Pupillometry to detect pain response during general anaesthesia following unilateral popliteal sciatic nerve block: A prospective, observational study

CONTEXT: Pupillary reflex dilation appears to be a reliable indicator of response to noxious stimulation even under general anaesthesia. The ability of pupillometry to detect the effects of extremity blocks during continuous infusion of opioids remains unknown. OBJECTIVE: To explore the performance of pupillometry to detect differences in pupillary reflex dilation response to a standardised noxious stimulus applied to each leg following unilateral popliteal sciatic nerve block during continuous infusion of remifentanil. DESIGN: Prospective, observational study. SETTING: University hospital anaesthesia department, between June 2010 and December 2010. PATIENTS: Twenty-four adult patients undergoing elective foot or ankle surgery under general anaesthesia who requested a peripheral nerve block. Unilateral popliteal sciatic nerve block with 0.75% ropivacaine and 1% lidocaine was performed awake. General anaesthesia was maintained with steady-state infusions of propofol and remifentanil. MAIN OUTCOME MEASURE: Video-based pupillometer was used to determine pupillary reflex dilation during tetanic stimulation (60 m, 100Hz) applied to the skin area innervated by the sciatic nerve for 5 s after the onset of general anaesthesia. RESULTS: Sensory nerve block led to a blunted maximal pupillary reflex dilation response to noxious stimulation compared with the non-blocked leg: median (interquartile range) change from baseline 2% (1 to 4%) versus 17%(13 to 24%), respectively (P<0.01). The differences in the response persisted throughout the 5-s stimulus and the recovery phase. CONCLUSION: These results are a proof of concept. The effects of peripheral nerve block can be detected via the measurement of pupillary reflex dilation response to noxious stimulation of the skin in patients receiving remifentanil.