needle and enter one of the foramina of the skull and injection of local anaesthetic agent can enter directly into the cerebrospinal fluid producing features of total spinal anaesthesia. Complications are known to occur even with smaller needle. The angle between the lateral walls of the two orbits is approximately 90° (Fig. 7). The angle between the lateral and medial walls of each orbit is nearly 45°. Thus, the medial walls of the orbit are almost parallel to the sagittal plane (sagittal plane passes directly from front to back of the body). The orbital rim is oblique laterally and inferiorly hence inferolateral (inferotemporal) angle is located near the equator plane. If a needle is inserted in the inferolateral quadrant, the needle may already have passed the equator.
Fig. 7: Horizontal CT scan of the orbit showing its general shape (pyramid) and orientation. Note that the main orbital axis (AO) is not strictly anteroposterior (visual axis, AV) but slightly oblique laterally, with an angle of about 30° between the visual axis and the orbital axis. A CT scan, B pictureSource: Ripart J. In Ophthalmic Anaesthesia, Editors: Kumar CM, Dodds C & Gayer S, Oxford University Press, 2012).
GLOBE
The shape of the globe is not perfectly spherical. The globe (Fig. 8) is situated in the anterior part of the orbital cavity closer to the roof than the floor and nearer the lateral than the medial wall. It may be represented by the juxtaposition of two segments of two different spheres posterior and anterior.7
Fig. 8: Sagittal schematic view of the globe contents with the insertion of rectus musclesSource: Ripart J. In Ophthalmic Anaesthesia, Editors: Kumar CM, Dodds C & Gayer S, Oxford University Press, 2012.
The posterior segment is the main part of the globe which has a diameter of about 23 mm. The part corresponding to the posterior segment is the sclera that gives to the eyeball its typical white aspect. The anterior segment is smaller 1/6th of the globe, transparent, vessel free, and is named the cornea. The junction between anterior and posterior segment is called the limbus (sclerocorneal junction).
The anterior pole of the eye is the centre of curvature of the transparent cornea. The posterior pole is the centre of the posterior curvature of the globe. Posterior pole is located slightly temporal to the optic nerve. The equator of the globe lies midway between the two poles.
The outer layer of the globe is sclera, a fibrous layer completely surrounding the globe except the cornea. It is thick posteriorly (1 mm) and become thinner anteriorly (0.3 mm). Although it is relatively very tough but can be easily perforated by needle. The optic nerve penetrates the sclera posteriorly 1–2 mm medial to and above the posterior pole. The central retinal artery and vein accompany the optic nerve. There are other vessels along outer surface of the sclera especially vortex veins which are known to be traumatised during sub-Tenon's block leading to orbital haemorrhage.17
The intermediate layer is called the uvea and consists of choroid, ciliary body and iris. In the posterior segment, it is constituted by choroid (vascular layer) a thin coating deep to the sclera. It has a rich blood supply and provides nutrients to most of the globe. Its circulation is important in the maintenance of intraocular pressure. Anteriorly, it continues with the ciliary body that secretes the aqueous humour into the posterior chamber. The iris is a diaphragm that separates the anterior from the posterior chamber. It is inserted peripherally on the ciliary body. The hole located in its centre is the pupil.
The inner layer is the retina, which is the sensory layer that allows light perception. The emergence of the optic nerve into the globe is called the optic nerve head (fundus). Near the posterior pole of the globe, lateral to the fundus is macula (vessel free) which is a part of the retina responsible for central vision. The visual part of the retina extends anteriorly approximately to 8the point of insertion of the medial and lateral rectus muscles. It has several layers which remain attached because of negative pressure created by absorption of fluid between layers, presence of viscous mucopolysaccharides and possibly an electrostatic force between two layers. Separation may occur after trauma or degenerative changes. The inner vitreal retina receives its blood supply from central retinal artery and out layer contains no blood vessels but supplied by choroid capillaries derived from ciliary vasculature.
LENS
The lens is a transparent biconvex structure situated in front of the vitreous body and behind the iris and the pupil. The convexity of anterior surface is less than its posterior surface. The centre point of the convexity anteriorly is called anterior pole and posteriorly is called posterior pole. The lens is made of three parts capsule, lens epithelium and lens fibre. The entire lens is enveloped by anterior and posterior capsule. The lens fibres constitute the main mass of the lens.
