Cortical Motor Threshold, a simple practical technique based on a Complex Motoneuronal System.

Posted on: December 23, 2021

The spinal cord has a complex network of neurons that handle movement of our bodies, these neurons connect directly to muscles and release neurochemicals that excite the muscle fiber to contract. The brain influence activity of the alpha motor neurons, to command voluntary movements.

In 1980 Merton and Morton were the first to report motor evoked potentials (MEPs) stimulating the scalp over M1 with electrical current to excite neurons with short duration pulses (50 to 100 μs). About a decade later Barker and collaborators reported the use of focal electromagnetic pulses with variable directions and intensities of 1.5 to 2.5 tesla. Given the fact that magnetic fields reach the cortex without activating nociceptive receptors in the scalp, trans-cranial magnetic stimulation (TMS) has been extensively used since then. The term resting motor threshold (rMT) is defined as the minimal stimulus intensity required to induce MEPs in a target muscle at complete relaxation in 50% of the stimulations (Rossini et al. 2015), the output MEPs are variable in amplitude and the MT would not evoke MEPs with every single stimulation (Kiers et al. 1993). Automated protocols have been described efficient to find the MT by sequences of programed sets of stimuli at different intensities, the inter stimulus intervals should be at least 5 seconds in clinical and research TMS applications (Kallioniemi et al. 2021).

Since the approval of TMS to treat major depressive disorder the interest on MT determination increased in the psychiatric community and currently is the daily practice of many clinics with the goal of finding the correct TMS dose. This follows the assumption that neuronal excitability in areas like dorsolateral prefrontal cortex is similar to that of motor cortex. The International Federation of Clinical Neurophysiology (IFCN) described the use of EMG MT determination to avoid inadvertent seizures. In 2014 Westin and collaborators compared EMG vs visual observation of muscle twitch (OM) as methods to define MT in a group of 20 participants without pathology, they reported an average of 11.3% higher intensity when OM was used, were 8 subjects showed 10% higher threshold and 2 subjects had 25% greater MTs. Exposing the potential danger of OM-MT to cause seizures in individuals with overestimated MTs.

While there are many new emerging concepts about TMS and its therapeutic applications including depression and other behavioral disorders, new alternative biofeedback techniques including TMS-EGG may better assist in defining the dose intensity needed to treat these disorders. In the meantime MT determination seems a viable alternative when executed correctly.

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