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 regarded as a misnomer since atrophy implies
disuse. Therefore, a better term for optic atrophy would be “optic
neuropathy.” However, that term is also controversial since, in certain
situations, such as 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
non-specific sign of optic disc pallor. While 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
axons of the optic nerve are heavily myelinated by oligodendrocytes and
reactive astrocytes, which express many inhibitory factors for axonal
regeneration. Thus, the optic nerve, with its 1.2 million fibers,
behaves more like a white matter tract rather than a true 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. It is this neuro-vascular degeneration which 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 cataract, red color is exaggerated, giving rise to a
hyperemic appearance of the disc. Conversely, in pseudophakic
individuals, the disc may appear to have some degree of pallor.
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 so that the white lamina cribrosa becomes 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
along with the rearrangement of astrocytes contributes to the disc
pallor. Cogan and Walsh, as well as Hoyt, have mentioned optic disc
pallor as a consequence of 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. The link to the article is given on the top of the post.
FURTHER READING: https://ourgsc.blogspot.com/2017/05/optic-atrophy-12.html
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