The most common neuropathy of the upper extremity is the carpal tunnel syndrome, with an estimated incidence of nearly 1% annually, or almost 2.8 million new cases per year, and prevalence of 0.125% to 5.8% . The syndrome is most often found in patients between 30 and 60 years of age, has a male/female ratio of 1:5, and is bilateral in as many as 50% of patients. Clinical complaints include often transient and reversible pain and paresthesia in the median nerve distribution.
In wrist flexion, in asymptomatic individuals, the median nerve moves radially and posteriorly and becomes interposed between the flexor tendons . The median nerve is more likely to remain adjacent to the flexor retinaculum during wrist flexion in patients who have carpal tunnel syndrome. This lack of motion of the median nerve may predispose it to compression and subsequent carpal tunnel syndrome. Other theories proposed to account for carpal tunnel syndrome include repeated compression of the median nerve with subsequent ischemia, subendoneurial edema, synovitis, and eventual fibrosis, as well as reduced gliding and tethering of the nerve due to scar tissue .
Increased incidence of carpal tunnel syndrome has been associated with repetitive flexion and extension of the wrist and has been described in swimmers, motocross riders, body-builders, and wheelchair athletes [77-80]. Many other pathologic processes may compress the median nerve within the tunnel, including anomalous muscles, ganglion cysts (Fig. 16), fracture fragments, bony spurs, inflammatory synovial pannus, amyloid deposits, and rice bodies.
Carpal tunnel syndrome is usually an easy clinical and electromyographic diagnosis. Clinical evidence of carpal tunnel syndrome includes a positive Pha-len's test (worsened paresthesia following 1 minute of maximal passive wrist flexion) and a positive Tinel's sign (paresthesia in the median territory elicited
Fig. 16. Carpal tunnel syndrome. (A) Axial fat-suppressed T2-weighted image at the proximal carpal tunnel demonstrates high signal intensity and enlargement of the median nerve (arrowhead). (B) Axial proton-density-weighted image at the level of the transverse carpal ligament shows increased bowing ratio, determined by dividing the palmar displacement of the retinaculum (short line) by the distance between the hook of the hamate and the tubercle of the trapezium (long line).
by gentle tapping over the carpal tunnel) . These clinical tests, however, are not foolproof, and some symptomatic patients fail to show decreased median nerve conduction velocity . MRI evaluation in this select group of patients can be useful [82,83]. MRI can also depict space-occupying lesions within the carpal tunnel, such as anomalous muscles, persistent median artery, carpal tunnel lipomatosis, ganglion cysts, and synovial hypertrophy.
Disagreement exists in the literature on the most sensitive and most specific MRI findings of carpal tunnel syndrome. Nevertheless, these findings may be divided into four major categories: (1) increased size of the nerve, (2) nerve flattening, (3) bowing of the flexor retinaculum, and (4) increased T2 signal within the median nerve (see Fig. 16A). Significantly increased cross-sectional area of the median nerve is noted in patients who have carpal tunnel syndrome as compared with asymptomatic individuals . Comparison of the cross-sectional area of the nerve at the radiocarpal joint and at the pisiform bone level is best performed on axial MR images. In patients who have carpal tunnel syndrome, the size of the nerve should be about two to three times bigger at the pisiform level compared with the radiocarpal joint level. The ratio between the major and minor axis of the nerve, both at the level of the distal radioulnar joint and at the level of the hook of the hamate, may be used to assess flattening of the nerve. A ratio of three is usually indicative of disease. In normal individuals, the flexor retinaculum at the level of the hook of the hamate should be flat or slightly convex. The degree of bowing is determined by dividing the distance of palmar displacement of the retinaculum by the distance between the hamate's hook and the tubercle of the trapezium (Fig. 16B). In normal patients, the ratio varies from 0 to 0.15 (mean 0.05); in carpal tunnel syndrome, the ratio varies from 0.14 to 0.26 (mean 0.18) . Increased T2 signal within the median nerve may be difficult to assess, because the normal median nerve, like many other nerves in the body, often reveals a slightly increased signal on fat-suppressed T2-weighted fast spin echo images. Intense increased signal, however, is fairly diagnostic of a diseased nerve.
