The number of stimuli required to reliably assess corticomotor excitability and primary motor cortical representations using transcranial magnetic stimulation (TMS): a systematic review and meta-analysis

BACKGROUND Transcranial magnetic stimulation (TMS) is a non-invasive means by which to assess the structure and function of the central nervous system. Current practices involve the administration of multiple stimuli over target areas of a participant's scalp. Decreasing the number of stimuli delivered during TMS assessments would improve time efficiency and decrease participant demand. However, doing so may also compromise the within- or between-session reliability of the technique. The aim of this review was therefore to determine the minimum number of TMS stimuli required to reliably measure (i) corticomotor excitability of a target muscle at a single cranial site and (ii) topography of the primary motor cortical representation of a target muscle across multiple cranial sites. METHODS Database searches were performed to identify diagnostic reliability studies published before May 2015. Two independent reviewers extracted data from studies employing single-pulse TMS to measure (i) the corticomotor excitability at a single cranial site or (ii) the topographic cortical organisation of a target muscle across a number of cranial sites. Outcome measures included motor evoked potential amplitude, map volume, number of active map sites and location of the map centre of gravity. RESULTS Only studies comparing the reliability of varying numbers of stimuli delivered to a single cranial site were identified. Five was the lowest number of stimuli that could be delivered to produce excellent within-session motor evoked potential (MEP) amplitude reliability (intraclass correlation coefficient (ICC) = 0.92, 95% CI 0.87 to 0.95). Ten stimuli were required to achieve consistent between-session MEP amplitudes among healthy participants (ICC = 0.89, 95% CI 0.76 to 0.95). However, between-session reliability was influenced by participant characteristics, intersession intervals and target musculature. CONCLUSIONS Further exploration of the reliability of multi-site TMS mapping is required. Five stimuli produce reliable MEP recordings during single-site TMS investigations involving one session. For single-site analyses involving multiple sessions, ten stimuli are recommended when investigating corticomotor excitability in healthy participants or the upper limb musculature. However, greater numbers of stimuli may be required for clinical populations or assessments involving the lower limb. SYSTEMATIC REVIEW REGISTRATION PROSPERO CRD42015024579.