Edema - types, pathogenesis and pathomorphology according to organ site
Edema is the accumulation of extra interstitial fluid within tissues. It can also be in cavities (e.g. pleural cavity = hydrothorax, pericardial cavity = pericardial effusion, peritoneal cavity = ascites), we use the term Anasarca for severe generalized edema.
We divide edema into inflammatory and non-inflammatory causes. Fluid that causes edema that is not due to inflammation is known as transudate, it has lower protein levels (less than 1.012 specific gravity) because the vessel walls don’t usually let proteins through. In contrast, exudate is when there is inflammation, during inflammation we have increased vascular permeability, and as a result, proteins can escape.
Edema can also be pitting and non-pitting. Non pitting edema is caused by lymphedema (blockage lymphatic vessels), Myxedema (advanced hypothyroidism) or lipedema. Everything else is pitting.
Pitting edema: firm finger pressure over the swollen shin leaves a dent that stays for some seconds after release.
Image: “Combinpedal” by James Heilman, MD, CC BY-SA 3.0, via Wikimedia Commons.
Let’s look at some of the main types:
- Increased hydrostatic pressure
- May be due to impaired venous return, congestive heart failure, water retention (hyperaldosteronism)
- Pushes fluid out of the veins into the interstitium
- Reduced plasma osmotic pressure
- Less fluid is drawn back into the vessels, so it accumulates in the tissues
- Lymphatic obstruction
- Less of an intake from lymphatic system causes fluid to build up in the tissues
- Water (and sodium) retention
- This reduces osmotic pressure in the vessels, as above, leading to more fluid diffusing across
Example: in right heart failure the raised venous pressure gives pitting ankle edema (hydrostatic); in nephrotic syndrome (albumin lost in urine) and liver cirrhosis (albumin not made) low plasma albumin lowers oncotic pressure, giving generalized edema and ascites; after axillary lymph node dissection for breast cancer the arm may develop lymphedema.
Edema can also be distinguished based on its location:
- Subcutaneous - often due to heart or liver
- Edema of loose connective tissue (e.g. Periorbital edema) - usually due to nephrotic syndrome
- Pulmonary edema - normally due to left ventricular failure
- Brain edema - can be local (due to abscess or tumour) or general