Within the intracranial portion of the optic nerve, myelin sheaths are produced by oligodendrocytes and surround the nerve fibers until they reach the lamina cribrosa within the optic nerve head. The appearance of myelinated nerve fibers within the fundus results from the anomalous myelination of nerve fibers anterior to the lamina cribrosa. Structural change is confined to the nerve fiber layer of the retina. The myelin sheaths may be produced by ectopic oligodendrocytes, or by intraretinal Schwann cells. (The latter occurs in the normal retina of the cat and rat.)
The vast majority of patients are asymptomatic. Patients with extensive myelination may have an increased incidence of myopia (nearsightedness), amblyopia (lazy eye) and strabismus (squint).
Myelinated nerve fibres are unilateral in 80 percent of cases. They appear as white-yellow patches with feathery borders in the superficial retina; the fibers follow the orientation of the nerve fiber layer and respect the horizontal raphe. Underlying retinal structures are often obscured from view, including the retinal vessels and the area may be totally opaque.
The location and extent of myelinated nerve fibers is highly variable. The patches often extend from the optic nerve for a short distance into the retina, and occasionally continue in an arcuate path to the peripheral fundus. Patches that partially obscure the disc margin occasionally resemble papilledema on cursory examination. Rarely there are peripheral patches of myelination only. Variable, but usually minimal, corresponding visual field defects are common.
Common – present in approximately 1 percent of the population.
Typically benign and stable.
Cotton wool spots, Astrocytic Hamartomas, Commotio Retinae, Papilloedema.
Routine visual field testing may be indicated if the myelination is extensive.
No treatment is required. Lesions have rarely been reported to disappear in patients following optic neuritis or in demyelination disease.
The left image shows a small patch of medullated nerve fibers continuous with the inferior optic disc. The right image shows a small area of medullation below the right optic disc in an adult Chinese male patient.
More extensive myelinated nerve fibers, showing pale opaque retina with feathery edges. The optic nerve head almost entirely obscured in the right image.