Carpal tunnel syndrome (CTS) is 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.
Prevalence of CTS is estimated to be 2-4% of the adult US population, translating to 4-10 million patients, with a lifetime incidence of 10-15%, dependant upon occupational risk4.
Carpal tunnel syndrome usually occurs between the ages of 36 and 60 years and is two to five times more common in women than in men.
CTS 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 test (paresthesias elicited by tapping the median at the wrist) and Phalen test (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 5
The dominant hand is affected more frequently, and bilateral involvement has been reported to occur in 8-50% of cases.
Clinical presentation can harbor some pitfalls. Sensory and pain symptoms of the pronator teres syndrome ( PTS ) and CTS can overlap :
distinguish the two by looking for numbness of the forearm, which does not occur in CTS, and asking about nocturnal exacerbation, which would atypical in PTS. Provocation tests as detailed above can help further.
There is the widest 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 few5.
Ultrasound and MRI are the two imaging modalities which best lend themselves to investigating entrapment syndromes. Next to directly visualizing direct causes [e.g. primary nerve or sheath tumors, ganglion cysts, osseous spurs, anatomical variants (e.g. Gantzer muscle), recognizing pathological muscle signal patterns on MRI can inversely point to the affected nerve.
In imaging median nerve syndromes, US is primarily useful in examining CTS, 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.
Enlargement of the nerve seems to be the most sensitive and specific criterion, but what cut-off value for pathological size remains debated; normal cross-sectional area is given at 9-11 mm², but the range of sizes deemed pathological is wide.
One study has calculated that 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 CTS 5.
In CTS, MRI can demonstrate 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 5. Although sensitivity and specificity of MRI in CTS are low (23-96% and 39-87%, respectively), MRI is especially well-suited for detecting masses, arthritic changes, or normal variants 6.
Treatment and prognosis
CTS is initially often treated conservatively with splinting and NSAR. 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 dysaesthesias and prolonged distal motor latency on electroneurography (>6 ms). Long-term recurrence rates reach 30% 5.
The differential diagnosis of carpal tunnel syndrome includes:
- other median nerve entrapment syndromes
- 1. Mesgarzadeh M, Schneck CD, Bonakdarpour A et-al. Carpal tunnel: MR imaging. Part II. Carpal tunnel syndrome. Radiology. 1989;171 (3): 749-54. Radiology (abstract) - Pubmed citation
- 2. Campagna R, Pessis E, Feydy A et-al. MRI assessment of recurrent carpal tunnel syndrome after open surgical release of the median nerve. AJR Am J Roentgenol. 2009;193 (3): 644-50. AJR Am J Roentgenol (full text) - doi:10.2214/AJR.08.1433 - Pubmed citation
- 3. Wong SM, Griffith JF, Hui AC et-al. Carpal tunnel syndrome: diagnostic usefulness of sonography. Radiology. 2004;232 (1): 93-9. Radiology (full text) - doi:10.1148/radiol.2321030071 - Pubmed citation
- 4. Miller TT, Reinus WR. Nerve entrapment syndromes of the elbow, forearm, and wrist. AJR Am J Roentgenol. 2010;195 (3): 585-94. AJR Am J Roentgenol (full text) - doi:10.2214/AJR.10.4817 - Pubmed citation
- 5. Miller TT, Reinus WR. Nerve entrapment syndromes of the elbow, forearm, and wrist. AJR Am J Roentgenol. 2010;195 (3): 585-94. AJR Am J Roentgenol (full text) - doi:10.2214/AJR.10.4817 - Pubmed citation
- 6. Dong Q, Jacobson JA, Jamadar DA et-al. Entrapment neuropathies in the upper and lower limbs: anatomy and MRI features. Radiol Res Pract. 17;2012: 230679. Radiol Res Pract (abstract) - doi:10.1155/2012/230679 - Free text at pubmed - Pubmed citation
Synonyms & Alternative Spellings
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