Night blindness, also called nyctalopia, 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.
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. 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. Dark adaptation takes about 15-30 minutes and, when complete, increases light sensitivity by about 100,000 times.
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.
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. Persons who experience night blindness should not drive during the evening or at night.