5.13  Myelinated Nerve Fibers

5.13.1  Introduction

Myelination or medullation of the optic nerve begins during fetal life and progresses from the lateral geniculate body toward the globe [178]. The optic tracts are the first portions of the visual pathway to be ensheathed by myelin; the chiasm and the optic nerve itself are the last portions to be myelinated. Normally, myelination is completed shortly after birth, at which time the myelin sheaths extend up to the posterior aspect of the lamina cribrosa.

area is characteristic (Fig. 5.25). Sometimes, the mylin will follow the nerve fibers extending far into the peripheral retina (Fig. 5.26).In most cases, macular is not involved. Because the superficial retina is opacified by the myelinated fibers, variably decreased visual acuity, visual field defects, amblyopia, strabismus, and nystagmus­ have been associated with extensive myelination of retinal nerve fibers [76]. Myopia accompanies the condition in up to 50% of cases [179]. Coloboma, polycoria, keratoconus, oxycephaly, and NF have been reported in conjunction with myelinated fibers.

5.13.2  Genetics

A familial tendency has been occasionally noted.

5.13.3  Pathophysiology

Myelin is a product of oliodendroglial cells, which are normally present only in the myelinated portion of the nerve behind the lamina cribrosa. However, these glial cells are present in the involved regions of the optic nerve head and retina in cases of myelinated nerve fibers. The myelinated fibers are well demonstrated histopathologically by specific special stains for myelin. The retina layers underlying affected portions of the retina are normal.

Fig. 5.25  Myelinated optic nerve fibers. Note feathered edges which follow retinal nerve fibers

Affected eye are usually otherwise normal in structure and function. The feathery edge of the affected

Fig. 5.26  Myelinated optic nerve fibers extending into the peripheral retina. Such cases are often associated with myopia and macular hypoplasia