Optic nerve hypoplasia (ONH) is a medical condition arising from the underdevelopment (hypoplasia) 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, 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 and signs
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.
Imbalances in sex hormones may result in a delay in sexual development (puberty) or precocious puberty. Sex hormones may be tested from birth to 6 months of age (during mini-puberty).
Hyperprolactinemia (an excess of prolactin) often occurs in conjunction with ONH and indicates either dysfunction of the hypothalamus or a disconnect between the hypothalamus and pituitary gland. Hyperprolactinemia often correlates with the development of obesity in children with ONH. To date, there have been few reports of ONH occurrence in Asian countries, although it is uncertain why this is so.
Gestational and exposure history
Although many perinatal and prenatal risk factors for ONH have been suggested, the predominant, enduring, most frequent risk factors are young maternal age and primiparity (the affected child being the first child born to the mother). Increased frequency of delivery by caesarean section and fetal/neonatal complications, preterm labor, gestational vaginal bleeding, low maternal weight gain, and weight loss during pregnancy are also associated with ONH.
DM:DD ratio has proven to be a clinically useful measurement to help diagnose optic nerve hypoplasia. Where "DM" represents the distance from Disk to Macula, and "DD" represents Disc Diameter.
The mean disc diameter (DD) is (Vertical diameter of Disc+Horizontal diameter of Disc) divided by 2. The distance between the center of the disc and the macula is DM.
Interpretation: When the ratio of DM to DD is greater than 3, ONH is suspected, and when it is greater than 4, Optic Nerve Hypoplasia is definite. The incidence of ONH is increasing, although it is difficult to estimate the true prevalence. Between 1980 and 1999, the occurrences of ONH in Sweden increased fourfold to 7.2 per 100,000, while all other causes of childhood blindness had declined. In 1997, ONH overtook retinopathy of prematurity as the single leading cause of infant blindness in Sweden, with 6.3 in every 100,000 births diagnosed with ONH. England's most recent prevalence report in 2006 is 10.9 per 100,000.