Carpal tunnel syndrome

Carpal tunnel syndrome results from compression of the median nerve (tunnel syndrome) 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. 

Epidemiology

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.

Clinical presentation

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: paresthesias elicited by tapping the median nerve at the wrist
• positive Phalen test: paresthesias 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.

Pathology

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:
  • mechanical overuse (considered the most common association)
  • osteoarthritis
  • trauma
  • acromegaly
• disease states leading to change of carpal tunnel contents:
  • masses, e.g. ganglion cysts, primary nerve sheath tumors
  • deposition of foreign material, e.g. amyloid
  • synovial hypertrophy in rheumatoid arthritis
  • bifid median nerve due to increased cross-sectional area ¹⁴

A useful mnemonic to remember these causes is MEDIAN TRAP.

Associations

• persistent median artery of the forearm

Radiographic features

Ultrasound and MRI are the two imaging modalities that best lend themselves to investigating entrapment syndromes. Next to directly visualizing direct causes and anatomical variants (e.g. a Gantzer muscle), recognizing pathological muscle signal patterns on MRI can point to the affected nerve.

Ultrasound

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 (CSA) is given at 9-11 mm² (0.09-0.11 cm²), but the range of CSAs deemed pathological is wide.

According to one study, a 2 mm² difference in nerve CSA 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 ^ref:

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

If a bifid median nerve is present, measuring the combined cross-sectional area can be performed with an increase in cross-sectional area of ≥4 mm² between the median nerve at the proximal-third pronator quadratus level and in the carpal tunnel providing >90% sensitivity/specificity for carpal tunnel syndrome ¹⁵.

MRI

MRI has good-to-excellent sensitivity (84-100%) and specificity (85-94%) for diagnosing carpal tunnel syndrome when using cross-sectional area >15 mm² as a cut-off ^12,13. MRI is especially well-suited for detecting masses, arthritic changes, and normal variants (e.g. bifid median nerve) ⁵.

In carpal tunnel syndrome, MRI can demonstrate ⁴:

• palmar bowing of the flexor retinaculum
• nerve thickening at the carpal tunnel inlet (level of the pisiform)
• nerve flattening at the carpal tunnel outlet (level of the hook of hamate)
• increased cross-sectional area
  • ultrasound values do not correlate with MRI values ¹¹
  • >15 mm² at the carpal tunnel inlet or outlet can be used as a diagnostic criterion or cut-off ^12,13
  • >19 mm² has been proposed as a marker for severe carpal tunnel syndrome ¹³
• edema or loss of fat within the carpal tunnel
• neural edema +/- contrast enhancement

Treatment and prognosis

It is initially often treated conservatively with splinting and non-steroidal anti-inflammatory drugs (NSAIDS). Corticosteroid injections into the carpal tunnel can alleviate symptoms temporarily for about 4 weeks. Median nerve injury is a very rare complication ¹⁰. 

Surgical release of the flexor retinaculum is indicated in cases of pronounced nocturnal pain, permanent dysesthesias and prolonged distal motor latency on electroneurography (>6 ms). Long-term recurrence rates reach 30% ⁴.

Differential diagnosis

Possible differential diagnoses of carpal tunnel syndrome include:

• other median nerve entrapment syndromes:
  • pronator teres syndrome
  • anterior interosseous nerve syndrome