Is the analgesic effect of motor cortex stimulation somatotopically driven or not?

Mechanisms of analgesic efficacy related to motor cortex stimulation (MCS) remain poorly understood. Specifically, it is unclear whether pain relief is somatotopically driven or not. We present three illustrative case-reports of MCS in which unintentional stimulation setting errors occurred, leading to differential (and reversible) pain relief outcomes across the hemi-body.

After successful preoperative rTMS trials, three patients suffering from post-stroke pain were selected for MCS. Stimulation was set with the aim of activating two epidural electrodes over the somatotopic representation of the lower and upper limbs. Data regarding pain relief were prospectively collected.

At the first follow-up, all three patients complained of a lack of pain relief in the lower limb, contrasting with good outcome in the upper limb. In fact, for each of them we diagnosed the same stimulation setting error, to which they were "blinded", i.e., the parasagittal electrode over the somatotopic representation of the lower limb was inadvertently turned off. Subsequently, six months after having the electrode turned on (still in a "blinded" fashion), all three patients described substantial pain relief in the lower limb, with a median improvement of 50% (range: 40-70%).

These incidental case reports argue in favor of a genuine and at least partly somatotopically-driven analgesic efficacy of MCS. Therefore, the parasagittal electrode seems crucial when treating lower limb pain with MCS.