It has long been observed that there is only a weak correlation between the severity of symptoms in carpal tunnel syndrome and the degree of abnormality of neurophysiological measurements. In an earlier study we found the pearson ‘r’ value for the correlation between Canterbury CTS grade and the symptom severity subscale of the Boston carpal tunnel questionnaire (SSS) to be only 0.26.(Sonoo, Menkes et al. 2018) This is often ascribed to the fact that nerve conduction studies primarily measure demyelination, which does not directly cause any symptoms, and that axonal loss is mainly a late feature which results only in ‘negative’ symptoms of numbness and weakness. We hypothesise that a major contributor to the low correlation is the intrinsic patient to patient variability of the instruments used to evaluate subjective severity.

To control for patient to patient and test-retest variability in the SSS we collected SSS values separately from the right and left hands of patients presenting for the first time with CTS and then compared the SSS difference between hands with the difference in nerve conduction severity using the Canterbury severity scale and the difference in ultrasound cross-sectional area (CSA) measurement between the hands.

13005 subjects were identified with available right and left side SSS scores, 65% female , mean age 59.7 (SD 16.2) years. After exclusion of patients with declared other hand pathology which might directly influence the SSS scores – arthritis, trigger digits, dupuytren’s disease, Raynaud’s phenomenon, de Quervain’s, Colles fractures and vibration white finger – there were 6521 subjects with bilateral nerve conduction results available and 433 subjects with bilateral ultrasound measurements. We transformed the SSS scores to Rasch corrected SSS (RSSS),(Jerosch-Herold, Bland et al. 2021) and carried out analyses using both the untransformed and transformed scores.

The pearson r-value for the correlation between difference in RSSS score between hands and difference in Canterbury grade between hands is 0.58 (p<0.0001). The pearson R-value for the correlation between RSSS and the difference in CSA is 0.30 (p<0.0001).

The analysis supports our hypothesis that the poor correlation between NCS severity and subjective severity as evaluated by the SSS results in large part from variability in the SSS. We also find that subjective symptom severity is more closely correlated with a measure of physiological performance (NCS) than with an anatomical one (CSA)