The Optic Disc

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Last modified 08/03/2015

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17 The Optic Disc

THE NORMAL OPTIC DISC

About 25 per cent of the sensory input to the brain is visual. The normal optic disc is oval and about 1.5 mm in vertical diameter. It is located in the nasal hemi-retina and the corresponding blind spot in the visual field lies between 10 and 15° from fixation in the temporal hemi-field, usually slightly below the horizontal meridian. Each optic nerve contains about 1.2 million afferent nerve fibres which are the axons of the retinal ganglion cells that synapse at the lateral geniculate body. In addition there are probably efferent fibres; their function is uncertain and they have been demonstrated conclusively only in nonprimates although they are likely to be present in humans as well.

The scleral opening is rigid and nonexpansible. Behind the cribriform plate the axons acquire a myelin sheath and the optic nerve enlarges to a diameter of 3–4 mm. The nerve fibres are arranged roughly topographically in both the retinal nerve fibre layer and the optic nerve (see Ch. 19). The optic nerve sheath is a continuation of the meninges and the axons in the nerve are divided into approximately 1000 bundles by fibrous septa (derived from the pia mater) which also carry centripetal blood vessels. The subarachnoid space communicates intracranially so that the nerves are surrounded by the cerebrospinal fluid (CSF) which transmits the intracranial pressure.

BLOOD SUPPLY

The blood supply of the optic disc is derived from the posterior ciliary arteries; the central retinal artery which pierces the disc does not contribute at all to the substance of the disc, supplying only a layer of superficial capillaries to the disc surface. Two branches of the posterior ciliary arteries, derived from the ophthalmic artery, pass forwards in the orbit medially and laterally and divide near the posterior aspect of the globe into two major trunks that subdivide again. One group supplies the choroid; the other trunk unites with its opposite number to form the circle of Zinn, a consistent feature in human eyes. Branches from this pass radially into the optic nerve head to supply the retrobulbar optic nerve and the cribriform plate as well as the prelaminar area by recurving branches. There are a few small direct branches that enter the prelaminar region radially from the choroid but these appear to be a relatively minor contribution to its overall blood supply. The orbital part of the optic nerve receives its blood supply from the ophthalmic artery by centripetal branches entering from the pia. The venous drainage of the disc is through the central retinal vein, the majority of capillaries in the disc are on the venous side of the circulation.

CONGENITAL ABNORMALITIES OF OPTIC DISC SIZE AND SHAPE

Normal optic discs can vary in size, shape and contour and an accurate appreciation of this is extremely important in clinical diagnosis. Anomalies of the disc can reflect defects in closure of the fetal fissure, degeneration of the hyaloid vascular system, the size of the globe (usually indicated by the refractive state but easily measured by A scan), developmental anomalies of the anterior visual pathways and the number of axons in the optic nerve.

Anomalous discs can appear swollen because of distortion and crowding of the normal nerve fibre pattern but careful scrutiny of the disc with white and ‘red-free’ light usually reveals whether a disc is truly pathologically swollen. When doubt remains, careful observation together with serial photography over time resolves the question. Congenitally anomalous discs can be associated with loss of either visual acuity or field which varies from normality to gross deficit, the deficit is, however, static.

COLOBOMAS OF THE DISC

A coloboma is a congenital defect resulting from a malclosure of the fetal cleft. Colobomas vary in size and shape; the disc is usually larger than normal although it may contain fewer nerve fibres. Colobomas usually occur inferiorly with varying involvement of the retina and uveal tract (see Ch. 9). Colobomas are occasionally inherited in an autosomal dominant fashion and may also be a feature of a number of genetic and maldevelopmental syndromes such as the CHARGE syndrome (coloboma, heart defects, anal atresia, retardation of growth, genital and ear defects) which is caused by dysgenesis at 5–6 weeks’ gestation.

OPTIC DISC HYPOPLASIA

Hypoplasia of the disc is an important physical sign as small optic discs transmit fewer axons than normal and a hypoplastic optic disc may be associated with poor acuity, field defect, strabismus or failure to improve with amblyopia treatment. Conversely, optic disc swelling can be simulated by a large number of axons passing through a small optic disc. Characteristically the optic disc looks ‘small’, especially when compared to a normal fellow eye and there may be a disparity between the retinal and scleral aperture. The visual deficit can vary from normal to severely defective. Severe forms of hypoplasia can also be associated with intracranial developmental defects of the anterior visual pathway.

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Fig. 17.18 These are the visual fields of the same patient. Fewer axons than normal enter the disc in the defective area and there is corresponding fundus ectasia (as shown by the thin retinal pigment epithelium in Fig. 17.16); together these features result in a visual field defect. As the defect is usually inferonasal this produces a superotemporal field defect simulating chiasmal compression. Such defects, however, cross the vertical meridian, a feature never seen with neurological causes of field loss. Furthermore, the refractive component can be corrected by additional myopic correction and these characteristics allow the correct diagnosis to be made.

ANOMALIES ASSOCIATED WITH AXIAL LENGTH

The axial length of the eye will also influence the size and shape of the optic disc. Hypermetropic eyes are smaller in size than normal and consequently optic discs are frequently smaller, do not have a physiological cup and have a crowded or ‘full’ appearance that can suggest swelling. Retinoscopy or A scan gives the clue to the underlying aetiology. Myopic discs tend to be larger and surrounded by a white crescent of bare sclera on the temporal side occasionally with some pigmentation along the border. The physiological cup is normally broad and shallow and the disc tends to be paler than in emmetropic eyes. These changes can mask glaucomatous changes for the unwary.

MYELINATED NERVE FIBRES

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