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

Area of Science:

• Neurology
• Radiology
• Biomarker Discovery

Background:

• Spinal muscular atrophy (SMA) causes progressive motor neuron loss, impacting lower motor neurons.
• Current neurophysiological methods indirectly assess motor unit number, with limited imaging biomarkers available.
• Facial nerve integrity is affected in SMA, but imaging correlates are underexplored.

Purpose of the Study:

• To investigate if MRI-derived facial nerve diameters can differentiate adult SMA patients from healthy controls.
• To determine the correlation between facial nerve diameters and clinical measures of motor function in SMA.
• To explore the potential of facial nerve diameter as an imaging biomarker for SMA.

Main Methods:

• A cross-sectional study involving 22 adult SMA patients (Types II/III) and 14 healthy controls.
• 3T brain MRI with bFFE sequences was used to measure facial nerve diameters at the brainstem root entry zones.
• Clinical assessments included ambulation, facial weakness, Hammersmith Functional Motor Scale Expanded (HFMSE), and Motor Function Measure (MFM-32).

Main Results:

• Facial nerve diameters were significantly smaller in SMA patients compared to controls.
• Reduced diameters correlated with lower ambulation status and presence of facial weakness.
• Nerve diameters showed significant correlations with HFMSE and MFM-32 scores, with the facial-to-trigeminal ratio showing stronger associations.

Conclusions:

• Facial nerve diameter and the facial-to-trigeminal ratio are promising MRI-based biomarkers in adult SMA.
• These parameters correlate with motor function, even in patients without apparent facial weakness.
• Further research is warranted to validate these easily measurable MRI parameters for assessing motor neuron involvement in SMA.