A 5-Year-Old Boy's Failing Vision

Sean M. Rodriguez; Madhuri Chilakapati, MD; Kimberly G. Yen, MD

Disclosures

August 28, 2018

Discussion

MRNFs may be readily visualized during funduscopic examination as one or more well-demarcated, gray-white, striated patches. These changes are often continuous with the optic disc, radiating in the distribution of the retinal nerve fibers along the course of the retinal nerve fiber layer.[1,2] These myelinated retinal nerve fibers are the result of aberrant myelination of the superficial retinal plexus by ectopic glial cells.[2,3]

MRNFs are relatively common ophthalmologic abnormalities present in approximately 1 in 150 individuals, 6% of whom may have bilateral involvement.[4] Previous studies have found a higher prevalence in females than males, but more recent studies have not been able to reproduce those findings.[2,4]

During normal development, myelination of the optic nerve occurs via oligodendrocytes from the lateral geniculate body to the lamina cribrosa.[2] The lamina cribrosa is a mesh-like network of collagen fibers continuous with the sclera through which retinal ganglion cells must pass as they exit the eye to travel along the optic pathway to the brain. Under abnormal circumstances, ectopic oligodendrocyte-like cells are present anterior to the lamina cribrosa and form a myelin sheath around a group of retinal nerve fibers. Although the source of these cells remains unknown, most authors postulate they either migrated prior to the development of the lamina cribrosa or were displaced during an early insult to the lamina cribrosa.[3] It is important to note that these myelinated nerve fibers are not continuous with myelinated optic nerve fibers.[2,3]

Although MRNFs often occur in isolation, they have been associated with certain ocular and systemic anomalies like neurofibromatosis, nevoid basal-cell carcinoma syndrome, craniofacial abnormalities, and vitreoretinopathy with skeletal malformations.[5,6] When the triad of MRNF, myopia, and amblyopia are present together, it may be referred to as Straatsma syndrome.[7] In addition to associations with skeletal abnormalities and vitreoretinal degeneration, certain MRNFs have demonstrated a genetic component and have been postulated to involve a variably expressed autosomal dominant mode of inheritance.[8]

Clinically, MRNF may present as an asymptomatic finding or with profound visual dysfunction that may or may not be recognized by the patient.[9] There are several routes by which MRNF can affect vision. First, the location and sheer size of the myelinated region has been found to greatly correlate with the degree of vision loss, with larger MRNFs and MRNFs overlaying the macular region being associated with greater visual deficits.[3,9] Another cause of visual deficits with MRNFs is due to their secondary effect on the developing morphology of the eye. Researchers theorize that if MRNF causes visual impairment, the developing globe will be induced to undergo axial enlargement. This causes significant axial myopia, leading to anisometropic amblyopia.[10] The theory has been strengthened by studies that demonstrate the role of visual deprivation in the morphological development of the eye.[11,12] As with many medical maladies in pediatrics, diagnoses may be made more difficult owing to the patients' inability to articulate a subjective complaint.

Treatment for patients with MRNF varies depending on their symptoms and origin of the MRNF. For asymptomatic patients, MRNF is not treated and can be monitored at subsequent visits. Patients with severe myopia are prescribed corrective lenses to improve visual acuity.[13] Amblyopia, including patching or penalization treatment, should be treated aggressively in those with MRNF;[3,14,15] however, variable results have been demonstrated.

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