PERIAQUEDUCTAL GRAY (PAG) / POSTRAL VENTROMEDIAL MEDULLA (RVM) ACTIVATION IN CHRONIC PAIN AND DEPRESSED PATIENTS WITH OFFSET ANALGESIA INDUCTION

Abstract

Offset analgesia (OA) is an endogenous pain inhibitory mechanism originating in the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), characterised by a prominent non-proportional reduction in pain perception following a discrete decrease in intensity of the noxious stimulus. This study aimed to investigate activity of PAG/RVM and other parts of the pain neuromatrix during offset, baseline and constant heat trials in 15 healthy, 15 depressed and 15 chronic pain subjects. fMRI brain activation during the third 12s of each trial was compared with activation during the first 12s and this difference was contrasted across trials. Both healthy and depressed subjects exhibited intact PAG activation with more activation throughout the pain neuromatrix for depressed than healthy subjects. Minimal activation was noted for chronic pain subjects. These findings suggest that OA was present in controls and depressed patients but not for chronic pain patients, and they affect the pain matrix differently.