Electrodiagnostic testing has an important role in the diagnostic workup for carpal tunnel syndrome. It can document a median nerve entrapment at the wrist and thereby support the diagnosis of carpal tunnel syndrome in the presence of appropriate symptoms. Simpson was the first to describe the increased distal motor latency of the median nerve stimulated at the wrist (Simpson 1956). Since then, increasingly refined nerve conduction techniques have been developed to demonstrate slowing of conduction in the median nerve across the carpal tunnel.
Electrodiagnostic testing usually includes motor and sensory nerve conduction studies of both median nerves and at least 1 ulnar nerve. Needle examination of the abductor pollicis brevis muscles is routinely performed to look for denervation. Needle examination of several other upper extremity muscles and the cervical paraspinal muscles is often necessary to evaluate for other entrapment neuropathies, plexopathy, or radiculopathy.
Sensory nerve conduction studies are the most sensitive in confirming the diagnosis. The most common finding is an increase in distal latency due to focal slowing of conduction across the carpal tunnel. Special adaptations may be necessary to demonstrate mild carpal tunnel syndrome, these include (Jablecki et al 2002):
The next most sensitive feature is a decrease in amplitude of the sensory response. Increased distal motor latency is seen less frequently and reduced amplitude of the median motor response is even less common. In more severe carpal tunnel syndrome there is acute or chronic denervation on needle examination of the abductor pollicis brevis, suggesting axon loss of median motor nerve axons. The severity of clinical weakness and sensory loss, but not of the complaints of tingling and pain, correspond in general with the severity of electrodiagnostic findings.
Median and sensory nerve conduction studies provide accurate and reproducible measurements that can confirm a clinical diagnosis of carpal tunnel syndrome with a high degree of sensitivity (66% to 82%) and specificity (82% to 97%) (Jablecki et al 1993).
MRI of the wrist can document abnormalities in the median nerve that are compatible with carpal tunnel syndrome (Britz et al 1995; Jarvik et al 2002). MRI may be helpful if tumors or other structural abnormalities are suspected or after surgery has failed. A routine wrist x-ray is not useful (Bindra et al 1997).
Ultrasound is another useful tool for assessment of carpal tunnel syndrome. Various measurements, including the wrist-to-forearm median nerve area ratio (WFR) and the cross-sectional area of median nerve at the wrist (CSA-W), have been shown to have high correlation with severity grade of carpal tunnel syndrome. WFR has been shown to be superior to CSA-W for diagnosis and grading of severity of carpal tunnel syndrome (Kang et al 2012). An evidence-based guideline was also recently published for use of ultrasound in the diagnosis of carpal tunnel syndrome. Ultrasound measurement of median nerve cross-sectional area has been found to be accurate and may be offered as a diagnostic test for carpal tunnel syndrome. The guideline also showed that ultrasound probably adds value to electrodiagnostic studies when diagnosing carpal tunnel syndrome and should be considered in screening for structural abnormalities at the wrist in those with carpal tunnel syndrome (Cartwright et al 2012).