In medicine, edema is "abnormal fluid accumulation in tissues or body cavities. Most cases of edema are present under the skin in subcutaneous tissue,"(i.e., as extracellular fluid.)  Not all edema is subcutaneous; some of the most critical are in or around organs, such as the eye, lungs, or chest cavity.
Similar terms are:
- Angioedema is "swelling involving the deep dermis, subcutaneous, or submucosal tissues, representing localized edema. Angioedema often occurs in the face, lips, tongue, and larynx."
- Lymphedema is "edema due to obstruction of lymph vessels or disorders of the lymph nodes.
- Myxedema is a "condition characterized by a dry, waxy type of swelling (edema) with abnormal deposits of mucopolysaccharides in the skin and other tissues. It is caused by a deficiency of thyroid hormones. The skin becomes puffy around the eyes and on the cheeks. The face is dull and expressionless with thickened nose and lips."
Extreme cases of edema are called anasarca.
- Diffuse edema, more commonly called peripheral edema or dependent edema.
- Focal edema from angiodema.
- Hydrostatic edema due to heart failure or venous insufficiency.
- Hypoprotenemic edema due to nephrotic syndrome, cirrhosis, protein-energy malnutrition (including Kwashiorkor and marasmus)
- Capillary lead edema including idiopathic edema
- Acute kidney injury or chronic kidney disease
- Medications such as vasodilator agents used for hypertension may cause edema by stimulating sodium reabsorption.
- Obesity may by associated with both lymphedema and edema. The odds ratio of having obesity are about 2 among people who complain of swollen legs. 75% of morbidly obese patients (body mass index >40) may have lymphedema. Obesity may be associated with edema due to obstructive sleep apnea and secondary pulmonary hypertension. 
Edema in one extremity but not the other may suggest a mechanical problem of blood or lymphatic circulation. Doppler ultrasonography may be informative; a number of additional examinations are specific to lymphedema. Most lymphedema is secondary to surgical or wound trauma, and the history will be informative. Venous and lymphadema can coexist.
On physical examination, when the examiner presses on the edematous tissue, the edema is pitting if the skin does not immediately spring back, leaving a depressed area. Not all edema is pitting. Fast recovery of pitting is associated with lower serum albumin levels. If the pitting lasts more than one minute, the serum albumin is probably over 3 (sensitivity=80%; specificity=76%). Presumably this is related to the viscosity of the interstitial fluid thus hypoalbuminemic interstitial fluid can reform more quickly.
Definitive care will address the underlying etiology, but various treatments can be quick interventions. Diuretics are the most important class of drugs. When the edema builds from ineffective heart pumping as in heart failure, positive inotropic agents help.
When fluid load overcomes the lungs, rotating tourniquets or even therapeutic venepuncture may be lifesaving in severe pulmonary edema.
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