blindness

Blindness

transient or continuing inability to see with one or both eyes. Transient

blindness, in this instance called blackout, affects such persons as aviators

or astronauts if they undergo acceleration that exerts its force on them in the

direction from head to foot and if the force reaches five or six times the force

of gravity. Transient blindness may also be a feature of kidney disease (glomerulonephritis).

Continuing blindness may arise from injury or disease that affects any of the

structures or substances that light passes through on its way to the retina, the

layer of light-sensitive tissue that lines the back and sides of the eye, or

the causes of the blindness may lie in injuries or disease of the retina itself,

of the optic nerve, or of the visual centres of the brain.

Night blindness

also called Nyctalopia, failure of the eye to adapt promptly from light to darkness

and reduced ability to see in dim light or at night. It occurs as a symptom of

numerous diseases that cause degeneration of the rods of the retina (the sensory cells

responsible for vision in dim light); as an inherited deficiency in visual purple, or

rhodopsin, which is the pigment of the rods; or as a result of vitamin A deficiency.

Congenital night blindness with or without myopia (nearsightedness) occurs either as a

dominant, recessive, or sex-linked hereditary trait and usually remains stable

throughout life. Vitamin A deficiency causes night blindness that is usually not

severe, and vision recovers when adequate levels of the vitamin are administered.

Night blindness also occurs in the light-sensitive condition known as xerophthalmia but

is treatable with vitamin A.