Facial Nerve Thinning Correlates With Motor Function in Adults With Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) leads to progressive lower motor neuron loss. While neurophysiological techniques such as CMAP and MUNIX indirectly estimate motor unit number, few imaging-based biomarkers exist. We aimed to assess whether MRI-derived facial nerve diameters distinguish adult SMA patients from controls and correlate with clinical status.

We conducted a cross-sectional study including 22 adult SMA patients (Types II/III) and 14 age and sex-matched healthy controls at a single center. All participants underwent 3T brain MRI with bFFE sequences. Diameters of facial nerves were measured bilaterally at their brainstem root entry zones. Clinical assessments included ambulation status, facial weakness, Hammersmith Functional Motor Scale Expanded (HFMSE), and Motor Function Measure (MFM-32).

There were four patients with SMA Type II and 18 with type III. Median age was 31.5 years. Facial nerve diameters were significantly reduced in patients compared with controls (right: median 1.15 vs. 1.40 mm; left: 1.14 vs. 1.35 mm). Within patients, ambulant individuals showed larger facial diameters than non-ambulant ones. Patients with clinical facial weakness had markedly smaller diameters. Diameters correlated with MFM (ρ up to 0.65) and HFMSE (ρ up to 0.68), while the facial-to-trigeminal ratio correlated even more strongly (ρ up to 0.73). No associations were found with disease duration.

In this exploratory study, facial nerve diameter and the facial-to-trigeminal ratio showed promising associations with motor function in adult SMA, including in patients without overt facial weakness. These easily measurable MRI parameters warrant further investigation as potential biomarkers of motor neuron involvement.