Night blindness is not a complete lack of vision at night, as the name implies. It is a below-average ability to see at night or in low light. Night blindness, unlike color blindness, is not a disorder in itself, but rather a symptom of an underlying condition. It can occur in people of all ages, even young children.
Your night vision naturally differs from your day vision in many ways. In darkness, the eye is basically color blind; visual acuity is poor, and the eye sees only a fraction of what it sees in daylight. A central scotoma (an area of diminished vision) appears in the center of the visual field; and the eye is unable detect stationary objects as well as it can detect moving objects.
If you have night blindness, you will have consistent difficulties in seeing at night, but will be able to see normally during the day or when an adequate amount of light is present. You will not be able to see objects in the dark that are easily visible to others, and your eyes may need more time to adjust after you go from a brightly lit space into a dark space, such as a movie theatre. People with night blindness often have problems driving at night. If you have a history of poor night vision, whether it is a recent occurrence or a long-standing problem, you should see your eye doctor for an evaluation.
Causes of Night Blindness
Night blindness, sometimes referred to as nyctalopia or impaired dark adaptation, is due to a disorder of the cells in the retina that are responsible for vision in dim light. It can be a symptom of a number of conditions that can be acquired or congenital (present at birth):
- Cataracts (clouded vision): people with cataracts often have difficulty reading, driving a car — especially at night — and seeing facial expressions.
- Myopia (nearsightedness): night blindness can be a sign of untreated myopia.
- Use of certain drugs: some glaucoma medications can cause constriction of the pupils, which can make it difficult to see at night.
- Vitamin A deficiency: Night blindness is one of the first signs of vitamin A deficiency, which is commonly associated with malnutrition. The vitamin deficiency often develops in malnourished children who are too young to recognize a problem with their night vision or report a vision problem.
- Congenital stationary night blindness with or without myopia: This stands for a group of genetically heterogeneous (mutation in gene) disorders. One type of congenital stationary night blindness (choroideremia) affects only males; females are carriers of the gene but do not have symptoms.
- Retinitis pigmentosa: This eye disease is caused by a number of genetic defects that result in damage to the retina. Affected persons may have decreased vision at night or in low light. They may also have problems with central and peripheral vision. Symptoms may detected in childhood but often become apparent in adulthood.
- Usher syndrome: This syndrome is characterized by hearing loss and retinitis pigmentosa, which, as noted above, can cause night blindness.
Testing for Night Blindness
If you have difficulty seeing at night, it is important to visit your eye care professional. He or she will perform tests to determine whether you have night blindness and whether it may be connected to an underlying disease. The eye examination will include the following:
- Tests to measure your visual acuity, ability to see colors, and your pupil light reflex
- Refraction test to measure your prescription for eye glasses or contact lenses
- Slit lamp examination to examine the structures in the front of the eye, including conjunctiva, cornea, eyelids, iris, lens, and sclera
- Retinal examination to look for any damage to the structures in the back of the eye ‒ the vitreous, retina, and choroid
Your eye doctor may order an electroretinogram, which measures the electrical responses of the rods and cones (the cells in the eye that sense light) when these cells exposed to light. This test can detect abnormal function of the retina, the light-detecting portion of the eye.
You may also undergo visual field testing, which can detect central and peripheral vision problems caused by glaucoma and other eye diseases or by conditions that affect the brain, such as stroke.
Treatment for Night Blindness
Night blindness caused by an acquired condition usually resolves after the underlying condition is treated. Cataracts are usually treated with surgery. Worsening myopia can be corrected with prescription glasses or contact lenses. Switching medications may reduce night blindness in people with glaucoma. Those with vitamin A deficiency usually respond well to nutritional supplementation and a healthy diet. Early treatment of vitamin A deficiency is critical; left untreated, the condition can lead to permanent blindness.
Night blindness as a result of a congenital disorder is permanent, and people who are afflicted in this way should be monitored by an eye care specialist. Affected persons need to take extra precautions to prevent injury that can occur as a result of poor vision at night.
Seeing your eye care professional for regular eye examinations is essential to managing night blindness. You may simply need prescription glasses for driving at night. Early detection is important to avoid the risk of injury to yourself and others while engaged in nighttime activities, such as driving.
How to Prevent Injury Due to Night Blindness
According to the National Safety Council, traffic death rates are three times greater at night than during the day. Driving at night is dangerous because a driver’s ability to react depends on vision, and normal vision is severely limited at night. Older drivers have even greater difficulty seeing at night. A fifty-year-old driver may need twice as much light to see as well as a thirty-year old.
The best way to protect yourself from injury due to night blindness is to avoid driving at night and drive only during the day. People with night blindness can have difficulty driving even in a well-lit city. If you must go out at night, you can take steps to protect yourself. Increase your visibility by cleaning your car windows and headlights, and slow down to give yourself more time to react to any unexpected hazards.
- R. L. Tomsak. Vision loss. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, eds. Neurology in Clinical Practice. 5th ed. Philadelphia: Butterworth-Heinemann; 2008:chap 14.
- P. A. Sieving, R. C. Caruso. Retinitis pigmentosa and related disorders. In: Yanoff M, Duker JS, eds. Ophthalmology. 3rd ed. St. Louis; Mosby Elsevier; 2008:chap 6.10.
- National Eye Institute. “Facts About Usher Syndrome,” October 2010, "http://www.nei.nih.gov/health/ushers/ushers.asp"
- MedlinePlus Medical Encyclopedia. “Vision - night blindness,” September 3, 2012, "http://www.nlm.nih.gov/medlineplus/ency/article/003039.htm
- National Safety Council, “Driving at Night Safety Facts and Tips,” "http://www.nsc.org/news_resources/Resources/Pages/DrivingatNight.aspx#.UeibmoY2TTI