Physiological Measures of Modulation in Individuals With Sensory Modulation Disorder (SMD)

SIGNIFICANCE: It is reported that individuals with sensory modulation disorder (SMD) demonstrate hyperalgesia lingering pain and no temporal summation. Habituation to pain is one of the endogenous analgesia mechanisms, serves to regulate pain, and is defined as response decrement resulting from repeated stimulation. Because there is evidence for central sensitization in individuals with SMD, studying habituation capacity is warranted in this population.

INNOVATION: Pain habituation and sensitization together form pain modulation. This study is the first to test habituation in adults with SMD, inferring that pain modulation in adults with SMD is less efficient. Less efficient pain modulation process adds valued perspective in understanding the effects of SMD on daily participation.

Taking into consideration that SMD is associated with an imbalance between centrally mediated inhibitory and excitatory processes, we assumed that pain modulation alterations characterize individuals with SMD.

Habituation to pain is one of the endogenous analgesia mechanisms, serves to regulate pain, and is defined as response decrement resulting from repeated stimulation. Another pain inhibitory mechanism is the diffuse noxious inhibitory control (DNIC), which relates to the pain-inhibits-pain counterirritation phenomenon and is testable psychophysically in the laboratory by applying the conditioned pain modulation (CPM) paradigm. In this study, we aim to delineate neurophysiological mechanisms of endogenous analgesia in individuals with SMD.

METHOD: Thirty young adults (15 adults with SMD and 15 healthy control adults) participated in this study. Group placement was determined with the Sensory Responsiveness Questionnaire—Intensity Scale (SRQ–IS). Mechanisms of endogenous analgesia were measured through psychophysical and psychophysiological measures: Brief noxious heat stimuli was delivered on the left volar forearm with a 572.5-mm thermode of the PATHWAY sensory evaluation system in two trains of 14 stimuli each, with interstimulus intervals (ISIs) of 8 to 12 s and an intertrain interval of 5 min. Participants provided a numerical pain scale (NPS) rating to each stimulus. The habituation capacity was calculated as the difference of pain ratings between the two trains. To test the CPM paradigm, we once again applied trains of 14 stimuli each while during one train simultaneously immersing the other hand in a hot (46.5 °C) water bath for a duration of 150 s. The CPM effect was calculated as the difference of pain ratings between these two trains, with and without the hot water immersion. During the habituation paradigms, electrophysiology-evoked potentials were recorded with 64 active electrodes referenced to the chin.

RESULTS AND CONCLUSION: Compared with control participants, adults with SMD demonstrated differences in central habituation capacity (p = .006). Although control participants showed a fading of CPM magnitude along the CPM paradigm (first two stimuli vs. middle two stimuli vs. last two stimuli) as expected, SMD demonstrated a gradual build-up of the CPM. Similar results were obtained by measuring the evoked potentials.