Hemimasticatory spasm (HMS) is a rare disorder that manifests as unilateral involuntary contractions of masticatory muscles involved in jaw closure, predominantly the masseter muscles. 1 , 2 HMS is visible on the side of the face, appearing as twitching of muscle fibers. Spasms usually last from seconds to several minutes and often cause jaw displacement and severe pain. Until now only unilateral cases of HMS have been described in the literature. We report a case of HMS with bilateral involvement.
Case Report
The patient is a 23‐year‐old female who presented with frequent involuntary contractions in her lower face. The onset was 2 years ago, with slight and infrequent contractions on the right side. Six months later she developed similar contractions on the left side. The frequency and severity of spasms increased with time. The patient described spasms as painless stiffness episodes. Contractions occurred more often on the right side, less frequently on the left, although infrequently both sides contracted simultaneously but, according to our observations, not synchronously (Video 1). Spasms were multiple during the day, brief, mostly lasting 2–10 seconds, less often 30–40 seconds. Bilateral mild masseter hypertrophy and several contractions were observed during examination. Neurologic examination was otherwise normal, including the jaw reflex. The patient had a normal brain MRI (1.5 T, standard T1‐, T2‐weighted, MR‐angiography) that did not show vascular compression or another sort of trigeminal nerve involvement. Routine blood and thyroid function tests were within normal limits. Concentric needle electromyography showed a spontaneous burst of motor unit potentials that correlated with muscle spasm of the right masseter muscle (Fig. 1) and multiple bursts of motor unit potentials during voluntary tonic contraction more apparent from the left masseter muscle (Fig. 2). Onabotulinum toxin A injections into masseter muscles on both sides were performed (20 U on the left and 30 U on the right) with excellent results. Spasms disappeared on the third day, partially restarting after 3 months, and remained mild for the next 4 months before the second injection.
Video 1.
Home video. Patient's self‐recording of involuntary contractions of masseter muscle fibers and bundles on the left, then on the right side of her face. Contractions occur simultaneously on both sides but are not synchronous. Video content can be viewed at https://onlinelibrary.wiley.com/doi/mdc313213
FIG. 1.
Concentric needle EMG of the right masseter muscle showing a spontaneous burst of motor unit potentials during an involuntary muscle spasm (courtesy of Dr. Armine Kteyan).
FIG. 2.
Concentric needle EMG of the left masseter muscle with bursts of motor unit potentials during voluntary tonic muscle contraction (jaw clenching) (courtesy of Dr. Armine Kteyan).
Discussion
HMS is a rare disorder. Like in the case of our patient, it usually occurs in the 3rd–4th decades of life with female preponderance. 1 Only unilateral cases have been described in the literature until now. We report the first case of bilateral HMS. HMS affects masticatory jaw muscles involved in jaw closure, mostly masseter muscles, less often medial pterygoid and temporal muscles, with lateral pterygoid very rarely involved. 2 Cases have been described with hemifacial atrophy and linear scleroderma or morphea. 3 Compression of the motor root of the trigeminal nerve by a superior cerebellar artery was observed on MRI in many cases, but others, similar to our case, remained idiopathic. Spasms in most cases were very painful, causing temporomandibular jaw subluxation, biting of oral mucosa or tongue in some patients. 1 , 4 Our patient describes stiffness rather than pain, although when spasms are severe, biting of oral mucosa is noted. Contractions in HMS may be provoked by talking, laughing, chewing, closing of the mouth. 4 Nevertheless, our patient did not report such an association.
Characteristic EMG findings of HMS include irregular high‐frequency bursts of motor unit potentials that correlate with masseter contractions. No simultaneous agonist–antagonist contraction or overflow phenomenon, characteristic of oromandibular dystonia, can be found. 5 The loss of the masseter silent period on EMG is considered typical and useful for the diagnosis of HMS. 1 This test was not performed on our patient. Also, EMG was performed 7 months after botulinum toxin injection when contractions possibly still did not have the same intensity as before.
The pathophysiology of HMS is thought to be quite similar to that of hemifacial spasm and classical trigeminal neuralgia, suggesting that involuntary muscle contractions occur due to ectopic firing from damaged trigeminal motor root fibers. Such lesion may originate from trigeminal nerve root compression by the superior cerebellar artery or its entrapment, and this notion is supported by the fact that vascular decompression has been successful for the treatment of HMS in many cases. 6 , 7 It is also supposed that irritation of the motor root of the trigeminal nerve may cause changes in the excitability of central reflex pathways that are responsible for the physiological characteristics of HMS. 8
Most of the cases described in the literature were pharmacologically treated for a while. Medications that have been used are carbamazepine, phenytoin, and clonazepam, but they only had a partial effect on the frequency of contractions. 3 , 5 , 9 Microvascular decompression of the trigeminal motor root from the superior cerebellar artery was beneficial when applicable. 6 According to many reports, botulinum toxin has a dramatic effect on contraction frequency and severity with the mean duration of the effect of 4 months. 8 In line with other reports, our patient was also successfully treated by bilateral botulinum toxin injections of masseter muscles.
To the best of our knowledge, we report the first case of bilateral HMS. It represents an interesting and important addition to the current clinical knowledge. Bilateral involvement could still be explained in the context of idiopathic HMS, but a possibility of bilateral neurovascular compression left undetected by MRI still exists. Finally, the current case could have a central mechanism, involving both masticatory nuclei of trigeminal nerves.
Author Roles
(1) Research project: A. Conception, B. Organization, C. Execution; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript Preparation: A. Writing of the first draft, B. Review and Critique.
ZDT: 1A, 1B, 1C, 3A
NSY: 1A, 1B, 3B
SGK: 1A, 3B
Disclosures
Ethical Compliance Statement
Approval of an institutional review board or ethics committee was not required for this work. Written informed consent was obtained from the patient to use her video and clinical case details in this manuscript. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines.
Funding Sources and Conflict of Interest
No specific funding was received for this work. The authors declare that there are no conflicts of interest relevant to this work.
Financial Disclosures for the previous 12 months
The authors declare that there are no additional disclosures to report.
Acknowledgments
We are grateful to Prof. Niall Quinn for his valuable opinion and advice. We would like to express our gratitude to Drs. Armine Kteyan and Mariam Isayan.
References
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