Judging pain sensitivity with subcutaneous lidocaine injections.

OBJECTIVE Pain perception is affected by psychological, social, medical, and environmental conditions, and contributes to the patient's treatment satisfaction and response. Better understanding of pain perception will likely improve pain assessment and treatment selection. The objective of this study was to define a range of verbal and nonverbal pain responses to a clinical stimulus in a clinical population. DESIGN Subjects were 165 patients with chronic pain conditions. The patients were scheduled for elective interventional pain procedures on the lumbar spine including lumbar interlaminar epidural steroid injections, lumbar transforaminal steroid injections, lumbar facet injections, lumbar medial branch nerve blocks, radiofrequency ablation of lumbar medial branch nerves, and lumbar discography. Intervention.  Prior to the procedure, subjects rated anxiety on a numerical rating scale (NRS) from 0 (no anxiety) to 3 (extreme anxiety), and received standardized subcutaneous injections of lidocaine (using 25-G needle to infiltrate 2 cc 1% lidocaine) as local anesthesia. Following the lidocaine injection, pain was rated on an NRS scale. Body movement detected during the injection was rated by an independent observer and recorded as none, less than 1 in., and more than 1 in. Body movement was defined as torso moving away from original prone position. RESULTS Patients were 37% men and 63% women, with average age of 53 years. The range of pain intensity responses fell within a normal curve (P < 0.01), with average pain intensity of 4.9 (standard deviation = 2.7). Patients with more body movement reported higher pain (P < 0.01). Anxiety predicted pain intensity ratings (P < 0.01). Use of opioids did not predict pain intensity, body movement, or anxiety. CONCLUSION This study shows normal distribution of verbal pain response to a clinical pain stimulus in a clinical population. Body movement and anxiety correlated with verbal pain intensity ratings. Subcutaneous injections of lidocaine may be a useful model for exploration of pain sensitivity in a clinical population.