Approach to compressive mononeuropathies: Clinical sciences

Compressive mononeuropathies occur as a result of mechanical compression or entrapment of peripheral nerves, which can eventually lead to demyelination and axonal injury, causing symptoms like pain and loss of function. Now, based on a focused history and physical exam, you can identify the most common compressive neuropathies, including median, ulnar, radial, peroneal, and tibial mononeuropathies.

Okay, if your patient presents with a chief concern suggestive of compressive mononeuropathy, your first step is to obtain a focused history and physical examination. First, let’s discuss a median neuropathy at the wrist, also known as carpal tunnel syndrome. Here, there’s a median nerve compression in the carpal tunnel, which is the area between the carpal bones and flexor retinaculum, also known as the transverse carpal ligament.

History will reveal a weak grip and decreased dexterity, such as dropping small objects and having trouble with buttons. Additionally, the patient will report numbness, tingling, and pain in the first three fingers, from the thumb to the middle finger, usually worse at night. Next, they will likely report daily activities with repeated or sustained wrist flexion, such as typing or housekeeping. The patient might also report that shaking the affected hand in the air will improve the symptoms. Finally, history might reveal associated medical conditions and risk factors such as diabetes, hypothyroidism, obesity, pregnancy, or rheumatoid arthritis.

These conditions can predispose a person to compressive neuropathy because they affect nerve health, causing nerves to be more susceptible to compression injury. However, in amyloidosis, the deposition of amyloid can directly compress the nerve and affect the conduction of signals. In some cases, this type of neuropathy might occur due to recent wrist trauma. In this case, direct force to the wrist can result in local edema and compression of the nerve within the carpal tunnel, eventually impairing normal signaling.

On physical examination, if the compression injury is severe enough, you will find the weakness of thumb abduction and opposition. You can test thumb opposition by asking the patient to touch the tip of their thumb to the tip or base of their pinky while you try to pull the thumb out with your index finger. You will also find sensory loss in the first three fingers and the lateral aspect of the fourth finger. You might find atrophy of the thenar muscles, which are the muscles at the base of the thumb on the palmar side.

Last but not least, you might elicit two positive signs on the exam with the Phalen and Tinel tests. With the Phalen test, ask the patient to press the dorsum of their hands together while flexing their wrists. If your patient reports sensory changes, the Phalen test is positive. On the other hand, with the Tinel test, you should tap on a patient’s wrist over the course of the median nerve. If the patient reports pain or paresthesias, the Tinel test is positive. With these findings, you should diagnose median neuropathy at the wrist.

Here’s a couple of clinical pearls to keep in mind! If the median nerve injury is more proximally than the wrist, you might see weakness in the flexion of the first three fingers as well as pronation of the forearm. Also, if you ask the patient to make a fist, they will be unable to flex the first three fingers but able to flex the fourth and fifth fingers. This is often referred to as the “hand of benediction”.

Now, management of carpal tunnel syndrome usually begins with conservative measures like wrist splinting at night to maintain a neutral wrist position. If the symptoms are mild and there isn’t evidence of muscle denervation on electromyography, you should also give nonsteroidal anti-inflammatory medications to treat local inflammation and reduce edema. If your patient does not respond to conservative measures, you can consider local corticosteroid injections. In severe and refractory cases, you should proceed with decompressive surgery by transecting the flexor retinaculum.

Alright, let’s switch gears and discuss ulnar neuropathy at the elbow. In this case, there’s compression of the ulnar nerve within the cubital tunnel, which lies on the dorsal surface of the elbow between the medial epicondyle of the humerus and the olecranon of the ulna. These patients will report weak grip and decreased dexterity in combination with numbness, tingling, and pain in the fourth and fifth fingers. Additionally, history will reveal activities with repeated or sustained resting on the elbows or flexion of the elbows, which is often seen in cyclists, and musicians, especially those who play string instruments, or following trauma to the elbow.

On the physical exam, you will find weakness in finger abduction and adduction, as well as weakness in the flexion of the fourth and fifth fingers. Also, this type of neuropathy affects muscles responsible for wrist flexion, so you will notice weakness in wrist flexion. Next, you will notice sensory loss in the medial aspect of the fourth finger and the entire fifth finger. Finally, the patient might present with a positive Froment sign.

Perform this test by asking the patient to hold a piece of paper between their thumb and index finger while you try to remove it. The test is positive if the patient flexes the thumb to try and hold onto the paper. This is because, in ulnar neuropathy, there's an impaired innervation of the adductor muscle of the thumb, so the patient will compensate by activating thumb flexors, which are controlled by the median nerve. With this set of findings, diagnose ulnar neuropathy at the elbow.

Time for another clinical pearl! Another common site of ulnar nerve compression is at the wrist in the Guyon canal, which is medial to the carpal tunnel. A compression here will still result in sensory loss in the fourth and fifth fingers, as well as weakness of finger abduction and adduction. However, wrist and finger flexor muscles are not affected, so there will be no weakness of flexion of the wrist, fourth, and fifth fingers.

Moreover, in this case, you might see flexion of the interphalangeal joints with the extension of the metacarpophalangeal joints at the fourth and fifth fingers with the hand at rest, which is known as the “ulnar claw”. You shouldn't confuse this finding with the “hand of benediction”, which is seen with proximal median nerve injury when the patient is actively trying to make a fist.