Motor Unit Number Index (MUNIX) as an Early Prognostic Biomarker in Acute Bell's Palsy: A Prospective Cohort Study

Recovery from acute Bell's palsy (BP) is variable and there are few predictors of response. We evaluated the usefulness of motor unit number index (MUNIX) to predict outcome in BP.

This prospective study evaluated the prognostic utility of MUNIX in 64 consecutive patients with acute unilateral BP. Within 7 days of symptom onset, participants underwent bilateral MUNIX testing of three facial muscles: orbicularis oculi muscle, zygomatic muscle, and orbicularis oris muscle. Clinical outcomes were assessed using the House-Brackmann Grading System (HBGS) by two blinded neurologists at baseline, 1 month, and 3 months. All patients received prednisolone treatment and regular rehabilitation.

At 1-month follow-up, 26 patients (65%) achieved good recovery (HBGS I-II). The zygomatic muscle demonstrated superior prognostic performance, with absolute value of affected-to-unaffected side MUNIX difference in the zygomatic muscle (ΔMUNIX zygomatic muscle) >14 predicting poor recovery (AUC =0.804, 95% CI 0.667-0.940; p =0.002), showing 85% sensitivity and 79% specificity. Three-month outcomes (n=20) confirmed ΔMUNIX zygomatic muscle >16 as the optimal cutoff (AUC =0.893, 95% CI 0.748-1.000; p =0.006).

These findings establish MUNIX, particularly zygomatic muscle measurements, as an objective, non-invasive prognostic tool for early BP management.