Compartment syndrome involves the compression of nerves and blood vessels within an enclosed space. This leads to impaired blood flow and muscle and nerve damage.

Thick layers of tissue called fascia separate groups of muscles in the arms and legs from each other. Inside each layer of fascia is a confined space, called a compartment, that includes the muscle tissue, nerves, and blood vessels. (They are surrounded by the fascia much like wires surrounded by insulation.)

Unlike a balloon, fascia do not expand, so any swelling in a compartment will lead to increasing pressure in that compartment, which will compress the muscles, blood vessels, and nerves. If this pressure is high enough, blood flow to the compartment will be blocked, which can lead to permanent injury to the muscle and nerves. If the pressure lasts long enough, the limb may die and need to be amputated.

Swelling leading to compartment syndrome is associated with high-energy trauma, such as from a car accident or crush injury, or surgery. Compartment syndrome may also occur due to tight bandages or casts; with significant swelling, pressure will build up and can cause compartment syndrome. Chronic compartment syndrome can be caused by repetitive activities like running that increase the pressure in a compartment only during that activity.

Compartment syndrome is most common in the lower leg and forearm, although it can also occur in the hand, foot, thigh and upper arm.

The hallmark symptom of compartment syndrome is severe pain that does not respond to elevation or pain medication. In more advanced cases, there may be decreased sensation, weakness, and paleness of the skin.

Typically, severe pain will occur when a muscle running through a compartment is passively moved. For example, when the doctor moves the toes up and down, a patient with compartment syndrome in the foot or lower leg will experience severe pain. The skin overlying the compartment will be tensely swollen and shiny. There will also be pain when the compartment is squeezed.

The test that will absolutely diagnose this condition involves directly measuring the pressure in the compartment by inserting a needle attached to a pressure meter into the compartment. When the compartment pressure is greater than 45 mmHg or when the pressure is within 30 mmHg of the diastolic blood pressure (the lower number of the blood pressure), then the diagnosis is made. When chronic compartment syndrome is suspected, this test must be performed immediately after the activity that causes pain.