Damage to the iris causes a disruption in the ability to regulate brightness. The visually impaired student will have difficultly adapting as light levels change in the environment. Adaptation problems can also result when there is damage to the retinal cells that are part of the iris-retina feedback loop, or to the brain level connections that regulate light adaptation. There are systemic diseases like atherosclerosis and diabetes that affect the blood vessels in the choroid and consequently reduce nutrition to retinal cells. These diseases also can release blood into retinal tissue or the vitreous, blocking light from reaching the photoreceptors.
Some disorders affect only the central processing system.
Optic atrophies, which appear commonly as a congenital condition, directly affect nerve transmission from the macula. Malnutrition can selectively destroy central vision at the retinal level. Other disorders impact primarily on the peripheral processing system. Retinitis pigmentosa destroys peripheral cells initially. Conditions like retinopathy of prematurity often damage portions of the peripheral retina sparing central vision.