Structural-functional dissociation in acute Bell's palsy: a comparative study of high-resolution ultrasound and...

Bell's palsy (BP) is the most common acute peripheral mononeuropathy worldwide. While electrodiagnostic studies are established tools for functional assessment, high-resolution ultrasonography (HRUS) has emerged as a complementary structural modality. Whether HRUS findings correlate with electrodiagnostic severity in the acute phase remains unclear.

A prospective within-subject paired longitudinal observational study enrolled 20 patients with mean age 30.6 ± 10.31 years, with unilateral acute idiopathic BP presenting within 7 days of onset. The unaffected contralateral side served as an internal control. All patients underwent clinical grading (House-Brackmann scale), facial nerve conduction studies (NCS), needle electromyography (EMG), and HRUS of the facial nerve, at baseline and at two-week follow-up.

CMAP amplitude on the affected side was significantly lower than the control at both baseline (1.37 ± 0.53 vs. 2.09 ± 0.63 mV; p = 0.002) and follow-up, with a progressive increase in degeneration percentage. HRUS demonstrated significant nerve enlargement on the affected side at baseline (0.36 ± 0.41 vs. 0.07 ± 0.08 cm; p = 0.001), with partial reduction at follow-up. Critically, no significant correlation was found between HRUS nerve diameter and any electrodiagnostic parameter at either points.

HRUS and electrodiagnostic testing capture distinct, non-redundant dimensions of facial nerve injury in acute BP: HRUS reflects structural inflammation, while NCS and EMG quantify functional axonal integrity. No significant correlation was observed, supporting the concept of structural-functional dissociation. Combined multimodal assessment is recommended for comprehensive evaluation of Bell's palsy.