Defects in vision can arise from problems with the optics of the eye (astigmatism, hypermetropia, keratoconus) or with the transparency of the cornea, which may be temporary (as in keratoscleritis and keratomalacia) or lens (see cataract), with deficiencies of particular classes of photosensitive cells (retinal rods and retinal cones) causing stationary night blindness or colour blindness of various sorts (see dichromatism, protanopia, tritanopia). In some cases, there is progressive loss of retinal cells (see age-related macular degeneration; cone–rod dystrophy; Doyne honeycomb retinal dystrophy; gyrate atrophy; Leber's optic atrophy; Oguchi's disease; retinitis pigmentosa), or the whole retina can become detached, a problem with neovascularization of the retina in diabetes and in Stickler's syndrome. Defects in the phototransduction system can occur at various levels in the signalling cascade (see Aland Island eye disease; arrestins; opsin; rhodopsin; Stargardt disease; transducin) Other vision-related problems include uveitis and glaucoma; in some cases the defect is not obvious (amaurosis). Infection with Onchocerca or Trachoma can cause blindness, and blindness can be a feature of more complex syndromes (hypotrichosis with juvenile macular dystrophy, Usher's syndrome). Problems can also occur with the eye muscles (Duane retraction syndrome). More complex disorders such as cortical blindness can occur at a higher level of processing of the visual image, usually due to damage of the relevant parts of the brain, typically the visual cortex.
Subjects: Medicine and Health.