Myelinated retinal nerve fiber layer
|Myelinated Retinal Nerve Fiber Layer|
|Classification and external resources|
Fundus photo of a 7 yo girl with myelinated RNFL, 20/20 vision, and mild hyperopia
- 1 Myelinated Retinal Nerve Fiber Layer
- 2 Pathophysiology
- 3 Histopathology
- 4 Genetics
- 5 Diagnosis
- 6 Ocular conditions associated with myelination of the RNFL
- 7 Management
- 8 References
- 9 Acknowledgements
Myelinated Retinal Nerve Fiber Layer
Myelinated retinal nerve fiber layers (MRNF) occur in 0.57- 1% of the population and can occur bilaterally in 7.7% of affected patients. MRNF are grey-white well-demarcated patches with frayed borders along the retinal nerve fiber layer, obscuring underlying retinal vessels. Clinically, MRNF appear continuous with the optic nerve, but they are often discontinuous with the optic nerve head as well.Most patients with MRNF are asymptomatic; however, visual function can become affected, resulting in axial myopia, amblyopia, and strabismus in the affected eye. Though rare, familial cases of MRNF have been reported both in isolation and in combination with ocular and systemic syndromes (Table 2).MRNF are typically present at birth and are static lesions, but a few cases of acquired and progressive lesions in both childhood and adulthood have been described. Disappearance of MRNF have also been reported after surgery and insults to the optic nerve.
International Classification of Disease
ICD-9 377.49 Other disorders of optic nerve
ICD-10 H35.89 Defect, defective retinal nerve bundle fibers
Virchow was the first to describe myelinated retinal nerve fibers as “chalk-white spots” around the optic disc 1856. However, his observation was predated by over a century of neuronal study. In 1717, Leeuwenhoek described seeing a subset of nerves covered with “fatty parts.” Ehrenberg described “cylindrical tubes” in the myelinated nerve fibers in 1833. German embryologist and neurologist Remak postulated that the cylinders originate from neuronal ganglion cells in 1838. Schwann stated that the white substance around the nerve fiber is a sheath in 1839. In 1858, Virchow described that the “medullary sheath is not an absolutely necessary constituent of the nerve, but that it confines electricity within the nerve itself and allows discharge to take place only at the non-medullated extremities of the fibers.” Cajal recognized that the axis cylinder is the axonal process and that the myelin sheath is external and secreted by the axon in 1909. In 1918, Gradle stated: “The cause of sporadic medullation of retinal nerve fibers falls among still unexplored phases of ophthalmology…”
Axonal myelination in the human central nervous system (CNS) is a complex, orderly process carried out by oligodendrocyte progenitor cells (OPC) which migrate under the influence of neuro-hormonal signals to generate oligodendrocytes, which produce myelin. Myelination generally occurs from the neuron along the direction of the impulse conduction down the axon. Retinal ganglion cell myelination, however, proceeds in the opposite direction from the optic tract toward the globe.
Myelinated fibers are not confined to a patch or fascicle, and single myelinated fibers can be found in between fascicles of unmyelinated fibers. Analysis of tissue within the MRNF shows no microscopic evidence of inflammation with few cell nuclei and reduced retinal ganglion cell population compared to retina outside of the patch, which is normal. MRNF have less condensed myelin along with larger axonal diameter causing the myelinated fiber to be greater than the respective optic nerve or retinal fibers in the same eye and can show degenerative signs of myelinolysis. The abnormal myelination results in increased thickness of the nerve fiber layer compressing the underlying retinal layers, reducing the width and altering the conformation of the inner and outer plexiform layers. This also causes an indistinct border between the inner nuclear layer and outer nuclear layer.Although MRNF appear macroscopically contiguous with the optic disk and vascular arcades, they are more likely to be discontinuous with the optic disk and rarely associated with the vascular supply. No abnormalities of the cribriform plate have been described in eyes with MRNF.
Familial cases associated with other disorders have also been described. GAPO syndrome is an autosomal recessive syndrome of growth retardation, alopecia, pseudoanodontia, optic atrophy, and MRNF associated with hypertelorism, severe end-stage glaucoma and myelinated retinal nerve fiber layer.
