The course of corticofacial projections in the human brainstem

Transcranial magnetic stimulation was used to investigate the corticofacial projections in 53 patients with (n = 28) and without (n = 25) central faci al paresis due to unifocal ischaemic lesions at different brainstem levels. Lesion topography documented by MRI studies was correlated with the electr ophysiological findings. In the majority of patients the corticofacial fibr es travel within the ventromedial base of the pons and cross the midline at the level of the facial nucleus. In some individuals, however, we found ev idence that corticolingual fibres form an `aberrant bundle' in a paralemnis cal position at the dorsal edge of the pontine base. In other patients the corticofacial fibres loop down into the ventral part of the upper medulla, cross the midline and ascend in the dorsolateral medullary region ipsilater ally to the facial nucleus. The findings suggest that facial paresis due to a brainstem lesion may present as contralateral supranuclear facial paresi s by a lesion of the cerebral peduncle, pontine base, the aberrant bundle a nd the ventral medulla. Supranuclear facial paresis ipsilateral to the lesi on side may result from a lesion in the lateral medulla, and facial paresis of the supranuclear type may be imitated by a lesion of the peripheral fac ial nerve in the dorsolateral medulla with involvement of the lower pons.