During normal prenatal development, retinal ganglion axon myelination starts centrally in the lateral geniculate body and proceeds anteriorly to the optic tracts, chiasm, and nerves. This process of myelination normally terminates shortly after birth at the level of the lamina cribrosa; however, occasionally myelination occurs in the retinal nerve fiber layer.1 Although generally considered to be a benign funduscopic finding, myelinated nerves have been associated with visual field defects, severe myopia, amblyopia, and strabismus.2,3
To our knowledge, all of the reported cases of amblyopia have occurred in the eye affected with myelinated fibers. In this article, we describe 2 patients with unilateral myelinated nerve fibers who had amblyopia in the eye not affected with nerve fiber layer myelination. We believe that these cases shed light on the causes of reduced visual acuity in patients with retinal nerve fiber layer myelination.
Case 1. A 5-year-old boy was referred for evaluation of a “wandering eye” that was first noted in infancy. He had received no previous eye treatment. Visual acuity without correction was 20/125 OD and 20/300 OS. Cycloplegic refraction revealed −2.50 diopters (D) OD and +7.50 D OS, correcting visual acuity to 20/40 OD and 20/160 OS. A 25–prism diopter, left-sided exotropia was present. There was no afferent pupillary defect. Funduscopic examination of the right eye showed extensive myelinated nerve fibers continuous with the optic nerve (Figure 1). The macula appeared normal. The left fundus was unremarkable. Axial length measurements were 25.22 mm OD and 21.32 mm OS. Keratometry showed regular astigmatism: right eye, 40.18 × 41.56 D (mean, 40.87 D); left eye, 39.66 × 41.56 D (mean, 40.69 D). Treatment with glasses and patching of the right eye 6 hours per day was recommended. At the last follow-up visit 10 months after the initiation of amblyopia treatment, his best-corrected visual acuity had remained 20/40 OD and improved to 20/50 OS.
Fundus photographs of patient 1 showing extensive myelination of the retinal nerve fiber layer of the right eye (A) and no myelination in the left eye (B).
Case 2. A 9-year-old boy was examined after failing a school vision screening. He had had no previous ocular examinations or treatment. His uncorrected visual acuity was 20/20 OD and 20/300 OS. Cycloplegic retinoscopy showed +1.00 D OD and +2.75 + 1.50 × 90° OS. There was no afferent pupillary defect. Funduscopic examination showed peripapillary retinal nerve fiber layer myelination in the right eye; the left eye showed no abnormality (Figure 2). Axial length measurements were 23.02 mm OD and 21.80 mm OS. Keratometry showed regular astigmatism: right eye, 41.93 × 42.99 D (mean, 42.46 D); left eye, 41.51 × 43.72 D (mean, 42.62 D). Eyeglasses were prescribed and occlusion therapy of 4 hours on weekdays and full-time on weekends was started. At the most recent follow-up visit after 8 months of treatment, visual acuity had improved to 20/100 OS.
Fundus photographs of patient 2 showing peripapillary myelination of the nerve fiber layer of the right eye (A) and no myelination in the left eye (B).
On funduscopic examination, myelinated retinal nerve fibers appear as white to gray-white patches corresponding to the distribution of the ganglion fibers. Lesions are often found incidentally or in patients referred for evaluation of visual disturbances or leukocoria. Frequently, visual acuity is not affected. Straatsma et al1 found myelinated fibers in roughly 1% of 3968 consecutive autopsies. Myelin was continuous with the optic nerve head in 81% and discontinuous in 19%.
The normal retina is not myelinated because it lacks oligodendrog lia, the cells responsible for myelination in the central nervous system. Anomalous distribution of oligodendroglia in the retina is believed to be responsible for retinal nerve fiber myelination.1 Reported cases demonstrating intact lamina cribrosa, considered to be a barrier to myelination, and the observation of myelination discontinuous with the optic nerve head refute speculation that myelination simply continues anteriorly through the posterior sclera.
It is not known whether myelination of retinal fibers leads to myopia or myopia leads to myelination of retinal fibers. Alternatively, myopia and retinal myelination may be associated with other factors. Schmidt et al4 proposed that myelinated fibers could blur retinal images and induce visual deprivation. Such deprivation at a critical stage of ocular development could lead to axial elongation, akin to structural changes seen in eyes with a unilateral congenital cataract or severe ptosis.5 However, scotomas in myelinated retinas are generally smaller than predicted by the extent of myelination, suggesting that light penetrates to the photoreceptor layer despite the myelin interference.1 Furthermore, many cases of myelinated fibers do not show visual deficits or have significant myopia.
It is also possible that the increased axial length of the eye predisposes to retinal myelination. If the process of lamina cribrosa development is prolonged, as could occur in axial myopia, myelination could continue down the optic nerve and into the retina. Although this theory has some merit, it cannot explain lesions discontinuous with the optic nerve head or myelination of eyes without increased axial lengths.
It is likely that our patients were genetically predisposed to be hyperopic. During ocular development one eye became myelinated as a cause of, or resulting in, disruption of the preprogrammed growth pattern of that eye. The resultant axial elongation led to 10 D of anisometropia in the first case and more than 2 D in the second case. As a consequence, these patients developed amblyopia in their more ametropic eyes. Myelination did not cause organic loss of vision from macular dysfunction as reported in some of the patients of Hittner and Antoszyk.3 The question as to whether myelination causes myopia or vice versa is left unresolved, but our cases do suggest that anisometropia can be a more powerful influence on the relative visual acuity of each patient’s eyes than the presence of retinal myelin. Lempert6 has proposed smaller optic disc size as an organic as opposed to functional reason for decreased vision in hyperopic anisometropia. Although the optic discs of our patients appeared normal in each eye, we cannot exclude this as an alternative explanation for their decreased visual acuity.
With an estimated amblyopia prevalence of 2% to 3%, much of which is anisometropic amblyopia, and a 0.9% prevalence of unilateral nerve fiber layer myelination, one would expect that the combination of these findings would be seen more frequently.1 We do not know why there have been no previous published reports about this finding, but do believe that these cases are unique in the literature on myelinated retinal nerve fibers.
Correspondence: Dr Ruttum, Eye Institute, 925 N 87th St, Milwaukee, WI 53226-4812 (firstname.lastname@example.org).
Financial Disclosure: None.
Funding/Support: This study was supported in part by an unrestricted grant from Research to Prevent Blindness Inc, New York, NY.
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