Optic nerve hypoplasia (ONH) is a medical condition arising from the underdevelopment of the optic nerve(s). This condition is the most common congenital optic nerve anomaly. The optic disc appears abnormally small, because not all the optic nerve axons have developed properly. It is often associated with endocrinopathies (hormone deficiencies), developmental delay, and brain malformations. The optic nerve, which is responsible for transmitting visual signals from the retina to the brain, has approximately 1.2 million nerve fibers in the average person. In those diagnosed with ONH, however, there are noticeably fewer nerves. Symptoms ONH may be found in isolation or in conjunction with myriad functional and anatomic abnormalities of the central nervous system. Nearly 80% of those affected with ONH will experience hypothalamic dysfunction and/or impaired development of the brain, regardless of MRI findings or severity of ONH. Vision ONH can be unilateral (in one eye) or bilateral (in both eyes), although it presents most often bilaterally (80%). Because the unilateral cases tend to have better vision, they are typically diagnosed at a later age than those with bilateral ONH. Visual acuity can range from no light perception to near-normal vision. Children diagnosed with ONH generally present with vision problems which include nystagmus (involuntary movement of the eyes), which tends to develop at 1 to 3 months and/or strabismus (inability to align both eyes simultaneously), manifested during the first year of life. The majority of children affected experience improvement in vision during the first few years of life, though the reason for this occurrence is unknown. There have been no reported cases of decline in vision due to ONH. Neuroradiographic abnormalities Estimates of cerebral malformations vary from 39% to 90% of children with ONH. Abnormalities evident via neuroradiography can include agenesis (absence) or hypoplasia of the corpus callosum, absence or incomplete development of the septum pellucidum, malformations of the pituitary gland, schizencephaly, cortical heterotopia, white matter hypoplasia, pachygyria, and holoprosencephaly. Hypoplasia of the corpus callosum, often in conjunction with other major malformations, is significantly associated with poor and delayed developmental outcome. ONH is often referred to as septo-optic dysplasia, a term that refers to agenesis of the septum pellucidum. It is now clear that the absence of the septum pellucidum does not correlate with the associated symptoms of ONH. Hypothalamic dysfunction Dysfunction of the hypothalamus results in loss of regulation over behavior and function of the pituitary gland (master gland).