edema, in medicine, an abnormal accumulation of watery fluid in the intercellular spaces of connective tissue. Edematous tissues are swollen and, when punctured, secrete a thin incoagulable fluid. This fluid is essentially an ultrafiltrate of serum but also contains small amounts of protein. Minor differences in composition are found in various diseases with which edema is associated. Generalized edema (also called dropsy, or hydrops) may involve the cavities of the body as well as the tissues with the excessive accumulation of fluid. Cerebral edema is the swelling of part or all of the brain, caused by the presence of excess fluid within either the cells or the extracellular tissues of the brain. Pulmonary edema is the buildup of excess fluid in the alveoli (air sacs) of the lungs.
Edema is most frequently a symptom of disease rather than a disease in itself, and it may have a number of causes, most of which can be traced back to gross variations in the physiological mechanisms that normally maintain a constant water balance in the cells, tissues, and blood. Among the causes may be diseases of the kidneys, heart, veins, or lymphatic system; malnutrition; or allergic reactions. The treatment of edema generally consists of correcting the underlying cause, such as improving kidney or heart function. Edema may be a purely local condition (e.g., hives), or it may be a general one (e.g., nephrotic edema).
The term dropsy is somewhat archaic, and edema has come to be the preferred term.