Myelination of the Nerve Fiber Layer Associated with Local Retinal Dysgenesis

Abstract

Abstract: : Purpose: Myelinated retinal nerve fibers occur in approximately 0.6% of the population. They can affect visual acuity and produce visual field scotomas. These effects are thought to be secondary to amblyopia, but few histopathologic studies have verified that the retina is otherwise normal. Methods: We evaluated the gross and microscopic appearance of the retina from a macaque monkey with myelinated retinal ganglion cell fibers. After intravascular fixation, the retina containing the myelinated nerve fibers was embedded in Epon–Araldite. Serial 1 micron sections were stained with toluidine blue and examined in the light microscope. Thin sections were examined using transmission electron microscopy. Results: A dense patch of myelinated nerve fibers was located immediately adjacent to the inferior pole of the left optic nerve. It measured 260 microns in width by 230 microns in thickness. It contained myelinated nerve fibers and oligodendrocytes, but most nerve fibers remained unmyelinated. There was local thickening of the nerve fiber layer, with marked attenuation of the remaining retinal layers. The ganglion cell layer and inner nuclear layer were nearly absent, and could not be clearly differentiated from each other. The outer nuclear layer was preserved, but contained only 40% of the normal cell population, with a corresponding reduction in rods and cones. Overall, the retinal layers were reduced by 70% in thickness, excluding the nerve fiber layer. Conclusions: In this macaque retina, a patch of myelinated nerve fibers was associated with structural abnormalities in the underlying retina. Lamination was disrupted, and there was severe thinning of the residual retina with loss of cells. These findings suggest that myelinated nerve fibers can be associated with local dysgenesis of the retina. Similar abnormalities have been reported in three human retinas containing myelinated nerve fibers (Fitzgibbon et al., 1997). In contrast, the largest series of cases has reported normal retinal structure, except for the presence of myelinated fibers (Straatsma et al., 1981). We conclude that in some cases, myelinated nerve fibers may occur in association with dysgenesis of the retina, implying that amblyopia due to occlusion of the light path by myelin is not the only cause of reduced vision in these patients.

Keywords: amblyopia • nerve fiber layer • neuro–ophthalmology: optic nerve