Ocular anatomy and physiology relevant to anaesthesia

OPHTHALMOLOGY

Ocular anatomy and physiology relevant to anaesthesia

Learning objectives After reading this article, you should be able to: C describe the fundamental anatomy of the orbit and understand its relevance to eye blocks C draw a labelled schematic diagram of the eyeball and anterior segment C understand basic optics, common visual defects and optic neural pathways.

Andrew Presland John Myatt

Abstract The orbit contains many delicate and vulnerable structures, but with a solid knowledge of the anatomy one can minimize the chance of complications and better understand how regional blocks work. This article discusses anatomy of the orbit and eye, and includes rudimentary ocular physiology.

Orbital blowout fractures Blunt trauma to the eyeball may result in a blowout fracture. This occurs most commonly on the relatively thin orbital floor. Inferior rectus sometimes becomes incarcerated with inferior blowout fractures and this may cause diplopia or (especially in children) fainting due to activation of the oculocardiac reflex.

Keywords Aqueous humour; iris; pupil; retina; Tenon’s fascia; vitreous body

holes (fissures and foramina) allow the passage of nerves and blood vessels between the cranial fossae and the orbital cavity (Figure 3). The inside of the orbital cavity is lined with a loosely attached fascial layer of periosteum. This layer is continuous anteriorly

The bony orbit Shape and structure The best way to understand the three-dimensional shape of the orbit is to look at a skull. It can be thought of as a socket in the skull that would accommodate a short ice-cream cone, with the tip innermost. How each orbit relates to the other can be seen by placing two ice-cream cones side by side. The volume of each orbit is around 30 ml, but varies between individuals. Medial to each orbital cavity lies the nasal cavity. The angle described by the medial and lateral walls from the posteriorly situated optic foramen is roughly 45 degrees, but can vary between about 40 and 60 degrees (Figures 1 and 2). The orbit has a roof, a floor, and medial and lateral walls. It is made up of frontal, zygomatic, sphenoid, ethmoid, lacrimal and maxillary bones (Figure 3). Superiorly, the orbital margin has a notch (which can be palpated at about 2.0e2.5 cm from the medial wall) that carries the supraorbital nerve. On the lateral orbital margin the notch formed by the suture line of the frontal and zygomatic bones can be palpated.

Long axis of muscle cone

22.5˚

Optic nerves

45˚

90˚

Periosteum and foramina There are a number of holes in the orbit, the biggest of which is at the front where the eyeball sits. At the back of the orbit the

Andrew Presland FRCA is Consultant Anaesthetist for adults and children at Moorfields Eye Hospital NHS Trust, London, UK. He graduated from the Royal Free Hospital School of Medicine, London University in 1992, and trained in anaesthetics at the North London (Central) School. Conflicts of interest: none declared. John Myatt FRCA is a Specialist Registrar in Anaesthesia on the Imperial School (Northwest Thames) rotation. He graduated from Imperial College School of Medicine in 2000. Conflicts of interest: none declared.

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Figure 1 Geometry of the bony orbit.

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Muscles and nerves of the orbit Trochlea

Levator palpebrae superioris

Infratrochlear nerve Anterior ethmoidal nerve

Superior rectus Lacrimal gland Lateral rectus

Superior oblique

Lacrimal nerves

Medial rectus

Short ciliary nerve Ciliary ganglion Abducent nerve

Long ciliary nerves Trochlear nerve Optic nerve

Superior rectus

Nasociliary nerve Oculomotor nerve, superior division

Levator palpebrae superioris

Figure 2

with the external periosteum of the skull and within the cranium with the dura mater.

The recti (superior, inferior, medial and lateral) arise from a ring-shaped thickening of periosteum surrounding the optic canal (Figure 3). They pass forward, getting wider as they do so, to attach onto the surface of the globe anterior to the coronal equator of the eyeball. These four muscles form the ‘muscle cone’ (or simply ‘the cone’). The distinction between intraconal and extraconal is important to anaesthetists when performing regional blocks (Table 1).

Orbital contents Muscles and their nerve supply There are four rectus muscles, two oblique muscles and a levator palpebrae superioris muscle situated within each orbit.

