Diagnosis of carpal tunnel syndrome with ultrasound: should we go more distal?

Objective: To assess the value of adding the ultradistal level to other more thoroughly studied levels of the carpal tunnel when measuring the cross-sectional area (CSA) of the median nerve (MN) by ultrasound (US) in diagnosing patients with primary carpal tunnel syndrome (CTS).

Methods: Patients clinically diagnosed with primary CTS and healthy controls were included. The MN-CSA was measured by US at three wrist levels: proximal, distal and ultradistal. The best cut-off to differentiate cases and controls was determined for the CSA and for the difference between levels of the same wrist. The performance of different definitions for US-CTS compared with the clinical diagnosis of CTS was evaluated: (1) CSA above cut-off at each level; (2) CSA-difference above cut-off at each level; (3) ≥1 level with CSA above cut-off and (4) ≥1 CSA-difference above cut-off. Definition 3, excluding the ultradistal level, and combinations of definitions were also tested.

Results: In total, 219 patients and 39 controls were included. The CSA was higher in patients (10.5-16.8 mm2) than controls (6.2-7.6 mm2). The difference between groups was maximal at the ultradistal level (right: 10.1 mm2; left: 8.3 mm2). The CSA cut-offs were 11 mm2, 9 mm2 and 10 mm2 at the right, and 10 mm2, 8 mm2 and 10 mm2 at the left, for the proximal, distal and ultradistal levels, respectively. Definition 3 yielded the best balance between sensitivity (98%) and specificity (95%) (right hand). Removing the ultradistal level from definition 3 decreased sensitivity to 90%, maintaining the same specificity.

Conclusions: Adding the ultradistal level improves the performance of US for diagnosing CTS. We suggest adding it in clinical practice when investigating CTS.