Have you ever woken up in the middle of the night and noticed your hand sleeping to the point where you had to get out of bed and shake or flick your fingers to alleviate the numbness? If the numbness was primarily on the thumb-side half of your hand, it may have been carpal tunnel syndrome that woke you up. So, the question is, why is it such an issue at night?
To properly answer this question, let’s get familiar with the anatomy of the wrist. There are 2 bones that make up the forearm – the ulna (on the pinky side) and the radius (on the thumb side). Just beyond that, there are two rows of four bones each called the carpal bones for a total of 8 small bones that make up the wrist joint. These carpal bones are arranged in a horseshoe or tunnel shape. When you look down at your wrist and wiggle your fingers quickly, you can see all the movement that occurs on the palm side of the wrist. That’s a lot of movement! You can also see the muscles on the upper half of the forearm moving rapidly as the fingers wiggle.
There are 9 muscle tendons that travel through the carpal tunnel, as well as some blood vessels and most important, the median nerve sits on top of all those moving tendons. Just beneath the floor of the tunnel is a ligament called the transverse carpal ligament. The tendons inside the tunnel are surrounded by lubricating sheaths that make it easier for the tendons to slide back and forth as we wiggle our fingers, grip to open a jar, type on a computer, play a musical instrument, or so on. Without the tendon sheaths, the friction between the rubbing tendons would quickly build up heat, resulting in swelling, pain and numbness. However, in spite of the lubricating function of the sheaths, when we work our fingers and hands too much, swelling and inflammation does occur.
So, why do we have numbness at night when we aren’t working, gripping and moving our fingers repetitively? The answer lies in how we sleep. Since we are asleep, we cannot control where we position our hands and wrists. Most of us curl up in a ball and tuck our hands under our chin or someplace cozy. Normally, when we bend our wrists, the pressure inside the carpal tunnel doubles. However, a carpal tunnel patient already has a higher level of pressure in their wrist. So, when a carpal tunnel patient bends their wrist in the exact same way, the pressure goes up even more – that is, 3, 4, 5, or more times than a normal person without their wrist bent. That is why a wrist “cock-up” splint works so well at night! It keeps the wrist straight so we can’t bend it. Often, this allows the CTS patient to sleep through the night instead of waking up 2, 3, or more times with numbness, tingling, and/or pain on the thumb half of the hand.