GLOBE CONTENTS
Globe contents are transparent to allow the light to travel to the retina. Anterior segments have two chambers anterior and posterior. The anterior chamber is a small cavity measuring 3 mm anteroposteriorly in its central portion lying behind the cornea and a small area of the sclera and posteriorly by the anterior surface of the iris, a small area of anterior surface of the lens exposed by the pupil and a part of the ciliary body. It is bounded in front by the cornea. The chamber is filled with 0.2 mL of aqueous humour. The posterior chamber is small 0.06 ml slit-like cavity filled with aqueous humour which communicates through the pupil with the anterior chamber. The aqueous humour is secreted by the ciliary process (part of the ciliary body). It passes through the pupil to reach the anterior chamber where it is then resorbed by the trabeculum in the irido-corneal angle and then evacuated through Schlemm's canal to the episcleral veins. The posterior chamber is bounded by iris anteriorly, posteriorly by the lens and the zonule and peripherally by the ciliary process. The posterior segment is filled with vitreous behind the lens it thus occupies the space between the lens and the retina. Vitreous transmits light and contribute slightly to the dioptric power of the eye. It supports the posterior surface of the lens and assists in holding the neutral part of the retina against the pigmented part of the retina. Vitreous is attached to the retina and can become easily detached. Vitreous, lens, aqueous, and the cornea are devoid of vessel, the drugs given systemically will poorly penetrate them.
TENON'S CAPUSLE
Tenon capsule (fascial sheath) is a fibroelastic layer that surrounds the entire scleral portion of the globe and separates it from the orbital fat. The space underneath the Tenon capsule is sub-Tenon's space, a potential space (Figs 9 and 10) with no actual volume although fluid can be injected into it. The inner surface is smooth and shiny and is separated from the outer surface of the sclera by a potential space called the episcleral space and thus forms a socket for the eyeball. Crossing the space and attaching the fascial sheath to the sclera are numerous delicate bands of connective tissue. Anteriorly the Tenon capsule merges with the conjunctiva before inserting on to the sclera 5–8 mm away from the limbus. The Tenon capsule is not completely sealed anteriorly therefore if volume of local anaesthetic agent is high; will flow toward the lids, thus may block the orbicularis muscle thus prevents from blinking. The tendons of all six extrinsic muscles of the eye pierce the sheath as they pass to their insertion on the eyeball. At the site of perforation, the sheath is reflected along the tendons of these muscles to form a tubular sleeve on each muscle.9
The superior oblique muscle sleeve extends as far as the trochlea, the inferior oblique muscle sleeve extends to the origin of the muscle. The tubular sleeves for the four recti muscles have expansions. Those for medial and lateral recti are strong and are attached to the lacrimal and zygomatic bones and are called medial and lateral check ligaments respectively. Thinner and less distinct expansion extends from the superior rectus tendon to that of levator palpebrae superioris and from the inferior rectus to the inferior tarsal plate. The inferior part of the fascial sheath is thickened and is continuous medially and laterally with the medial and lateral check ligaments. Near the equator, the Tenon capsule is perforated by the tendons of the oblique and rectus muscles before they insert into the sclera. At this point there is continuity between Tenon capsule and the fascial sheath of these muscles. Posteriorly, the sheath fuses with the meninges around the optic nerve and with the sclera around the exit of the optic nerve. Sub-Tenon's block is effective because the local anaesthetic agent spreads toward the intraconal area shown by ultrasounds imaging that the local anaesthetic does not stay more than 2 minutes in the sub-Tenon's space.14 A major textbook of anatomy also suggests that the space under the Tenon capsule is a lymph space and this follows the optic nerve and continues with subarachnoid space.17 This may explain how local anaesthetic injected into the sub-Tenon's space although rarely can reach the central nervous system.18
Fig. 10: Sub-Tenon's space shows multiple connective tissue bandsSource: Gray H. Anatomy of the Human Body. Philadelphia: Lea & Febiger; 1918; Bartleby.com, 2000. Available at www.bartleby.com/107/.)
