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Some people lose the ability to see depending on the lighting conditions.

Hemeralopia, commonly known as day blindness, is the inability to see under bright light. Day blindness is more than just not being able to see due to glare. In most cases it is due to a disorder of the cone cells of the retina. The retina is the part of the eye that is responsible for capturing the image you’re looking at. Within the retina are cells called cones. Cones are especially good at daytime vision, but are less sensitive than their rod counterparts. Cones are clustered near the macula, and thus are responsible for central vision. There are three types of cones that receive different spectra of wavelengths. Together, they are responsible for colored vision.

Day blindness is characterized by the failure to see in high levels of light when normal eyes are able to distinguish objects. Light sources may appear more intense and glare may overwhelm vision. Additionally, those with day blindness may take a longer time to adjust from dark to brighter conditions. Central visual acuity may also decrease and patients may have a problem distinguishing colors.

Hemeralopia is characterized more by an avoidance of light than sensitivity to it. This is called heliophobia as opposed to photophobia. Patients prefer dusk and dim lighting to anything else. Hemeralopia does not actually cause pain, but patients tend to avoid bright light and going outside. In rare cases hemeralopia leads to a fear of going outside or going out in public at all.

The condition rarely appears on its own, and is normally a symptom of another disease. Hemeralopia is frequently associated with cone dystrophy, where the cones of the retina begin to disintegrate. The severity of hemeralopia will continue throughout one’s lifetime. Achromatopsia is a syndrome that is associated with color blindness and hemeralopia. Cataracts can cause hemeralopia if not operated on. Cancer that metastasizes to the retina can cause hemeralopia as well. Iris complications may cause more light to enter the eye than necessary, and thus could trigger hemeralopia. It can also be a side effect of certain medications or brain surgeries. Mild hemeralopia can occur from severe allergies.

In general, there is no cure for hemeralopia besides for treating underlying diseases. Removing cataracts helps light enter the eye properly, and can reduce or eliminate day blindness. Wearing specially made eyeglasses or sunglasses can help a patient go out during daytime conditions. Normal vision will return when medicines causing hemeralopia are stopped. There is no cure for hemeralopia caused by a congenital disease such as achromatopsia.