The Optic Nerve, which contains 1.2 million fibers each, carries the neural impulses created by the retina to the brain to enable us to see. Disorders of the optic nerve are caused either by developmental (genetic or abnormal development) or acquired factors (trauma or disease). Optic nerve disorders will always impact vision in some way and can affect one or both eyes.
The optic nerve fibers from each eye combine at the optic chiasm located above the pituitary gland, located in the skull about 1 inch behind the bridge of the nose. Defects to the optic nerve in front of the chiasm will affect vision in only that eye, while defects at the chiasm and beyond (toward the back of the head) will affect vision in both eyes.
Optic nerve atrophy can be diagnosed during an examination of the eyes. In optic atrophy patients will show a:
- Reduction in visual acuity
- Loss or reduction in color vision (especially to red)
- Pupil response abnormality (the pupil of the affected eye will not perform normally- called an afferent pupillary defect)
- The Optic Nerve will appear pale in color
There are a number of optic nerve disorders that can be inherited or can occur due to abnormal development.
Congenital optic atrophy is usually hereditary. The milder form is autosomal dominant and has a gradual onset of deterioration in childhood but little progression thereafter. The more severe form is autosomal recessive and is present at birth or within 2 years; this form is accompanied by nystagmus.
Leber’s Hereditary Optic Neuropathy has an unclear mode of inheritance but is suspected of being X-linked, since it rarely occurs in women; optic neuropathy occurs more commonly in 20-30 year old males; usually some level of vision is retained however there are varying degrees of impairment, sometimes with marked vision loss.
Optic Nerve Hypoplasia (ONH) is an undeveloped optic nerve due to a neurological insult early in the prenatal developmental period; the optic nerve has started to develop, but regresses. Visual acuity ranges from normal to severely impaired.
ONH is most often (approximately 80%) present in both eyes (bilateral) but can also be in one eye (unilateral). Because the unilateral cases tend to have better vision, they are typically diagnosed at a later age than those with bilateral ONH.
Children diagnosed with ONH generally present with vision problems which include nystagmus (involuntary pendular movement of the eyes), which tends to develop at 1 to 3 months and/or strabismus (inability to align the eyes), that becomes noticeable during the first year of life.
Acquired Optic Atrophy
The acquired type of optic atrophy may be due to vascular disturbances (occlusions of the central retinal vein or artery or arteriosclerotic changes within the optic nerve itself), may be secondary to degenerative retinal disease (e.g., papilledema or optic neuritis), may be a result of pressure against the optic nerve, or may be related to metabolic diseases (e.g., diabetes), trauma, glaucoma, or toxicity (to alcohol, tobacco, or other poisons). Loss of vision is the only symptom.
Optic Neuritis (ON)
The most common cause of Optic Neuritis is multiple sclerosis (MS). Up to 50% of patients with MS will develop an episode of optic neuritis, and vision loss is the presenting symptom of the disease in 20-30% of cases.
Optic neuritis is an inflammation of the optic nerve. It may affect the part of the nerve and disc within the eyeball (papillitis) or the portion behind the eyeball (retrobulbar optic neuritis) in which case the optic nerve may appear normal.
The onset of optic neuritis is usually in one eye (unilateral) rather than both eyes (bilateral). There usually is a marked but temporary decrease in vision for several days or weeks, and there is pain in the eye when it is moved. Single episodes generally do not result in permanent vision loss; however, multiple episodes may.
Optic neuritis typically affects young adults, especially females, ranging from 18–45 years of age. In the U.S. the prevalence of ON is estimated to be 115/100,000 population.
Visual symptoms include a sudden loss of vision (partial or complete), or sudden blurred or “foggy” vision, and pain on movement of the affected eye. The vision might also look blacked-out rather than blurry. Color vision can also be reduced (especially to red), with colors appearing washed-out when compared to the other eye.
Ischemic optic neuropathy can be a severely blinding disease resulting from loss of the arterial blood supply to the optic nerve (usually in one eye). Optic neuropathy is divided into anterior, which causes a pale swelling (edema0 of the optic nerve, and posterior, in which the optic nerve is not swollen and the abnormality occurs between the eyeball and the optic chiasm. Vision loss from ischemic anterior optic neuropathy can be sudden or occur over several days. Ischemic posterior optic neuropathy is uncommon, and the diagnosis depends largely upon exclusion of other causes, chiefly stroke and brain tumor.
Vision loss from ION often presents with an altitudinal visual field loss where either the top or bottom half of the visual field is affected. Individuals may complain of difficulty walking and also that their vision appears dull and washed out.
Glaucoma is usually a slowly progressing disease which damages the optic nerve fibers and can cause a reduction in the visual field. It ultimately can produce a visual field constriction (tunnel vision) and a loss of visual clarity (visual acuity). It is usually associated with an increased intraocular pressure (IOP) that is higher than the optic nerve fibers can tolerate. Untreated glaucoma can eventually lead to optic atrophy and blindness.
Generally, magnification and illumination control are used to enhance visual functioning for individuals with optic nerve disorders.
Strong reading glasses, optical and electronic magnifiers, and software to enlarge text on computer screens can all be helpful to read.
Proper lighting that increases contrast but reduces glare can be very helpful in many circumstances. Specially tinted wrap-around sunglasses that reduce brightness but increase contrast are often helpful out of doors. These are also available in versions that fitover prescription eyewear that may also be required.
Conventional eyeglasses, while maximizing the focus on the back of the eye, may no longer provide adequate vision for seeing at a distance. The only options are to move close or to use telescopic devices to bring things closer optically.
Telescopic devices can be either handheld or spectacle mounted (bioptics). They can be very helpful to improve distance vision for TV, signs, the classroom, recognizing faces and social engagement, and even for driving.