Dystonia

INDEX

 

The muscles involved in opening the jaw are:


Anatomy

The lateral pterygoidmuscle has superior and inferior heads. Involvement of the inferior head is the usual cause of jaw opening dystonia, and for practical purposes, the superior head is not typically injected.

Action: 
The two heads have separate, reciprocal functions: when the inferior head contracts, the superior head of the pterygoid muscle is inhibited.
When the inferior head of the muscle is activated unilaterally, a contralateral deviation of the jaw is produced, usually  in concert with the medial pterygoids.

Conversely, during jaw closure, the superior head contracts, and the inferior head relaxes. The superior head acts to stabilize the condyle during jaw closure.

When acting bilaterally, the inferior head of the pterygoid muscle pulls the condyle forward and slightly downwards, and produces jaw protrusion and jaw opening  (acting in conjunction with the suprahyoid muscles). 

 

Figure 1. Superior and inferior heads of lateral pterygoid, with zygomatic arch removed (left) and in place (right). 

 

 

From: Blitzer, A., Benson, B. E., & Guss, J. (2012). Botulinum Neurotoxin for Head and Neck Disorders. Thieme Medical.

 

 

Figure 2. Anatomy of the mandible, and the insertions of the superior and inferior heads of the lateral pterygoid.
 

TREATMENT

Injection of the inferior lateral pterygoid. There are two approaches: 
1. External injection
The examiner should palpate for the mandibular notch (the gap between the coronoid process anteriorly and the mandibular condyle posteriorly), which is found immediately beneath the zygomatic arch.  This is easily felt as the patient opens and closes their jaw. The needle is passed horizontally through the notch into the lateral pterygoid, which is activated by jaw opening, typically with EMG guidance.

Figure 3. Injection of lateral pterygoid by the external approach

 

Potential side-effects: The maxillary artery is relatively superficial and is therefore potentially prone to trauma from the injecting needle, potentially resulting in haematoma formation.

Figure 3. Relationship of the maxillary artery to the inferior head of the lateral pterygoid muscle

2. Intraoral injection

The muscle may be injected intraorally, also with EMG guidance. This allows for placement of toxin to a wider part of the muscle.

In the intraoral injection, the patient should be semi-reclined with their mouth slightly open and deviated to the contralaterally to the side of the procedure.
The insertion of the needle electrode to inject the inferior head is above the second molar mucobuccal fold (the fold formed by the oral mucosa where it passes from the maxilla to the cheek). The electrode is directed medially (by 200), upward (by 300 to the occlusal plane (the plane formed by opposition of the teech of the upper and lower jaw), and backward sliding close to the maxillary tuberosity until hitting the pterygoid plate.The direction of the injection should be oriented towards the middle point of a virtual line connecting the ipsilateral ear’s tragus and lobe.

Identification of the 2nd molar

Figure 4. Intraoral injection of lateral pterygoid.

The needle must penetrate (at least 30–40 mm) through the masseter muscle and temporalis tendon before reaching the inferior lateral pterygoid head.
Once the needle is in position, the patient is asked to produce lateral excursions of the jaw to confirm proper placement.

 

Figure 5. Intraoral injection of the inferior head of the lateral pterygoid muscle

 

From: Yoshida K. How Do I Inject Botulinum Toxin Into the Lateral and Medial Pterygoid Muscles? Mov Disord Clin Pract. 2016 Dec 14;4(2):285. doi: 10.1002/mdc3.12460. 


Dose
7.5 u Botox

 

Video 1. Injection of lateral pterygoid via intraoral route


(vv)Lay Pty.mp4 (tt)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

Yoshida K. How Do I Inject Botulinum Toxin Into the Lateral and Medial Pterygoid Muscles? Mov Disord Clin Pract. 2016 Dec 14;4(2):285. doi: 10.1002/mdc3.12460. PMID: 30838273; PMCID: PMC6353475.