Optic Atrophy

Excerpt

Optic atrophy is a pathological term referring to optic nerve shrinkage caused by the degeneration of retinal ganglion cell (RGC) axons. The term "optic atrophy" is a misnomer since atrophy implies disuse. Therefore, a better term for optic atrophy is "optic neuropathy." However, this term is also controversial since, in certain situations (eg, primary optic atrophy or traumatic brain injury), optic neuropathy may not occur.

Optic atrophy is the end stage of a disease process affecting the retinogeniculate portion of the visual pathway, characterized by a nonspecific sign of optic disc pallor. Although the peripheral nervous system has an intrinsic ability for repair and regeneration, the central nervous system, for the most part, is incapable of such processes. The optic nerve axons are heavily myelinated by oligodendrocytes and reactive astrocytes, which express many inhibitory factors for axonal regeneration. Thus, with its 1.2 million fibers, the optic nerve behaves more like a white matter tract rather than an actual peripheral nerve. The optic nerve head is supplied by pial capillaries that undergo degeneration, contributing to the pallor of the optic disc seen in optic atrophy. This neurovascular degeneration forms the foundation for the development of optic atrophy.

When light is thrown on the fundus from a light source, it undergoes total internal reflection through the axonal fibers. Subsequently, reflection from the capillaries on the disc surface gives rise to the characteristic yellow-pink color of a healthy optic disc. In eyes with cataracts, the red color is exaggerated, giving rise to a hyperemic appearance of the disc. Conversely, the disc may appear to have some degree of pallor in pseudophakic individuals.

Usually, 4-6 weeks are required following axonal damage for the optic disc pallor to start developing. In severe cases, the disc ultimately becomes chalky white. The overlying axons and capillaries degenerate, making the white lamina cribrosa visible. This contrasts sharply with the surrounding red-colored retina. The exact mechanisms responsible for the optic disc pallor seen in optic atrophy are not clearly elucidated. It is assumed that the loss of axonal fibers and the rearrangement of astrocytes contribute to disc pallor. Cogan and Walsh, as well as Hoyt, have mentioned optic disc pallor due to the loss of smaller blood vessels and the variable amount of reactive gliosis and fibrosis as the optic nerve shrinks due to various factors. The degenerated axons also lose the optical property of total internal reflection, leading to the pale optic disc seen in this condition.

Recognition of optic atrophy might prove to be life-saving for the patient. Therefore, it is imperative to have adequate knowledge regarding this fairly common condition. This review presents the basic concepts of optic atrophy.