Edema

Definition

Soft tissue swelling due to abnormal expansion of interstitial fluid volume.

Edema fluid is a plasma transudate that accumulates when movement of fluid

from vascular to interstitial space is favored. Since detectable generalized edema

in the adult reflects a gain of _3 L,renal retention of salt and water is necessary

for edema to occur. Distribution of edema can be an important guide to cause.

LOCALIZED EDEMA 

Limited to a particular organ or vascular bed;

easily distinguished from generalized edema. Unilateral extremity edema is usually

due to venous or lymphatic obstruction (e.g.,deep venous thrombosis, tumor

obstruction,primary lymphedema). Stasis edema of a paralyzed lower extremity

may also occur. Allergic reactions (“angioedema”) and superior vena

caval obstruction are causes of localized facial edema. Bilateral lower extremity

edema may have localized causes,e.g.,inferior vena caval obstruction, compression

due to ascites,abdominal mass. Ascites (fluid in peritoneal cavity) and

hydrothorax (in pleural space) may also present as isolated localized edema,

due to inflammation or neoplasm.

GENERALIZED EDEMA Soft tissue swelling of most or all regions of

the body. Bilateral lower extremity swelling,more pronounced after standing

for several hours,and pulmonary edema are usually cardiac in origin. Periorbital

edema noted on awakening often results from renal disease and impaired Na

excretion. Ascites and edema of lower extremities and scrotum are frequent in

cirrhosis or CHF. In CHF,diminished cardiac output and effective arterial blood

volume result in both decreased renal perfusion and increased venous pressure

with resultant renal Na retention due to renal vasoconstriction,intrarenal blood

flow redistribution,direct Na-retentive effects of norepinephrine and angiotensin

II,and secondary hyperaldosteronism.

In cirrhosis,arteriovenous shunts lower effective renal perfusion, resulting

in Na retention. Ascites accumulates when increased intrahepatic vascular resistance

produces portal hypertension. Reduced serum albumin and increased

abdominal pressure also promote lower extremity edema.

In nephrotic syndrome,massive renal loss of albumin lowers plasma oncotic

pressure,promoting fluid transudation into interstitium; lowering of effective

blood volume stimulates renal Na retention.

In acute or chronic renal failure,edema occurs if Na intake exceeds kidney’s

ability to excrete Na secondary to marked reductions in glomerular filtration.

Severe hypoalbuminemia [_25 g/L (2.5 g/dL)] of any cause (e.g.,nephrosis,

nutritional deficiency states,chronic liver disease) may lower plasma oncotic

pressure sufficiently to cause edema,if accompanied by low levels of nonalbumin

proteins [total protein _54 g/L (5.4 g/dL)].

Less common causes of generalized edema: idiopathic edema,a syndrome

of recurrent rapid weight gain and edema in women of reproductive age; hypothyroidism,

in which myxedema is typically located in the pretibial region;

drugs such as glucocorticoids,estrogens,thiozolidinediones, and vasodilators;

pregnancy; refeeding after starvation.

TREATMENT

Primary management is to identify and treat the underlying cause of edema

(Fig. 47-1).

Dietary Na restriction (_500 mg/d) may prevent further edema formation.

Bed rest enhances response to salt restriction in CHF and cirrhosis. Supportive

stockings and elevation of edematous lower extremities will help mobilize interstitial fluid. If severe hyponatremia (_132 mmol/L) is present,water intake

should also be reduced (_1500 mL/d). Diuretics (Table 47-1) are indicated for

marked peripheral edema,pulmonary edema,CHF,inadequate dietary salt restriction.

Complications are listed in Table 47-2. Weight loss by diuretics should be limited to 1–1.5 kg/d. Distal (“potassium sparing”) diuretics or metolazone

may be added to loop diuretics for enhanced effect. Note that intestinal edema

may impair absorption of oral diuretics and reduce effectiveness. When desired

weight is achieved,diuretic doses should be reduced.

In CHF (Chap. 126),avoid overdiuresis because it may bring a fall in

cardiac output and prerenal azotemia. Avoid diuretic-induced hypokalemia,

which predisposes to digitalis toxicity.

In cirrhosis and other hepatic causes of edema,spironolactone is the diuretic

of choice but may produce acidosis and hyperkalemia. Thiazides or

small doses of loop diuretics may also be added. However,renal failure may

result from volume depletion. Overdiuresis may result in hyponatremia,hypokalemia,

and alkalosis,which may worsen hepatic encephalopathy.