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.
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