Orofacial Applications of Botulinum Neurotoxin

By Dr. Evan Friedman | June 8, 2022

The following is the first article in a series to address applications of Botulinum neurotoxin. The following article is presenting a short review of the Orofacial applications of BoNT, with a focus on the benefit of using the Myoguide™ EMG/ESTIM BoNT injection guidance system. The information within, is simply informative and not considered to be instructive, in any way. Please have a look at the details within the appropriate reference materials for more details.

Botulinum toxin (BoNT) is a neurotoxin produced by the bacterium Clostridium Botulinum. There are seven types of BoNT (A–G) which bind and cleave one of several neuronal cellular proteins including SNAP-25, Syntaxin or VAMP/Synaptobrevin. 

The outcome of BoNT application results in the inhibition of release of acetylcholine (Ach) from the motor neuron terminals resulting in muscle weakness or paralysis.  Duration of action depends on cellular regeneration mechanisms, which can take 3-6 months.

The orofacial complex consists of the muscles of:

  • The jaw
  • Lips
  • Tongue
  • Soft palate
  • Larynx

BoNT has several orofacial applications and is effective in reducing pain and/or muscle activity in chronic migraine, myofascial pain, orofacial movement disorders, neuropathic pain, trigeminal neuralgia and bruxism.  Other applications include control of excess salivation (sialorrhea) and gustatory sweating (Frey Syndrome), and spasmodic dysphonia (1-9,27-28).

BoNT was originally thought to reduce orofacial pain through inhibition of muscle activity, however, current evidence suggests that there may be analgesic properties as a result of its ability to inhibit the release of substance P, calcitonin gene-related peptide and transient receptor potential, vanilloid receptor type 1 (31).

The net result may translate into reduced pain and neurogenic inflammation in the peripheral and central nervous system. This may be synergistic with the ability of BoNT to block neuromuscular transmission (29).

Injection techniques are usually fairly straight forward and can be performed with or without electromyography (EMG) guidance, depending on the indication.  

We recommend using the Myoguide Injection guidance system, regarding superior design and access to many useful built-in features, to support both EMG and STIM guidance procedures. Visit the Myoguide Store! to get started.

Orofacial movement disorders

Orofacial movement disorders include orofacial dystonia and orofacial dyskinesia (32).

These conditions can affect multiple orofacial muscles including platysma, orbicularis oris, buccinator, genioglossus, geniohyoid, digastric and intrinsic tongue muscles. The net result can be involuntary jaw opening, clenching, tongue thrusting and facial tics. They have the potential to result in Temporomandibular (TM) dysfunction when the masseter, temporalis, medial pterygoid muscles are involved. It may also result in TM joint dislocation when the lateral pterygoid muscle is involved.

BoNT remains an effective treatment particularly for focal dystonia and dyskinesia.

There are two basic options for accessing these muscles, including either intraoral or extraoral approaches.  Both require EMG guidance (11-15).  

Temporalis and Masseter muscles injections are fairly straight forward, whereas injection for the Medial Pterygoid muscle is more challenging, and more subject to iatrogenic effects.

The techniques for accessing the Lateral Pterygoid muscle is less challenging, however, the extraoral approach does have risks (33).

Neuropathic pain

BoNT is known to inhibit neuromodulator and transmitter secretion that may reduce central neuropathic pain.

The proposed mechanism of action includes a reduction in the release of excitatory glutamate, SP, and CGRP. Furthermore, BoNT reduces the expression of TRPV1 that is associated with capsaicin-evoked and calcium channel responses (16,17).

The injection technique in the management of neuropathic pain is unique in that the BoNT is injected subcutaneously in the dermatome overlying the neuropathic pain. This procedure does not require EMG guidance.

Trigeminal Neuralgia

Trigeminal Neuralgia (TN) is not synonymous with neuropathic pain. The etiology of TN remains unclear, although compression of the trigeminal nerve root by the superior cerebellar artery is thought to result in focal demyelination and paroxysms of pain. Additional causes include tumors, multiple sclerosis and post-viral syndromes.

The mainstay of treatment is medication including carbamazepine, oxycarbamazepine, phenytoin, gabapentin, pregabalin and baclofen. Surgical treatment involves Gasserian ganglion rhizotomy, gamma knife and microvascular decompression (18-20).

The mechanism of action of the BoNT is thought to be similar to that of neuropathic pain. The injection technique is also identical. The time to maximal pain reduction for both neuropathic pain and TN appears to be 4–8 weeks. This is much longer than the 1–2 weeks for muscular injection and may reflect the need for retrograde axonal transport to the CNS (18-20).

Bruxism

The beneficial effect of BoNT on sleep-related bruxism has been reported (21,22). It appears to reduce the intensity of muscle activity within the masseter and temporalis muscles with a reduction in self-reported pain and jaw stiffness. The injection technique and dosing is identical to that for myofascial pain.

Frey syndrome

Frey syndrome is a rare complication following parotid and temporomandibular joint surgery. It results in gustatory sweating during eating. The parotid gland receives parasympathetic secretory innervation via a complicated neural pathway that involves the auriculotemporal nerve and the release of Ach.

The sweat glands of the skin are innervated by the sympathetic fibers originating from superior cervical ganglion traveling via the same auriculotemporal nerve. Parotid and temporomandibular joint surgery are thought to cause an injury to the auriculotemporal nerve leading to inappropriate parasympathetic innervation of the overlying sweat glands.

This can result in profuse sweating during mastication. The diagnosis of Frey syndrome can be made using the minor iodine/starch test. This test will also delineate exact area of skin that is affected. This should be outlined with a skin marker. BoNT can be injected subcutaneously. The therapeutic effect is generally apparent within a week and should last 3–6 months (23).

Sialorrhea

The excessive production of saliva or the inability to adequately swallow saliva may result in drooling. This is often seen in patients with neurological disorders including those with developmental delay, cerebral palsy, amyotrophic lateral sclerosis, Parkinson’s disease, cerebrovascular accident and traumatic brain injury.

Antisialagogues have limited efficacy and result in systemic side effects. Surgical procedures including salivary gland duct ligation and sialodochoplasty to reposition ducts has also been described. The use of BoNT to reduce salivation has grown in popularity with promising results (24-26).

The technique for parotid and submandibular gland injection is relatively simple. The use of ultrasound to identify the precise location of the gland and appropriate intraglandular injection is encouraged. The ideal dose varies depending on the gland size and the severity of hypersalivation and drooling.

Spasmodic Dysphonia

The disorder is due to the situation where the (laryngeal) muscles that generate a person’s voice go into periods of spasm. This results in breaks or interruptions in the voice, often every few sentences, which can make a person difficult to understand (27,28).  See our animated video illustrating the approach for vocal cord injections using Myoguide.

Myoguide™ EMG/ESTIM BoNT injection guidance system is designed to amplify EMG signals from muscle and provide audio and visual feedback to assist clinicians in locating areas of muscle activity.

Myoguide also has an integrated and well featured stimulator, capable of stimulation in either 1.0 mA or 0.1 mA steps, for muscle, nerve, and motor endplate location procedures.

Myoguide supports injection of neuromodulators and both chemodenervation and neurolytic procedures, as well as, peripheral nerve stimulation. Learn more about how to use Myoguide

References

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