Mrs. V is 62-year-old woman with leg edema for the past 2 weeks.
|What is the differential diagnosis of edema? How would you frame the differential?|
Edema is defined as an increase in the interstitial fluid volume and is generally not clinically apparent until the interstitial volume has increased by at least 2.5–3 L. It is useful to review some background pathophysiology before discussing the differential diagnosis:
Distribution of total body water
67% intracellular; 33% extracellular
Extracellular water: 25% intravascular; 75% interstitial
Regulation of fluid distribution between the intravascular and interstitial spaces
There is constant exchange of water and solutes at the arteriolar end of the capillaries
Fluid is returned from the interstitial space to the intravascular space at the venous end of the capillaries and via the lymphatics.
Movement of fluid from the intravascular space to the interstitium occurs through several mechanisms
Capillary hydrostatic (hydraulic) pressure pushes fluid out of the vessels
Interstitial oncotic pressure pulls fluid into the interstitium
Capillary permeability allows fluid to escape into the interstitium
Movement of fluid from the interstitium to the intravascular space occurs when opposite pressures predominate
Intravascular (plasma) oncotic pressure from plasma proteins pulls fluid into the vascular space
Interstitial hydrostatic pressure pushes fluid out of the interstitium
In skeletal muscle, the capillary hydrostatic pressure and the intravascular oncotic pressure are the most important.
There is normally a small gradient favoring filtration out of the vascular space into the interstitium; the excess fluid is removed via the lymphatic system.
Edema formation occurs when there is
An increase in capillary hydrostatic pressure (for example, increased plasma volume due to renal sodium retention)
An increase in capillary permeability (for example, burns, angioedema)
An increase in interstitial oncotic pressure (for example, myxedema)
A decrease in plasma oncotic pressure (for example, hypoalbuminemia)
Although it is possible to construct a pathophysiologic framework (Figure 15–1) for the differential diagnosis of edema, it is more useful clinically to combine anatomic, pathophysiologic, and organ/system frameworks:
Generalized edema due to a systemic cause and manifested by bilateral leg edema, with or without presacral edema, ascites, pleural effusion, pulmonary edema, periorbital edema
Systolic or diastolic dysfunction, or both
Advanced renal failure of any cause
The most common systemic causes of edema are cardiac, renal, and hepatic diseases as well as anemia.
Antidepressants: Monoamine oxidase inhibitors
Calcium channel blockers, especially dihydropyridines
Direct vasodilators (hydralazine, minoxidil)
Nonselective nonsteroidal antiinflammatory drugs (NSAIDs) and cyclooxygenase-2 inhibitors
Limb edema due to a venous or lymphatic cause, manifested by unilateral or bilateral edema
Deep venous thrombosis (DVT) (see Chapter 14, Dyspnea for a full discussion of lower extremity DVT)
Lymphatic obstruction (lymphedema)
Primary (idiopathic, often bilateral)
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