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narcolepsy
a sleep disorder characterized by excessive daytime sleepiness and recurring unwanted episodes of sleep (“sleep attacks”). People with narcolepsy may abruptly fall asleep at almost any time, including while talking, eating, or even walking. The attacks may range from embarrassing or inconvenient to severely disabling, interfering with a person's daily life. An estimated 125,000–250,000 people in the United States have narcolepsy; it occurs about equally in males and females.
Most people with narcolepsy also experience cataplexy, sudden muscular weakness without loss of consciousness, which usually accompanies laughter or anger. Other symptoms, occurring just after falling asleep or upon awakening, include sleep paralysis (a feeling that one cannot move) and vivid hallucinations.
The cause of narcolepsy is not known with certainty, and there is no cure. Treatment, including regular planned naps and the use of stimulant drugs (e.g., amphetamines) plus antidepressants for cataplexy, can help to control its symptoms.
nausea
sensation of discomfort, or queasiness, in the stomach. It may be caused by irritation of the stomach by food or drugs, unpleasant odors, overeating, fright, or psychological stress. It is usually relieved by vomiting. Nausea is frequently present during the early months of pregnancy, and it is a concomitant of motion sickness. However, nausea may also be the symptom of a serious illness; thus persistent nausea should receive medical attention.
nearsightedness
nearsightedness or myopia is the defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it. Corrective eyeglasses with concave lenses compensate for the refractive error and help to focus the image on the retina. Hard corneal contact lenses or soft hydrophilic contact lenses are another option, usually offering better acuity and peripheral vision than do eyeglasses. (Contact lenses may be troublesome for people who tend to get eye infections or have hand tremors, however.) Nearsightedness can also be corrected by using laser cornea surgery and a device called a microkeratome to flatten the eye, or by surgically implanting a corrective lens behind the iris.
neoplasm
neoplasm or tumor is a tissue composed of cells that grow in an abnormal way. Normal tissue is growth-limited, i.e., cell reproduction is equal to cell death. Feedback controls limit cell division after a certain number of cells have developed, allowing for tissue repair but not expansion. Tumor cells are less responsive to these restraints and can proliferate to the point where they disrupt tissue architecture, distort the flow of nutrients, and otherwise do damage.
Tumors may be benign or malignant. Benign tumors remain localized as a discrete mass. They may differ appreciably from normal tissue in structure and excessive growth of cells, but are rarely fatal. However, even benign tumors may grow large enough to interfere with normal function. Some benign uterine tumors, which can weigh as much as 50 lb (22.7 kg), displace adjacent organs, causing digestive and reproductive disorders. Benign tumors are usually treated by complete surgical removal. Cells of malignant tumors, i.e., cancers, have characteristics that differ from normal cells in other ways beside cell proliferation. For example, they may be deficient in some specialized functions of the tissues where they originate. Malignant cells are invasive, i.e., they infiltrate surrounding normal tissue; later, malignant cells metastasize, i.e., spread via blood and the lymph system to other sites.
Both benign and malignant tumors are classified according to the type of tissue in which they are found. For example, fibromas are neoplasms of fibrous connective tissue, and melanomas are abnormal growths of pigment (melanin) cells. Malignant tumors originating from epithelial tissue, e.g., in skin, bronchi, and stomach, are termed carcinomas. Malignancies of epithelial glandular tissue such as are found in the breast, prostate, and colon, are known as adenocarcinomas. Malignant growths of connective tissue, e.g., muscle, cartilage, lymph tissue, and bone, are called sarcomas. Lymphomas and leukemias are malignancies arising among the white blood cells. A system has been devised to classify malignant tissue according to the degree of malignancy, from grade 1, barely malignant, to grade 4, highly malignant. In practice it is not always possible to determine the degree of malignancy, and it may be difficult even to determine whether particular tumor tissue is benign or malignant.
