The neurosurgical community has witnessed significant changes in the intraoperative motor mapping paradigms since the first descriptions of Penfield and Boldrey in 1937[16]. Our knowledge about the human motor network has evolved thanks to the advances produced by animal-based studies[11], such as viral tracing, and human-based studies, such as cadaveric, neurophysiological and imaging studies[1, 2, 8]. Such knowledge has now taken us forward beyond the anatomical localization of structures towards understanding the concept of functional reserve[19] and its implication for impairment as well as their connections[14].
Preoperative knowledge of patient-centred anatomy and function of motor network improve surgical planning and enhance the odds of identifying and preserving eloquent structures. The initial developments of intraoperative neuromonitoring (IONM) of the motor network started with awake craniotomy techniques – 1st phase - where the primary motor cortex (PMC, M1) was identified with low-frequency mapping techniques[16]. Improvements in anaesthesiology alongside both surgeon and patient preferences, drove the IONM research towards development of reliable asleep techniques – 2nd phase, using high frequency mapping techniques, a train of 5 and the classic rule of 1mA-1mm for corticospinal tract (CST) identification[17]. The emerging 3rd phase of IONM for motor network is focused on a patient-centred approach integrating both awake and/or asleep mapping techniques to preserve not only the M1-CST but also the “higher motor functions”. It is now widely recognized that the human motor function requires more than the preservation of the M1-CST complex.
Intra and interlobar networks in both frontal and parietal lobes are responsible for the fine tune control and fluency of the generated movement, overall perceived as motor cognition[19]. The supplementary motor area (SMA) is involved in the fluency and initiation of the movement – volition - with a potential involvement of the SMA-M1 association fibres, Fronto-Aslant tract and U-fibres from the cingulum[4]. The premotor cortex is in direct connection with the parietal lobe via the superior longitudinal fascicular (SLF) system[20] and the inferior fronto-occipito-fasciculus[12]. The contribution of the U-fibers to this network are of increasing interest. The U-fibers connecting the pre and post central gyri are not uniformly distributed, clustering around the hand-knob area – superior, middle and inferior U-tracts – but also in the paracentral lobe – Paracentral U-tract – and in the functional area of the face – face superior and inferior U-tracts[3, 4, 7]. Intra-parietal short fiber tracts may also be responsible for multimodal sensory integration as they provide connection between the inferior parietal lobule and the postcentral gyrus: to the areas of the face and hand – Parietal inferior-to-Postcentral Tract – and the lower limb – Parietal Superior-to-Postcentral Tract, providing auditory and visual sensory information to the human motor output[5].
The possible interhemispheric influence on motor function adds a further level of complexity. Clinically, interhemispheric control in the motor network has been established mainly in stroke patients[15]. To the best of our knowledge, no intraoperative report of M1-M1 connectivity has been published. We present here an intraoperative report of cortico-cortico-evoked potentials in an asleep patient between both M1s and M1-SMA bilaterally, providing first-in-human data supporting bilateral connectivity between both primary and supplementary motor areas.