Experimental acute muscle pain and itch induce similar inhibitory effects on corticospinal excitability

Pain and itch provoke different motor behaviors. Pain induces motor inhibition, whereas itch triggers an active scratching response. Acute pain is known to decrease corticospinal excitability, assessed by transcranial magnetic stimulation (TMS) through motor-evoked potentials (MEPs), but the effects of acute itch on bilateral corticospinal excitability are unknown.

We hypothesized that acute pain and itch differentially affect corticospinal excitability, decreasing and increasing MEPs, respectively.

Nineteen healthy participants completed a 2-session, crossover study. Transcranial magnetic stimulation was applied targeting the right and left first dorsal interosseous (FDI) muscles. Motor-evoked potentials were recorded before, during experimentally induced pain (intramuscular hypertonic saline injection in the right FDI) or induced itch (cutaneous histamine application above the FDI), immediately after, and 30 minutes after provocation. Transcranial magnetic stimulation pulses were delivered every ∼7 seconds, and participants rated the pain and itch intensities using a numeric rating scale every ∼30 seconds.

Pain numeric rating scale scores were higher than itch scores from 0 to 120 seconds but lower at 390 seconds (all P < 0.05). Compared with baseline, both modalities reduced affected-hand MEPs (P = 0.003) with no changes in nonaffected-hand MEPs. Correlations between affected- and nonaffected-hand MEP reductions were found during pain (P = 0.009) and itch (P = 0.025).

Opposite to the hypothesis, acute, experimental pain and itch induced similar corticospinal inhibition, suggesting a comparable effect on the motor output of the affected limb. This study found that the effects of acute pain and itch showed similar inhibition on bilateral corticospinal tract excitability.