Anatomy

The carpal tunnel is open-ended proximally and distally, but it behaves like a closed compartment physiologically and maintains its own distinct tissue fluid pressure levels. It is a fibro-osseous canal that is bounded by the concave arch of the carpal bones dorsally and the flexor retinaculum (FR) palmarly. The hook of the hamate, the triquetrum, and the pisiform form the ulnar border, whereas the radial border consists of the scaphoid, trapezium, and the fascial septum overlying the flexor carpi radialis (FCR). The flexor retinaculum consists of three zones: a proximal zone that is continuous with the deep forearm fascia, a central zone that is composed of the transverse carpal ligament (TCL), and a third zone that consists of the aponeurosis between the thenar and hypothenar muscles. The median nerve at the wrist has approximately 30 fascicles. The motor recurrent branch often consists of two fascicles situated in a volar position, with the various sensory groups in the radial, ulnar, and dorsal positions. The motor branch can be separated from the main trunk without harm for up to 100 mm proximal to the thenar muscles. The sensory fibers travel within the common digital nerves to the thumb, index, and middle fingers, as well as the communicating branch to the third web space.

Pathophysiology

There are two potential sites of compression anatomically. The first is at the proximal edge of the TCL, where compression may be produced by acute wrist flexion. This accounts for the positive Phalen’s test (wrist flexion test) in carpal tunnel syndrome (CTS). The second site is adjacent to the hook of the hamate, in which an hourglass deformity of the median nerve may be seen. In this case, the patient has a positive median nerve compression (Durkan’s) test result, but a negative Phalen’s test result. Compression within the carpal tunnel may also result from any lesion that takes up space within the canal such as flexor tenosynovitis, hematoma, palmar carpal dislocation, distal radius fractures, tumors, and ganglia. Although many cases have been attributed to a nonspecific synovitis, synovial biopsies typically fail to show evidence of inflammation. They do reveal edema and vascular sclerosis, which may by themselves be secondary to compression rather than the primary event.

History

The patient with CTS typically complains of numbness and paresthesia in the median nerve distribution. Initially, symptoms occur at night as a result of a combination of wrist flexion during sleep and fluid shifts that occur with the horizontal position, all of which increase the carpal canal pressure. In this early stage of nerve compression, the symptoms are of a vascular nature, which culminate in endoneurial edema. With early compression, the symptoms are intermittent, and the edema is reversible. As the symptoms progress, they become more frequent during the day and are precipitated by gripping and pinching activities as well as tasks requiring repetitive wrist flexion. When symptoms are constant, there is usually myelin damage and/or chronic endoneurial edema.

Physical Examination

CTS represents a constellation of signs and symptoms in which no single test absolutely confirms its diagnosis. A positive Tinel’s sign may be present over the median nerve at the wrist and produces paresthesia in the thumb and radial two and a half digits. Phalen’s test consists of passive wrist flexion for 1 minute, which, when positive, produces subjective paresthesia in a median nerve pattern. This is best performed with the elbows extended, since simultaneous wrist and elbow flexion may reproduce ulnar nerve symptoms as well. Direct compression of the nerve, or the Durkan’s test, is thought to be more sensitive. Szabo and colleagues found that a patient with an abnormal hand diagram, abnormal sensibility by Semmes-Weinstein test (SWT) testing, a positive Durkan’s test result, and night pain had a probability of CTS of 0.86. They found that when all four of the latter conditions were normal, the probability of having CTS was 0.0068.

Electrodiagnostic Studies

The nerve conduction study can yield useful information, but the severity of the preoperative nerve conduction deficit does not provide significant data for prediction of the final outcome or return to work after carpal tunnel release (CTR). There are some caveats for nerve conduction studies in CTS. First, sensory abnormalities usually occur before motor abnormalities do. In other words, the distal sensory latencies often slow before the distal motor latency. This is not surprising, since 94% of the axons in the median nerve at the wrist level are sensory. The sensory nerve axons are larger than the motor axons and hence more susceptible to compression. If the distal motor latency (DML) is abnormal in the presence of normal sensory nerve action potentials (SNAPs), extra care must be taken to rule out anterior horn cell disease or a C8 radiculopathy, although isolated recurrent motor branch compression has been reported. Second, the nerve conduction studies may not return to normal after decompression because of retrograde fiber degeneration or incomplete remyelination, even with a full clinical recovery.