Bones of the orbit (left); extraocular muscle attachments and nerve supply (right) Supratrochlear nerve Levator palpebrae superioris Supraorbital nerve Superior rectus Ophthalmic vein Lacrimal nerve

Orbital plate of frontal Superior orbital fissure Zygomatic Greater wing of sphenoid

Frontal nerve Trochlear nerve Upper division of oculormotor nerve Inferior orbital fissure Abducent Nasociliary Lower division of oculomotor nerve

Optic canal Lesser wing of sphenoid Maxilla Ethmoid Lacrimal Superior oblique

Ophthalmic artery Inferior rectus Optic nerve

Medial rectus Reproduced with permission from Snell RS. Clinical anatomy by systems. Philadelphia: Lippincott, Williams and Wilkins, 2007

Figure 3

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the sclera at the corneoscleral junction (limbus). From its origin, Tenon’s fascia is adherent to the overlying conjunctiva but as it passes backwards and continues closely overlying the sclera, the two layers separate. The conjunctiva also arises at the limbus but is reflected onto the inner surface of the eyelids and does not continue posteriorly within the orbit. Between Tenon’s fascia and the sclera is a potential space, called the sub-Tenon’s space.

Eye blocks Block Site of anaesthetic delivery Notes Retrobulbar Intraconal Rapid onset Peribulbar Extraconal Relies on spread into cone Delayed onset Sub-Tenon’s Sub-Tenon’s space Often spreads into cone Rapid onset

The eyeball The eyeball is made up of three main layers: a fibrous exterior layer, a vascular/muscular layer and a neural layer (Figure 4). The outermost fibrous layer comprises the sclera (or white of the eye) and the clear cornea. This layer is made up of collagen and elastin. The composition of the sclera and cornea is identical; however, one layer is clear and the other opaque. This is because of the structural organization of the collagen fibres: in the cornea, collagen fibres are arranged in highly regular laminae; in the sclera, the fibres appear interwoven and extend in all directions. If the fibrous layer of the eye is broken, there is said to be a penetrating eye injury (Table 2). There is a high risk of subsequent visual loss through expulsion of the contents of the eye, retinal detachment or infection. Inside the fibrous layer is the vascular/muscular layer. Posteriorly is the choroid (Figure 4). Richly vascular, the choroid supplies oxygen and nutrients to outer layers of the retina and the structures of the anterior chamber. It is loosely adherent to the sclera. Anteriorly the choroid becomes continuous with the ciliary body, which, in turn, is continuous with the iris.

Table 1

A retrobulbar block aims to deliver local anaesthetic within the muscle cone and close to the nerves, and works instantly. A peribulbar block delivers local anaesthetic outside the cone and relies on spread of anaesthetic to within the cone. Hence, the block takes several minutes to develop fully and requires a greater injectate volume. A sub-Tenon’s block achieves anaesthesia within the sub-Tenon’s space (described below), but local anaesthetic often spills into the cone. The superior, inferior and medial recti, and the inferior oblique muscles are all supplied by the oculomotor nerve (III cranial nerve). The lateral rectus is supplied by the abducent nerve (VI cranial nerve). These nerves all reside within the cone. The inferior oblique muscle arises close to the nasolacrimal canal on the floor of the orbit. It passes backwards and laterally beneath the inferior rectus and rises to insert below the belly of the lateral rectus in a broad tendon behind the coronal equator of the eyeball. The superior oblique muscle arises from the body of the sphenoid, close to the origin of the recti. As it passes forward it becomes rounded and tendinous and loops through the fibrocartilagenous trochlea, which is medial to the supraorbital notch just inside the orbital rim. It then passes backwards and laterally and inserts onto the sclera beneath the superior rectus muscle and behind the coronal equator of the eyeball. The superior oblique muscle is supplied by the trochlear nerve (IV cranial nerve) which is situated outside the muscle cone (Figure 3). Commonly with retrobulbar and sub-Tenon’s blocks the trochlear nerve is spared, leading to some remaining rotational movement of the eyeball.

Anatomy of the eyeball Vitreous Lens

Sclera

Iris Pupil Cornea

Ocular movement The long axis of the muscle cone (and orbit) is approximately 22.5 degrees (45/2) from the sagittal plane. It is useful to remember this when thinking about the action of the muscles (Figure 1). Medial and lateral rectus muscles move the eye to the left and right. The superior and inferior recti do not move the eye ‘straight’ up and down, owing to the 22.5 degree angle of the long axis of the cone. The action of looking straight up and down is a complex interaction of the recti and other extraocular muscles.

