Optic nerve hypoplasia

Optic nerve hypoplasia (ONH) is a medical condition that results in underdevelopment of the optic nerve(s). This condition is the most common congenital optic disc anomaly in which the optic disc appears abnormally small, due to the fact that not all the optic nerve axons have developed properly.

Development of the optic nerve
During the second month of pregnancy, a structure called the optic stalk develops into a pair of optic nerve bundles. These bundles send signals from the eyes to the occipital lobe of the brain, naturally undergo pruning as the fetus develops. In some individuals, however, either this pruning process is too complete, or the nerves simply fail to develop fully. Such an occurrence causes a congenital condition called optic nerve hypoplasia.

Symptoms
Optic nerve hypoplasia can appear in one or both eyes (then known as bilateral optic nerve hypoplasia ), causing anywhere from a mild to serious visual impairment in the form of decreased visual acuity and visual fields. People with this condition are also more likely to present with photophobia and nystagmus.

Because optic nerve hypoplasia involves the underdevelopment of structures located within the brain, the condition may also be found in conjunction with a constellation of hormonal imbalances and midline brain defects known as septo-optic dysplasia.

This condition is also sometimes known to cause a loss in the sense of smell or taste.

Cause
No one is certain as to what causes optic nerve hypoplasia. The condition is usually not hereditary.

Clinical presentation
Patients suffering from optic nerve hypoplasia have extremely variable visual acuities, ranging everywhere from normal to no light perception (blind); however, vision tends to be stable (unless amblyopia, or “lazy eye,” develops). Clarity of vision has been found to correlate with the integrity of the papillomacular bundle rather than with the size of the optic disc itself.

Segmental optic nerve hypoplasia may also occur. This is a condition in which only a portion of the optic nerve is affected. For example, patients presenting with superior optic nerve hypoplasia may retain visual acuity yet concurrently display inferior visual field defects. However, inferior segmental optic nerve hypoplasia is also possible. One example of this is found in patients suffering from optic disc coloboma, in which the surviving neuroretinal tissue is only seen in the superior rim of the optic disc.

Diagnosis
Optic nerve hypoplasia is diagnosed by direct ophthalmologic examination. Direct ophthalmoscopy allows the doctor to see a magnified view of the fundus, or the back part of the eye, which includes the optic disc, retina, macula, fovea, and posterior pole.

The optic discs of patients with this disease look small and pale. They also commonly show a “double-ring sign,” in which the optic disc is surrounded by a “yellowish-white mottled ring.” Furthermore, the blood vessels in the retina typically look normal; however, they are found to originate in a more central location, as opposed to the typical nasal location. The nerve fiber layers of the retinas of these patients also tend to appear to be thinner than normal. Finally, the foveal reflex may be reduced or absent.

Prognosis
The visual prognosis in optic nerve hypoplasia is quite variable. Occasionally, optic nerve hypoplasia may be compatible with near-normal vision; in other cases, one or both eyes may be functionally, or legally blind. Although most patients with optic nerve hypoplasia lead normally productive lives, those with septo-optic dysplasia may experience non-visual problems, for example, with growth retardation.

Associated central nervous system abnormalities
There are a number of central nervous system (CNS) abnormalities that are often associated with optic nerve hypoplasia. Septo-optic dyplasia, or de Morsier syndrome, is characterized by congenital, bilateral (both eyes) optic nerve hypoplasia and diminished or absent septum pellucidum. Other possible CNS anomalies that are associated include abnormal pituitary infundibulum or hemispheric migration anomalies, and hypogenesis or agenesis of the corpus callosum. There are also a number of endocrinologic diseases that may present with optic nerve hypoplasia, including pan-hypopituitarism, hypothyroidism, hypocortisolism, diabetes insipidus, and hyperprolactinemia. Due to the prevalence of these associations, it is critical that endocrine evaluation and MRI are included in the workups of patients presenting with optic nerve hypoplasia.