Optic atrophy means loss of nerve tissue on the disc, and the resulting abnormal pallor of the disc must be accompanied by a defect in the visual field, but not necessarily by a reduction in the visual acuity. It must be remembered that the disc tends to be somewhat pale and the cup of disc tends to be larger in short-sighted eyes and care must be taken in diagnosing optic atrophy in such cases. The number of small vessels, which can be counted on the disc, is sometimes used as an index of atrophy in difficult cases.
Classification of the causes of optic atrophy usually includes the term "consecutive optic atrophy", referring to atrophy following retinal degeneration. The terms primary and secondary atrophy are also used but because these terms are confusing a simple aetiological classification will be used here. It should be borne in mind that it is not usually possible to determine the cause of optic atrophy by the appearance of the optic disc. Even the cupped, pale disc of chronic glaucoma can be mimicked by optic atrophy because of chiasmal compression. When optic atrophy follows swelling of the optic disc, there is more gliosis than when it is "primary", that is, caused by disease in the nerve itself. Gliosis makes the appearance of the disc more grey or yellowish-grey than white and the cribriform markings often seen in optic atrophy might not be evident.
The following are the important causes of optic atrophy:
• Vascular. Following obstruction of the central retinal artery or vein, giant cell arteritis and nonarteritic anterior ischaemic optic neuropathy.
• Following disease in the optic nerve, for example optic neuritis, or compression of the nerve by an aneurysm or tumour (Figure 22.2).
• Following papilloedema. The disc can become atrophic as a direct result of the chronic swelling, irrespective of its cause.
• Inherited. Retinitis pigmentosa is an inherited retinal degeneration in which there is a progressive night blindness, constriction of the visual field and scattered pigmentation in the fundus. As the condition advances toward blindness, the discs become atrophic. Optic atrophy might also appear in certain families without any other apparent pathology, for example Leber's hereditary optic neuropathy and autosomal dominant optic atrophy. It is also seen in the rare but distressing cerebroretinal degeneration, which presents with progressive blindness, epilepsy and dementia.
• Toxic. A number of poisons can specifically damage the optic nerve; methyl alcohol is a classical example. Tobacco amblyopia is a type of progressive atrophy resulting from excessive smoking of coarse tobacco, usually in a pipe and often in association with a high ethyl alcohol intake. Reversal can be achieved by abstention in the early phases of the disease. Other toxic agents include ethambutol, isoniazid, digitalis and lead.
• Trauma. The optic nerve can be damaged by indirect injury if bleeding occurs into the dural sheath. This can result from a fracture in the region of the optic foramen or rarely, from contusion of the eye itself. After the nerve has been damaged, a period of a few weeks elapses before the nerve head becomes atrophic, so that initially the eye could be blind but the fundus normal. The pupil reaction to direct light is impaired from the time of the injury. Such an injury can result in complete and permanent blindness in the affected eye but a degree of recovery is achieved in a small proportion of cases, if decompression of the nerve sheath is undertaken early.
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