MRI Manifestations of Facial Paralysis Following Retroauricular Hyaluronic Acid Injection.

BACKGROUND

Retroauricular hyaluronic acid (HA) injection can enhance the cranio-auricular angle, thereby reducing perceived facial width and enhancing aesthetic appeal. Although this is generally a safe procedure, acute facial nerve paralysis is a rare but devastating complication.

OBJECTIVE

The aim of this study is to characterize the MRI features of acute facial paralysis following retroauricular HA injection, including manifestations of nerve injury and the distribution of the injected material, and to infer the potential etiology of the facial paralysis based on imaging findings.

METHODS

15 patients presenting with acute complete unilateral facial paralysis following retroauricular HA filler injection between December 1, 2023, and August 10, 2025 were identified. No perceivable improvement was seen despite non-surgical interventions for 2 weeks. MRI using a 3.0-T scanner was conducted to acquire gadolinium-enhanced T1-weighted and T2-weighted images. Each segment of the facial nerve was evaluated, and the diffusion range of the HA injection was observed by examining the contralateral side.

RESULTS

MRI demonstrated significant enhancement of the facial nerve on the affected side compared to the unaffected side in the tympanic (P = 0.006), mastoid (P = 0.004), and parotid segments (P < 0.001) on contrast-enhanced T1-weighted images. Additionally, the HA filler exhibited extensive diffusion (46.7 ± 12.2 mm horizontally, 53.1 ± 12.2 mm vertically) and infiltrating adjacent tissue spaces.

CONCLUSIONS

This group of patients showed widespread distribution of HA beyond the injection site. Additionally, persistent facial nerve edema and enhancement were mainly localized to the tympanic segment and its distal end of the facial nerve, indicating a possible need for facial nerve decompression in this cohort.

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