Abstract
Background
The aim of this study was to investigate if intramuscular injection of sterile water can be used as a human experimental pain model that resembles clinical craniofacial muscle pain, and to analyze if the effects differ between sexes.
Methods
This randomized, double‐blind, placebo‐controlled cross‐over study included 30 healthy age‐matched women and men (23.6±2.4 years). At three sessions, with at least one week of wash‐out in between, 0.2mL of either sterile water (test‐substance), hypertonic saline (58.5mg/mL; active‐control) or isotonic saline (0.9mg/mL; passive‐control) were randomly injected into the right masseter muscle. Pain intensity (VAS) was continuously assessed during 5min whereafter pain duration (s) and pain area (au) were calculated pressure pain thresholds (PPT;kPa) were recorded every 5‐min during 30min.
Results
Sterile water evoked pain of similar intensity (74.5±49.9) as hypertonic saline (74.0±50.5), whereas isotonic saline evoked low‐intensity pain (11.4±23.4). The pain induced by sterile water and hypertonic saline had higher intensity (P<0.001), longer duration (P<0.001) and larger pain area (P<0.001) than isotonic saline. There were no significant differences in any pain variable between sterile water and hypertonic saline. The PPT did not change significantly after any substance, except for in women 5 min after sterile water injection (P<0.002). Pain duration was longer in the men for all substances (P<0.006), while the pain area was larger in women after injection of hypertonic saline (P<0.003).
Conclusion
These results indicate that pain evoked by sterile water resembles clinical muscle pain and may offer a novel and simpler alternative to hypertonic saline injections.
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