A syndrome of vitreoretinal degeneration, posterior subcapsular cataract, and skeletal abnormalities (missing digits) was also described. Goltz-Gorlin (multiple basal cell nevus) syndrome has also been reported to be associated with acquired myelination of the RNFL. A patient with Albright hereditary osteodystrophy was also found to have a focal area of myelination of the RNFL (unreported, seen at UCSD). Other systemic syndromes that have been associated include Turner syndrome, epilepsy, Downs Syndrome, and craniosynostosis.
The majority of MRNF cases are incidentally diagnosed in asymptomatic, healthy children by ophthalmoscopy. The distinct peripapillary white striated patches with feathered borders involving from one disc diameter in size to larger areas of the retina, often found located in the superior sector of the optic nerve head, is observed on examination (see Figure below).
Figure 1. Multicolor image of myelination of the RNFL.
Ophthalmic Imaging Characteristics
Imaging characteristics of MRNF have been reviewed recently, and the results are summarized here. The myelin in MRNF has a high lipid content affecting imaging modalities. Infrared and red-free imaging are sensitive to, lipids resulting in the white appearance of MRNF. Myelin blocks detection of fluorescent material appearing dark on fundus autofluorescence. On optical coherence tomography (OCT), thickened MRNF (red arrow in Figure 2) causes hyper-reflectivity with back-scattering resulting in decrease visibility of retinal layers behind MRNF (see Figure 2).
Figure 2. OCT of myelination of the RNFL
Ocular conditions associated with myelination of the RNFL
MRNF are often isolated but may be associated with ipsilateral myopia, amblyopia, strabismus and other ocular conditions (Table 1). Strabismus was found in 66% with MRNF in one review. Visual acuity is usually not affected; however, if a sufficient number of myelinated fibers are present a relative scotomas may develop depending on the location of the MRNF. If the myelination involves the optic disk, macula or peripheral retina an enlarged blind spot, ringed scotomas, or isolated peripheral scotomas may develop respectively. These scotomas are smaller than the size of the MRNF patch would suggest. Visual acuity and prognosis depends on the severity of ocular complications or associated syndromes (Table 1).
Ocular associations with myelination of the RNFL
|Refractive and Sensorimotor:|
|Optic nerve associated:|
|Uveal and Retinal:|
Conditions associated with acquired and progressive myelination of the RNFL
There are a few case reports of acquired and progressive myelination in childhood, adolescence, and adulthood. These cases are associated with blunt trauma, optic nerve sheath fenestration for chronic papilledema, Optic nerve drusen, Family history of optic nerve hypoplasia (unaffected child), Arnold-Chiari malformation associated with hydrocephalus, Von Recklinhausen’s disease.
Conditions associated with loss of myelination of the RNFL
The disappearance of MRNFL has also been reported and associated with multiple neurologic, inflammatory and retinal diseases. The neurological disease associations include pituitary adenoma, optic neuritis, acute optic neuropathy, and primary open angle glaucoma.    The inflammatory disease associations are Bechet’s disease after recurrent papillitis and vitritis and plaque radiotherapy for choroidal melanoma. Retinal disease such as BRAO, CRAO, diabetic retinopathy, pars palna vitrectomy for epiretinal membrane have been associated with disappearance of MRNFL.
MRNF are typically benign but can be mistaken for other potentially serious conditions. A complete blood count may be helpful in differentiating from a neoplastic infiltrate. Ophthalmoscopy and fluorescein angiography can help determine if an embolic phenomenon is in the differential. If gross visual defects are present, formal visual field testing to rule out a concomitant neuro-ophthalmologic issue, as visual defects in MRNF are usually mild. Management of MRNF is focused on serial eye examinations assessing for and treating associated complications such as retinal vascular complications, including neovascularization and recurrent vitreous hemorrhage, with possible argon photocoagulation. Myopia should be treated with corrective lenses. If significant anisometropia is present, correction with glasses may not be tolerated secondary to aniseikonia, and contact lenses should be prescribed. Amblyopia must be treated aggressively to optimize visual outcomes. Strabismus should be managed with the usual protocols, and patients often respond well to surgical realignment. It is important to recognize the generally benign nature of MRNF to avoid additional diagnostic evaluations increasing healthcare costs and causing undo patient anxiety.
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The authors thank Penny Coppernoll-Blach and the UCSD Library staff for their help in facilitating the literature review necessary for this paper. Special thanks to Giulio Barteselli and Gabriel Balea for the ophthalmic photography presented here. We thank Christopher Rugaber, OD, Faculty of Optometry, Rangsit University, Pathum Thani,Thailand for his valuable suggestions to improve this wiki page.