Mechanism of Pitting and Non-Pitting edema

This was a topic which I found very difficult to comprehend even though the concept behind it was very easy. It is customary to mention about edema in general examination and if it is present, examiners can take the viva to any system quite easily. The differential diagnosis of both pitting and non-pitting edema was difficult to remember. Later when I understood the mechanism, it became easy. I will here explain in simple words the mechanism of both types of edemas.
When fluid flows through the vessel, a small amount of it reaches the extra-vascular space and this fluid is drained back into the venous system by the lymphatic vessels. Also, it is important to mention that along with fluid, some proteins also leak out into third space. This is the normal process happening in our body. The amount of fluid reaching the interstitial space depends on the hydrostatic pressure and oncotic pressure present inside the blood vessels.  Hydrostatic pressure forces the fluid to go out and oncotic fluid pulls fluid into the blood vessel. In normal condition, lymphatics can drain back both the fluid and proteins reaching the third space.
Fluid accumulates more due to various causes which, in one way or the other involve renal (ex. Nephrotic syndrome, due to loss of proteins through urine thereby decreasing the oncotic pressure), hepatic (liver produces albumin and in case of chronic liver injury, hypoalbuminemia can occur which will eventually result in decreased oncotic pressure) or cardiovascular system (ex. Congestive heart failure where heart is unable to pump the blood leading to fluid accumulation thereby causing increased hydrostatic pressure). When this fluid accumulation exceeds the capacity of lymphatics, edema appears. But the protein leak is not totally dependent on hydrostatic pressure and oncotic pressure. It is dependent on size and charge of the proteins. So the pressure changes won’t cause more protein accumulation. We get pitting edema in this case, because the tissues filled with fluid yield easily on pressing.

When there is lymphatic obstruction due to any cause (filariasis, malignancy), the drainage of both fluid as well as proteins back to the venous system is affected. Now more and more fluid and proteins accumulate in the interstitial space leading to edema. In this case, the swelling won’t yield on pressing because of the increased protein content. So we call it as non-pitting type of edema.

So, by differentiating edema into these two kinds, we can rule out or rule in some diseases. Cardiovascular, renal and hepatic causes usually presents as pitting edema.  So, the take home message is that, lymphatic obstruction causes non-pitting type of edema whereas hydrostatic and oncotic pressure changes cause pitting edema.

It is due to increased levels of TSH (ex. In hypothyroidism) in the body which will stimulate fibroblasts, resulting in increased deposition of glycosaminoglycans and mucopolysaccharides. This will lead to swollen appearance of the tissue. It is usually a non-pitting type of edema. It can be seen in Graves’ disease also.

P.S: This article was first published in my wordpress blog(May 2016).

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