With compartment syndrome, compartment refers to separate sections of the body that contain muscles, nerves, and blood vessels surrounded by a layer of fibrous connective tissue, called fascia.
When the pressure within these compartments rises, normal blood flow can be cut off, leading to tissue damage due to hypoxia, or the lack of oxygen.
Compartment syndrome typically happens in the limbs, usually in the lower leg or the forearm.
Now, if we remove the skin and then we remove the fat tissue from the lower leg, we would uncover the fascia.
Fascia surrounds the muscles, keeping them tightly together while they contract to move the limb, and it also help attach these muscles to the bones.
Now if we look at a cross section of the lower leg, we can see that the fascia sends intermuscular septa that together with the interosseous membrane between the tibia and fibula divide the lower leg into four compartments.
The anterior, lateral, deep posterior and superficial posterior compartments all contain their own muscles, and blood vessels.
For example, the anterior compartment holds the muscles that perform dorsiflexion of the foot and also aid in it’s inversion and eversion; the deep peroneal nerve that innervates them; and its blood supply comes from the anterior tibial artery and veins.
Since the fascia is not elastic it can’t stretch much. Therefore any increase of the cellular and extracellular volume or a decrease of the volume capacity, like with some external compression, will lead to the increase of the pressure inside the compartment.
This will compress the structures within, and the first ones to feel the effect of compression are the veins, because the pressure inside their lumen is normally low.
As the pressure increases, the arteries will become compressed next and this obstructs the normal blood flow.
This cuts off the oxygen supply and hypoxia develops, which causes cells to start releasing substances like histamine and nitric oxide.
These substances cause capillaries to become leaky so fluid leaks out, causing extracellular edema that further increase intra-compartment pressure.
Okay, let’s look at the muscle cells! Hypoxia prevent their mitochondria from producing adenosine triphosphate, or ATP, which is the cell’s main source of energy.
Inside the cell membrane is a sodium potassium pump that needs ATP in order to pump sodium out of the cell and potassium inside.
So without ATP, there’s an increase of intracellular sodium levels which cause water to move into the cells causing intracellular edema.
If hypoxia lasts long enough, intracellular edema can lead to cell membrane rupture, causing proteins to leak out into the extracellular space.
These proteins draw more water from the capillaries and we get a vicious cycle of increased compartment pressure, tissue hypoxia, edema and cellular death and even more pressure.
The most common cause of compartment syndrome is bleeding inside the compartment.
This typically occurs with long bone fracture, like the tibia or the forearm bones, and penetrating wounds or surgical procedures that injure blood vessels.
Other causes are swelling of the tissue after severe burns, intravenous drug injection, repetitive use of the injured muscles, or a vigorous muscle contractions like in a tetany or seizures.
Any limb compression, like from a crush injury or from an inappropriately placed cast can also lead to compartment syndrome.
Another potential cause can be reperfusion injury. This occurs with the reestablishment of normal blood flow to hypoxic cells.