Spatial and temporal summation of pain evoked by mechanical pressure stimulation

Chronic pain patients often suffer from widespread and long lasting pain. The integrative effect of combined spatial and temporal summation on pain intensity has not been quantitatively tested. The present study was designed to investigate: (1) if the size of the stimulation area would facilitate the temporal summation of pain to repetitive pressure stimulation, and (2) if temporal summation is effective when stimulating separated sites, repetitively. Twenty healthy male subjects participated in this study. The test sites were located on the bilateral upper trapezius and tibialis anterior muscles. The ten stimuli (each with a duration of 1 s) were applied to a single site at three inter-stimulus intervals (ISI: 1, 5, 30 s) using five different probe sizes (0.5, 1, 2, 4 and 8 cm2). The stimulation intensity was equal to the pressure pain threshold (PPT) determined for each probe size. Similar repetitive stimulations at two inter-stimulus intervals (5 s and 30 s) using two sizes of probes (0.5 cm2 and 2 cm2) were applied to ten separate sites. The PPT at the trapezius muscle decreased significantly with the increase in stimulus area from 0.5 cm2 to 8 cm2 (P < 0.001) due to spatial summation. Temporal summation of pain was evoked by repetitive pressure stimuli on the same site for all ISI and was more pronounced at 5 s and 30 s ISI with larger probe areas (2, 4, and 8 cm2) compared to smaller probe areas. There was no temporal summation of pain to stimuli with ISI 5 and 30 s when stimulating the separated sites. The current study indicated that spatial summation facilitated the temporal summation of pain for stimuli given at 5 s and 30 s ISI. The combination of temporal and spatial integration of nociceptive input facilitates the pain intensity, suggesting that temporal summation is clinically relevant in conditions with widespread pain.