Dictionary

hyperactivity A higher-than-normal level of activity. An organ can be described as hyperactive if it is more active than is usual. Behavior can also be described as hyperactive. See also attention deficit hyperactivity disorder.

hyper- Prefix meaning high, beyond, excessive, or above normal, as in hyperglycemia (high sugar in the blood) and hypercalcemia (high calcium in the blood). The opposite of hyper- is hypo-.

hyoglossus The muscle that permits the tongue to be held on the floor of the mouth.

hymen A thin membrane that may completely or partially cover the vaginal opening before first sexual intercourse but that usually disappears before puberty.

hygiene The science of preventive medicine and the preservation of health. Also commonly used as a euphemism for cleanliness and proper sanitation.

hydroxyapatite crystal disease Inflammation caused by hydroxyapatite crystals. Hydroxyapatite molecules can group together (crystallize) to form microscopic clumps. If the tiny crystals of hydroxyapatite are deposited by mistake in or around joints, they may cause inflammation of the joints and nearby tissues, such as tendons and ligaments. Hydroxyapatite crystal disease is sometimes the cause of…

hydroxyapatite An essential ingredient of normal bone and teeth that makes up bone mineral and the matrix of teeth and gives them their rigidity. See also hydroxyapatite crystal disease.

hydrops fetalis A serious and potentially fatal condition of a fetus characterized by gross edema (swelling) from abnormal accumulation of fluids in the body. Hydrops fetalis has both immune-related and non-immune-related causes. Immune-related hydrops fetalis can be due to hemolytic disease such as Rh blood group incompatibility, in which antibodies crossing the placenta from the…

hydronephrosis Distention of the kidney with urine. Hydronephrosis is caused by obstruction of urine outflow (for example, by a stone blocking the ureter).

hydrogen breath test A test that uses the measurement of hydrogen in the breath to diagnose several conditions that cause gastrointestinal symptoms. Large amounts of hydrogen may be produced by anaerobic bacteria in the colon when there is a problem with the digestion or absorption of food in the small intestine that allows more unabsorbed…

hydrocephaly See hydrocephalus.

hydrocephalus ex-vacuo Hydrocephalus that occurs when there is damage to the brain caused by stroke or injury, in which there may be an actual shrinkage of brain substance. The CSF pressure itself is normal. See also hydrocephalus; hydrocephalus, normal pressure.