nephritis
The inflammation of the kidney. The earliest finding is within the renal capillaries (glomeruli); interstitial edema is typically followed by interstitial infiltration of lymphocytes, plasma cells, eosinophils, and a small number of polymorphonuclear leukocytes. The disease is thought to be immunological, but immunoglobulins and drug antigens have been found in only a few cases. The damage is usually reversible when the cause is recognized and removed, but severe cases can progress to fibrosis and renal failure. The disease can take several forms. Pyelonephritis is usually associated with a bacterial infection transmitted from the bladder or blood; it affects the renal pelvis and is treated with antibiotics. Glomerulonephritis, or Bright's disease, causes degenerative changes in the glomeruli and is believed to be an allergic response to infection elsewhere in the body. Symptoms include headache, mild fever, puffiness of the eyes and face, high blood pressure, and discoloration of the urine. Treatment includes bed rest and limiting the intake of water, sodium, and proteins, and, if necessary, dialysis; antibiotics are given to halt infections (e.g., streptococcal) invasion. The disease occurs more frequently among the young. About 95% of patients recover from the acute phase of the disease; however, if glomerulonephritis becomes chronic, renal damage results after many years, causing kidney failure..
nephrosis
kidney disease characterized by lesions of the epithelial lining of the renal tubules, resulting in marked disturbance in the filtration function and the consequent appearance of large amounts of protein (albumin) in the urine (see urinary system). The nephrotic syndrome can result from a number of conditions including streptococcal infection in children leading to chronic glomerulonephritis, reaction to toxins, diabetes, collagen disease, and other end-stage kidney diseases. The major symptom is massive edema. Corticosteroid therapy has been successful in treating certain forms of the disease.
neuralgia
An acute paroxysmal pain along a peripheral sensory nerve. Unlike neuritis, there is no inflammation or degeneration of nerve tissue. Neuralgia occurs commonly in the area of the facial, or trigeminal, nerve and brings attacks of excruciating pain at varying intervals. Often no cause can be found for trigeminal neuralgia, and in severe cases deadening of the nerve with novocaine or alcohol, or even surgical interruption of the nerve, is necessary to bring relief. Neuralgia can be caused by such disturbances as diabetes, infections, diseases of the nervous system, anemia, and extreme cold. The pain may occur for many months after an attack of shingles (see herpes zoster), and it is one of the symptoms of syphilitic involvement of the central nervous system. In many cases, pain can be relieved by hot applications, drugs, and various kinds of physiotherapy.
neuritis
The inflammation of a peripheral nerve, often accompanied by degenerative changes in nervous tissue. The cause can be mechanical (injury, pressure), vascular (occlusion of a vessel or hemorrhage into nerve tissue), infectious (invasion by microorganisms), toxic (metallic or chemical poisoning, alcoholism), or metabolic (vitamin deficiencies, pernicious anemia). Symptoms of neuritis that arise from involvement of sensory nerves are tingling, burning, pin-and-needle sensations, or even loss of sensation. If motor nerves are involved, symptoms may range from a slight loss of muscle tone to paralysis. Since neuritis is regarded as a condition that results from a number of disorders, rather than a disease in itself, treatment is directed first at the underlying cause.
night blindness
An inability to see normally in subdued light. It is usually a result of vitamin A deficiency. The rod cells, one of two light-sensitive areas of the retina of the eye, are impaired in their capacity to produce a chemical compound called rhodopsin, or visual purple, that is necessary for the perception of objects in dim light. Consequently, the visual threshold, or the minimum intensity of light necessary for sight, is greatly increased. Folk medicine has long recognized the role of the ingestion of liver in alleviating the condition, but it was not until the first quarter of the 20th cent. that vitamin A was identified as the crucial element. Treatment of night blindness consists of the oral or intravenous administration of vitamin A.
nosebleed
nasal hemorrhage occurring as the result of local injury or disturbance. Most nosebleeds are not serious and occur when one of the small veins of the septum (the partition between the nostrils) ruptures. These will usually stop without treatment or when pressure is applied to the nose. A nosebleed may also occur in association with infections, heart failure, hypertension (high blood pressure), arteriosclerosis, scurvy, leukemia, hemophilia, and other disorders. Persistent nosebleeds should be brought to the attention of a physician, who can stop the bleeding with vasoconstrictors and topical anesthetics.