MRI is particularly useful for identifying space-occupying lesions within the carpal tunnel. A variety of median nerve sheath tumors can be identified and often have distinct MR characteristics, including intermediate signal on T1-weighted images and relative homogeneous increased signal on fluid-sensitive sequences. The target sign indicating central low-signal postcontrast enhancement is a fairly specific sign for neurilemmomas.
Another lesion of the median nerve is the fibrolipomatous hamartoma, which has a pathognomic striated increased signal, dubbed ''coaxial cablelike,'' compatible with fat. Other masses within the carpal tunnel, such as ganglions, bursa, fracture fragments, proliferative synovitis, and aberrant muscles, can also be easily displayed on MRI. Aberrant muscles may be identified by their isointense to muscle signal on all pulse sequences.
MRI is useful in identifying persistent median artery or vein within the carpal tunnel. This finding is important to recognize to avoid inadvertent injury to the vessels during surgery. MRI has also been used in the assessment of persistent carpal tunnel syndrome after surgical release. Scar tissue engulfing the nerve, adhesions, persistent tenosynovitis, and incomplete release of the ret-inaculum may be identified.
Because the diagnostic accuracy of MRI signs of carpal tunnel syndrome is still controversial, particularly when no mass effect is identified, and disagreement exists as to the specificity of nerve swelling and flattening, the presence of MRI indications of carpal tunnel syndrome should not be taken as sole proof of the condition. It should be reviewed in light of the clinical history, physical examination, and electrodiagnostic studies.
Compression of the ulnar nerve, although more commonly seen at the elbow, can occasionally occur at the wrist in Guyon's canal. The loss of sensation on the dorsal ulnar hand, present in proximal ulnar compressive neuropathy, can aid in distinguishing elbow and forearm ulnar neuropathy from Guyon's or ulnar tunnel syndrome.
The ulnar nerve can undergo compression at three potential sites within Guyon's canal: (1) zone 1, extending from the proximal edge of the palmar carpal ligament to the bifurcation of the ulnar nerve into the deep motor and superficial sensory branches; (2) zone 2, extending from the bifurcation of the ulnar nerve just distal to the fibrous arch of the hypothenar muscles and containing the deep motor branch of the ulnar nerve; and (3) zone 3, which is parallel to zone 2 and contains the superficial sensory branch of the ulnar nerve.
The clinical presentation can provide clues to the site of ulnar nerve compression: combined motor and sensory deficit is seen in zone 1 lesions, zone 2 lesions display pure motor deficit, and isolated sensory deficits are noted in zone 3 lesions. Most patients undergo ulnar nerve compression at either zone 1 or zone 3; in more than 50% of the cases, the compression occurs in more than one zone .
Ulnar nerve compression within Guyon's canal can either be idiopathic (45%) or secondary to trauma (26%) . A displaced hook of the hamate fracture or enlarged hook of the hamate are well-known offending entities (Fig. 17).
Isolated compressive neuropathy of the deep terminal motor branch of the ul-nar nerve in bikers has been termed handlebar palsy  and is related to prolonged riding with the hands pressed against the handlebars. Because no sensory fibers are affected, the patients are not aware of the ongoing nerve compression until a severe nerve lesion develops.
Other causes for ulnar tunnel syndrome include space-occupying lesions, such as tumors, musculotendinous variants, aberrant fibrous bands, and enlarged bursae, and vascular lesions, such as ulnar artery aneurysm and thrombosis .
MRI findings of Guyon's canal include increased size and increased signal in the nerve and its branches, best appreciated on axial MR images. The nerve can usually be distinguished from the adjacent bright signal vessels by its closer proximity to the pisiform and hamate bones. Space-occupying lesions such as ganglions, lipomas, and hemangiomas, posttraumatic scarring, and obliteration of the fat around the nerves may be seen. Careful attention should be paid to the presence of aberrant muscles, a common cause of Guyon's canal syndrome. Hook of the hamate fractures are often associated with edema and scarring in the adjacent Guyon's canal. Muscle denervation, edema, and atrophy in the hypothenar and intrinsic muscles should be sought, but this is a less common finding.
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