Imaging Studies
The diagnosis of carpal tunnel syndrome in the ED is made presumptively. The clinician who follows the patient after the acute presentation usually orders the imaging studies and other tests discussed below. Plain radiography is low-yield. [24]
Magnetic resonance imaging (MRI) is reasonably accurate in diagnosing CTS. This imaging modality is only recommended when the clinical picture is confusing or when nerve conduction studies are equivocal or contradictory. Dynamic MRI imaging may be useful in identifying dynamic CTS (CTS symptoms brought on only by repetitive wrist motion). [25] MRI may also identify causative lesions in carpal tunnel. [26]
(See the images below.)
Carpal tunnel syndrome. Axial fast spin-echo T2-weighted MRI with fat saturation. Note the increased T2-weighted signal within the median nerve (arrow). A slightly increased cross sectional area of the nerve is noted but the nerve architecture is preserved, consistent with early or mild inflammation.
Carpal tunnel syndrome. Fast spin-echo T2-weighted MRI illustrates more pronounced increased signal within the median nerve (arrow). Note the small amount of fluid within the carpal tunnel, a secondary sign of inflammation. Slightly less optimal fat saturation is noted than on other images, which is a common occurrence.
Carpal tunnel syndrome. Axial fast spin-echo T2-weighted MRI with greater increase in signal and loss of definition within the nerve (arrow). Inflammatory change is noted within the carpal tunnel, adjacent to the flexor digitorum superficialis tendons. The appearance is consistent with pronounced inflammatory change within the carpal tunnel.
Carpal tunnel syndrome. Normal findings of isointense-to-hypointense appearance of the median nerve on fast spin-echo T2-weighted MRI (arrow). Note the fairly well-defined nerve fascicles within the median nerve sheath.
Carpal tunnel syndrome. Normal findings on an axial spin-echo T1 MRI of the carpal tunnel showing the intermediate signal intensity of the median nerve (arrow).
High-resolution ultrasonography (US) has received increased attention in the evaluation of CTS. US as a modality is more widely available than electrodiagnostic studies and is noninvasive and has lower costs. Some studies have shown that patients with CTS have an increased cross-sectional area of the median nerve in the carpal tunnel. One study showed that both the cross-sectional area of the median nerve in the wrist (CSA-M) and of the ratio of the area of the median nerve between the wrist and the forearm (R-WF) were useful measures in the diagnosis of CTS, using electroneuromyography (ENG) as a reference test. [27]
Superb microvascular imaging (SMI) is a new imaging mode to detect blood flow. When compared with conventional color Doppler and power Doppler, the sensitivity and finer detail of the microvessels visualized with SMI appears significantly better. In one series, SMI provided improved results for the evaluation of the blood flow in the MN compared to color Doppler ultrasonography (CDUS) and power Doppler ultrasonography (PDUS). [28]
There has been recent recognition of the diagnostic accuracy of ultrasound approaching that of nerve conduction studies [29] . Ultrasound has now been proposed as the initial diagnostic modality due to the added benefit of increased acceptability to patients, decreased cost, and the ability to assess carpal tunnel anatomy and guide corticosteroid injections. [30]
Other Tests
Electromyographic (EMG) and nerve conduction studies
EMG and nerve conduction studies help confirm the diagnosis of CTS. They are most helpful in the determination of the site and severity of nerve compression.
Nerve conduction studies show a high specificity (95%) with a wide range of sensitivity depending on the choice of cutoff values (75-92%). Due to this high specificity, nerve conduction studies are recommended prior to considering surgical intervention [14] .
Clinically symptomatic CTS may have normal nerve conduction findings. [31]
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Carpal tunnel syndrome. Carpal and Guyon tunnels. Drawing showing the proximal level of the carpal tunnel delimited by the pisiform (P) and the scaphoid (S). The flexor retinaculum (medium gray region) forms the roof of the carpal tunnel and the floor of the Guyon tunnel. The palmar carpal ligament (dark gray region) forms the volar boundary of the Guyon tunnel. * = flexor pollicis longus tendon, * = flexor carpi radialis tendon. From Martinoli C, Bianchi S, et al. US of nerve entrapments in osteofibrous tunnels of the upper and lower limbs. Radiographics 2000; 20:S199-S217. Used by permission of the authors and RSNA.
