March 3, 2026Open Access

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

Key Points

Good recovery was seen in 65% of patients at one-month follow-up, emphasizing the significance of early prognosis.
The zygomatic muscle showed a ΔMUNIX cutoff of >14 as predictive of poor recovery, with 85% sensitivity.
Assessment used bilateral MUNIX testing in 64 patients, revealing strong prognostic capability, especially for the zygomatic muscle.
These findings underscore the potential of MUNIX as a reliable, non-invasive tool for managing acute Bell's palsy.

Abstract

Objective: 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. Methods: 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. Results: 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). Conclusion: These findings establish MUNIX, particularly zygomatic muscle measurements, as an objective, non-invasive prognostic tool for early BP management. Plain Language Summary: MUNIX was evaluated for predicting outcomes in BP.ΔMUNIX values in the zygomatic muscle were identified as the most significant predictive indicator of BP recovery.Zygomatic muscle MUNIX serves as a sensitive prognostic tool for acute BP recovery, aiding in the development of effective management strategies. Keywords: facial nerve, Bell’s palsy, MUNIX, zygomatic muscle, prognosis

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