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

Nyctalopia, commonly known as night blindness or less commonly as moonblink, is the inability to see in low light. Night blindness is more than just not being able to see because it is dark. It most cases it is due to a disorder of the rod cells of your 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 rods. Rods are especially good at nighttime vision, but lack the ability to distinguish color. Since they are on the edges of the retina, they are also responsible for peripheral vision.

Night blindness is characterized by the failure to see in low levels of light when normal eyes are able to distinguish objects. Light sources appear dimmer and dimly lit areas can appear pitch black. Additionally, those with night blindness may take a longer time to adjust from bright to dark conditions. Peripheral and contrast vision are also decreased.

The condition rarely appears on its own, and is normally a symptom of another disease. Nyctalopia is most frequently associated with retinitis pigmentosa, a retinal dystrophy that causes the deterioration of the rod cells.  Those with RP have continually progressing nyctalopia throughout their lifetime. Vitamin A deficiency is the next leading cause of night blindness and causes more cases of preventable childhood vision loss than any other disorder. Zinc deficiency is also a possibility. Cataracts, myopia, macular degeneration, and retinal detachment are other common causes of nyctalopia. It can also be a side effect of certain medications or laser surgeries.

Nyctalopia can appear as a primary condition where it is the main symptom of the disease known x-linked congenital stationary night blindness (CSNM). Unlike other cases of nyctalopia, the level of night blindness remains constant throughout a patient’s life. Type 1 CSNM is when the rods show no response to light at all; this is known as complete CSNM. Type 2 is when the rods show a small response to light; this is known as incomplete CSNM. Both types of patients may also experience blurred vision, strabismus, nystagmus, or myopia. Those with type 1 may have cone deterioration as well. Those with type 2 may also be hyperopic instead of myopic.

Some forms of nyctalopia are treatable. Removing cataracts, for example, will reduce night blindness. Normal vision will return when medicines causing nyctalopia are stopped. Taking vitamin A supplements and getting corrective eyewear will also improve vision at night when deficiency or extreme myopia is the problem. Congenital or RP induced nyctalopia are non-treatable at the moment. Visual therapy can help improve vision or help one adapt to progressive night blindness.

Some mammals, such as horses and dogs, also experience night blindness.