View source for 10. Pathophysiology of the lymphatic circulation

We don’t normally read a lot about the lymphatic system, and there’s not a lot to say about its pathophysiology either. The book doesn’t write a lot about it either. However, oedema is a process which is related to the lymphatics, so we’ll start there.

''In retrospect I don’t think oedema is what’s important about this topic, but I don’t know what else is important here, so I’ll leave it like it is.''

Oedema can occur internally or peripherally, but when we talk about oedema we usually mean peripheral oedema. Internal oedema occurs as pulmonary oedema, cerebral oedema, ascites, etc. Peripheral oedema most frequently occurs in the lower legs, feet, ankles and periorbital area, but can occur anywhere. All types of oedema are caused by an imbalance in fluid homeostasis that causes fluid to accumulate in the interstitium.

Peripheral oedema can be divided into local and general types, according to where it occurs. Local oedema is seen in relation to local inflammation, like burns, trauma, allergy, thrombosis, or lymphoedema. Generalized oedema can affect larger areas, like the whole lower leg.

Peripheral oedema can also be divided into pitting and non-pitting types, according to the clinical behaviour. We’ll focus on the generalized types of peripheral oedema, as those are the types we usually mean when we talk about oedema.

Fluid is lost from the plasma to the interstitium in any type of oedema. The decreased plasma volume can activate the RAAS-system, which causes an increase in aldosterone called ''secondary hyperaldosteronism.'' Recall that aldosterone causes potassium loss, so all types of oedema run the risk of hypokalaemia.

== Pitting oedema ==
It’s called “pitting” because if you press on the oedema a “pit” will remain. This happens because the fluid below the pressed point is moved around in the interstitium and needs some time to come back. 1 – 1.5 L of fluid must enter the tissues for pitting oedema to be visible. Pitting oedema is mostly seen in the lower extremities (due to gravity) and is most frequently seen due to:

* Heart failure
* Hypoalbuminaemia
* Kidney failure
* Inflammation, burns, allergic reactions, trauma

There are multiple mechanisms by which pitting oedema can occur, depending on the cause:

* Fluid retention (increased fluid content in the body)
** Occurs due to heart failure, kidney failure
* Decreased oncotic pressure
** Albumin is important in fluid homeostasis, and hypoalbuminaemia causes fluid to enter the interstitium
** Occurs due to protein deficiency, nephrotic syndrome, cirrhosis, etc.
* Increased hydrostatic pressure
** Occurs due to heart failure, venous thrombosis, etc.
* Increased capillary permeability
** Occurs due to inflammation, trauma, allergy, etc.

The role of cardiac failure in pitting oedema formation was also discussed in topic 6.

Pitting oedema is often treated with diuretics; drugs which increase the production of urine, thereby “draining” the interstitium of fluid.

The extreme form of generalized oedema is called anasarca. This is rarely seen nowadays as the oedema and the underlying cause are treated before it reaches that stage.

== Non-pitting oedema ==
In non-pitting oedema the extra fluid isn’t freely movable because the fluid contains proteins or mucopolysaccharides that don’t allow the fluid to move. Therefore, the fluid isn’t moved when the oedema is pressed, so no pit is formed. Two types of non-pitting oedema exist: lymphoedema and myxoedema.

Lymphoedema occurs due to a blockage or impairment of the lymphatic circulation, so that the circulation isn’t sufficient to remove the extra fluid. It’s often due to secondary causes like removal of lymph nodes during surgery, as is procedure in mastectomy for example.

True myxoedema occurs only in hypothyroidism. For reasons not well understood will mucopolysaccharides be deposited in the interstitium in this disorder, which binds fluids in the interstitium. These patients have a characteristic “puffy” appearence.

What was previously called “pretibial myxoedema” is nowadays called simply “dermopathy”. It’s not a true myxoedema like in hypothyroidism. It occurs in Graves disease, a type of hyperthyroidism. These patients have inflamed and swollen skin, often around the legs but also other places. Intraorbital dermopathy is what causes the characteristic exophthalmos (outbulging of the eyes) in Graves’ hyperthyroidism.
[[Category:Pathophysiology 1]]