Experimental muscle pain provokes long-lasting alterations of thermal sensitivity in the referred pain area.

This study explores thermal sensitivity and thermal nociception for signs of central sensitization in the area of referred muscle pain. Two groups of 24 healthy subjects (ss) each, and with mean ages of, respectively, 27 and 55 years, were first trained in quantitative sensory testing and pain rating. Then, in a second session, referred pain was evoked by injection of 6% hypertonic saline into the infraspinatus muscle. Cold and warm thresholds, synthetic heat threshold (SHT--evoked by an alternating pattern of adjacent cold and warmth), and thermal pain thresholds were measured within the referred pain area at a rate of 1/20 min for 60-120 min. All ss of both groups experienced referred pain mostly in the upper arm and of medium intensity. Pain lasted for approximately 12min with a shorter duration in the older group (p<0.02). The cold threshold increased significantly (p<0.001), and the warm threshold slightly, after the injection and remained high for the whole observation period (i.e. lower and higher temperatures were necessary to elicit cold and warmth, respectively). Threshold recovery was more delayed in the older age group. Of those 28 ss in whom cold pain threshold could be followed during the whole observation period, 18 ss showed an immediate threshold decrease of average 6 degrees C which outlasted the observation period. Four ss responded with a threshold increase. Heat pain thresholds were not affected in the referred pain area. Average synthetic heat threshold did not change; there were, however, distinct and lasting individual threshold shifts in either direction. Ss with lowered cold pain thresholds or evident threshold shifts for synthetic heat had also higher pain ratings. The results demonstrate that experimental muscle pain can induce long-lasting changes in thermal sensitivity and nociception. The unexpected cold threshold increase may tentatively be explained as an expression of long-term depression. The decrease of cold pain threshold or SHT in subgroups of ss may indicate central sensitization. However, the observed changes in this experiment do not provide an unambiguous indicator for central sensitization which seems to be rather individual and might depend on pain intensity and proneness to express central mechanisms of sensitization. Therefore in clinical pain states the individual pattern of sensory abnormalities has to be analysed and interpreted in addition to the pain parameters to assess central involvement.