Dystonia

INDEX

Jaw closing is caused by activation of the masseter, the temporalis and – to a lesser extent – the medial pterygoid muscles.  
The function of the medial pterygoid muscle is primarily jaw closure and it may be considered a functional analog of the masseter muscle. 

Anatomy of the Mandible


Anatomy

The muscles of mastication are innervated by the trigeminal nerve and include the temporalis, masseter and medial and lateral pterygoids. All of these muscles may develop compensatory overactivity if the others are injected (a whack-a-mole phenomenon)1.

  1. The temporalis muscle originates in the temporalis fossa of the temporal bone and inserts on the coronoid process and anterior surface of the ramus of the mandible. It functions to elevate and retract the mandible. The muscle may be divided into three fascicles: anterior (vertical orientation), middle (diagonal orientation), and posterior (horizontal orientation).  The anterior and middles fascicles are primarily concerned with closure of the jaw.
  2. The masseter originates from the zygomatic arch and attaches to the angle and ramus of the mandible. It has two divisions, superficial and deep2. The superficial division is quadrangular; the deep portion is covered anteriorly by the superficial division, and posteriorly by the parotid gland.
  3. The medial pterygoid originates from the medial surface of the lateral pterygoid plate (deep head) and the tuberosity of the maxilla (superficial head). The deep and superficial heads of the muscle attach by a tendinous lamina to the medial surface of the angle of the mandible and ramus. 
    The functions of the medial pterygoid are:
    - Jaw closure
    - Jaw protrusion (combined medial and lateral pterygoid contraction)
    - Contralateral excusion of the jaw (in combination with the lateral pterygoid of the same side)

The masseter and medial pterygoid together form a U-shaped sling around the inferior border of the mandible. The two muscles elevate and forcibly close the jaw.

Anatomy of Medial Pterygoid
From: Gilroy, Anne M., et al. Atlas of anatomy. New York: Thieme, 2016.

TREATMENT
1. Masseter Injection

Figure 1. Anatomy of the masseter and temporalis muscles

Note that the parotid gland overlies ther muscle, particularly at the angle of the jaw.

 

 

Figure 2.  Inferior margin of the mandible shown as dotted horizontal line and the muscle is shown having been divided into three. Injection sites are shown (three): black circles with white central dots.

Typically unnecssariy, but the examiner may wish to approach masster injections as follows:
Draw an imaginary line between the corner of the mouth and the intertragal notch (Figure 2)3.
Ask the patient to clench their jaw and palpate the anterior border of the masseter muscle which can be felt up to the zygomatic arch
Divide muscle into thirds and inject 1 cm above the line of the mandible at three locations; one in between and slightly above.the other two sites
(this technique avoids the marginal mandibular and buccal branches of the facial nerve)/

Dose
12.5-50 u; Max 50 u Botox
 

2. Temporalis muscle injection

The muscle belly is palpated while the patient clenches their teeth. The needle is placed through the skin overlying the temporalis muscle and position is confirmed with EMG activity with clenched teeth. The needle is usually inserted along the anterior or superior border of the temporalis muscle, and subsequently advanced posteriorly or inferiorly in the sagittal plane, once the correct depth of the needle placement is confirmed with the EMG. This allows multiple aliquots to be injected with one pass of the needle, a technique that is impossible if the needle is passed in a lateral to medial direction. 0.1 mL (5 units each) aliquots are delivered at four or five points within the temporalis muscle. These aliquots are spaced at 1-cm intervals, preferably through one to three skin puncture sites.
 

Anatomy of the Temporalis muscle

From: Gilroy, Anne M., et al. Atlas of anatomy. New York: Thieme, 2016.

Figure 3. Injection of temporalis muscle

 

From: Cultrara A, Chitkara A, Blitzer A. Botulinum toxin injections for the treatment of oromandibular dystonia. Oper Tech Otolaryngol - Head Neck Surg 2004; 15: 97–102.

3. Medial Pterygoid Injection

The muscle may be injected either intraorally or from below; a combination of injections is desirable.
Typically easier, with the patient supine, and their head turned away from the side which is to be injected.

From below:

The needle is placed through the skin in the submandibular area (about 0.5-1 cm anterior to the angle of the mandible), directed superiorly deep to the mandible and the U-shaped sling which the masseter forms with the medial pterygoid, and along the inner aspect of the mandibular ramus, thus entering the muscle4.  One approach is to insert needle until it contacts the inferior margin of mandible, then retract the needle, move slightly inwards, and push up: one should angle the needle to be as close to the inner aspect of the mandible as possible.
The needle should be parallel to the direction of the muscle, and perpendicular to the ramus (with the mouth open, see Figure  5). Note that the facial artery is found in the vicinity of the injection.

The position is confirmed by brisk EMG activity when the patient is asked to clench their teeth, and the muscle is injected at two to three points.

 

Figure 4. Injection of medial pterygoid from below

From: Cultrara A, Chitkara A, Blitzer A. Botulinum toxin injections for the treatment of oromandibular dystonia. Oper Tech Otolaryngol - Head Neck Surg 2004; 15: 97–102.

Figure 5. Approach to injection of medial pterygoid

From: Skármeta, N. P., Espinoza-Mellado, P., & Chana, P. (2018). Orofacial Dystonia and Other Oromandibular Movement Disorders. In Dystonia - Different Prospects. InTech. https://doi.org/10.5772/intechopen.78607

 

Video 1. How to inject the medial pterygoid: Intraoral approach

 

The needle is angled posteriorly and superiorly at 20° to the occlusal plane, and laterally by 20°; needle should be inserted about 15-20 mm.

(vv)Med Pterygoid.mp4(tt)

From: Yoshida K. How Do I Inject Botulinum Toxin Into the Lateral and Medial Pterygoid Muscles? Mov Disord Clin Pract 2017; 4: 285.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

  1. Yoshida K. How Do I Inject Botulinum Toxin Into the Lateral and Medial Pterygoid Muscles? Mov Disord Clin Pract 2017; 4: 285.
  2. Corcoran NM, Goldman EM. Anatomy, Head and Neck, Masseter Muscle. [Updated 2021 Jun 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539869/
  3. IAPRD Botulinum Toxin in Movement Disorders Webinar Series; Dr M Pathak. Oromandibular Dystonias and Blepharospasm. February 2021.
  4. Cultrara A, Chitkara A, Blitzer A. Botulinum toxin injections for the treatment of oromandibular dystonia. Oper Tech Otolaryngol - Head Neck Surg 2004; 15: 97–102.