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ANATOMY

Pierre Rautenbach

In order to develop an understanding of the glaucomatous disease process and the treatment options available, it is imperative to have a good grasp of the basic anatomy and physiology of certain relevant parts of the eye.

The eye is an approximate sphere 2.5 cm in diameter (equivalent to an axial length of 25 mm) with a volume of 5 ml (it fills one-sixth of the orbit whose volume is 30 ml).

It is a highly specialised organ of photoreception. This is a process by which light energy from the environment produces changes in specialised nerve cells in the retina (rods and cones). These changes result in action potentials (the electrical voltage across a cell) that are subsequently relayed to the optic nerve and then to the brain where the information is processed and consciously appreciated as vision.

THE LAYERS OF THE EYE

The eye consists of 3 basic layers.

Fig. 1.1. The eye in cross section.

The Fibrous Corneoscleral Coat

It consists of the cornea and sclera.

•The Cornea

This is the anterior most transparent window of the eye. The cornea meets the sclera at the limbus, which is also where the conjunctiva ends. The conjunctiva covers the sclera but does not cover the cornea. The cornea is kept transparent by its avascularity and the innermost monolayer of cells (endothelium) which pumps fluid out of the corneal stroma. The cornea presents a tough barrier to trauma and infection and is responsible for about two-thirds of the eyes refractive power (the other one-third coming from the lens).

•The Sclera

This is an opaque white fibrous coat that also protects the eye and maintains its shape due to inherent structural integrity.

The Uvea (or Uveal Tract)

The middle vascular pigmented layer of the eye and consists of the iris, ciliary body and choroid.

•The Iris

This is a thin contractile circular disc, which is analogous to the diaphragm of a camera. The iris separates the anterior and posterior chambers, which are filled with aqueous humour and are in continuity through an opening, the pupil. The iris is attached by its root at the ‘angle’ (iridocorneal) of the anterior chamber where it merges with the ciliary body and trabecular meshwork. Aqueous humour drains mainly through the trabecular meshwork, which is visible using a mirror within a contact lens called a gonioscope.

•The Ciliary Body

This is approximately 6 mm in width and is responsible for the production of aqueous humour. It also contains muscles, which are attached to the zonular ligaments of the lens (changing its shape on contraction to focus or accommodate). It has two parts, the pars plicata and the pars plana. The pars plicata is the anterior part. It is 2 mm long (measured from the limbus) and contains about 70 ciliary processes which are the site of attachment for the aforementioned zonular ligaments. The pars plana is a posterior flat area which is 4 mm long. As the sclera and cornea are relatively rigid, excess production/reduced drainage of aqueous humour or injection of substances into the eye leads to raised intra-ocular pressure (normal is up to 21 mmHg). Intraocular pressure is high immediately after intravitreal injections (can be as high as 60 mmHg). Normalisation of the pressure usually occurs over 30 minutes after injection and is dependent on aqueous humour outflow through the trabecular meshwork.

•The Choroid

This highly pigmented and vascular posterior portion lies between the sclera and the retina and extends forward to the ciliary body. Its principal function is to nourish the outer layers of the retina and prevent unwanted light from reflecting back through the retina. It is composed of an outer layer of large calibre blood vessels, which divide into smaller diameter vessels and ultimately form the choriocapillaris (a network of capillaries). These drain into the vortex veins, which ultimately drain into the superior and inferior ophthalmic veins.

The Retina (Neural Layer)

This is where photoreception occurs and consists of two primary layers, the inner neurosensory retina and an outer layer called the retinal pigment epithelium. Anatomically the following regions are described:

•The Macula (Latin ‘patch’, same as macula lutea)

The area within the main vascular arcades and is 5–6 mm in diameter. Cone photoreceptors are mostly concentrated here for fine resolution (maximum density in the fovea).

•The Fovea (Latin: ‘pit’)

The central 1.5 mm diameter area of the macula. The foveola is the central 0.35 mm diameter area of the fovea.

•The Optic Disc

1.5 mm in diameter, it contains no normal retinal layers or photoreceptors (thus causing the blind spot) and is the area where nerve fibres of retinal ganglion cells pierce the sclera to enter the optic nerve. The central pale thinned area of the disc forms the cup, which becomes progressively enlarged through loss of ganglion cells in glaucoma. The cups vertical diameter is measured in relation to the disc diameter when monitoring patients with glaucoma (referred to as the cup to disc ratio).

•The Peripheral Retina

Rich in rod photoreceptors that provide acuity in low levels of illumination.

•The Ora Serrata

This is where the peripheral retina ends. It is approximately 7 mm from the limbus.

AQUEOUS HUMOUR

The aqueous humour is a clear fluid that fills the anterior segment of the eye. It has many vital functions. It provides nutrients and removes toxic waste products from all surrounding structures. It is clear, allowing light to pass unhindered and acts as a vehicle for important immunological cells and chemicals. It inflates the globe to maintain structural and functional integrity to all eye structures. The degree to which this is done can be measured as the intra ocular pressure (IOP). The IOP is therefore a delicate balance between the production and drainage of aqueous humour. This is normally regulated automatically by various mechanisms to produce an ideal IOP and good blood flow around the optic nerve head. In glaucoma there is an imbalance in this system. All glaucoma treatments are therefore designed to optimize and modify this pathway, specific to the patient being treated. Controlling the IOP is the only risk factor modification proven to prevent progression in glaucoma.

