Transcranial magnetic stimulation in ALS: Utility of central motor conduction tests

ALS is diagnosed by finding clinical upper motor neuron (UMN) and lower motor neuron (LMN) signs. LMN dysfunction can be confirmed objectively through electromyography, whereas UMN dysfunction lacks a comparable established marker. Detection of subclinical UMN dysfunction would be helpful for diagnostic purposes, and objective markers of UMN dysfunction may also help clarify the relationship between ALS and its variants, including progressive muscular atrophy (PMA). PMA has been diagnosed by the absence of clinical UMN signs in the presence of LMN findings for several years. However, autopsies on patients with clinically diagnosed PMA show corticospinal tract degeneration in half of the patients. Therefore, clinical examination alone does not disclose all relevant UMN pathology.1–3 Transcranial magnetic stimulation (TMS) is a neurophysiologic technique used to assess the function of central motor pathways. Standard TMS measures include motor threshold, central motor conduction time (CMCT), and motor evoked potential (MEP) amplitudes. There have been numerous studies of TMS in ALS for over 20 years, investigating a multitude of issues on UMN physiology, cortical excitability and inhibition, the utility of the silent period and other responses to detect early changes in ALS, and the relationship between the UMN and LMN, many of which have been contradictory.4–11 Threshold, CMCT, and MEP are three useful measures of single pulse TMS; however, the diagnostic success of these nonspecific UMN measures varies widely from 16% to 100%.5,9,12–17 Reasons include different muscle recording sites, different methods of calculating CMCT,4,18 and different thresholds for defining diagnostic success (i.e., 1/4 sites prolonged vs 4/4). The TMS MEP is activated through both UMN and LMN pathways after magnetic stimulation of the cortex. In ALS, TMS amplitudes are often attenuated or absent19–21; even patients with pseudobulbar features are prone to have small, desynchronized MEPs.12 Reduced TMS MEP/M-wave amplitude ratio may be more closely correlated with pyramidal tract involvement than prolonged CMCT.21 There are fewer investigations of TMS amplitude compared to CMCT. One study found TMS amplitude abnormalities in only 16 of 54 patients with ALS (30%),5 whereas another found a significant difference between patients and controls at three recording sites, with amplitude reductions in almost all patients.19 The sensitivity of baseline TMS measures in ALS has been studied extensively, with mixed results, but there have been few longitudinal studies of TMS changes in ALS. Most reports showed no change in TMS threshold with time6,22,23 but one noted significant increase.17 CMCT has generally been reported to not increase with time.6,22,24 In a small series, TMS MEP amplitude and MEP/peripheral compound motor action potential (CMAP) ratio did not change with disease progression.22 In this study, we report on TMS threshold, CMCT, and amplitude data that were not included in an overarching report on neurophysiologic, brain imaging, and clinical methods to quantify and track progression in patients with ALS.25