Comparison of Autonomic Nervous System Activity in Subjects with Chronic Neck Pain and Healthy Controls

Objective: This study investigated if differences in autonomic nervous system activity exist in patients with chronic neck pain as compared to controls measured by using a fully automated pupillometry system. Methods: 100 chronic neck pain subjects and 50 controls participated in this quasi-experimental study design evaluating the effect of chronic pain on the autonomic nervous system using a fully automated method of pupillometry. The Chisquare test for categorical data was used to establish homogeneity of baseline characteristics. The Mann-Whitney U test determined the difference in pupil diameter between the chronic neck pain group and the healthy control group. The Spearman's rho test was used to relate the pupil diameter to the Neck Disability Index (NDI) scores, subject's age, and the duration of chronic neck pain. Results: This study demonstrated that the chronic neck pain group had a statistically significant smaller pupil diameter than the healthy control group (p=0.022). This study showed that there was no relationship between the NDI scores and the pupil diameter change. However, there was a weak to moderate correlation between age and pupil size (p<0.001), indicating that there is a significant negative relationship and that the pupil diameter decreases with age. There was a weak but non-significant correlation between the pupil diameter and the duration of chronic neck pain (p>0.05). Discussion: The results of this study demonstrate that subjects with chronic neck pain exhibited a smaller pupil diameter than healthy controls. This is a direct indication of an altered autonomic balance.