Tractographic description of the course of cortico-facial fibers in healthy humans: evidence for a dorsal pathway in the brainstem

The precise anatomical course of the human corticofacial tract (CFT) in the brainstem is debated. Classical anatomy indicates that CFT trajectory overlaps with the corticospinal tract in the ventral brainstem, but clinical evidence suggests the co-existence of a dorsal paralemniscal pathway. This study aims to delineate the CFT trajectory in vivo by means of diffusion-MR tractography in a population of healthy humans and to compare it with the anatomy of individual brainstem lesions known to cause central facial palsy. We applied multi-shell diffusion imaging with constrained spherical deconvolution tractography to 57 participants, employing custom facial nucleus regions of interest and Freesurfer cortical parcellations. CFT reconstructions were validated against 16 published brainstem lesion locations associated with central-type facial palsy. Probabilistic tractography consistently reconstructed both uncrossed and crossed CFT bilaterally, revealing evidence for a dorsal pathway component in the brainstem, compatible with the dorsal CFT hypothesized in the literature. Reconstructed tracts significantly overlapped with clinically relevant lesion sites, with the uncrossed CFT intersecting 81.2% of lesion ROIs and 93.7% in the right side, and crossed CFT intersecting 93.7% of lesion ROIs in both sides. Furthermore, a distinct medullary loop variant was identified in 40.4% of subjects (N = 23/57). These findings provide in vivo tractographic evidence supporting the systematic presence in all participants of a non-canonical dorsal CFT, clearly separated from the corticospinal tract. The current data support the redrawing of human corticofacial anatomy, reconciling anatomical models with clinical observations from brainstem lesions and serving as possible guidance for clinical decision-making in neurology and neurosurgery.