Anterior chamber

Choroid Retina

Optic nerve Optic disc

Ciliary body

Tenon’s fascia and the conjunctiva Tenon’s fascia is a layer of connective tissue that envelops the eyeball, and is pierced by the muscles and nerves that penetrate and attach to it. It is not present over the cornea but arises from

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Extraocular muscle

Posterior chamber

Macula Blood vessels of the retina

Figure 4

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the eye (the optic disc seen at retinoscopy). This results in a small blind spot on the retina. At the posterior pole of the eye, medial to the optic disc, the macula lutea (yellow spot) can be seen. This is the position of the fovea, a small area where the colour photoreceptors (cones) are intensely concentrated. The fovea is the point of greatest visual acuity, where light rays are concentrated when fixing on an object.

Penetrating eye injury Causes Management Complications

Fibrous layer of eye disrupted by sharp object, high velocity missile or blunt trauma Surgery, antibiotics Prolapse of eye contents, infection, cataract, retinal detachment, visual loss

The chambers of the eye and the lens Table 2

Within the eyeball are two ‘fluid’ media: the aqueous humour and vitreous body. They are separated by the lens and its suspensory ligament. The aqueous humour, as its name suggests, is a watery substance. It is secreted by the ciliary processes into the posterior chamber, which is anterior to the lens but behind the iris. The anterior chamber lies anterior to the iris and behind the cornea (Figure 5). In the angle between the cornea and iris lies the trabecular meshwork. This is not normally visible to the naked eye but contains the canal of Schlemm, into which the aqueous humour drains and is removed. Glaucoma is a condition in which there is reduced drainage of aqueous humour. This results in rising intraocular pressure causing damage to the optic nerve. The vitreous body is a gelatinous substance that gives the eye structural support. It lies behind the lens and fills the bulk of the space within the globe. The vitreous may be removed during repair of retinal detachment (vitrectomy e see Table 3). The lens is clear, biconvex in shape and is contained within an elastic capsule. It is attached to the ciliary body by the suspensory ligament (zonules). The lens has no blood supply but receives nutrients from the aqueous humour. A cataract occurs when opacity develops within the lens.

The ciliary body is made up of ciliary muscle, which controls the shape of the lens (accommodation, discussed below), and the ciliary processes, which produce aqueous humour (Figure 5). The iris is the ring-shaped, muscular, coloured part of the eye. At the edges of the ring are longitudinal muscle fibres, which cause the pupil to dilate when they contract. ‘Dilator pupillae’ is supplied by postganglionic sympathetic nerve fibres originating in the cervical plexus. The innermost portion of the iris comprises the circular sphincter muscle fibres, which cause pupillary constriction. ‘Sphincter pupillae’ is supplied by postganglionic parasympathetic nerve fibres originating in the ciliary ganglion. The innermost layer of the eyeball is the neural layer, or retina. The retina has many layers of cells, and contains the lightsensitive photoreceptors, the rods and cones, which detect light impulses from the environment and relay them to the visual cortex. There are no photoreceptors where the optic nerve enters

Blood supply to the eye The ophthalmic artery is a branch of the internal carotid artery. It enters the orbit through the optic canal alongside the optic nerve, and its branches supply the eyeball and extraocular muscles (Figure 6a and b). The most clinically significant branch of the ophthalmic artery is the central retinal artery, which pierces the dural coat of the

Vitrectomy Definition

Indicated during treatment of

Complications

Figure 5

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Surgery to repair retinal detachment, and to prevent further detachment and visual loss. Vitreous is removed to facilitate access to the retina Vitreous floaters obscuring vision Retinal detachment Epiretinal membranes Diabetic retinopathy (bleeding/retinal detachment/scar tissue) Macular holes Vitreous haemorrhage Retinal detachment, infection, bleeding, cataract

Table 3

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optic nerve and travels within the nerve itself. It supplies the innermost layer of the retina and is an end-artery. Branches of the central retinal artery may be viewed radiating out from the optic disc at fundoscopy.

Central retinal artery occlusion Because the terminal branches of the central retinal artery are end arteries, obstruction by an embolus results in sudden onset blindness in the affected area of usually only one eye.

Central retinal vein occlusion Thrombophelebitis of the cavernous sinus or central retinal vein can result in blockage of the small retinal veins, usually resulting in slow, painless loss of vision.