Quantitative Sensory Testing

Static two-point discrimination (2PD) testing remains normal in mild cases. With more severe and/or longer-standing median nerve compression, the 2PD becomes abnormal, that is, more than 5 mm in variable combinations of the thumb, index, middle, and radial ring fingers. Loss of protective sensation occurs when the 2PD is more than 15 mm, and more than 25 mm correlates with complete anesthesia. 2PD is relatively insensitive in picking up changes compared with testing using the pressure-specified sensory device (PSSD). In CTS, testing of the index pulp may initially show an abnormal two-point static (2PS) pressure threshold. With progression, one sees a widening of the 2PS distance, followed by an abnormal 1PS threshold.

Weber and Rude evaluated 54 patients with CTS, including 26 control subjects, comparing nerve conduction studies (NCS) with PSSD testing. The NCS test had a sensitivity of 80% and specificity of 77%, whereas the PSSD testing had a sensitivity of 91% and a specificity of 82%. The difference was not statistically different however.

A prospective side-by-side comparison of NCS and PSSD testing was undertaken by the author in 69 patients with clinical signs and symptoms of CTS. A total of 102 tests were performed (28 bilateral). Twenty patients (21 hands) underwent a CTR and were re-tested at 4 to 6 months. The Symptom Severity Score (SSS) was calculated before and after surgery. A control group of 20 hands in 10 asymptomatic patients underwent identical testing. The average age was 52 years (range 35 to 77). Tinel’s sign was positive in 25 of 102 hands. 2PD was abnormal in 22 of 102. Positive Phalen’s test and/or Durkan’s test results were seen in 77 of 102 hands. The NCS sensitivity was 87.3% (95% confidence interval [CI]: 79.2% to 93.0%) with a specificity of 90%, whereas the PSSD sensitivity was 81.4% (95% CI: 72.5% to 88.4%) with a specificity of 65%. This difference was not statistically significant. The combined sensitivity of the two tests was 93.1%. The PSSD and/or the NCS was positive in all of the 19 hands with negative physical signs.

In the postoperative group, the SSS improved from an average of 3.34 preoperatively (range 2.9 to 5.72) to 1.95 postoperatively (range 1.0 to 3.57). The NCS improved in 19 of 21 hands (91%), whereas the PSSD improved in 16 of 19 hands (84%). This study demonstrated that the addition of noninvasive tests including the SSS and PSSD can increase the diagnostic yield in CTS, especially when the NCS results are normal.

Nonoperative Management

Nonoperative therapy includes splinting the wrist in a neutral position, steroid injections, and management of any underlying systemic diseases. Steroid injection offers transient relief in 80% of patients, but only 20% will be symptom-free 12 months later. Those most likely to benefit from conservative management have had symptoms for less than 1 year, only intermittent numbness, normal 2PD, less than 1 to 2 ms prolongation of distal motor and sensory latencies, and no motor findings. Forty percent of this group will remain symptom free for longer than 12 months.

Surgical Indications

Indications for surgery for CTS can be divided into four different categories of severity:

Mild CTS

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  Failed trial of conservative treatment with splints, nonsteroidal anti-inflammatory drugs (NSAIDs), and activity modification for at least 1 month

  
  
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  No sensory or motor loss, nocturnal symptoms, and/or transient paresthesia only with prolonged gripping or pinching

  
  
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  No thenar wasting, no change in 2PD

  
  
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  May have a prolonged 2PS, normal NCS, increased comparative sensory index (CSI), or slowing of distal median SNAP amplitudes but with a normal distal motor latency.

  
  
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  Normal electromyograph (EMG)

Moderate CTS

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  Failed trial of splinting and/or cortisone injections

  
  
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  Frequent daytime symptoms even without gripping

  
  
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  May have abnormal 2PS and 1PS, 2PD greater than 15 mm

  
  
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  No wasting of APB but may have weak abduction

  
  
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  NCS may show slowing of distal SNAPs

  
  
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  Distal motor latency slowing of less than 1 to 2 ms, but no drop in amplitude

  
  
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  Normal EMG

Severe CTS

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  No indication for conservative treatment

  
  
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  Frequent to constant symptoms

  
  
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  Abnormal 2PS, 1PS, 2PD greater than 15 mm

  
  
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  Slowed to absent SNAPs, prolonged DML with amplitude loss

  
  
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  EMG showing membrane instability, decreased recruitment, and fibs/psw (fibrillations/positive sharp waves)

Acute CTS

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  Secondary to distal radius fracture, bleeding disorder, burn, or other cause of massive swelling

Contraindications

Contraindications include untreated hypothyroidism, diabetes, or other metabolic neuropathy.

