OPTIC NERVE HEAD HYPOPLASIA

SIGNS AND SYMPTOMS
Because optic nerve head hypoplasia is congenital, it is typically diagnosed in younger patients at their initial eye examination. Visual acuity may range from normal to no light perception.

If the condition is unilateral, a relative afferent pupillary defect may be noted. Other dysfunctions of the afferent system, such as diminished color vision, red desaturation and brightness perception will likewise be present. Visual field defects may also be elicited but vary considerably-altitudinal loss, central and cecocentral scotomas, and other field defects have been documented. In addition, up to 50 percent of patients manifest a constant strabismus, most often esotropia.

Examination reveals a smaller-than-expected optic nerve, with the vasculature appearing large relative to the disc. If unilateral, there is a notable size difference in the nerve heads. A circumpapillary ring of white or yellowish scleral tissue is typically evident ("double-ring sign"). The normally bright reflex from the nerve fiber layer is diminished. A review of the patient's history may reveal associated brain disorders (e.g., absence of the septum pellucidum, pituitary dysfunction, porencephaly) and/or gestational disease, as well as a history of maldeveloped growth.

PATHOPHYSIOLOGY
The exact mechanism of optic nerve head hypoplasia is not completely understood. But the condition is believed to represent a dysplasia of the retinal ganglion cell layer with an associated loss of the nerve fiber layer, secondary to some interruption in fetal development. Underdevelopment of the optic nerve results, and the posterior scleral foramen "fills in" with connective and scleral tissues.

Many disorders have been implicated in this disorder, including gestational diabetes, maternal infection by cytomegalovirus, syphilis, rubella, fetal alcohol syndrome and other drug use by the mother while pregnant. ONH hypoplasia may be part of larger clinical syndromes such as septo-optic dysplasia, which is marked by short stature, congenital nystagmus and a hypoplastic disc. The majority of patients, however, have no associated systemic abnormalities.

MANAGEMENT
Remember, ONH hypoplasia is a congenital condition. Appropriate management begins with proper diagnosis. Use visual field testing to confirm your suspicions.

In addition, many clinicians photograph the posterior pole of the affected eye and measure the disc-macula/disc-disc (DM/DD) ratio; this allows them to compare the horizontal diameter of the nerve head to the distance between the fovea and the center of the nerve. In a normal eye, the DM/DD measures between 2:1 and 3.2:1. Ratios greater than this suggest hypoplasia.

In uncomplicated unilateral cases, manage the condition through patient education and protective eyewear. If the history or examination indicates any associated neurological manifestations, refer the patient for studies to rule out forebrain disease. Studies may include physical and neurologic evaluation, neuroimaging and endocrinologic assessment. In more profound cases where both eyes are affected, consider visual rehabilitation services.

CLINICAL PEARLS

Make sure to clearly distinguish this condition from amblyopia. Although both amblyopia and ONH hypoplasia can present with reduced acuity, strabismus and variable refractive error, the former remains a diagnosis of exclusion. In cases of ONH hypoplasia, the associated refractive and/or binocular findings are secondary findings rather than primary causes of the problem. Furthermore, conventional amblyopia therapy will not be helpful in ONH hypoplasia. Vision therapy is a lengthy, costly, yet fruitless option for these patients.