The superior and inferior ophthalmic veins collect blood from the structures within the orbit. They exit the orbit via the superior orbital fissure and empty into the cavernous sinus.

Essential ocular physiology Basic optics When parallel light rays from a distant object enter the eye they are refracted (bent), inverted and directed onto the retina. Emmetropia or normal vision occurs if a focused image forms on the retina without the aid of glasses or contact lenses. If an individual’s gaze moves from a distant object to a close object the light rays must be bent further in order to produce a focused image. In humans this is achieved by contraction of the ciliary muscle which causes the lens to become more rounded. This increases the refractive power of the lens and is called accommodation. In myopia (short sight) the eyeball is too long and the refracted image focuses in front of the retina. Myopia is corrected by wearing biconcave lenses. People with myopia can usually see nearby objects unaided. In hyperopia (long sight) the eyeball is too short and the refracted image focuses behind the retina. People with hyperopia can see distant objects clearly by the mechanism of accommodation. However, this may result in eye strain if the vision remains uncorrected. The optical solution is to wear a biconvex lens (Table 4). With age the lens becomes less elastic and accommodation causes less change in the curvature of the lens. This in turn

Figure 6

Common visual defects and their correction Condition Emmetropia Myopia Hyperopia Presbyopia Pseudophakic eyes

Image focus On retina In front of retina Behind retina Behind retina No accommodation

Common term Normal vision Short sight. Can usually see nearby objects unaided Long sight. Can usually see distant objects unaided ‘Old sight’ (near objects only) Can see distant objects unaided, usually need reading glasses

Corrective lens No correction Concave Biconvex Convex Convex

Table 4

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vice versa (Figure 7). The neural pathway continues posteriorly to the geniculate body of the thalamus and on to the visual cortex in the occipital lobe.

Visual neural pathway Left eye Left field

Right eye Right field

Lesions of the visual pathways and visual field defects

Retina Optic nerve

The anatomical arrangement of the visual pathways leads to specific visual field loss patterns determined by the site of a lesion. Optic nerve lesions can lead to scotomas or complete blindness in the affected eye. Bitemporal hemianopia (loss of temporal field of vision in right and left visual fields) can be produced by a lesion at the optic chiasm. Homonymous hemianopia (loss of either right or left field of vision) can be produced by a lesion at the optic tract, optic radiation or visual cortex.

Short ciliary nerves Ciliary ganglion Optic tract Lateral geniculate nucleus of thalalmus Edinger-Westphal nuclei

Optic radiation

Pupillary reflexes When we shine a bright light into the eye the pupil constricts. This is known as the pupillary light reflex. Normally, the opposite pupil also constricts. This is the consensual light reflex. The afferent neurones involved in the integrity of this reflex travel in the optic nerve to the lateral geniculate body of the thalamus. Impulses then pass into the midbrain, deviating from the bulk sensory afferents. Fibres from the midbrain pass to both the ipsilateral and contralateral EdingereWestphal nuclei, and efferent neurones return via the ciliary body to cause pupillary constriction. Lesions at any point along the pathway may result in loss of or anomalous reflexes (Figure 7). A

Visual cortex

Figure 7

means that the nearest point we can focus clearly on moves gradually further from the eye with time. This condition is called presbyopia or ‘old sight’ and is the reason why many older people eventually need to wear glasses with a convex lens for reading and close work. In pseudophakic eyes (i.e. when a lens prosthesis is in place) accommodation is not possible. It is usual to require reading glasses for close work in this situation too.

FURTHER READING Ganong WF. Review of medical physiology. 21st edn. New York: McGrawHill, 2003. Hamilton RC. Techniques of orbital regional anaesthesia. Br J Anaesthesia 1995; 75: 88e92. Johnson RW. Anatomy for ophthalmic anaesthetists. Br J Anaesthesia 1995; 75: 80e7. Romanes GJ. Cunningham’s manual of practical anatomy. 15th edn. Oxford: Oxford University Press, 1986.

Neural pathways Light in the visible spectrum is converted into action potentials by the rod and cone cells. These action potentials are relayed back to the optic nerve and pass back to the optic chiasm. At the optic chiasm impulses from the nasal side of the retina decussate, while impulses from the temporal retina remain ipsilateral. In the optic tract the impulses are generated by each hemi-field; hence the left optic tract carries the image from the right visual field and

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