Carpal Tunnel Syndrome: An investigation of the impact of neuropathic Pain phenotype on post-operative outcome (CAPS)

Abstract

This study investigated the association of neuropathic pain phenotype including median nerve somatosensory function, conditioned pain modulation (CPM), pain parameters, and psycho-social state with outcome of surgery for carpal tunnel syndrome (CTS). Methods: With ethical approval (14/LO/36) and consent, this prospective observational study recruited patients from two London hospitals. Measures prior to, 3 and 6 months post-surgery included quantitative sensory testing (QST), CPM, pain parameters, insomnia, catastrophising and mood. Pain in median nerve distribution with electrophysiologically confirmed compression and DN4 score ≥ 4 was defined as neuropathic. Primary outcome was patient-rated change at 6 months; “worse” or “no change” dichotomised as poor outcome; “slightly better”, “much better” or “completely cured” good outcome. Two additional studies recruited healthy volunteers for QST and CPM reference data. Results: Seventy-six participants were included. At baseline, 75% had neuropathic pain, 25% had sensory loss to thermal stimuli, 59% to mechanical stimuli, 16% gain to mechanical stimuli. CPM inhibition was seen in 17%. 33% had high levels of pain catastrophisation, 64% had clinical insomnia. At 3 months post-surgery there was significant improvement in QST, pain parameters, psychosocial and quality of life measures; change thereafter was not significant. At 6 months QST showed recovering thermal and mechanical function (p<0.001) but persisting mechanical hyperalgesia (p>0.05). CPM was not restored and did not correlate with outcome (p=0.85). Pain catastrophising diminished (p<0.001), scores improved for insomnia (p<0.001), anxiety (p=0.02), symptom and pain severity (p<0.001) but not depression (p=0.42). Surgical outcome was good in 92% of participants, poor in 8%. Baseline pain catastrophizing, anxiety, pain interference and functional severity correlated with outcome (p≤0.05). Conclusion: Sensory profiles pre and post-surgery correlated with symptom severity however baseline sensory profile was not associated with post-surgical symptom severity. In patients with CTS, pain catastrophizing, anxiety, pain interference and functional severity scores at baseline correlated with patient-rated outcome.