blindness

Complete absence or impairment of sight. It may be caused by heredity, accident, disease, or deterioration with age. Blind people can be trained to use echolocation and in certain cases blindsight to navigate around obstacles. Other aids include electronic devices that convert print to recognizable mechanical speech, and sonar devices.

Globally, the leading cause of blindness is trachoma. Other important causes include:

Age-related macular degeneration (AMD), the commonest form of blindness, occurs as the retina gradually deteriorates with age. It affects 1% of people over the age of 70, with many more experiencing marked reduction in sight.

Retinitis pigmentosa, a common cause of blindness, is a hereditary disease affecting 1.2 million people worldwide.

Education of the blind was begun by Valentin Haüy, who published a book with raised lettering in 1784, and founded a school. Aids to the blind include the use of the Braille and Moon alphabets in reading and writing. Guide dogs for the blind were first trained in Germany for soldiers blinded in World War I.

In the early 21st century the implantation of electronic light-sensor chips as an artificial retina began to be investigated. Results of a first clinical trial on six patients with retinitis pigmentosa were published in 2004 and revealed that the implants caused no side effects such as rejection or inflammation. All patients reported some improvement in their vision. As of 2006, this procedure was still at an experimental stage and not available commercially.