Night Blindness

Night blindness is defined as vision that is defective in reduced light. It can be caused by genetic disease, certain drugs, or vitamin A deficiency.

Night blindness can be mild and merely an inconvenience, or it can be severe and limiting, especially in certain occupations such as piloting an airplane or in the military.

Terminology
The medical term for night blindness is nyctalopia. This is in contrast to hemeralopia, which is the inability to see in bright light.



Night Vision
The eye sees using the retina of the eye. The retina is a net of nerve cells located at the back of each eye that enable vision. The retina is made up of two main kinds of cells: rods and cones. Cones are responsible for color vision and work best in bright light. Rods work in low light but are unable to see color. The cones are located mainly in the center of the retina, while the rods are located around the edges. This is why our vision at night is less colorful. This is also why we can see better in very dim light when we look slightly away from what we are looking at. In complete darkness (an absolute absence of light), neither the cones nor the rods can function, and we would see absolutely nothing. However, in dim light, we can see amazingly well, mainly because of the rods in our retina. A rod can register a single photon of light. This means that a human with perfect vision can see a lit match several miles away on a clear night.

Symptoms
Night blindness is generally caused by impaired functioning of the rods. This can result in a longer time to adapt to light and in a higher threshold of light needed to see at all. People with night blindness see poorly at night, and they also require more time for their eyes to adjust from bright areas to dim ones. In addition, people with night blindness may have poor contrast vision, especially in low light situations.

Causes
The causes of night blindness can be categorized as genetic or acquired, stationary (not changing) or progressive.

Genetic causes

 * Congenital stationary night blindness is a general term referring to a genetic condition of night blindness that arises from birth. It does not get better or worse over time. It is due to an inherited genetic defect that affects the retina. It is sometimes associated with myopia (short-sightedness) and nystagmus (an involuntary tremor of the pupil of the eye). The genetic basis of congenital stationary night blindness is either X-linked, autosomal recessive, or autosomal dominant.


 * X-linked congenital stationary night blindness is passed from mother to son on the X chromosome. For a woman to be affected, she must have an affected father and a mother who is a carrier of the gene.  The condition comes in two forms--complete and incomplete.  The complete form is characterized by a complete absence of function of the rod cells.  Each of the two forms is caused by a separate genetic defect of the X chromosome.  X-linked congenital stationary night blindness is also associated with myopia, nystagmus, and strabismus (a deviation of the eye, otherwise known as squint or cockeye).


 * Oguchi disease is the autosomal recessive form of congenital stationary night blindness. It is associated with a yellow or gray discoloration of the fundus (the back of the eye, where the retina is located).  People suffering from Oguchi disease adapt very slowly to darkness. After two to three hours in total darkness, the normal color of the fundus returns, and the individual slowly adapts to night vision. This disease is more common in individuals of Japanese ethnicity.


 * Retinitis pigmentosa is the most common genetic cause of night blindness. Retinitis pigmentosa is a neurocutaneous syndrome, a genetic condition affecting the skin and the nerves of the eye (the retina). The first sign of this disease is a loss of night vision in early childhood. After that, there is a slow, progressive loss of peripheral vision, until only tunnel vision remains. This eventually leads to total blindness, which usually does not set in until the 40's or 50's.  However, the progression varies.  Some people never lose their sight, and some people become blind early in childhood. Another sign of this disease is a change in the pigmentation of the skin of the lower abdomen, which also occurs in early childhood. There is no definitive cure for this disease, but high doses of vitamin A can slow its progression, and may delay the onset of blindness by ten years.  Future treatments may include retinal transplant, artificial retinal implants, and stem-cell treatment of the genetic defect.

Acquired Causes
Most of the acquired causes of night blindness (night blindness which is not present since birth and does not have a genetic basis), generally fall into the category of progressive night blindness. This condition is usually caused by progressive degeneration of the retina.

The most common acquired cause of night blindness is vitamin A deficiency. Vitamin A deficiency is rarely discovered, diagnosed, and treated, and so it is considered rare in developed countries. However, milder forms of this condition may be vastly underreported and underdiagnosed, as people generally make do without ideal night vision, or may not need good night vision. Dependence on night vision varies among individual habits and work conditions. As such, vitamin A deficiency may be the most common cause of night blindness.

The retina requires vitamin A for many reasons. Vitamin A forms a much needed nutrient for the nerve cells of the retina. Vitamin A is needed in significant amounts for light detection. In this respect, it is a component of the protein rhodopsin, which switches between the "off" and "on" positions in the presence of light. Vitamin A is needed to turn rhodopsin back to the off configuration. Without adequate vitamin A, the function of the rod cells deteriorate, and night blindness sets in.

