Pericollicular approaches to the rhomboid fossa. Part II. Neurophysiological basis

Object. The authors describe their technique of electrophysiological mappin g to assist pericollicular approaches into the rhomboid fossa. Methods. Surgical approaches to the rhomboid fossa can be optimized by dire ct electrical stimulation of superficially located nuclei and fibers. Elect rophysiological mapping allows identification of facial nerve fibers, nucle i of the abducent and hypoglossal nerves,motor nucleus of the trigeminal ne rve, and the ambiguous nucleus. Stimulation at the surface of the rhomboid fossa performed using the threshold technique allows localisation above the area that is located closest to the surface. Simultaneous bilateral electr omyographic (EMG) recordings from cranial motor nerves obtained during stim ulation document the selectivity of evoked EMG responses. With respect to s timulation parameters and based on morphometric measurements, the site of s timulation can be assumed to be the postsynaptic fibers at the axonal cone. Strict limitation to 10 Hz with a maximum stimulation intensity not exceed ing 2 mA can be considered safe. Direct side effects of electrical stimulat ion were nor observed. Conclusions. Electrical stimulation based on morphometric data obtained on superficial brainstem anatomy defines two safe paramedian supra- and infrac ollicular approaches to the rhomboid fossa and is particulary helpful in tr eating intrinsic brainstem lesions that displace normal anatomical structur es.