What is injection facial nerve paresis?

What is Injection Facial Nerve Paresis?

Injection facial nerve paresis is a rare complication where facial weakness or paralysis occurs after local anesthetic injection, most commonly during dental procedures, with onset typically immediate after injection and duration usually less than 12 hours. 1

Definition and Clinical Characteristics

Injection-related facial nerve paresis represents a specific subset of peripheral facial nerve paralysis (PFNP) that occurs as a direct consequence of local anesthetic administration. 1 This differs from Bell's palsy, which the American Academy of Otolaryngology-Head and Neck Surgery defines as acute unilateral facial nerve paresis or paralysis with onset in less than 72 hours without an identifiable cause. 2, 3

Key Distinguishing Features:

• Timing: Paralysis begins immediately after the injection of anesthesia, not gradually over hours 1
• Duration: Facial weakness typically resolves within 12 hours in most cases 1
• Identifiable cause: Unlike Bell's palsy, there is a clear temporal relationship to the injection procedure 1
• Location: Most commonly reported after mandibular anesthesia, though rare cases occur after maxillary procedures 1

Proposed Mechanisms

Multiple pathophysiologic mechanisms have been associated with injection-related PFNP: 1

• Direct trauma to the facial nerve from needle penetration
• Vasospasm causing temporary ischemia of the nerve
• Edema and compression of the nerve within its bony canal
• Demyelination from the anesthetic agent itself
• Viral reactivation triggered by the procedure (though less likely with immediate onset)

Clinical Assessment

When evaluating suspected injection facial nerve paresis, distinguish it from other causes by examining: 2, 3

• Complete vs. incomplete paralysis: Patients with complete paralysis (inability to wrinkle forehead, raise eyebrow, or close eye) have higher risk of incomplete recovery compared to those with mere paresis 2, 4
• Unilateral vs. bilateral: Bilateral facial weakness is extremely rare in injection-related cases and should prompt investigation for systemic conditions 4, 3
• Associated symptoms: Isolated facial weakness without other cranial nerve deficits supports peripheral nerve injury rather than central pathology 5

Critical Pitfall to Avoid:

Do not assume all post-injection facial weakness is benign and self-limited. If symptoms persist beyond 12-24 hours or worsen, consider alternative diagnoses including: 2, 3

• Stroke (though forehead should be spared in central lesions) 5
• Viral reactivation (Herpes zoster, Lyme disease) 3
• Underlying tumor or structural lesion 3

Management Approach

For typical injection facial nerve paresis with immediate onset and expected short duration: 1

• Observation is appropriate for cases with onset immediately post-injection and improving trajectory
• Corneal protection is essential even for brief paralysis to prevent exposure keratitis 4
• Multidisciplinary consultation should be considered if symptoms persist beyond expected timeframe 1

When to Escalate Care:

If facial weakness persists beyond 72 hours or is complete paralysis, treat as potential Bell's palsy: 4, 3

• Corticosteroids within 72 hours of symptom onset (increases recovery from 70% to 94%) 4
• Electrodiagnostic testing (ENoG/EMG) at approximately 7 days after onset to quantify nerve damage and provide prognostic information 2, 4
• MRI imaging if symptoms are atypical, involve multiple cranial nerves, or persist beyond 2-4 months 4, 5