What are the ultrasound criteria for diagnosing carpal tunnel syndrome?

Ultrasound Criteria for Carpal Tunnel Syndrome

Ultrasound demonstrates high sensitivity and specificity for diagnosing carpal tunnel syndrome, with median nerve cross-sectional area measurement at the carpal tunnel inlet being the primary diagnostic criterion, using a threshold of ≥10 mm² (or 9-10.5 mm² depending on measurement technique). 1, 2

Primary Diagnostic Criterion

Median Nerve Cross-Sectional Area (CSA)

• Measure at the carpal tunnel inlet (proximal carpal tunnel at the level of the pisiform bone) 3, 4, 5
• Cut-off values with optimal diagnostic performance:
  • ≥10 mm² using direct tracing technique (sensitivity 85%, specificity 92.1%, accuracy 89.3%) 3
  • ≥9 mm² using indirect technique (sensitivity 88.5%, specificity 82.5%) 3
  • ≥10.5 mm² at pisiform level (sensitivity 89%, specificity 94.7%) 4
  • ≥6.5 mm² at tunnel inlet (sensitivity 89.5%, specificity 93%) 5

The American College of Radiology confirms that multiple systematic reviews and meta-analyses demonstrate ultrasound to be highly sensitive and specific compared with clinical assessment and electrophysiologic studies, though measurements used in the literature are heterogeneous. 1

Additional Supportive Ultrasound Findings

Median Nerve Morphology

• Enlargement and flattening of the median nerve 1, 2
• Increased flattening ratio (anteroposterior to mediolateral diameter) 3, 4
• Decreased echogenicity (hypoechogenicity) of the nerve 3, 6
• Reduced median nerve mobility during dynamic finger flexion/extension 3
• Loss of normal fascicular pattern 6

Carpal Tunnel Structural Changes

• Bowing (palmar displacement) of the flexor retinaculum >1 mm at the outlet 7
• Thickening of the flexor retinaculum 3, 6
• Increased anteroposterior diameter of the carpal tunnel 3, 6

Extended Measurement Sites

• CSA proximal to the carpal tunnel ≥14 mm² (sensitivity 75%, specificity 87.5%) 7
• CSA distal to the carpal tunnel ≥14 mm² (sensitivity 63.6%, specificity 100%) 7
• Using either proximal OR distal CSA OR retinacular bowing >1 mm yields combined sensitivity of 100%, specificity 84.3% 7

Anatomic Variants and Space-Occupying Lesions

Ultrasound can identify conditions that may cause or contribute to carpal tunnel syndrome: 1, 2

• Bifid median nerve
• Persistent median artery
• Space-occupying lesions within the carpal tunnel
• Tendon pathology (tenosynovitis)

Scanning Technique

Patient Positioning

• Sitting position with hand resting on thigh or examination table 1
• Perform dynamic examination with active finger flexion/extension to assess nerve mobility 1

Standard Scanning Protocol

• Volar (palmar) transverse scan at carpal tunnel - primary view for CSA measurement 1
• Volar longitudinal scan 1
• Use high-frequency transducers (≥10 MHz) for optimal resolution 1

Clinical Context and Limitations

Important caveat: The 2016 American Academy of Orthopedic Surgeons guidelines report limited evidence supporting routine ultrasound use for CTS diagnosis, indicating disagreement among hand surgeons regarding necessity of diagnostic testing. 1 However, the American College of Radiology acknowledges that some authors advocate for ultrasound as the confirmatory test of choice or complementary test due to its ability to identify anatomic variants and the false-positive rate of electrodiagnostic studies. 1

MRI correlation: There is good correlation between median nerve CSA measured by ultrasound and MRI, though MRI is typically not indicated for routine CTS evaluation due to cost and availability considerations. 1, 2