Carpal tunnel syndrome results from compression of the median nerve within the carpal tunnel. It is a cause of significant disability and is one of three common median nerve entrapment syndromes, the other two being anterior interosseous nerve syndrome and pronator teres syndrome.
The prevalence of carpal tunnel syndrome is estimated to be 2.7-5.8% of the general adult population, with a lifetime incidence of 10-15%, depending on occupational risk 4.
Carpal tunnel syndrome usually occurs between ages 36 and 60 and is more common in women, with a female-to-male ratio of 2-5:1.
Carpal tunnel syndrome is primarily defined by pain and sensory symptoms:
- brachialgia paraesthetica nocturna, or nocturnal ascending pain emanating from the wrist, is typical
- sensory symptoms affect the first three digits and, depending on innervation patterns, the radial aspect of the fourth digit
- positive Tinel test: paraesthesias elicited by tapping the median nerve at the wrist
- positive Phalen test: paraesthesias caused by wrist flexion over 30-60°
Hand weakness, as a rule, is a late and often functionally irrelevant symptom 5.
The dominant hand is affected more frequently, and bilateral involvement has been reported to occur in ~30% (range 8-50%) of cases.
The clinical presentation can harbor some pitfalls. Sensory and pain symptoms of the pronator teres syndrome (PTS) and carpal tunnel syndrome can overlap; one can distinguish the two by examining for numbness of the forearm, which does not occur in carpal tunnel syndrome, and ask about nocturnal exacerbation, which is atypical in PTS. Provocation tests as detailed above can help further.
There is a wide spectrum of causative pathologies, converging on two mechanisms of disease, both of which lead to entrapment 5:
- a decrease in the size of the carpal tunnel caused by such conditions as:
- disease states leading to augmentation of carpal tunnel contents:
A useful mnemonic to remember these causes is MEDIAN TRAP.
Ultrasound and MRI are the two imaging modalities which best lend themselves to investigating entrapment syndromes. Next to directly visualizing direct causes and anatomical variants (e.g. a Gantzer muscle), recognising pathological muscle signal patterns on MRI can point to the affected nerve.
In imaging median nerve syndromes, ultrasound is useful in examining carpal tunnel syndrome, 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
- enlargement of the nerve proximal to the flexor retinaculum
Enlargement of the nerve seems to be the most sensitive and specific criterion, but what cut-off value for pathological size remains debated; the normal cross-sectional area is given at 9-11 mm2 (0.09-0.11 cm2), but the range of sizes deemed pathological is wide.
According to one study, a 2 mm2 difference in nerve cross-section between the level of the pronator quadratus and the carpal tunnel has a 99% sensitivity and 100% specificity for carpal tunnel syndrome 9.
Some of the other proposed findings include:
- a flattening ratio of over x 3
- bowing of the flexor retinaculum of >4 mm
In carpal tunnel syndrome, MRI can demonstrate palmar 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 hamate.
Other signs are edema or loss of fat within the carpal tunnel, and increased size or edema of the nerve on fluid-sensitive sequences, and, in some cases, contrast enhancement of the nerve 4.
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, and normal variants 5.
Treatment and prognosis
It is initially often treated conservatively with splinting and non-steroidal anti-rheumatic drugs (NSAR). Corticosteroid injections into the carpal tunnel can alleviate symptoms temporarily for about 4 weeks.
Surgical release of the flexor retinaculum is indicated in cases of pronounced nocturnal pain, permanent dysaesthesias and prolonged distal motor latency on electroneurography (>6 ms). Long-term recurrence rates reach 30% 4.
Possible differential diagnoses of carpal tunnel syndrome include:
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- 6. Tsujii M, Hirata H, Morita A et-al. Palmar bowing of the flexor retinaculum on wrist MRI correlates with subjective reports of pain in carpal tunnel syndrome. J Magn Reson Imaging. 2009;29 (5): 1102-5. doi:10.1002/jmri.21459 - Pubmed citation
- 7. Mesgarzadeh M, Triolo J, Schneck CD. Carpal tunnel syndrome. MR imaging diagnosis. Magn Reson Imaging Clin N Am. 1995;3 (2): 249-64. Pubmed citation
- 8. Buchberger W. Radiologic imaging of the carpal tunnel. Eur J Radiol. 1997;25 (2): 112-7. Pubmed citation
- 9. Klauser AS, Halpern EJ, De Zordo, T, et-al. Carpal tunnel syndrome assessment with US: Value of additional cross-sectional area measurements of the median nerve in patients versus healthy volunteers. Radiology. 2009;250 (1): 171-7. Radiology (full text) - doi:10.1148/radiol.2501080397 - Pubmed citation
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