Early Diagnosis of Carpal Tunnel Syndrome:
Comparison of Digit 1 with Wrist and Distoproximal Ratio

Khema R. Sharma MD
Francisco Rotta MD
Jose Romano MD
D. Ram Ayyar MD

Published: March 29, 2001

Our objective in this study was to compare the sensitivity and specificity of the median sensory nerve conduction velocity (SNCV) from digit 1 to wrist with those of the distoproximal (D/P) ratio of the median SNCV from palm to digit 3/palm to wrist in the diagnosis of mild carpal tunnel syndrome (CTS) by using a receiver operating characteristic (ROC) curve. To achieve this objective, we studied prospectively (January 1997–October 1998) 370 patients referred for CTS. One hundred forty-two patients (38.4%) with moderate to severe CTS and 15 patients (4.1%) with multiple ( 3) compressive neuropathies in upper limbs with subclinical peripheral neuropathy were excluded. The remaining 213 patients (302 hands with mild CTS; 167 women; mean age, 50 y ± 12 y) and 38 controls (71 hands; 25 women; mean age, 47 y ± 13 y) had median and ulnar nerve conduction studies. ROC curves were constructed for median SNCV digit 1 to wrist and median SNCV D/P ratio from the patients' and controls' data. The median SNCV at 45.9 m/s, corresponding to an optimal cutoff point on ROC curve, discriminated 89.5% of mild CTS from controls with specificity of 98.6%. The median D/P ratio at 1.12, corresponding to an optimal cutoff point on ROC curve, discriminated 67.2% of mild CTS from controls with specificity of 97.2%. Of the 10.3% (31/302) of hands in which digit 1 to wrist was within normal limits at the selected optimal cutoff value ( 45.9 ms), 7% (21/302) had an abnormal D/P ratio ( 1.12), and 3.3% (10/302) had a normal electrophysiologic examination. The likelihood ratio (true-positive ratio to false-positive ratio, assessing the discriminative power of a test) of the median SNCV digit 1 to wrist, at an optimal point on ROC curve (63.9), was higher than that of the median SNCV D/P ratio (23.9, X ² = 36.9, P < .001). These findings suggest that the median SNCV digit 1 to wrist is more sensitive than the median SNCV D/P ratio in the diagnosis of mild CTS.

Keywords: carpal tunnel syndrome, early diagnosis, median sensory nerve conduction velocity, digit one, distoproximal ratio, receiver operating characteristic curve, sensitivity, and specificity