The arm nerves arise from the spinal cord in the lower neck, forming a nerve bundle known as the brachial plexus. This nerve bundle splits, giving rise to the ulnar, radial and median arm nerves. Smaller branches from these nerves supply specific muscles and skin regions of the arm. Arm nerve damage at any point from the shoulder to the hand can cause varying loss of arm function and sensation.
Arm Paralysis
The most severe arm nerve injury is a complete cut or tear of the brachial plexus. This injury results in arm paralysis and loss of sensation, reports the Neurosurgery Division of UCLA Health System. Incomplete brachial plexus tears and stretch injuries cause partial arm paralysis and numbness, tingling or burning of the arm and hand. The degree of arm weakness varies according to the severity and site of the injury.
Brachial plexus injuries may occur during a difficult birth. The position of the baby during delivery or the need to forcibly pull the baby from the birth canal may cause brachial plexus stretching or tearing. Traumatic neck injuries may also cause brachial plexus damage with an associated loss of arm function and sensation. Mild to moderate brachial plexus injuries often slowly improve over time. Surgical intervention may afford partial recovery of function with certain types of severe brachial plexus injuries.
Hand Numbness
Distinctive patterns of hand numbness occur with damage to each of the arm nerves. The ulnar nerve branches from the brachial plexus near the collarbone, courses the upper arm and crosses the outer elbow as it continues to the hand. "Funny bone" pain occurs due to striking the ulnar nerve where it crosses the elbow joint. Ulnar nerve damage causes tingling or numbness of the little finger and the outer half of the ring finger, according to the American Academy of Orthopaedic Surgeons.
The median nerve arises near the collarbone, crosses the inner bend of the elbow and enters the hand through the carpal tunnel at the wrist. Damage or injury of the median nerve causes palmar numbness of the thumb, index finger, second finger and the inner half of the ring finger. Carpal tunnel syndrome commonly presents with this characteristic pattern of hand numbness due to pressure on the median nerve.
The radial nerve arises near the collarbone and travels close to the upper arm bone before crossing the inner elbow bend and entering the hand. Damage to the radial nerve causes numbness over the back of the thumb and first two fingers, extending across the back of the hand and wrist.
Wrist and Finger Drop
Fractures of the upper arm bone, or humerus, may cause radial nerve damage due to the proximity of the nerve to the bone. A radial nerve injury at this level typically causes wrist and finger drop, notes MedlinePlus. The wrist hangs toward the palm and cannot be voluntarily raised due to extensor muscle paralysis. Similarly, the fingers curl toward the palm and cannot be intentionally straightened or lifted.
