Edema refers to the clinical condition in which an excessive amount of fluid accumulates in the extravascular interstitial space of the body. The balance of competing Starling forces (hydrostatic and oncotic pressure) between plasma and the interstitium normally favors a small net movement of fluid from the capillary lumen into the interstitial space. This fluid is then collected by the lymphatic system and returned to the venous system via the thoracic duct. Edema occurs when these different forces become unbalanced either from alterations to capillary hemodynamics (change in hydrostatic or oncotic pressure gradients across the capillary wall), an increase in capillary wall permeability, or impaired lymphatic function. Edema may be localized or generalized. Localized edema is found in diseases of the lymphatic system, with venous obstruction, and in conditions associated with increased permeability of the capillary wall (e.g., infection, burns, trauma, and allergic reactions). Allergic reaction is the most common cause of localized edema in childhood. Generalized edema usually develops secondary to hypoalbuminemia (the main source of plasma oncotic pressure) or increased hydrostatic pressure from sodium and water retention. Minimal-change nephrotic syndrome (MCNS), although uncommon in and of itself with an annual incidence of 2-7 cases per 100,000 children, accounts for most cases of generalized edema. Severe generalized edema is referred to as anasarca. Additionally, localized edema may be seen in the abdomen (ascites) or chest (pulmonary edema or pleural effusions), but are typically associated with generalized edema.

A variety of diseases may cause localized or generalized edema, with some contributing to both. They may be classified according to the pathophysiologic mechanism, although for localized edema the location of the edema may be more helpful as an approach to the diagnosis.

The general approach to children with edema is to first determine the character of the edema, localized versus generalized, as their subsequent etiologies are usually quite different. Localized edema can be unilateral or even fixed (not extending further) in distribution, whereas generalized edema would not. Generalized edema
often is dependent in nature, involving those areas affected by gravity most. This is typically the lower legs and feet in ambulatory children but may include the sacral area in nonambulating children. Generalized edema may also involve tissues that are easily distensible such as the eyelids, but also areas less easily examined like the scrotum or labia. However, localized edema may also be dependent, especially if it involves the lower extremity or scalp, or involve distensible tissues.

As both localized and generalized edema can be associated with life-threatening conditions, the immediate evaluation should always be to insure that the patient is stable from a cardiorespiratory standpoint before proceeding. Localized facial edema could concurrently involve the airway while patients with generalized edema could have pulmonary edema and/or cardiac compromise, all potential medical emergencies. An initial inquiry into how the patient is breathing while assessing their respiratory effort (tachypnea, retractions), breath sounds (stridor, wheezing, rales), and heart rate (tachycardia) and heart sounds (S3 or S4 gallop) should be completed before proceeding with any extended work-up.