Transient facial nerve palsy following dental local anaesthesia

Abstract

Facial nerve palsy is a rare but known complication of dental local anaesthesia and may be underreported. We describe a case of a transient facial nerve palsy following the administration of an inferior alveolar nerve block and discuss the immediate practical management. Knowing the likely transient nature of this complication means the patient can be reassured and unnecessary referral avoided. While the blink reflex is inhibited, steps are needed in order to protect the cornea and prevent secondary infection and scarring.

Background

The inferior alveolar nerve block is a common technique to ensure pain relief during and after many common dental procedures. Transient facial nerve palsy has been described following this technique. When faced with such a scenario, it is important to know when to refer for further investigation and how to manage the immediate issues associated with facial nerve palsy such as corneal exposure.

Case presentation

A 30-year-old woman underwent extraction of the left maxillary and bilateral mandibular wisdom teeth under general anaesthetic, due to recurrent episodes of pericoronitis.

She was otherwise well. There was no relevant medical history. Of note, there was no previous history of facial weakness or Bell’s palsy and no history of migraine or other neurological disorders.

She was on no regular medication other than the combined oral contraceptive pill. There were no allergies. She had recently finished a course of metronidazole.

The left upper wisdom tooth was extracted uneventfully with simple elevation and dental forceps. The lower mandibular wisdom teeth were extracted by raising buccal full thickness mucoperiosteal flaps, undertaking bone removal and sectioning the teeth. Resorbable 4-0 Vicryl sutures were placed.

For postoperative pain management, a long acting local anaesthetic was given at the end of the surgical procedure. This was by the administration of bilateral inferior alveolar nerve blocks. A total of 10 mL (5 mL at each site) 0.5% bupivacaine local anaesthetic was administered using a 34 mm 27g needle.

On recovering from the general anaesthetic, a generalised left-sided weakness of the facial muscles was noted. There was asymmetry on facial expression, with associated drooping of the lip and the inability to close the left eye (figure 1). Of note, the left frontalis function was absent. No other neurological deficit was found. Due to the associated lagophthalmos, an ophthalmological opinion was immediately sought.

Figure 1.

Photo showing left-sided facial weakness with inability to close the left eye A poor Bell's phenomenon is shown (limited upwards rotation of the globe on attempted eye closure).

Discharge was not delayed but clear instructions were given to seek advice if facial function did not return by the next day. There was a gradual improvement in facial expression, with full facial nerve function including eyelid closure returning within 5 hours following the procedure.

Investigations

As the patient’s symptoms began to resolve during the expected time period, no further investigations were undertaken.

Differential diagnosis

The main concern with regards to any patient waking up from a general anaesthetic with a neurological deficit is whether the cause is central in origin. Any additional neurology would have been caused for alarm and necessitated immediate neuroimaging.

As this was a unilateral isolated lower motor neuron lesion (LMNL) of the facial nerve, with no additional neurology found after a comprehensive neurological examination, a decision was made not to refer for imaging at that time. The key to determining an upper motor neuron lesion (UMNL) verses an LMNL lies in the forehead. Crossed innervation means a sparing of frontalis movement with an UMNL and involvement of the frontalis with a LMNL.

Due to the recent surgery and administration of a local anaesthetic in close proximity to the course of the facial nerve through the parotid gland, a peripheral insult or injury to the facial nerve secondary to either the surgery or the anaesthetic was suspected.

Treatment

After exclusion of other cranial nerve palsies, treatment of a facial nerve palsy must include prompt management of the cornea to prevent complications due to associated lagophthalmos. Failure of eyelid closure leads to rapid drying of the cornea, which can lead to epithelial breakdown and risks subsequent infection and scarring. This is more of an issue in patients such this case when there is a poor Bells phenomenon—meaning little or no upwards rotation of the eyeball on attempted closure (this reflex helps protect the cornea, even with lack of lid closure).

