Chapter 593

### Optic Nerve Hypoplasia

Optic nerve aplasia is extremely rare, and most often the diagnosis represents an extreme example of optic nerve hypoplasia. Optic nerve hypoplasia is characterized by a reduction in the number of axons within the optic nerve. The nerve head is small and often pale. There may be a white or yellow peripapillary halo surrounded by a ring of pigment (double ring sign) corresponding to the size of a normal disc (Fig. 593-1). The normal distance from the temporal disc edge to the macula is 3 to 3.4 disc diameters. If the optic nerve is small, the number of disc diameters increases to greater than 4 disc diameters. The retinal vasculature may be tortuous, and the fovea pit may be underdeveloped. On radiographic study, the optic canal is small. The diagnosis can be difficult if optic nerve hypoplasia is mild. Disc photography and quantitative assessment of optic nerve head features by a nerve head analyzer may be helpful in establishing a diagnosis.

###### Figure 593-1.

Optic nerve hypoplasia. Arrow points to optic nerve. Surrounding hypopigmentary area (*) makes up the “double ring” sign and represents the scleral canal through which a normal-sized optic nerve would have run.

Visual acuity ranges from normal to no light perception but is stable over time. It is difficult to predict vision outcome based on the appearance of the optic nerve head or with neuroimaging. Most cases are sporadic. Familial instances are rare. Optic nerve hypoplasia has been associated with fetal alcohol syndrome; maternal diabetes (usually a hemihypoplasia affecting the upper or lower half of the disc); maternal use of quinine, anticonvulsants, or aminopterin; maternal illicit drug use; in utero cytomegalic infection; toxemia; and adolescent pregnancy. Accompanying ocular disorders may include aniridia, albinism, coloboma, Duane retraction syndrome, high myopia, and numerous neurological and pediatric conditions.

Up to 78% of affected patients have bilateral optic nerve hypoplasia, and 75% have neurodevelopmental problems that accompany bilateral involvement. In bilateral optic nerve hypoplasia, particularly in severe forms, poor vision and nystagmus may be apparent early in life, and there may be associated abnormalities such as mental retardation, delayed development, seizures, deafness, cerebral atrophy, hemiparesis, ventricular defects, porencephalic cyst, hypopituitarism, hypothyroidism, and diabetes insipidus. Sudden death has been reported in response to a viral illness likely due to unrecognized pituitary insufficiency. Septo-optic dysplasia, also known as de Morsier syndrome, is one of the more common bilateral optic nerve hypoplasia syndromes. It is characterized by midline central nervous system abnormalities such as hypothalamic dysfunction, pituitary dysfunction, and agenesis of the septum pellucidum; malformation of the optic chiasm; and agenesis of the corpus callosum. Patients have a characteristic facies with a broad forehead and large anterior fontanelle. Mutations in the autosomal dominant HESX1 gene at 3p21.2-p21.1 may be responsible in many affected patients.1

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