Consistent interindividual increases or decreases in muscle sympathetic nerve activity during experimental muscle pain

Abstract

We recently showed that long-lasting muscle pain, induced by intramuscular infusion of hypertonic saline, evoked two patterns of cardiovascular responses across subjects: one group showed parallel increases in muscle sympathetic nerve activity (MSNA), blood pressure, and heart rate, while the other group showed parallel decreases. Given that MSNA is consistent day to day, we tested the hypothesis that individuals who show increases in MSNA during experimental muscle pain will show consistent responses over time. MSNA was recorded from the peroneal nerve, together with blood pressure and heart rate, during an intramuscular infusion of hypertonic saline causing pain for an hour in 15 subjects on two occasions, 2-27 weeks apart. Pain intensity ratings were not significantly different between the first (5.8 ± 0.4/10) and second (6.1 ± 0.2) recording sessions. While four subjects showed significant decreases in the first session (46.6 ± 9.2% of baseline) and significant increases in the second (159.6 ± 8.9%), in 11 subjects, there was consistency in the changes in MSNA over time: either a sustained decrease (55.6 ± 6.8%, n = 6) or a sustained increase (143.5 ± 6.1%, n = 5) occurred in both recording sessions. There were no differences in pain ratings between sessions for any subjects. We conclude that the changes in MSNA during long-lasting muscle pain are consistent over time in the majority of individuals, reflecting the importance of studying interindividual differences in physiology.

Muscle sympathetic nerve activity‐coupled changes in brain activity during sustained muscle pain

“…Hypertonic saline infusion into the tibialis anterior muscle induced pain that was described as dull, aching, and throbbing. Consistent with our previous investigations, we found that tonic muscle pain evoked increases in MSNA amplitude in some individuals and decreases in others (Fazalbhoy et al., , ; Kobuch et al., , , ). These changes began approximately 20 volumes (160 s) after the start of the hypertonic infusion, at approximately the same time as subjects began to perceive pain, and remained fairly stable for the duration of the pain period.…”

“…We have since shown that prolonged (~45 min) muscle pain in humans evokes a mixed cardiovascular response, with some individuals showing a sustained MSNA increase, and others a sustained decrease (Fazalbhoy, Birznieks, & Macefield, , ; Kobuch, Fazalbhoy, Brown, Henderson, & Macefield, ; Kobuch, Fazalbhoy, Brown, & Macefield, , ). These individual differences are reliable across multiple experimental sessions in individuals (Fazalbhoy et al., ) and are not influenced by sex, age, anxiety levels, attitudes to pain, or resting MSNA, BP, or HR levels (Kobuch et al., , ). Using concurrent functional magnetic resonance imaging (fMRI) and microneurography, we recently found that areas of the prefrontal, cingulate and precuneus cortices, hypothalamus, midbrain, and medulla displayed sustained increases in BOLD (blood oxygen level dependent) signal intensity in those individuals who displayed sustained increases in MSNA compared with those who displayed sustained decreases (Kobuch et al., ).…”

Autonomic responses to exercise: Cortical and subcortical responses during post-exercise ischaemia and muscle pain

“…Of course, such an interpretation is problematic, given that both isometric exercise and pain can cause increases in blood pressure. Interestingly, our recent work has shown that long-lasting muscle pain, induced by infusion of hypertonic saline, causes sustained increases in MSNA, blood pressure and heart rate in some individuals but sustained decreases in others (Fazalbhoy et al, 2012(Fazalbhoy et al, , 2014; we are currently undertaking concurrent fMRI and recordings of MSNA using this paradigm in an attempt to disambiguate cortical and subcortical changes related to the sensory input and the generation of the autonomic changes. Fig.…”

Vital Signs Are Not Associated with Self-Reported Acute Pain Intensity in the Emergency Department

“…A small sample can bias the finding toward a positive association if, by chance, patients with similar responses to pain are included in the study. Fazalbhoy et al, 44 employing intramuscular infusion of hypertonic saline to evoke pain, determined that muscle sympathetic nerve activity, blood pressure, and HR were increased in some subjects, while others presented decreased variables. Furthermore, these responses were constant over time.…”