Preservation of motor maps with increased motor evoked potential amplitude threshold in RMT determination

TMS Motor Mapping Methodology and Reliability: A Structured Review

Motor cortical representation can be probed non-invasively using a transcranial magnetic stimulation (TMS) technique known as motor mapping. The mapping technique can influence features of the maps because of several controllable elements. Here we review the literature on six key motor mapping parameters, as well as their influence on outcome measures and discuss factors impacting their selection. 132 of 1,587 distinct records were examined in detail and synthesized to form the basis of our review. A summary of mapping parameters, their impact on outcome measures and feasibility considerations are reported to support the design and interpretation of TMS mapping studies.

Primary hand motor representation areas in healthy children, preadolescents, adolescents, and adults

The development of the organization of the motor representation areas in children and adolescents is not well-known. This cross-sectional study aimed to provide an understanding for the development of the functional motor areas of the upper extremity muscles by studying healthy right-handed children (6-9 years, n = 10), preadolescents (10-12 years, n = 13), adolescents (15-17 years, n = 12), and adults (22-34 years, n = 12). The optimal representation site and resting motor threshold (rMT) for the abductor pollicis brevis (APB) were assessed in both hemispheres using navigated transcranial magnetic stimulation (nTMS). Motor mapping was performed at 110% of the rMT while recording the EMG of six upper limb muscles in the hand and forearm. The association between the motor map and manual dexterity (box and block test, BBT) was examined. The mapping was well-tolerated and feasible in all but the youngest participant whose rMT exceeded the maximum stimulator output. The centers-of-gravity (CoG) for individual muscles were scattered to the greatest extent in the group of preadolescents and centered and became more focused with age. In preadolescents, the CoGs in the left hemisphere were located more laterally, and they shifted medially with age. The proportion of hand compared to arm representation increased with age (p = 0.001); in the right hemisphere, this was associated with greater fine motor ability. Similarly, there was less overlap between hand and forearm muscles representations in children compared to adults (p<0.001). There was a posterior-anterior shift in the APB hotspot coordinate with age, and the APB coordinate in the left hemisphere exhibited a lateral to medial shift with age from adolescence to adulthood (p = 0.006). Our results contribute to the elucidation of the developmental course in the organization of the motor cortex and its associations with fine motor skills. It was shown that nTMS motor mapping in relaxed muscles is feasible in developmental studies in children older than seven years of age.

TMS motor mapping in brain tumor patients: more robust maps with an increased resting motor threshold

OBJECTIVE

Navigated transcranial magnetic stimulation (nTMS) has found widespread usage across many clinical centers as part of their surgical planning routines. NTMS offers a non-invasive approach to delineation of the motor cortex, in which the region is outlined through electromagnetic stimulation and electromyographic recordings of target muscles. Several neurophysiological parameters such as the motor evoked potential (MEP) and its derivatives, the resting motor threshold (RMT) and motor latency, are collected. The present study investigates the clinical feasibility and reproducibility of increasing the MEP threshold in brain tumor patients, with the goal to improve the robustness of the procedure.

MATERIALS AND METHODS

Twenty-three subjects with peri-motor cortex tumors underwent motor mapping with nTMS. RMT was calculated with both conventional 50-μV and experimental 500-μV MEP amplitude thresholds. Motor mapping was performed with 105% of both RMTs stimulator intensity using the FDI as the target muscle.

RESULTS

Motor mapping was possible in 20 patients with both the conventional and experimental thresholds. No significant differences in area size were found between motor area maps generated with a conventional 50-μV threshold in comparison to those generated with the higher 500-μV threshold (50 μV 272.56 mm² [170.47-434.31] vs. 500 μV 240.54 mm² [169.77-362.84], P = 0.34). Latency time was significantly reduced in 500-μV recordings relative to 50-μV recordings (50 μV 23.38 ms [22.55-24.51] vs. 500 μV 22.57 ms [21.41-23.70], P < 0.001). Both electric field intensity (50 μV 63.81 V/m [54.26-76.11] vs. 500 μV 77.83 V/m [65.21-93.94], P < 0.001) and RMT (50 μV 33 MSO% [28-36] vs. 500 μV 39.5 MSO% [32-44], P < 0.001) were significantly greater with the higher 500-μV threshold.

CONCLUSIONS

Our study demonstrates the feasibility of increasing the MEP detection threshold to 500 μV in brain tumor patients for RMT determination and motor area mapping with nTMS.