Velopharyngeal dysfunction following botulinum toxin type A injection to the lateral pterygoid muscles for recurrent jaw dislocation

Abstract

Complications related to lateral pterygoid muscle (LPM) botulinum toxin A (BtA) injection for recurrent temporomandibular joint dislocation are uncommon. No cases of velopharyngeal dysfunction (VPD) following LPM BtA injection have been reported to date. This report details the perioperative and follow-up findings for a patient developing VPD following LPM BtA injection.

Background

Lateral pterygoid muscle (LPM) botulinum toxin A (BtA) injection is a minimally invasive treatment for recurrent temporomandibular joint (TMJ) dislocation. It operates on the effects of botulinum neurotoxin (BoNT) produced by the bacterium Clostridium botulinum.¹ BoNT induces muscle paralysis by binding to presynaptic cholinergic receptors at the neuromuscular junction reducing acetylcholine release necessary for muscular contraction. The muscle fibre regains its strength with nerve regeneration denoting its effects to be temporary.²

Bilateral LPM BtA injection has been shown to alleviate recurrent TMJ dislocation up to 6 months.¹ BtA injections may be aided with electromyography (EMG) where electrical impulses applied to contract the LPM result in synchronous lateral excursion to the contralateral side. This verifies correct needle placement in the LPM.³

LPM resides in the infratemporal fossa (ITF). It originates from the lateral pterygoid plate forming ITF’s medial border along with tensor veli palatini (TVP), levator veli palatini (LVP) and the superior constrictor muscle (see figure 1). Some important ITF structures include the maxillary artery, pterygoid venous plexus and the parotid gland.⁴ LPM BtA needle injection traverses the ITF potentially damaging any structure within it. BoNT deposits have been postulated in the literature to paralyse any muscle within the ITF through passive diffusion.³ Depending on the muscle(s) involved, this may result in rare transient complications including dysphagia, dysarthria and velopharyngeal dysfunction (VPD) symptoms: nasal regurgitation, nasal emissions and hypernasality.² There are no reported cases of VPD symptoms following LPM BtA injection in the literature.

Figure 1.

LPM lying adjacent to TVP and LVP at ITF’s medial border. ITF, infratemporal fossa; LPM, lateral pterygoid muscle; TVP, tensor veli palatine.

Case presentation

A 25-year-old woman reported of a 10-year history of recurrent left-sided TMJ dislocation. Other than a previous history of three presentations to the emergency department requiring TMJ relocation over the course of 10 years, she has neither been treated nor undergone any previous TMJ procedures. She reports an otherwise unremarkable medical history. Examination findings showed left TMJ clicking and pain on palpation with normal mandibular range of motion. An orofacial functional assessment showed no abnormalities detected.

Investigations

CT and functional TMJ MRI showed no significant findings.

Treatment

Bilateral intra-articular autologous blood injection to the TMJs and LPM BtA injection was performed under general anaesthesia. Bilateral arthrocentesis using 100 mL of good flow compound sodium lactate (CSL), followed by 3 mL of bilateral autologous blood injection and intermaxillary fixation, was undertaken. Bilateral LPM BtA injection was subsequently approached extraorally and verified with EMG guidance. Fifty units of BtA in 2 mL of normal saline was injected per side using a spinal needle and flushed with 0.5 mL of normal saline.

Outcome and follow-up

The patient was reviewed 10-days postoperatively, at which time she reported increasing nasal regurgitation and hypernasality. She denied any pain. Clinical examination revealed normal mouth opening, a midline uvula and an intact gag reflex. Flexible nasendoscopy showed grossly normal soft palate movement and symmetrical vocal cords. A speech pathologist evaluation was undertaken at 2 months postoperatively. This delayed evaluation was subsequent to appointment availability during the COVID-19 outbreak. The speech pathologist identified symmetrical palate and uvula at rest; however, the patient was unable to perceive palatal elevation or phonation. Additionally, bilateral nasal emission, nasalised consonants and weak plosive sounds were evident, in keeping with VPD. A trial of straw blowing exercises was advised by the speech pathologist to encourage palatal movement. VPD symptoms gradually improved with full resolution 6 months postoperatively.

Discussion

TMJ dislocation is a nonreducing condylar displacement anterosuperior to the articular eminence.⁵ Dislocation may occur as either subluxation, acute, chronic or recurrent. Recurrent TMJ dislocations are repeated episodes of TMJ disarticulation secondary to joint laxity, anatomical variation or dystonic LPM condylar head pull.¹

LPM BtA injection can be approached either intra or extraorally with EMG guidance. The extraoral approach involves palpating the ITF and inserting a vertical percutaneous needle 20–25 mm through the mandibular notch. BoNT is deposited once EMG activity confirms correct needle placement. The most common reported complication of LPM BtA injection in the literature is transient dysphagia (8%) subsiding within 2-to-4 weeks. Others include arterial bleeding and/or haematoma following injury to the maxillary artery. Rarer postulated complications include dysarthria, painful chewing and VPD symptoms.^{2 6} VPD is the inability to achieve velopharyngeal closure (VPC) necessary for the production of oral phonemes and is only achieved by contracting the velopharyngeal muscles.^{7 8} The primary effects of VPD are nasal emissions and hypernasality where oral phonemes resonate nasally.^{8 9}

Given the proximity of TVP and LVP to LPM lying together at ITF’s medial border, the authors of this report hypothesise the patient’s postoperative symptoms that are subsequent to the diffusion of BoNT, eliciting impaired VPC and VPD symptoms. This complication has been previously hypothesised, however, to our knowledge, this is the first report of its occurrence. In conclusion, VPD is a rare adversity following LPM BtA injection with its effects being temporary. Precautionary measures to minimise BoNT diffusion through using ultrasound guidance and/or mixing BoNT in smaller volumes may prevent its occurrence.

Learning points.

• Velopharyngeal dysfunction is a rare complication that may occur following botulinum toxin A (BtA) injection to the lateral pterygoid muscles for the treatment of recurrent jaw dislocation.

• The self-limiting nature of this complication is in line with the temporary effects of BtA.

• Precautionary measures to minimise botulinum neurotoxin (BoNT diffusion through using ultrasound guidance and/or mixing BoNT in smaller volumes may prevent the occurrence of this complication.