MRI measurements of facial nerve may detect motor neuron damage in SMA

MRI measurements of the facial nerve, which controls the muscles used for facial expression, may offer a simple way to detect damage to motor neurons — nerve cells that control movement — in adults with spinal muscular atrophy (SMA), according to a study.

They may also reflect physical weakness even when facial weakness is not yet obvious.

“These measurements are simple, feasible with basic imaging tools, and do not require advanced neuroimaging techniques,” researchers wrote in “Facial Nerve Thinning Correlates With Motor Function in Adults With Spinal Muscular Atrophy,” which was published as a short communication in the Journal of Neuroimaging.

Recommended Reading

Aug. 5, 2026 News by Marisa Horak, MS

SMA-related gene activity affects nerve cells differently in worm study

MRI may detect facial nerves changes before weakness is obvious

SMA causes the gradual loss of motor neurons. As these nerve cells are lost, muscles become weaker. Disease-modifying therapies for SMA have increased the need for biomarkers, which are measurable features that can indicate disease progression. This is especially important in adults, because SMA may progress slowly and other health problems can affect standard measures of physical function.

Current tests can estimate the number of functioning motor units, which are individual nerve cells and the muscle fibers they control. These include the compound muscle action potential, an electrical measurement of muscle response to nerve stimulation, and the motor unit number index, which estimates how many motor units are working. However, these tests provide indirect information and take significant time.

Researchers in Brazil sought to determine whether MRI could detect changes in the facial nerve in adults with SMA and whether those changes were related to how well patients could move. The researchers also measured the trigeminal nerve, which is mainly responsible for sensation in the face. Measuring both nerves allowed the researchers to calculate a facial-to-trigeminal nerve diameter ratio.

The study included 22 adults with SMA type 2 or type 3, and 14 healthy adults of similar age and sex. All participants had a brain MRI. A specialized MRI sequence called balanced fast field echo was used to obtain detailed images of the cranial nerves, which include the facial and the trigeminal nerves. A neuroradiologist, a doctor specializing in nerve system imaging, reviewed all scans.

These easily measurable MRI parameters warrant further investigation as potential biomarkers.

Doctors also recorded whether each patient could walk independently and whether they had noticeable facial weakness. Physical function was measured using the Hammersmith Functional Motor Scale Expanded (HFMSE) and the Motor Function Measure-32 (MFM-32). Higher scores on these scales indicate better motor function.

Both right and left facial nerves were significantly thinner in patients than in healthy controls. Regarding the trigeminal nerve, only the left side was significantly different. These findings suggest that facial nerve thinning is more closely related to SMA than changes in the trigeminal nerve.

Among patients, those who could walk had facial nerves with larger diameters than those who could not walk. Patients with obvious facial weakness also had much smaller facial nerves. Importantly, however, the MRI changes were not limited to patients with visible facial weakness. This suggests that MRI may detect changes in facial nerves before weakness becomes obvious during a clinical examination.

Larger facial nerves were associated with better scores on both HFMSE and MFM-32. The facial-to-trigeminal nerve diameter ratio showed even stronger association with motor function. These measurements can be obtained from routine high-resolution MRI and may detect changes even without obvious facial weakness.

“These easily measurable MRI parameters warrant further investigation as potential biomarkers,” the researchers wrote.