Carpal tunnel syndrome on MRI
Carpal tunnel syndrome, the most common entrapment syndrome of the median nerve, has a wide spectrum of causative pathologies, converging on two mechanisms of disease. Osteoarthritis, trauma, acromegaly, and mechanical overuse, among others, cause the carpal tunnel to decrease in size, whereas other disease states lead to augmentation of the contents of the carpal tunnel, both leading to entrapment: masses (e.g. ganglion cysts, primary nerve sheath tumors), deposition of foreign material (e.g. amyloid), or synovial hypertrophy in rheumatoid arthritis, to name a few.
Carpal tunnel syndrome is primarily defined by pain and sensory symptoms. A typical symptom is brachialgia paresthetica nocturna, or nocturnal ascending pain emanating from the wrist. Sensory symptoms affect the first three digits and, depending on innervation patterns, the radial aspect of the fourth digit. Positive Tinel (paresthesias elicited by tapping the median at the wrist) and Phalen tests (paresthesias caused by wrist flexion over 30-60 s) are typical, as well. Hand weakness, as a rule, is a late and often functionally non-relevant symptom.
In imaging median nerve syndromes, ultrasound is primarily useful in potentially revealing, in fully developed cases, a triad of palmar bowing of the flexor retinaculum (>2 mm beyond a line connecting the pisiform and the scaphoid), distal flattening of the nerve, and enlargement of the nerve proximal to the volar skin crease. MRI can demonstrate the same changes seen in US: bowing of the flexor retinaculum, enlargement of the median nerve at the level of the pisiform, and flattening of the median nerve at the level of the hook of the hamate. Other signs are a loss of fat or edema within the carpal tunnel and increased size, edema of the nerve on water-sensitive sequences, and, in some cases, contrast enhancement of the nerve 1. Although sensitivity and specificity of MRI in carpal tunnel syndrome are low (23-96% and 39-87%, respectively), MRI is especially well-suited for detecting masses, arthritic changes, or normal variants 2.
Carpal tunnel syndrome is initially often treated conservatively with splinting and non-steroidal anti-inflammatory drugs . Corticosteroid injections into the carpal tunnel can alleviate symptoms temporarily over about 4 weeks. Surgical release of the flexor retinaculum is indicated in cases of pronounced nightly pain, permanent dysesthesias, and prolonged distal motor latency on electroneurography (>6 ms). Long-term recurrence rates reach 30% 1.