The degree of impairment of the facial nerve may yield important insight regarding the disease subtype in patients with spinal muscular atrophy (SMA), according to a study recently published in Muscle & Nerve.

“In the present study we explore facial nerve response, motor unit number index (MUNIX), and motor unit size index (MUSIX) of the orbicularis oculi muscle in a cohort of patients with SMA,” the authors wrote.

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This cross-sectional observational study included 37 patients previously diagnosed with SMA and 27 healthy individuals that served as the control group. Among the SMA cohort, most (n=21) patients were diagnosed with SMA type 2, while the remaining 16 had SMA type 3.

The researchers performed measurements of the compound muscle action potential (CMAP) of the facial nerve, along with the analysis and identification of the MUNIX and MUSIX of the orbicularis oculi muscle in both the SMA and control groups. The aim was to compare parameters from each group and observe differences among them that could yield important clinical information.

The MUNIX score and the CMAP amplitude significantly decreased in the SMA patients compared to the control group (P <0.0001). Meanwhile, the MUSIX score did not showcase significant variations among these groups.

Moreover, the MUNIX score and the CMAP amplitude also exhibited a different pattern between the 2 subtypes of SMA. Patients with SMA type 3 had the lowest values of these parameters when compared to those with SMA type 2. Similarly, the MUSIX score did not vary between SMA subtypes.

These results translate into a deep insight into SMA and its clinical presentation. The feasibility and well-tolerated characteristics of these techniques suggest a potential use in the diagnosis and/or management of patients with this condition, the study team noted.

“CMAP of the facial nerve and MUNIX of orbicularis oculi showed high accuracy in discriminating between the various subtypes of SMA and in quantifying the motor unit loss of the facial nerve,” the authors concluded.

Reference

Barp A, Carraro E, Salmin F, et al. Facial nerve vulnerability in spinal muscular atrophy and motor unit number index (MUNIX) of the orbicularis oculi muscle. Muscle Nerve.  Published online March 1, 2023. doi:10.1002/mus.27809

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