edema
Summary
Edema is the abnormal accumulation of fluid in the interstitial space, resulting from disruption of Starling forces. Common causes include increased hydrostatic pressure, decreased oncotic pressure, increased vascular permeability, and lymphatic obstruction. It can be localized or generalized (anasarca) and signals underlying cardiac, renal, hepatic, or venous/lymphatic pathology.
Detail
Edema results from an imbalance in Starling forces governing fluid movement across capillary walls: capillary hydrostatic pressure, plasma oncotic pressure, interstitial hydrostatic pressure, and interstitial oncotic pressure, along with lymphatic drainage capacity.
Mechanisms: 1. Increased hydrostatic pressure - e.g., heart failure (right-sided HF causes systemic edema, left-sided causes pulmonary edema), venous obstruction (DVT), volume overload. 2. Decreased oncotic pressure - due to hypoalbuminemia from nephrotic syndrome, liver cirrhosis (decreased synthesis), protein-losing enteropathy, or malnutrition. 3. Increased vascular permeability - inflammation, burns, allergic reactions, sepsis (cytokine-mediated capillary leak). 4. Lymphatic obstruction - lymphedema from malignancy, surgery (post-mastectomy), filariasis, or radiation; causes protein-rich edema fluid. 5. Sodium retention - renal failure, heart failure activate RAAS leading to volume expansion.
Clinical distinctions: - Pitting edema: seen in heart failure, nephrotic syndrome, cirrhosis; indicates fluid in interstitium that can be displaced. - Non-pitting edema: classic for lymphedema and myxedema (hypothyroidism, due to glycosaminoglycan deposition). - Transudate vs exudate: edema fluid from hydrostatic/oncotic causes is low-protein transudate; inflammatory edema is high-protein exudate.
Specific presentations: - Pulmonary edema: dyspnea, orthopnea, crackles; cardiogenic (elevated pulmonary capillary wedge pressure) vs non-cardiogenic (ARDS, increased permeability). - Peripheral edema: lower extremity in heart failure/venous insufficiency; periorbital in nephrotic syndrome. - Ascites: fluid in peritoneal cavity, seen in cirrhosis (portal hypertension + hypoalbuminemia), heart failure, malignancy. - Anasarca: severe generalized edema, seen in nephrotic syndrome, severe hypoalbuminemia.
Diagnostic workup: assess JVP, BNP (heart failure), albumin/liver function, urinalysis for proteinuria (nephrotic syndrome), thyroid function.
Treatment targets underlying cause: diuretics (loop diuretics for heart failure/renal edema), sodium restriction, treating underlying hypoalbuminemia or lymphatic obstruction, compression therapy for lymphedema.
Sources
- Robbins & Cotran Pathologic Basis of Disease
- Guyton and Hall Textbook of Medical Physiology
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
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