Surgical Technique

The procedure is performed under tourniquet control. A 3- to 5-cm incision is made in the palm parallel with the thenar crease and in line with the ring finger axis to protect the palmar cutaneous branch of the median and ulnar nerves. Tenotomy scissors are used to spread down to the palmar aponeurosis. This is divided exposing the transverse carpal ligament (TCL). The TCL is divided from distal to proximal. A hemostat may be used to protect the median nerve. The skin is retracted, and the deep flexor retinaculum is divided under direct vision for an additional 2 cm. The nerve and tendons are retracted to the radial side, and the floor of the canal is inspected for masses. The recurrent motor branch is inspected and decompressed separately if necessary. The same considerations apply when using a mini-incision technique ( Fig. 62-1 A–E). The tourniquet is released and hemostasis is obtained. The wound is closed with 4-0 nylon mattress sutures after injection of local anesthetic.

Carpal tunnel release. A, The standard carpal tunnel incision is parallel with the thenar crease along the ring finger axis to prevent injury to the palmar cutaneous branches of the median and ulnar nerves. B, The palmar aponeurosis is identified by the longitudinally oriented fibers. C, The fibers of the transverse carpal ligament (TCL) are transversely oriented. Arrows are pointing to a partial incision in the ligament. D, Release of the TCL exposes the median nerve. The tip of the scissors is behind the recurrent motor branch.

(Copyright, David Slutsky, MD, 2010.)

Carpal tunnel release. E, Mini-incision technique.

(Copyright, David Slutsky, MD, 2010.)

Postoperative Care

Finger motion begins immediately, which also aids in median nerve excursion. A below-elbow splint is applied for comfort for the first week, followed by desensitization and progressive strengthening.

Ancillary Procedures

In advanced cases of nerve compression, internal neurolysis and epineurotomy have been described. No significant differences were found in comparisons of patients with and without this procedure; hence these procedures are no longer recommended. Small finger numbness due to coexistent Guyon’s canal compression often improves after CTR alone, since magnetic resonance imaging (MRI) studies have demonstrated an increase in the volume of Guyon’s canal after a CTR. Routine tenosynovectomy does not provide better results than CTR alone and is mostly recommended with associated proliferative tenosynovitis from some other cause, such as rheumatoid arthritis or granulomatous infection. Ketchum did a comparison of open CTR, CTR with flexor tenosynovectomy, and tenosynovectomy alone. He noted a significant decrease in pillar pain and earlier return to work in the isolated synovectomy group.

Complications

Pillar pain is an oft-cited complication. The cause of this is murky, and the treatment is debatable. This pain has been the impetus for procedures that range from Z-plasty lengthening of the TCL to mini-incision and endoscopic techniques. Injury to the palmar cutaneous branch of the median nerve is a cause of persistent scar tenderness and led to the plea for a more ulnar-based incision. Blind release of the TCL, which was commonplace in the 1960s, occasionally resulted in laceration to the deep motor branch. Injury to branches of the superficial palmar arch are repaired or tied off as indicated. Median nerve anomalies or altered anatomy may lead to inadvertent nerve laceration. Any recognized fascicular lacerations should be repaired under microscopic magnification immediately or as soon as recognized postoperatively, since this maximizes the chances for recovery. Acute infection is treated aggressively with antibiotic treatment and/or drainage as necessary. Wound coverage problems consisting of exposed tendons or nerves may be covered with pedicled or free flaps. Causalgia should be treated with stellate blocks and aggressive therapy, including edema control and dynamic finger splinting.

Outcomes

Open median nerve decompression leads to symptomatic relief in most patients. When there is clinical evidence of demyelination, patients should be informed of the possibility of residual symptoms and delayed improvement. After open CTR, patients typically regain their preoperative baseline grip strength within 3 months and their pinch strength within 6 weeks. Mini-incision open techniques have similar outcomes but carry a higher risk of incomplete TCL release.