EXTRAOCULAR MUSCLES
There are four rectus muscles; superior, inferior, medial and lateral as well as two oblique muscles; superior and inferior (Fig. 11). The combined actions of four rectus and two oblique muscles on each eyeball allow elevation, depression, adduction and abduction. Under normal circumstances unmodified activity of one muscle is rare, but testing individual muscle function becomes necessary after local anaesthetic block to identify the unblocked nerve when some movement is still present.
Superior rectus muscle arises from the annulus of Zinn (Fig. 12). It passes forwards and laterally and then inserts into the sclera about 7.7 mm behind the limbus. It moves the eye upward.
The inferior rectus muscle arises from the annulus below the optic foramina and passes forward and laterally then insert into the sclera 6.5 mm from the limbus. It moves the eye downward. The lateral rectus runs from the lateral part of the tendinous annulus and inserts into the sclera about 6.9 mm from the limbus. It moves the eye outward. The medial rectus passes from the medial aspect of annulus along the medial orbital wall and then inserts into the sclera about 5.5 mm from the limbus. The superior oblique is a long and slender muscle arises from the body of the sphenoid bone superomedial to the optic canal. It travels through a small pulley (the trochlea) in the orbit near the nose and then attaches to the top of the eye. The superior oblique rotates the eye inward around the long axis of the eye (front to back). The superior oblique also moves the eye downward. The inferior oblique arises from the front of the orbit near the nose (arising from the floor posterior to the orbital margin and just lateral to the nasolacrimal duct). It then travels laterally, posteriorly and superiorly to pass inferiorly to the inferior rectus muscle to insert into the sclera at the posterolateral aspect of the eyeball. It rotates the eye outward along the long axis of the eye (front to back). The inferior oblique also moves the eye upward.
Fig. 11: Rectus muscles of the eyeSource: Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy, WB Saunders Company, 1994.
NERVE AND VASCULAR SUPPLY
Motor Innervation (Fig. 13)
Oculomotor nerves (III nerve both superior and inferior divisions), abducent nerve 11(VI nerve), nasociliary nerve (a branch of V nerve), ciliary ganglion and vessels lie within the muscle cone. Oculomotor nerve enters the orbit through the superior orbital fissure and splits into two divisions, superior and inferior. The superior division of oculomotor nerve supplies the superior rectus and the levator palpebrae muscles. The inferior division of the oculomotor nerve splits into at least three trunks within the intraconal space, and these in turn divide into eight to ten branches as they course forward, lateral to the optic nerve. They supply medial rectus, inferior rectus, and inferior oblique muscles. The trochlear nerve (IV nerve) supplies motor fibres to the superior oblique muscle. It enters the orbit through the superior oblique fissure above the annulus, along with the frontal and lacrimal branches of the ophthalmic division of the trigeminal nerve. It crosses the superior rectus origin above the levator and enters the superolateral surface of the superior oblique muscle. The abducent nerve (VI nerve) supplies the lateral rectus. The trochlear nerve (IV nerve) runs outside and above the annulus, which supplies the superior oblique muscle and the retained activity of this muscle is frequently observed as anaesthetic agent may not completely block this nerve.
Fig. 12: Annulus of Zinn (Most rectus muscles arise from this place)Source: Dutton JJ, Atlas of Clinical and Surgical Orbital Anatomy, WB Saunders Company, 1994.
Fig. 13: Motor nerves supplying rectus musclesSource: Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy, WB Saunders Company, 1994.
Sensory Innvervation (Fig. 14)
The sensory supply is mainly from the ophthalmic division of V nerve (trigeminal). The V nerve (trigeminal) has three divisions: ophthalmic, maxillary, and mandibular. The ophthalmic division has three components (frontal, lacrimal, and nasociliary).12
Fig. 14: Sensory supply to the orbit is complex. Sensory nerves are present in both intraconal and extraconal areasSource: Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy, WB Saunders Company, 1994.
