Optic Nerve Hypoplasia
Optic nerve hypoplasia (ONH) is the most common type of congenital blindness. Persons with ONH have small, underdeveloped optic nerves (cranial nerve II) as well as small optic discs known to affect one or both eyes. This condition can occur alone or in occurrence with additional physiological abnormalities, such as a dysfunctional pituitary gland. Depending on the severity of the abnormalities, ONH has been associated with septo-optic dysplasia or de Morsier’s disease. However, recent findings have shown that septo-optic dysplasia is a different disease and is not ONH (Garcia-Filion & Borchert, 2013).
ONH is a congenital defect in children, but can become an event in adults. The main characteristic of ONH is the abnormal color and shape of the optic nerve, which looks similar to a doughnut with a pink rim and white depression when viewed through an ophthalmoscope. Abnormal characteristics of the optic nerve are an indication of unusual function. A normal functioning optic disc should look orange-pink when viewed through an ophthalmoscope.
In the event of ONH occurring, the affected optic nerve seizes function and becomes white in color. Some common causes of optic nerve fiber failure result from inflammation, increased pressures (as in glaucoma), and/or reduced blood flow to the tissue. In cases of ONH associated with glaucoma, ophthalmologists and optometrists use a measurement called the cup-to-disc ratio to determine the progression of the disease. This is a measurement of the optic disc’s (the area where the optic nerve and associated blood vessels enter the retina, creating an anatomical “blind spot”) flatness to its natural cupping formation. The normal cup-to-disc ratio should not exceed 0.5 in diameter. However, if the cup-to-disc ratio measures greater than 0.5, this is an indication that the optic nerve is most likely pathological in nature. These characteristics are a result of a patient likely experiencing glaucoma. Furthermore, an abnormal optic disc will display a shade of white, as the dead nerve fibers are replaced by a pale appearance.
The first documented case of ONH was in 1915, but previous ONH cases could have occurred at a much earlier time. In 1915, German physician O. Schwarz produced the first drawing of a postmortem patient with ONH, which contained an illustration of the eye’s optic disc. Since then and in the past 30 years, ONH has been recognized as an epidemic cause of congenital blindness, but current research in endocrinology has developed treatments to improve this syndrome.
Eye Function and Brain Function
A person’s eyes take in outside light and send pictures through the optic nerves to the brain. In normal eye function, more than 1 million nerve fibers make up a single optic nerve that connects the brain to the eye. However, an individual who has ONH will have smaller or absent optic nerves traveling from the brain to the eye. Therefore, individuals with this condition have a reduced connection between the brain and the affected eye(s). In some cases, people with ONH have normal vision in only one eye, others experience reduced vision in each eye, and some people are nearly or completely blind.
A patient’s vision will need to be monitored on a semiannual or annual basis. Children who have been diagnosed with ONH by an optometrist or ophthalmologist should be referred to an endocrinologist who specializes in hormone treatment. This is to determine if any other physiological functions are being impaired or if their pituitary gland is functionally normally. For adults with an event of ONH, an ophthalmologist should monitor them regularly, particularly if glaucoma is suspected.
Treatment for OHN is provided to patients with the outcome of having increased vision. Patching therapy—where a patient wears a patch over the good eye during the daytime—has been used as an old method of treatment. In severe cases, this old style of treatment may result in developmental and learning setbacks in pediatric patients. Physiological and/or endocrine conditions that are correlated with ONH can be treated with daily medications. However, even with proper medical care, the lack of endocrine deficiencies can have adverse affects in certain patients, and thus they will need to be monitored very closely.
See also: Blindness; Optic Nerve; Visual Perception; Visual System
Borchert, Mark, & Pamela Garcia-Filion. (2008). The syndrome of optic nerve hypoplasia. Current Neurology and Neuroscience Reports, 8, 395—403. Retrieved from http://www.chla.org/sites/default/files/migrated/ONH_Review_Article.pdf
Garcia-Filion, Pamela, & Mark Borchert. (2013). Optic nerve hypoplasia syndrome: A review of the epidemiology and clinical associations. Current Treatment Options in Neurology, 15(1), 78—89. http://dx.doi.org/10.1007/s11940-012-0209-2. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3576022/#R1
Kaufman, Francine, et al. (n.d.). Optic nerve hypoplasia: A guide for parents. One Small Voice Foundation. Retrieved from http://www.onesmallvoicefoundation.org/pdf/ONH%20pamphlet_sm.pdf