Transcranial Magnetic Stimulation in Alzheimer’s Disease and Cortical Dementias

Transcranial magnetic stimulation (TMS) has become a safe, noninvasive, and promising tool to assess specific cortical circuits in the central nervous system. Since its introduction, the use of TMS in clinical neurophysiology, neurology, neuroscience, and psychiatry has spread widely, leading to important findings on cortical function in physiological and pathological conditions. Indeed, numerous studies have described abnormalities in specific cortical circuits using particular TMS stimulation paradigms, which allow the indirect assessment of inhibitory and excitatory interneuronal activity, mainly dependent on GABA receptors, of central cholinergic activity, and of cortical plasticity. The objective of the present work is to examine the utility of TMS as a means to support and predict the clinical diagnosis of Alzheimer ’s disease and other cortical dementias, in a setting where our understanding of neurodegenerative diseases is far from adequate. *Corresponding author: Barbara Borroni, MD, Clinica Neurologica, Università degli Studi di Brescia, P.le Spedali Civili 1, 25123, Brescia, Italy, Tel: 0039 0303995632; E-mail: bborroni@inwind.it Received September 07, 2015; Accepted October 16, 2015; Published October 23, 2015 Citation: Benussi A, Padovani A, Borroni B (2015) Transcranial Magnetic Stimulation in Alzheimer’s Disease and Cortical Dementias. J Alzheimers Dis Parkinsonism 5: 197. doi: 10.4172/2161-0460.1000197 Copyright: © 2015 Benussi A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. The objective of the present review is to examine the utility of TMS as a means to aid and predict the early diagnosis of AD and other cortical dementias, bearing in mind that a precise and prompt diagnosis will be critical in the prospect of future disease-modifying therapies. Transcranial Magnetic Stimulation Techniques Single-pulse stimulation Motor threshold (MT): MT is defined as the minimal intensity of motor cortex stimulation required to elicit a reliable motor evoked potential (MEP) of minimal amplitude in the target muscle. It is considered a reliable global measure of corticospinal excitability and depends on the excitability of axons activated by the TMS pulse, as well as the excitability of synaptic connections at both the cortical and spinal level [11-14]. Resting MT (RMT) is determined while the target muscle is completely at rest, while the active MT (AMT) is usually determined during a slight tonic contraction of the target muscle at approximately 20% of the maximal muscle strength [11,12]. RMT is considered as a global parameter of human brain excitability as it mainly reflects the membrane excitability of corticospinal neurons, cortico-cortical axons and their excitatory synaptic contacts, mediated by voltage-gated sodium channels and by ionotropic glutamatergic non-NMDA receptors [15]. Motor evoked potential amplitude: The MEP is typically recorded over the target muscle using surface electrodes in a bipolar bellytendon arrangement, applying a single TMS pulse to the contralateral primary motor cortex (M1) at adequate stimulator intensity [16]. Extrinsic factors, as conditioning stimuli preceding a test TMS Journal of Alzheimer’s Disease & Parkinsonism J o u r n a l o f A lzh eim ers ease & Prkin s o n i s m