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Alternative Names: Nyctalopia.
Night blindness is the inability or reduced ability to see in dim light or darkness. It also refers to the condition in which the time it takes for the eyes to adapt to darkness is prolonged.
Light travels through the cornea and lens and lands on the retina at the back of the eye. The retina is composed of photoreceptors - specialized nerve cells which convert light rays into electrical signals and transmit them to the brain, which then creates an image.
There are two types of photoreceptors, rods and cones. There are some 3 million cones and 100 million rods in each eye. Rods see only black and white, but cones see colors. Rods also enable the eyes to detect motion and provide peripheral vision. Rods are responsible for vision in dim light and cones are responsible for vision in bright light. The rods are spread throughout the retina, but the cones are only in the center of the retina.
Vision in dim light or darkness is blurry because of the connections between the photoreceptors and the nerve cells which are linked to the brain. Each rod must share this connection to the brain with several other rods so the brain does not know exactly which rod produced the signal. Vision in bright light is sharp because each cone has its own connection to the brain so the brain can determine exactly where on the retina the signal originated.
Another feature of rods is that they must adapt to darkness. This is best exemplified by walking into a dark movie theater. At first, one can see very little. With time, vision improves and one is able to discern objects. Ultimately, one can see moderately well. This dark adaptation process occurs because of the chemical nature of rhodopsin. Rhodopsin is decomposed in bright light, making the rods nonfunctional. In darkness, rhodopsin is regenerated faster than it can be decomposed. Full dark adaptation takes about 15-30 minutes and, when complete, increases light sensitivity by about 100,000 times.
Persons who experience night blindness should not drive during the evening or at night.
Causes & Development
Night blindness is a symptom of several different diseases or conditions, sometimes caused by unlikely diseases or medications. All of the possible causes of night blindness are associated with the way in which the eye receives light rays.
Diagnosis & Tests
Night blindness can be diagnosed and treated by an ophthalmologist, a physician who specializes in eye disorders. Opticians can only dispense eye glasses but optometrists may be able to diagnose and treat vision problems.
Diagnosis begins with a detailed medical history regarding the night blindness. Questions include: severity of night blindness, when night blindness began and whether it occurred gradually or suddenly. An eye examination is performed and a slit lamp examination - in which a narrow beam of intense light is used to examine the internal components of the eye - may also be performed. Additional testing may be performed based upon the results of these standard tests.
Seek medical attention if
Changes in vision should never be taken lightly. Because night blindness can be a symptom of a serious disease, an ophthalmologist should be consulted before a person embarks on self treatment.
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Signs, symptoms & indicators of Night Blindness:
Risk factors for Night Blindness:
Night Blindness suggests the following may be present: | | | | | Organ Health |  Retinitis Pigmentosa | Retinitis pigmentosa results in visual rods being destroyed early in the course of disease, resulting in night blindness. Night blindness in children may be an early indicator of retinitis pigmentosa. |
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KEY | | Weak or unproven link |  | | Strong or generally accepted link |  | | Proven definite or direct link |
GLOSSARY
Cataract (Cataracts)
A steadily worsening disease of the eye in which the lens becomes cloudy as a result of the precipitation of proteins. Most cataracts are caused by the functions of the body breaking down. Eye trauma, such as from a puncture wound, may also result in cataracts.
Cornea
Transparent structure forming the anterior part of the eye.
Liver (Hepatic)
The largest and one of the most complex organs of the body, the liver is responsible for much of the metabolism of fats, proteins and carbohydrates. It is the site of much of the body's detoxification. It is connected very closely with digestion and the regulation of blood sugar, among many other functions. Found behind the ribs on the right side of the abdomen, it has many important functions such as removing harmful material from the blood, making enzymes and bile that help digest food, and converting food into substances needed for life and growth. Hepatic: Pertaining to the liver.
Metabolism (Metabolic, Metabolize, Metabolizes, Metabolizing)
The chemical processes of living cells in which energy is produced in order to replace and repair tissues and maintain a healthy body. Responsible for the production of energy, biosynthesis of important substances, and degradation of various compounds. Also defined as the sum total of changes in an organism in order to achieve a balance (homeostasis): Catabolic burns up, anabolic stores and builds up; the sum of their work is metabolism.
Retina (Retinal)
A 10-layered, frail nervous tissue membrane of the eye, parallel with the optic nerve. It receives images of outer objects and carries sight signals through the optic nerve to the brain.
Vitamin A
A fat-soluble vitamin essential to one's health. Plays an important part in the growth and repair of body tissue, protects epithelial tissue, helps maintain the skin and is necessary for night vision. It is also necessary for normal growth and formation of bones and teeth. For Vitamin A only, 1mg translates to 833 IU.
Zinc
An essential trace mineral. The functions of zinc are enzymatic. There are over 70 metalloenzymes known to require zinc for their functions. The main biochemicals in which zinc has been found to be necessary include: enzymes and enzymatic function, protein synthesis and carbohydrate metabolism. Zinc is a constituent of insulin and male reproductive fluid. Zinc is necessary for the proper metabolism of alcohol, to get rid of the lactic acid that builds up in working muscles and to transfer it to the lungs. Zinc is involved in the health of the immune system, assists vitamin A utilization and is involved in the formation of bone and teeth.
Last updated: Dec 01, 2008
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