Aqueous Production

Aqueous fluid is actively produced by the ciliary body. Enzymes like Carbonic Anhydrase play an important role in this process. The fluid then circulates from the posterior to the anterior chamber through the pupil.

Aqueous Outflow

Drainage of the fluid takes place via two routes:

•Trabecular (Conventional) Route

This accounts for the majority (90%) of the outflow. The trabecular meshwork is a sieve-like structure in the angle between the peripheral iris and the cornea (also known as the iridocorneal angle). Aqueous passes through the trabecular meshwork into the Schlemm’s Canal and then into the episcleral veins (Fig. 1.2). This route can be affected by changes in pressure in the eye and venous drainage around the eye. The higher the IOP the more the drainage, and the higher the episcleral venous pressure the less the drainage.

•Uveoscleral (Unconventional) Route

This accounts for the minority of the outflow (10%). Aqueous passes across the face of the ciliary body and iris into the suprachoroidal space and is thereafter drained by the venous system of the ciliary body, iris, choroid and sclera.

THE RETINA AND THE OPTIC NERVE HEAD

The retina’s role in vision is to convert light energy as it falls on it into electrical energy. This is then transported from the eye along the optic nerves to the brain, so it can be appreciated. The various layers of the retina are shown in Fig. 1.4. About 1.2 million retinal ganglion cells are present in each eye. The innermost layer of the retina is known as the nerve fiber layer and is composed of axons of the ganglion cells. These are supported by glial cells without any myelin sheaths. Blood vessels associated with the central retinal artery and vein are also present in this layer and supply the inner retina.

Fig. 1.2. Anatomy of the iridocorneal angle.

Fig. 1.3. Production of aqueous by the ciliary body and drainage through the Trabecular route (black arrows).

The nerve fibres come together to form the neuroretinal rim of the optic disc before they exit the globe. They are arranged around a depression called the optic cup which does not contain any neural tissue. The bundles of nerve fibres then pass through a sieve-like structure in the sclera known as the lamina cribrosa and form the optic nerve.

Fig. 1.4. The retina in cross section (note the Nerve Fibre Layer has been labelled as the Axon Layer).

Clinically, the neuroretinal rim is seen as an area between the edge of the cup and the margin of the disc. It is pink in colour and represents the nerve fibre layer changing course by ninety degrees to enter the sclera (Fig. 1.5).

The central retinal artery and vein enter and exit the disc centrally and then course nasally before diving into Superior, Inferior, Nasal and Temporal branches.

Fig. 1.5. Optic disc.

The cup to disc ratio (CDR) describes the diameter of the cup compared to the disc. It is expressed as a fraction; e.g. 0.5 means 50% of the diameter of the disc is occupied by neuroretinal rim. This is an important clinical sign as an increase in this raises the suspicion of glaucoma. Traditionally, a ratio of above 0.5 is regarded as suspicious of glaucoma. It is however important to note that the size of the scleral canal is often proportional to the size of the globe. In myopia (short sightedness) the globe and the scleral canal tend to be larger, but the amount of ganglion cells remains constant. Retinal nerve fibres therefore pass out of the eye through a more generous space at the periphery of the disc, which can lead to a physiologically increased CDR ratio. Similarly, hypermetropes (long sightedness) with smaller eyes can give the impression of a reduced CDR.

A temporal crescent of chorio-retinal atrophy around the optic disc can occur in up to eighty percent of the normal population and is common in high myopia. However, this phenomenon is more common, and the area tends to be larger, in glaucoma.

Detailed understanding of the optic nerve heads blood supply is not required here, but it is important to have some knowledge since interruption to it can result in damage to the optic nerve. The main source of blood supply is from the short posterior ciliary arteries, which in turn are branches of the ophthalmic artery. However, the nerve fibre layer visible within the eye is supplied by the retinal circulation. The flow of blood around the nerve is automatically regulated to ensure normal fluctuations in IOP, blood pressure and cerebrospinal fluid pressure have a minimal effect on blood flow. If this regulation is interrupted or the changes are too extreme to cope with, blood supply to the optic nerve head can be compromised and ischemic irreversible damage to the nerve fibres could occur. This often results in disc pallour, and the astute clinician must be able to differentiate this from glaucomatous changes, as further investigations and treatment will vary according to the underlying condition.

THE LENS

The lens is a highly organised system of specialised cells within a transparent capsule. Situated in the anterior segment of the eye it provides a third of the refractive power of the eye. Zonules from the ciliary body hold the lens in place.

A cataract (Latin for ‘waterfall’) is regarded as a visually significant opaqueness of the lens for which age is the most common risk factor. Development of a cataract can lead to narrowing of the iridocorneal angle, which, in turn, can cause both acute or chronic angle closure glaucoma.