Fundamentals Of Glaucoma: A Guide For Ophthalmic Nurse Practitioners, Optometrists And Orthoptists - Группа авторов

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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.

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      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.

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