Methods of Detecting Lesions of Upper Motor Neuron in Amyotrophic Lateral Sclerosis using Transcranial Magnetic Stimulation

Introduction: Coexistent involvement of upper and lower motor neurons is a characteristic feature of amyotrophyc lateral syndrome (ALS) necessary for the diagnosis. Diagnosis of upper motor neuron involvement in ALS is based solely on clinical features, which may not be detected at the disease onset and in rare forms manifesting clinically as the pure lower motor neuron syndrome (LMNS). The main method of assessment of the functional state of the upper motor neuron in ALS is transcranial magnetic stimulation (TMS). It allows assessing the excitability of motor cortex, corticospinal tract function, and mapping of cortical representation of the muscles. In patients with ALS changes of various indicators demonstrating hyperexcitability as well as degenerative lesions of the motor cortex and the corticospinal tracts are recorded on TMS. Objective: to discuss changes in the TMS in patients with ALS, pathophysiological mechanisms of their formation and possible diagnostic value. Results: In 22 patients with LMNS, navigated TMS revealed disturbances of intracortical inhibition on paired stimulation and recording cortical silent period, increase of motor threshold in dominant hemisphere, decrease of the weighted area and reorganization of cortical representations of the hand muscles. Conclusion: The data obtained allow to consider navigated TMS as a promising technology for identifying upper motor neuron involvement in patients with ALS.