Cortical and subcortical representation of the intercostal muscles: Is it time to revise the motor homunculus?
EANS Academy. Lavrador J. Sep 26, 2019; 276077; EP03099
Mr. Jose Lavrador
Mr. Jose Lavrador

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Introduction: We report the first case series of cortical and subcortical stimulation of the cortex and subcortical region leading to stimulation and contraction of intercostal muscles during intraoperative mapping and neuromonitoring for brain tumour resection. Our findings define this previously under-studied region of the motor homunculus and its relevant projections, paving the way forward for routine intra-operative mapping of the intercostal muscles during tumour surgery for the optimal post-operative outcome.
Methods: Pre-operative MRI, motor TMS, advanced DTI were performed to establish the relationship of brain tumours with the motor cortex and corticospinal tract (CST). Intraoperative motor mapping and monitoring were performed with monopolar stimulation ('train of 5' technique). Cortical and subcortical stimulation demonstrated a selective response from intercostal muscles separate from the lower limb and upper limb. PubMed literature search was performed to identify any case reports with similar findings.
Results: 4 patients were included (2 gliomas, 2 metastases): isolated subcortical stimulation - 1 patient and cortical and subcortical stimulation - 3 patients - one with confirmation by intraoperative muscle evoked potential recordings and co-registration with intraoperative tractography (isolated cortical and subcortical activation with no involvement of other muscle groups). The location of the cortical response defined the intercostal muscle representation within the motor homunculus and that of the subcortical response confirmed the somatotopy of the CST.
Conclusion: Intercostal muscles are controlled by clear cortical and subcortical motor pathways. Damage to these areas can lead to paralysis of voluntary respiratory muscles and affect post-operative outcome. Redefinition of the motor homunculus to highlight this region is warranted to alert surgeons of the importance of preserving these structures when operating in the eloquent brain.
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