Ulnar nerve entrapment occurs when the ulnar nerve, a main nerve in the arm, experiences compression or irritation. This can cause pain, numbness, and tingling, often affecting the hand and fingers. Understanding this condition involves examining the nerve’s path and factors contributing to its compression or stretching.
The Ulnar Nerve’s Course
The ulnar nerve originates from the brachial plexus, a nerve network in the neck, and extends down the arm into the hand. It provides sensation to the little finger and half of the ring finger, and controls muscles in the forearm and hand for fine motor movements and grip strength.
Along its path, the ulnar nerve passes through areas where it is exposed and vulnerable. For example, it travels behind the medial epicondyle, the bony prominence on the inside of the elbow, in the “funny bone” area. Its superficial location here means it lacks significant protection, making it susceptible to pressure or stretching. The nerve then continues into the forearm and wrist through other narrow spaces.
Key Entrapment Locations
The most frequent site for ulnar nerve entrapment is at the elbow, known as cubital tunnel syndrome. Here, the nerve passes through the cubital tunnel, a narrow passageway of bone and ligament. When the elbow bends, the ulnar nerve stretches around the medial epicondyle, and the cubital tunnel’s volume decreases, increasing pressure on the nerve.
Entrapment can also occur at the wrist, less commonly, in Guyon’s canal. This small tunnel, formed by wrist bones and ligaments, allows the ulnar nerve and artery to pass into the hand. Compression here can result from repetitive trauma or cysts. Less common sites include areas beneath the collarbone or where the nerve emerges from the spinal cord in the neck.
Mechanisms of Compression and Stretching
The ulnar nerve can be compressed by several mechanisms. Sustained external pressure, like habitually leaning on the elbow, directly squeezes the nerve against bone. This pressure can impede blood flow and cause irritation. Direct trauma, such as a blow to the elbow, can also injure the nerve.
Repetitive bending or flexing of the elbow and wrist also contributes. When the elbow is bent for extended periods or repeatedly flexed, the ulnar nerve stretches and may even slide out of its normal position, leading to irritation. This stretching increases tension on the nerve, reducing its blood supply and causing symptoms. Anatomical variations, such as bone spurs, thickened ligaments, anomalous muscles, or cysts, can also narrow nerve pathways, leading to compression.
Common Risk Factors and Activities
Certain activities and conditions increase susceptibility to ulnar nerve entrapment. Occupations with repetitive elbow or wrist movements, such as assembly line work, typing, or playing musical instruments, contribute to the condition. Prolonged computer use, especially leaning on elbows or maintaining bent elbow positions, is another common risk factor. Athletes in sports with repetitive throwing or swinging motions, like baseball or tennis, also face increased risk due to valgus stress on the elbow.
Medical conditions can also predispose individuals. Arthritis, including osteoarthritis and rheumatoid arthritis, can lead to bone spurs, swelling, or joint changes that narrow nerve pathways. Diabetes is a significant risk factor, as it can cause nerve damage and make nerves more vulnerable to compression. A history of elbow fractures or dislocations, fluid buildup, or cysts near the elbow or wrist can also lead to entrapment.