Intra-operative neurophysiological mapping to identify distorted functional anatomy of the 4th ventricle in a 5-month-old infant.

Background: Neurophysiological brainstem mapping techniques facilitate the intra-operative localisation of cranial nerve nuclei amidst distorted anatomy. Neurophysiological recording in young infants can be limited due to immature myelination and synaptogenesis, as well as an increased sensitivity to anaesthetic agents.

Case Report: A 5-month-old boy was diagnosed with a cystic brainstem lesion located dorsally within the pons and upper medulla. An open surgical biopsy was undertaken via a posterior fossa craniotomy, revealing a grossly distorted fourth ventricular floor. Intra-operative neurophysiological mapping produced oculomotor, facial, glossopharyngeal and vagal muscle responses allowing a deviated functional midline to be identified. Direct stimulation was used to identify an area in the floor of the fourth ventricle eliciting no cranial nerve responses and allow safe entry into the tumour cavity and biopsy. Transcranial motor evoked responses (TcMEPs), short-latency somatosensory evoked potentials (SSEPs) and brainstem auditory evoked potentials (BAEPs) were all successfully recorded throughout the procedure, despite the use of halogenated gaseous anaesthesia.

Conclusions: We describe the use of brainstem mapping techniques for identification of a distorted midline on the floor of the 4th ventricle in an infant, with reproducible recordings of intra-operative TcMEPs, SSEPs and BAEPs.