Leber hereditary optic neuropathy (LHON) is a form of progressive blindness. It is genetic in origin and is extremely rare. The symptoms typically first occur when those who are afflicted are more than 10 years old.
When a person who has Leber hereditary optic neuropathy first suffers from the symptoms of the disease, he or she experiences blurred sight or cloudiness of vision. This can occur in both eyes or just one, although both eyes typically are affected eventually. As the disease progresses, the person develops problems seeing well enough to recognize other people by sight, drive a car or read. Color perception also worsens.
All of these symptoms are caused by the fact that the nerve that transmits visual data to the brain from the eyes is slowly dying. Generally, the loss of vision does not cause any pain, and sometimes, the vision can improve. Some people who have Leber hereditary optic neuropathy, however, also have trouble moving properly, have erratic heartbeats or develop a multiple sclerosis-type of illness.
Mutations in certain genes are responsible for the nerve deterioration. Four main genes are involved: MT-ND1, MT-ND4L, MT-ND4 and MT-ND6. All of these genes are located in particular areas of a human cell.
The parts of the cell that contain the genes are called the mitochondria. These are small structures that help the cell make usable energy from food. Most of the genetic information inside a cell is wrapped up in a different part of the cell, called the nucleus, but the mitochondria also have little packets of genes.
Normally, the genes involved in LHON disease perform an essential function for the mitochondria. When one of these genes is mutated, however, the gene does not do its jobs properly, and disease results. As of 2011, it was unknown exactly how the individual abnormal genes cause problems.
Every human inherits mitochondria from his or her mother and none from the father. This is because the egg contains mitochondria, but a sperm does not. When an embryo is formed, it has only the mother's mitochondria and therefore only the mother's mitochondrial genes.
Inheritance of one of the abnormal genes, therefore, can come only from the mother's side. On rare occasions, a person can have a spontaneous mutation in his or her genes that is not inherited. Not all people who have an abnormal gene will develop Leber hereditary optic neuropathy, although the reasons for this are unknown. Smoking, drinking alcohol, a head injury or stress might be factors in determining who develops this disease, but these causations had not been proved as of 2011.