Controversies and Clinical Applications of Non-Invasive Transspinal Magnetic Stimulation: A Critical Review and Exploratory Trial in Hereditary Spastic Paraplegia

Abstract

Magnetic stimulation is a safe, non-invasive diagnostic tool and promising treatment strategy for neurological and psychiatric disorders. Although most studies address transcranial magnetic stimulation, transspinal magnetic stimulation (TsMS) has received recent attention since trials involving invasive spinal cord stimulation showed encouraging results for pain, spasticity, and Parkinson's disease. While the effects of TsMS on spinal roots is well understood, its mechanism of action on the spinal cord is still controversial. Despite unclear mechanisms of action, clinical benefits of TsMS have been reported, including improvements in scales of spasticity, hyperreflexia, and bladder and bowel symptoms, and even supraspinal gait disorders such as freezing and camptocormia. In the present study, a critical review on the application of TsMS in neurology was conducted, along with an exploratory trial involving TsMS in three patients with hereditary spastic paraplegia. The goal was to understand the mechanism of action of TsMS through H-reflex measurement at the unstimulated lumbosacral level. Although limited by studies with a small sample size and a low to moderate effect size, TsMS is safe and tolerable and presents consistent clinical and neurophysiological benefits that support its use in clinical practice.

Keywords: magnetic stimulation; neuromodulation; spinal cord.

Figure 1

The patient 1 recovery curves of H-reflex obtained by delivering paired stimulus at interstimulus intervals (ISI) of 50 ms and 100 ms, respectively, pre (A,B) and post (C,D) TsMS inhibitory protocol. Note that in figure (A), before inhibitory modulation with TsMS, the conditioned H-reflex was present and with high amplitude at ISI of 50 ms (arrow). This provides neurophysiological evidence of neuronal hyperexcitability in this patient. After the TsMS inhibitory protocol, the conditioned H-reflex at 50 ms was almost completely inhibited (arrow head) (C). There was no evident difference in conditioned H-reflex with ISI of 100 ms before (B) and after (D) neuromodulation (arrow).

Figure 2

Transspinal magnetic stimulation induced electric fields, as suggested by the exploratory literature and computational models. (A). Generated current is concentrated on neuroforamina with estimated three to ten-fold greater intensity than at the spinal cord. (B). Current distribution in the spinal cord favors the posterior column, sparing the anterior horn and antero-lateral columns. (C). Generated current orientation on neuroforamina and spinal canal depends on coil type and positioning. Intraspinal transversal currents are substantially weaker than longitudinal currents when generated with similar coils. Root stimulation is optimal, leading to lower resting motor threshold and higher peak amplitudes when induced current is better aligned with the neuroforamina (orange and red arrows), preferably in an outwards direction (red arrow). mV/mm: millivolts per millimeter.