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A 54-year-old man comes to your office because of worsening "leg swelling." The swelling began 3 months ago and has steadily increased since that time. He also notes polyuria and generalized fatigue.

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  • What additional questions would you ask to characterize the edema?
  • What are the 5 pathophysiologic categories that induce peripheral edema?
  • Can a definitive cause of the edema be determined by historical information alone?
  • What historical questions will be useful in determining the appropriate evaluation of the edema?

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Edema is the accumulation of fluid in the interstitial space between cells. It is categorized into 5 subtypes based on the Starling law of fluid flow across a membrane:

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where K = vessel permeability

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Pin = intravascular hydrostatic pressure

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Pout = interstitial hydrostatic pressure

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Oncin = intravascular oncotic pressure

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Oncout = interstitial oncotic pressure

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Fluid is kept in the intravascular space by the capillary walls that selectively allow small amounts of fluid to leave the vascular space to deliver water, oxygen, and nutrients to the body cells. Under normal circumstances, only a small amount of fluid leaves the vascular space and is returned to the vascular space via lymphatic drainage. The permeability constant of the capillary membrane regulates how much fluid leaks out (K). The high protein concentration in the blood also prevents excessive fluid from leaving the intravascular space by osmotically retaining water in the vessels (Oncin).

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Edema typically results when the pressure in the vessels (Pin) overrides the semipermeable capillary membrane, pushing more volume into the extravascular space.

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Edema can also arise when the lymphatic drainage of the tissues is obstructed, the capillary membrane permeability is increased (K), or the blood protein concentration is decreased (Oncin). Increased protein concentration in the interstitium (Oncout) is rarely a cause of edema, although excess fat in the interstitium may draw and hold water into the interstitial space causing edema (lipedema).

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The most efficient method of diagnosing the etiology of edema is to sequentially consider each of these 4 forces that augment fluid leaving the intravascular space and entering the interstitial space.

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AnasarcaEdema involving all aspects of the body: upper and lower extremities and the face.
AscitesCollection of fluid in the peritoneal cavity.
LipedemaEdema caused by fluid retained in the interstitial space by lipids in the dermis.
LymphedemaEdema caused by obstruction of lymphatic drainage of the tissues.
MyxedemaEdema resulting from hypothyroidism (see below).
Pretibial myxedemaNot technically edema, the swelling on the anterior shins is due to coalescing of subcutaneous plaques due to Graves disease antibodies infiltrating dermal tissue.
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Patients with edema but no additional symptoms are most likely to have venous stasis ...

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