The peripheral nerves in the arms and legs deliver sensation and information to the brain. In return, the brain sends messages back to the muscles through the same nerves. Damage to these nerves can disrupt some or all of these functions. If you've injured your hand, your doctor may order tests during your rehabilitation process to assess your nerve function.
Peripheral nerve injuries that affect hand function are caused by deep cuts, high pressure, overstretching, tearing and crushing injuries. They are classified based on the severity of the injury. Neuropraxia is irritation causing nerve function to be blocked temporarily. Axonotmesis is where the inner layers of the nerve are damaged, typically due to a crushing injury, while the outer layers are mainly intact. Neurotmesis is damage to the nerve and its protective outer layers. You can get nerve function back in all 3 classes, but full recovery is not possible with neurotmesis. Peripheral nerve injury in the arm causes loss of sensation, movement and strength.
Your hand function will be assessed immediately after the nerve injury, and at regular intervals during rehabilitation, typically once per month. Probe testing measures the size of stimulus detected using calibrated nylon monofilaments with varying diameters. These hairlike fibers are used to determine the smallest diameter a patient can feel with his eyes closed. Testing begins with the largest monofilament and progressively moves to smaller sizes until the patient is unable to detect it. The last detectable size is considered to be the patient's pressure threshold. Semmes-Weinstein monofilaments -- ranging from 2.83 to 6.65 mm -- are most commonly used for this testing. A score of 2.83 is normal, and progressively higher scores correlate with decreased sensation.
Probe testing is done on the fingers and areas of the hand affected by the specific damaged nerve. For example, testing for ulnar nerve damage in the hand is performed on the pinky finger and the muscles in the palm directly below the finger, called the hypothenar eminence.
Two-point discrimination helps determine a person's ability to detect two separate pressure points along the arm or hand. This information assesses how well the sensory nerves are functioning in a particular area of the hand or arm.
The test is performed using adjustable calipers with 2 pointed probes. The person is tested with his eyes closed. The examiner touches the patient with 1 or 2 probes at a time, in a random pattern. As the patient answers correctly, the probes are moved closer together, 1 mm at a time. The results of this exam are determined by the smallest distance a patient can correctly discern 2 distinct points of contact. Normal 2-point discrimination in the fingertips ranges from 2 to 4 mm. Data for ranges for the forearm and upper arm aren't as well studied. Measurements taken on the forearm and upper arm of the injured side can be compared to the individual's uninjured arm. Peripheral nerve injury will cause these numbers to be higher.
Grip and Pinch Strength
Peripheral nerves supply the muscles in the hand that perform gripping and pinching. These muscles become weak with peripheral nerve injury. A hand dynamometer measures grip strength. The test is performed with the person's elbow bent to 90 degrees, held against the side of his body. He then squeezes the handle of the dynamometer as hard as possible for 3 seconds. The handle does not move, but measures the amount of force applied against it. This is recorded digitally or on a dial that is read by the examiner. Strength is tested 3 times on each hand for comparison.
Pinch strength is also affected by peripheral nerve injury. A pinch gauge assesses 3 types of pinch: key, or lateral pinch, tip pinch and 3-jaw chuck. Key pinch is tested with the thumb on top of the device, tip pinch with the index on top and thumb on bottom, and 3-jaw chuck with both the index and middle fingers on top and thumb on the bottom. Strength measurements are taken during the initial evaluation and every month to measure progress.
Manual Muscle Testing
Peripheral nerve injury can cause weakness in specific hand and wrist muscles powered by the damaged nerve. Manual muscle testing helps assess strength in a particular muscle. For example, the extensor digitorum muscle straightens the fingers -- index, middle, ring and small -- and is powered by the radial nerve. To test this muscle, the person holds his wrist bent backward and the large knuckles straight. The clinician applies pressure against the back of the fingers, just above the large knuckles, as the patient attempts to keep his fingers from moving. The test is also performed on the uninjured hand to compare strength between the 2 hands.