Carpal tunnel syndrome happens when the median nerve, which runs through a narrow passageway in your wrist, gets squeezed by the surrounding tissues. This passageway, called the carpal tunnel, is rigid on all sides, so when anything inside it swells or thickens, the nerve has nowhere to go. The compression disrupts blood flow to the nerve, causes swelling, and progressively interferes with the nerve’s ability to send signals to your hand and fingers.
The Space Inside Your Wrist
The carpal tunnel is a small channel on the palm side of your wrist. The floor and walls are formed by a curved row of wrist bones, and the roof is a thick band of connective tissue called the transverse carpal ligament. This creates a tube roughly the diameter of your index finger.
Packed inside that tube are ten structures: nine tendons that bend your fingers and thumb, plus the median nerve. The median nerve controls sensation in your thumb, index finger, middle finger, and half of your ring finger. It also powers the small muscles at the base of your thumb. Because the tunnel is surrounded by bone and a tough ligament, it can’t expand. Any increase in the size of its contents, or any decrease in the space available, puts pressure directly on the nerve.
What Happens to the Nerve Under Pressure
When pressure builds inside the carpal tunnel, the first thing affected is the nerve’s blood supply. Even light compression reduces blood flow to the tiny vessels running along and through the nerve. Short-term compression causes the outer layers of the nerve to swell as fluid leaks from damaged blood vessels. At this stage, symptoms come and go because the nerve recovers once pressure drops.
With prolonged or repeated compression, the damage goes deeper. Fluid accumulates inside the nerve itself, and because the nerve’s protective sheath has no way to drain excess fluid, pressure keeps building in a self-reinforcing cycle: swelling restricts blood flow, restricted blood flow causes more swelling, and the nerve becomes increasingly starved of oxygen. Over time, the insulating coating around nerve fibers begins to break down, the fibers themselves degenerate, and scar tissue forms. This is when numbness becomes constant rather than intermittent, and the muscles at the base of the thumb start to visibly waste away. That stage of muscle atrophy signals severe, prolonged compression and is much harder to reverse.
Why the Tissue Inside the Tunnel Swells
The most common physical change found in people with carpal tunnel syndrome isn’t inflammation in the traditional sense. It’s a noninflammatory thickening of the connective tissue surrounding the tendons. This tissue slowly bulks up, taking up more room in the tunnel and increasing the pressure on the median nerve. The thickened tissue also becomes less permeable to fluid, which traps swelling around the tendons and creates a cycle of reduced blood flow and further tissue damage.
What triggers that thickening varies from person to person, and it’s often a combination of factors rather than a single cause.
Wrist Position and Repetitive Movement
The angle of your wrist has a direct, measurable effect on pressure inside the carpal tunnel. Research measuring real-time pressure during typing found that a neutral wrist position (flat, no bend) produced the lowest tunnel pressure. Bending the wrist back by 30 degrees roughly doubled the pressure, and at 45 degrees of extension, pressure more than doubled compared to neutral. The lowest-pressure positions were neutral and a slight 15-degree downward bend.
This is why activities that hold your wrist at an angle for extended periods are a problem. Typing with your wrists cocked upward, gripping a steering wheel, holding a phone, or sleeping with bent wrists all sustain higher pressure inside the tunnel. Repetitive finger movements compound the issue because the tendons slide back and forth through the tunnel, creating friction that contributes to tissue thickening over time.
Vibrating tools add another layer of risk. A large Swedish registry study found that workers exposed to hand-arm vibration had about 60% higher odds of developing carpal tunnel syndrome compared to unexposed workers. At the highest vibration levels, the risk increased to roughly 84% higher. The vibration is thought to cause microscopic damage to blood vessels and nerve tissue inside the tunnel.
Health Conditions That Raise Your Risk
Several systemic conditions make carpal tunnel syndrome significantly more likely by changing the environment inside the tunnel from the inside out.
Diabetes is the most strongly linked condition. In one study, over 71% of people with diabetes showed signs of carpal tunnel syndrome on nerve testing, compared to 4-6% of the general population. Diabetes damages nerves in two ways: excess blood sugar gets converted into a sugar alcohol that accumulates inside cells, drawing in water and causing swelling; and it damages the tiny blood vessels that supply the nerve itself, making it more vulnerable to even mild compression.
Hypothyroidism (underactive thyroid) is another significant contributor, with roughly a third of hypothyroid patients showing evidence of carpal tunnel syndrome. The mechanism is different: low thyroid function leads to deposits of a gel-like substance called mucin in the soft tissues around nerves, physically compressing them. Some researchers believe the thyroid dysfunction also directly damages nerve insulation.
Pregnancy and Fluid Retention
Carpal tunnel symptoms are common during pregnancy, particularly in the third trimester, and the reasons are both mechanical and hormonal. Blood volume increases by nearly 50% during pregnancy, and fluid retention causes widespread tissue swelling. In the rigid confines of the carpal tunnel, even a small amount of extra fluid creates meaningful pressure on the nerve.
Pregnancy also triggers a sharp increase in a hormone called relaxin starting around the 18th week. Relaxin loosens ligaments throughout the body to prepare for delivery, but it also relaxes the carpal ligament. This destabilizes the tunnel’s structure and can lead to inflammation and flattening of the median nerve. The good news is that pregnancy-related carpal tunnel typically resolves after delivery as fluid levels return to normal, though it can take weeks to months.
How Symptoms Progress
Carpal tunnel syndrome follows a fairly predictable pattern. Early on, you notice tingling or numbness in the thumb, index, and middle fingers, often at night. Nighttime symptoms are common because most people sleep with their wrists flexed, which sustains higher tunnel pressure for hours. Shaking or flicking your hand relieves it temporarily because it restores blood flow.
As compression continues, numbness becomes more frequent during the day, especially during activities that involve gripping or bending the wrist. You might notice you drop things more often or have trouble with fine tasks like buttoning a shirt. Grip and pinch strength decrease as the nerve’s motor signals weaken.
In advanced cases, the muscles at the base of the thumb (the thenar muscles) visibly shrink, creating a flattened appearance on the palm side of the hand below the thumb. This atrophy represents severe, prolonged nerve compression and indicates significant nerve fiber loss. At this stage, even surgical release of the tunnel may not fully restore strength or sensation, which is why earlier intervention tends to produce better outcomes.
How It Gets Confirmed
A nerve conduction study is the standard test for confirming carpal tunnel syndrome. It measures how quickly electrical signals travel through the median nerve at the wrist. In a healthy nerve, sensory signals cross the wrist in under 3.5 milliseconds. In carpal tunnel syndrome, the damaged or compressed nerve conducts signals more slowly. A sensory delay beyond about 3.5 to 3.6 milliseconds, or a motor delay beyond about 4.2 to 4.4 milliseconds, is considered abnormal. The sensory test picks up carpal tunnel syndrome with roughly 87% accuracy, while the motor test is better at confirming severe cases.
Pressure measurements tell a more dramatic story about what’s happening during daily activities. In people with carpal tunnel syndrome, making a fist or gripping an object can drive pressure inside the tunnel above 1,000 mmHg, far beyond what nerve tissue can tolerate. These pressures drop significantly after surgical release, which involves cutting the transverse carpal ligament to permanently open the roof of the tunnel and give the nerve room to recover.