Vitamin A is fat-soluble, and as a result it is stored by the body. A deficiency results only after months of foregoing any food with vitamin A. As such, it does not occur often. It may occur in settings with a very restricted availability of food, such as in some settings in the military or in developing countries. Children are susceptible to a deficiency either because they may not have built up large reserves, or because they are excessively picky in their diet. Pregnant women are also more susceptible to night blindness, especially during the seventh month of pregnancy when the demand for vitamin A by the fetus increases. Approximately 127 million preschool-aged children and 7 million pregnant women are deficient in vitamin A, and 6 million pregnant mothers worldwide have night blindness.

Diseases or conditions that result in poor absorption of fat will result in a deficiency of fat-soluble vitamins, including vitamin A, and can lead to night blindness. As such, night blindness may be associated with cystic fibrosis and Crohn's disease.

Drugs are another cause of night blindness. Drugs that can damage the retina and cause night blindness are called retinotoxic. Use of Quinine is often associated with night blindness. isotretinoin(Accutane) also has been associated with night blindness, particularly in people who have a low level of vitamin A before starting the medication.

Since the rods of the retina are located mainly on the outer edges of the retina, any condition that causes degeneration or damage to the outer edges of the retina may cause night blindness. For example, Peripheral Cortical Cataract can be associated with night blindness. Cataracts, chronic glaucoma, and choroidal dystrophy are other such conditions.

Impact
Night blindness has likely caused numerous automobile accidents, although this has been difficult to document. Most accidents occur around sunset, and this may be due to poor contrast vision as well as poor night vision. Generally, even at night in most places, headlights and streetlights provide enough light to see. However, this is not always the case in rural areas. Someone with documented night blindness may be restricted from driving at night.

Night vision is especially important for pilots, both commercial and in the military where vision is routinely checked. Night blindness can be disqualifying in these jobs. A military occupation that particularly relies on night vision is the scout. Although soldiers in most positions in the military depend on night vision, the scout specializes in having good night vision. Night blindness, even if temporary, during times of war can prove to be dangerous or fatal.

Some children who are afraid of the dark may in fact have night blindness. Fear of the dark, or nyctophobia, in children is not a fear of the dark per se but a fear of partially visible things in the room heightened by imagination. Night blindness may add to this effect, as the child may not be able to clearly see objects in the room for what they are.

Diagnosis
Diagnosis of night blindness in adults can be straightforward. A drug history and a diet history may be all that is needed to determine the cause. A trial of treatment with vitamin A may help as well. Occasionally, night blindness thought to be caused by vitamin A deficiency may actually be due to an underlying malabsorption disorder of the intestines and an inability to absorb fat-soluble vitamins.

In children, the diagnosis is difficult or impossible. It is possible that there are many children who have temporary night blindness due to vitamin A deficiency but go undiagnosed. As mentioned above, a new-onset fear of the dark may be a clue. Another clue is increased falls and bumps at night after the lights are out.

Testing
Testing can be done in a clinic or office. It is important to remember that a small amount of dim light is needed in order to see at all. Turning off the lights and holding a few fingers up is all that is needed to test for night blindness. It is important to test at what level of light the patient can see the object. It is equally important to test the amount of time it takes to see the object.

If it is not obvious from the history that the night blindness is due to vitamin A deficiency, ophthalmologist can perform further tests to detect retinal defects and other eye abnormalities.

Treatment
Treatment of the most common cause of night blindness, vitamin A deficiency, may be as simple as eating a diet rich in vitamin A, or taking supplements. Liver is a very good source of vitamin A because it stores the nutrient. Vitamin A is orange and is generally found in the orange fruits and vegetables (but not in oranges), such as carrots, sweet potatoes, pumpkin, cantaloupe, apricots, papaya, mangoes, and winter squash. Other good sources are egg yolks and butter. Fortunately, this cause of night blindness is quickly reversible.

Genetic causes of night blindness currently have no treatment, although high doses of vitamin A may still delay the progression of night blindness. Future treatments of genetic diseases causing night blindness (and blindness in general) may include retinal transplant, artificial retinal implants, and stem cell treatment of the genetic defect.

Trivia
Night blindness was also once known as moonblink, which was a temporary night blindness believed to be caused by sleeping in moonlight.

Treatment of night blindness with vitamin a was recognized as early as 30 ACE. At that time, the physician Aulus Cornelius Celsus recommended that night blindness be treated with liver, either by applying the fluid extracts of a cooking liver to the eyes, or by consuming the liver itself.

Other Resources
Nelson Textbook of Pediatrics 16th Edition, Behrman, Kliegman, Jenson; WB Saunders Company; Philadelphia, PA; 2000.
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