Carpal tunnel syndrome

Radswiki et al.

Carpal tunnel syndrome (CTS) 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 carpal tunnel syndrome is estimated to be 2-4% of the adult ultrasound population, translating to 4-10 million patients, with a lifetime incidence of 10-15%, dependent upon occupational risk 4.

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.

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 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 ~30% (range 8-50%) of cases.

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 asking about nocturnal exacerbation, which would be 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.

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 tumours), deposition of foreign material (e.g. amyloid), or synovial hypertrophy in rheumatoid arthritis, to name a few 5.

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), recognising pathological muscle signal patterns on MRI can inversely 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; normal cross-sectional area is given at 9-11 mm² (0.09-0.11 cm2), but the range of sizes deemed pathological is wide. 

One study has calculated that a 2 mm ² 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 4.

Some of the other proposed findings include

  • a flattening ratio of over 3
  • bowing of the retinaculum > 4 mm

In CTS, 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 the hamate

Other signs are oedema or loss of fat within the carpal tunnel, and increased size/oedema of the nerve on water-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, or normal variants 5.

It 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 dysesthesias and prolonged distal motor latency on electroneurography (>6 ms). Long-term recurrence rates reach 30% 4.

Possible differential diagnoses of carpal tunnel syndrome include:

Ultrasound - general index
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Article Information

rID: 12632
Section: Syndromes
Synonyms or Alternate Spellings:
  • Carpal tunnel syndrome (CTS)
  • CTS

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