The frontal nerve branches into two more divisions supraorbital and supratrochlear. These nerves course outside the muscle cone; hence patient may experience pain during surgery if not blocked. The lacrimal nerve carries sensory input from the skin and conjunctiva of the lateral aspect of the upper lid. The nasociliary nerve carries sensory fibres from the cornea, iris, ciliary body, perilimbal bulbar conjunctiva.
Injection of local anaesthetic solution into the lateral adipose compartment from an inferotemporal needle insertion normally blocks the nasociliary, lacrimal, frontal, supraorbital and supratrochlear branches of the ophthalmic division of the trigeminal nerve and the infraorbital branch of the maxillary division. Injection into the medial compartment (medial peribulbar block) usually blocks the medial branches of the nasociliary nerve, the long ciliary nerves, the infratrochlear nerve and medial components of the supraorbital and supratrochlear nerves.
The parasympathetic supply is from the Edinger-Westphal nucleus accompanying the third nerve to synapse with the short ciliary nerves in the ciliary ganglion. The sympathetic fibres are from T1 and synapse in the superior cervical ganglion before joining the long and short ciliary nerves.
Vascular Supply
Arterial blood supply to the globe and orbital contents is mainly from the ophthalmic artery which is a branch of the internal carotid artery as it passes into the orbit through the optic canal inferolateral to the optic nerve and within the meningeal sheath of that nerve. It is tortuous and vulnerable to needle trauma in the elderly and hypertensive patient. Retinal blood supply comes from choroidal plexus and the retinal arteries which are branches of the central retinal artery. The venous drainage is via the superior and inferior ophthalmic veins.
LACRIMAL APPARATUS
The lacrimal gland has orbital and palpebral components. The orbital part is almond shaped lies in the lacrimal fossa on the anterolateral part of the orbital roof. The palpebral part (1/3rd the size of orbital part) is situated below the levator palpebrae superioris aponeurosis and extends into the upper eyelid secreting 13tear fluid into the superior conjunctival fornix. The lacrimal gland receives both autonomic and sensory nerves fibres. Lacrimal drainage occurs through superior and inferior lacrimal puncta near the medial ends of both lid margins which form entrances to the 10 mm long lacrimal canaliculi medially passing through the lacrimal fascia to enter the lacrimal sac. The nasolacrimal duct connects the inferior end of the lacrimal sac to the inferior meatus of the nose.
EYELIDS
The eyelids serve to protect the globe and especially the cornea from the outer aggressions (dust, insects, dryness, injury, excess light). Eyelids also serve to spread the tears on the anterior surface of the eyeball automatically blinking around 20 times per minutes. Cornea has no blood supply and receives oxygen only from the air, needing the tears to dissolve it first. The eyelids close the orbit anteriorly. The levator palpebrae superioris inserts on the superior tarsa. The orbital septum that closes the orbit also inserts on the tarsa. The conjunctiva covers the anterior sclera and the posterior side of the lids. The lacrimal gland, located behind the lids in the superomedial angle secretes the tears. The tears are then evacuated medially through the lacrimal canals that open on the border of the lids, at the junction between free and lacrimal part of the lids.
ORBICULARIS MUSCLE
The orbicularis muscle is a flat elliptical muscle surrounding the orbital margin and extending into the lids and onto the temporal region and cheek. It is composed of striated voluntary muscles. The orbital part of the muscle is under voluntary control but it may act reflexly when a threat to eye demands protective activity. The palpebral part effects reflex blinking. The innervation is from temporal and zygomatic branches of the facial nerve.
LEVATOR PALPEBRAE
The levator palpebrae superioris is a strong striated muscle which originates on the lesser wing of the sphenoid bone, just above the optic foramen. It broadens and becomes the levator aponeurosis inserts into upper eyelid deep to the orbital septum. It is innervated by the superior division of the oculomotor nerve. An adjoining smooth muscle, the superior tarsal muscle, is sympathetically innervated and is occasionally considered to be part of the levator palpebrae superioris. Levator raises the upper lid.
SUMMARY
The knowledge of anatomy of the orbit and its surrounding contents helps in performing ophthalmic regional anaesthesia. Ophthalmic regional anaesthesia based on scientific and sound anatomical knowledge surely can make the technique effective and reduce life and sight threatening complications.
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