The patient was instructed to use thick paraffin-based ointment (Xailin Night, VISUfarma) four times per day to lubricate and protect the cornea, to manually close the eyelids regularly and to tape the lids closed horizontally at night (vertical lid taping and/or padding is not advised).¹

Outcome and follow-up

In keeping with our suspicion that this was secondary to the local anaesthetic, there was a gradual improvement in facial power after several hours, and full facial nerve function including eyelid closure had returned within 5 hours. No neurological defect remained.

Discussion

Facial nerve palsy is a rare but described the complication of inferior alveolar nerve block anaesthesia. The exact incidence is unknown. Literature suggests most cases have an immediate palsy with quick recovery usually resolving within 7 hours. Occasionally onset is delayed and associated with a prolonged recovery.^{2 3}

Immediate type, rapid recovery

Most of the described cases of facial nerve palsy secondary to inferior alveolar nerve block anaesthesia are of the immediate type with rapid recovery. A number of different possible underlying mechanisms have been described^3–6:

1. Direct trauma from the needle.

2. Air blast during surgery.

3. Haematoma formation with subsequent compression of the nerve.

4. Infiltration of the peripheral branches of the facial nerve with local anaesthetic.

As our case had immediate onset and prompt recovery, we suspect infiltration of the peripheral branches of the facial nerve with local anaesthetic was the underlying cause. As our patient developed the palsy just on the left side, we suspect the local anaesthetic may have reached the facial nerve due to the injection being too posterior on this side and entering the parotid gland. An alternate reason would be if there was an anatomical variant of the facial nerve on the left side, which resulted in it being located in the retromandibular space.⁷ The left side would additionally have had more manipulation, which may have played a contributory role, possibly by aiding the spread of the local anaesthetic.

In cases secondary to local anaesthetic, the paralysis would be expected to last as long as the local anaesthetic agent (although this may be prolonged if epinephrine is used). Bupivacaine is reported to have a duration of action of 3–5 hours, which would be in keeping with the timeframe of resolution of symptoms in this case.⁸

Delayed type

The following have all been suggested to be the possible causes of delayed facial nerve palsy following inferior alveolar nerve block anaesthesia:

1. Reactivation of a latent viral infection such as herpes simplex or herpes zoster.

   • Local trauma could act as a precipitant to viral replication leading to neural sheath inflammation and dysfunction.^{9 10}

2. Axonal ischaemia secondary to a delayed reflex spasm of the vasa nervorum of the facial nerve.

   • This is thought to be due to a sympathetic vascular reflex triggered by stimulation of the sympathetic plexus of the external carotid artery. Mechanical action of the needle during the nerve block, infiltration of the anaesthetic agent and/or epinephrine or the breakdown products of the anaesthetic agent have all be suggested as causative factors.^{7 11 12}

3. Excessive stretching of the facial nerve from prolonged oral instrumentation leading to direct damage or ischaemia.²

4. Inadvertent intra-arterial injection of the anaesthetic agent.

   • This has been shown to cause retrograde flow along the arterial system and can lead to distal spread of the anaesthetic agent, including centrally, and subsequent complications secondary to this.^{7 13}

Unrelated pathology must also be considered, such as a coincidental idiopathic facial nerve palsy (Bell’s palsy), especially if there is a significant delay in onset of symptoms.

Patient’s perspective.

Once I had come round from the general anaesthetic, I could tell my left eye was not shutting. I was not bothered at the time as my whole face felt weird and numb but I could tell the doctors were concerned and I got worried that I would had a stroke. Fortunately, it got better pretty soon.

Learning points.

• Ensure this is an isolated lower motor neuron lesion facial nerve palsy with no additional neurology by performing a comprehensive neurological examination, paying particular attention to the forehead/frontalis involvement.

• Protect the cornea:

  • Instillation of thick paraffin-based eye ointment four times per day, for example, Lacrilube (Allergan), Xailin Night (VISUfarma) or ocular chloramphenicol 1% ointment (generic), which are available without prescription in the UK.

  • Regular manual blinking (demonstrate to the patient how to bring the upper lid downwards resulting in full closure using gentle movements with the index finger).

  • Horizontal eyelid taping may be used to close the eye fully at night.

• The patient should be re-examined or instructed to seek advice if the facial weakness does not recover promptly (within 7 hours).