Facial nerve thinning is a hallmark and correlates with motor function in adults with spinal muscular atrophy

Spinal muscular atrophy (SMA) leads to progressive degeneration of lower motor neurons, yet reliable imaging biomarkers remain limited. We evaluated whether MRI-derived facial nerve morphometry distinguishes adult patients from healthy controls and reflects clinical status. Twenty-two adults with SMA types II and III and fourteen matched controls underwent 3T brain MRI using high-resolution balanced fast field echo sequences. Facial nerve diameters were measured bilaterally at the brainstem root entry zone and compared with clinical assessments including ambulation, facial weakness, the Hammersmith Functional Motor Scale Expanded (HFMSE) and the Motor Function Measure (MFM-32). Patients showed consistently reduced facial nerve diameters, with the smallest values observed in non-ambulant individuals and in those with facial weakness. Facial diameters correlated with motor function, and the facial-to-trigeminal ratio demonstrated even stronger associations, emerging as a sensitive indicator of motor neuron loss. These findings support facial nerve morphometry as a promising and accessible biomarker for assessing disease involvement in adults with SMA.