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Carpal tunnel syndrome. Carpal and Guyon tunnels. Transverse 5-12-MHz ultrasound scan corresponding to the image above shows the proximal level of the carpal tunnel delimited by the pisiform (P) and the scaphoid (S). The flexor tendons and median nerve (MN) extend through the carpal tunnel, with the nerve lying palmar and radial. The flexor retinaculum (open arrowheads) forms the roof of the carpal tunnel and the floor of the Guyon tunnel. At the level of the pisiform, the ulnar nerve (U) courses medial to the ulnar artery (solid arrowhead) within the Guyon tunnel. * = flexor pollicis longus tendon. From Martinoli C, Bianchi S, et al. US of nerve entrapments in osteofibrous tunnels of the upper and lower limbs. Radiographics 2000; 20:S199-S217. Used by permission of the authors and RSNA.
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Carpal tunnel syndrome. Carpal and Guyon tunnels. Drawing showing the distal level of the carpal tunnel delimited by the hook of the hamate (H) and the tubercle of the trapezium (T). The flexor retinaculum (medium gray region) forms the roof of the carpal tunnel. From Martinoli C, Bianchi S, et al. US of nerve entrapments in osteofibrous tunnels of the upper and lower limbs. Radiographics 2000; 20:S199-S217. Used by permission of the authors and RSNA.
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Carpal tunnel syndrome. Carpal and Guyon tunnels. Transverse 5-12-MHz ultrasound scan corresponding to the image above shows the distal level of the carpal tunnel delimited by the hook of the hamate (H) and the tubercle of the trapezium (T). The flexor retinaculum (open arrowheads) forms the roof of the carpal tunnel. The flexor tendons and median nerve (MN) extend through the carpal tunnel, with the nerve lying palmar and radial. At the level of the pisiform, the ulnar nerve courses medial to the ulnar artery (solid arrowhead) within the Guyon tunnel. At the level of the hamate, the ulnar nerve divides into two terminal branches, a deep motor branch (curved arrow) and a superficial sensory branch (straight arrow). From Martinoli C, Bianchi S, et al. US of nerve entrapments in osteofibrous tunnels of the upper and lower limbs. * = flexor pollicis longus tendon. Radiographics 2000; 20:S199-S217. Used by permission of the authors and RSNA.
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Carpal tunnel syndrome. Normal findings on an axial spin-echo T1 MRI of the carpal tunnel showing the intermediate signal intensity of the median nerve (arrow).
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Carpal tunnel syndrome. Normal findings of isointense-to-hypointense appearance of the median nerve on fast spin-echo T2-weighted MRI (arrow). Note the fairly well-defined nerve fascicles within the median nerve sheath.
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Carpal tunnel syndrome. Axial fast spin-echo T2-weighted MRI with fat saturation. Note the increased T2-weighted signal within the median nerve (arrow). A slightly increased cross sectional area of the nerve is noted but the nerve architecture is preserved, consistent with early or mild inflammation.
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Carpal tunnel syndrome. Fast spin-echo T2-weighted MRI illustrates more pronounced increased signal within the median nerve (arrow). Note the small amount of fluid within the carpal tunnel, a secondary sign of inflammation. Slightly less optimal fat saturation is noted than on other images, which is a common occurrence.
-
Carpal tunnel syndrome. Axial fast spin-echo T2-weighted MRI with greater increase in signal and loss of definition within the nerve (arrow). Inflammatory change is noted within the carpal tunnel, adjacent to the flexor digitorum superficialis tendons. The appearance is consistent with pronounced inflammatory change within the carpal tunnel.
