Medullated Nerve Fibers Retinopathy: Its Symptoms, Diagnosis, and Treatment

What Is Medullated Nerve Fibers Retinopathy?

The word retinopathy indicates diseases of the retina. The retina is a layer at the back of the eyeball that converts the light entering the eye into electric signals, which are then relayed to the brain by the optic nerve to create an image.

The optic nerve that transports the electrical signals comprises numerous subunits called neurons, encased individually by an insulating layer called the myelin sheath. This sheath allows the transmission of electrical impulses efficiently and effectively. The myelin sheath, however, is not continuous; it disappears once the neurons reach the retina; however, in a few individuals, it is retained, which leads to a condition called medullated nerve fibers retinopathy, and the neurons which retain the myelin sheath are called as myelinated retinal nerve fibers (MRNF).

What Causes Medullated Nerve Fibers Retinopathy?

Although the exact cause of the myelination is unknown, several observations and theories have been put forward since its discovery in 1856 by a German pathologist, Robert Virchow. MRNF can be congenital (present at birth) or acquired.

1. Congenital MRNF - In individuals with congenital MRNF, it is believed that an error in the myelination process or the formation of the sclera normally starts in the fifth month inside the womb and ends at the seventh month seems to cause MRNF. MRNF, when present since birth, can be an isolated condition or seen in association with other genetic conditions like Turner syndrome, epilepsy, trisomy 21, and craniosynostosis.

2. Acquired MRNF - Although rare, MRNF can be acquired through trauma where sufficient damage to the eye triggers the myelin-forming cells (oligodendrocytes) to reach the retina, where they reach the nearest area of the fairly loose nerve fibers and myelinate them. Other scenarios where MRNF can be acquired are optic nerve sheath fenestration, optic nerve drusen, optic nerve hypoplasia, Arnold-Chiari malformation, neurofibromatosis type 1, and Gorlin-Goltz (basal cell nevus) syndrome.

What Are the Symptoms of Medullated Nerve Fibers Retinopathy?

The impact of MRNF on visual functioning depends on the extent of myelination; symptoms range from white patches to severe photophobia. Some of the frequently seen symptoms associated with medullated nerve fibers retinopathy (either congenital or acquired) are mentioned below-

1. Myopia - It is a visual defect where the near objects appear clear, but the farther objects are blurry; patients with extensive MRNF are seen to suffer from severe myopia.

2. Scotoma - It is a blind spot in vision that develops when sufficient myelinated nerve fibers are present. However, these scotomas are smaller than the size of an MRNF patch.

3. Amblyopia - Also known as the lazy eye. The condition causes reduced vision in the eye due to abnormal visual development early in life. Amblyopia in patients with MRNF is refractory to treatment, differentiating it from other forms of amblyopia.

4. Strabismus - It is a disorder in which both eyes do not line up in the same direction. Almost 66 percent of MRNF patients exhibit strabismus; hence, it is considered an important tool for diagnosing medullated nerve fibers retinopathy.

5. Monocular Nystagmus - nystagmus is a condition in which the eyes make rapid, repetitive, and uncontrolled movements, and when it occurs in only one eye, it is known as monocular nystagmus.

6. Polycoria - It is a condition characterized by more than one pupillary opening in the iris.

7. Keratoconus - Thinning and gradual outward bulging of the cornea into a cone shape resulting in blurred vision and light sensitivity is known as keratoconus.

8. Congenital Cataracts - Clouding of the lens in the eye is known as a cataract, and when it is present since birth, it is known as a congenital cataract.

9. Uveal Coloboma - It is a condition where part of the tissue in the eye (iris, pupil, cornea, or any other part) is missing due to the failure in the closure of the optic fissure.

10. Choroiditis - Inflammation of the choroidal tissue is known as choroiditis.

11. Uveitis - Inflammation of the middle layer (uvea) of the eye is known as uveitis.

Apart from these, decrease in acuity, retinal detachment, macular thickening, telangiectasias, neovascularization, and recurrent vitreous hemorrhage are some rarely seen symptoms in patients with MRNF.

Not all the symptoms mentioned above are present in patients with medullated nerve fibers retinopathy; the number and extent of the symptoms depend on the severity of the myelination and the associated conditions.

How Is Medullated Nerve Fibers Retinopathy Diagnosed?

Medullated nerve fibers retinopathy mimics conditions like cotton wool spots, papilledema, hard exudates, etc., which is why it is best to conduct necessary investigations before arriving at a diagnosis; some of the commonly performed imaging studies to diagnose it are-

1. Fundus Photography - It is helpful to highlight the white myelinations and feathery borders along the neurons.

2. Fundus Autofluorescence (FAF) - FAF is a non-invasive imaging modality that works by detecting natural fluorescence, such as lipofuscin. In MRNF, the myelination prohibits the detection of the underlying fluorescent material, making the myelinated areas look dark in appearance.

3. Optical Coherence Tomography (OCT) - It is helpful to demonstrate the thickened retinal nerve fibers in myelinated areas.

In the absence of the above-mentioned testing facilities, formal investigations like ophthalmoscopy or fundoscopy can also be used, showing feathery white clumps usually connected to the optic disc indicating the presence of myelination.

How Is Medullated Nerve Fibers Retinopathy Treated?

MRNF is a benign condition that does not affect vision; early recognition can prevent dense amblyopia later in life. Treatment involves relieving the symptoms, which include:

• Refractive correction for myopia can be done with eyeglasses, contact lenses, or surgery.

• However, contact lenses are preferred for severe anisometropia because eyeglasses correction is insufficient.

• Surgical realignment for strabismus along with usual protocols.

• Initiation of anti-amblyopia therapy involves patching up the non-amblyopic eye for a couple of hours each day.

Conclusion:

Medullated nerve fiber retinopathy is a non-progressive benign condition often diagnosed accidentally in a clinical setting. Many patients will be asymptomatic, and present common symptoms include myopia, amblyopia, and strabismus. Definitive diagnosis must always be made with proper investigations because it is easy to mistake MRNF for retinal infiltrate, retinoblastoma, leukokoria, etc. The condition has no cure, and treatment always involves relieving the symptoms.