The best way to understand carpal tunnel syndrome is to start by understanding the carpal tunnel. As its name implies, it is an actual tunnel, with a floor, two walls, and a roof. The carpal bones of the wrist make up the floor and both walls of the carpal tunnel, and the roof is a tough, rigid sheet of connective tissue called the transverse carpal ligament. The tunnel is 2-4 cm long, and 10 structures travel through it to get from the forearm to the hand: nine flexor tendons and the median nerve (Figure 4-1).
A. The transverse carpal ligament. B. The carpal tunnel.
Carpal tunnel syndrome is actually a vascular problem!
Yes, it’s true. Compressive neuropathies are the result of compromised blood flow.
Question: What tissue in the human body is most sensitive to ischemia?
Answer: Nerve (muscle is a close second).
Why? Just remember membrane physiology. Both nerve and muscle have electrically active cell membranes. They have ATP-driven transmembrane pumps that are hard at work pumping charged ions across the cell membrane against both electrical and concentration gradients (Figure 4-A).
That takes a lot of energy! When a nerve fires, that energy is released. It’s like setting off a mousetrap: Transmembrane channels snap open, allowing the ions to rush back into the cell, depolarizing the cell membrane. This creates the action potential that nerves use to transmit electrical impulses down their axons. A fraction of a second later, all of these mousetraps need to be reset, a job for the ATP-driven transmembrane pumps. Running these pumps 24/7 is an expensive proposition. It requires a great deal of energy, which requires a rich blood supply, and that blood supply can be compromised by pressure. (Press your thumb firmly into your palm. The skin blanches because the pressure drives the blood out of the tissue. Release your thumb, and the skin turns pink again as the blood flow normalizes.) Pressure on the carpal tunnel compromises blood flow in the small vessels that nourish the axons of the median nerve. Under even transient ischemic conditions, the transmembrane pumps run out of fuel and quit working. The nerve starts to malfunction, which we perceive as numbness, tingling, and paresthesias. Most of us have experienced this firsthand. If you sit too long on the hard edge of a toilet seat, your leg will go numb from pressure and local ischemia as your sciatic nerve is pinched between the toilet seat and your femur bone. Relieve the pressure and sensation returns to normal.
The nerve cell membrane potential.
Question: What tissue in the human body is least sensitive to ischemia?