Hemimasticatory spasm

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This chapter and the series of sub-chapters will be mainly dedicated to the clinical case of our poor patient Mary Poppins who had to wait 10 years to have a certain and detailed diagnosis of Hemimasticatory Spasm, finding herself between two foci, that of the dental context and the neurological one beyond all the other branches of medicine encountered in the clinical path such as dermatology, otolaryngology, internal medicine, etc. It would be too hasty to dismiss this event by confirming the diagnosis of 'Hemimasticatory Spasm' without understanding the reason for the diagnostic delay and much less neglecting the elements that could help the clinician to formulate a rapid and detailed diagnosis. In this section of Masticationpedia, therefore, we would like to start laying the foundations for a more formal language in medical diagnostics and not the classic model in which ambiguity and vagueness can complicate the diagnostic process. We will therefore resume some contents already proposed in the 'Introduction' section and make them practical and clinically essential in the diagnosis of our patient Mary Poppins.

Focal demyelination of the trigeminal motor fibers

Unlike hemifacial spasm, hemimasticatory spasm (HMS) is a very rare disorder. Since Gowers' original description in 1897, very few cases have been described in the world literature. From a recent study in a movement disorder clinic conducted between 2000 and 2010, there was only one patient with HMS out of 215 patients with hemifacial spasm. Corina Christie et coll confirmed that only 36 cases of HMS were reported in the literature between 1980 and 2013 (see Table 1 of chapter: Hemimasticatory Spasm: Report of a Case and Review of the Literature ).

From the article by Corina Christie et coll. some interesting elements emerge:

1. Typically, HMS involves the masseter and temporal muscles including the rarely involved medial pterygoid muscle. There is usually no involvement of the mandible opening muscles, but there are at least two cases describing involvement of the lateral pterygoid, one with associated lateral deviation of the mandible. This data highlights an essential feature that it is specific for the trigeminal motor tract and mainly affects the levator muscles rich in neuromuscular spindles. Could there be an involvement of these nerve structures in hemimasticatory spasm as occurs in dystonias and spasticity?
2. The most frequent triggers that precipitate spasms are talking, laughing or chewing. These triggers are always voluntary movements rather than sensory stimuli, as happens in trigeminal neuralgia.
3. Neurological examination should be normal in HMS except for spasm, hypertrophy of the involved muscles or subcutaneous tissue atrophy which may occur in cases associated with localized scleroderma. Facial sensitivity is always spared and no other cranial nerve must be compromised.
4. The pathophysiological mechanisms that produce HMS are not fully understood. There is a reduced inhibition of muscle contraction which can be evidenced electrophysiologically by the loss of the masseterian silent period, which is almost exclusive to HMS, and therefore can be a very useful aid for differential diagnosis. Characteristic EMG findings of HMS include irregular discharges of motor unit potentials (MUPs) that are related to involuntary spasms of the masseter. MUPs are often morphologically normal but with a very high frequency. Cruccu et al. noted a delay in the conduction velocity of the motor branch of the trigeminal nerve located in the infratemporal fossa between the lateral pterygoid and the cranial surface. This could explain focal demyelination of trigeminal motor fibers as well as hemifacial atrophy observed in nearly 70% of cases.

«Does 'Carbamazepine' have an effect in clinical situations of focal demyelination of trigeminal motor fibers?»

In the sub-chapter 'Long-Term Outcome of Hemimasticatory Spasm' it is clear that in the study by Somdattaa Ray et coll. two patients maintained long-term improvement in symptoms after being treated with botulinum toxin for 4-7 years, while one patient reported symptom improvement with carbamazepine treatment which subsequently completely regressed after pregnancy. This further confuses the clinical picture in the sense that carbamazepine pharmacologically has little indication for hemimasticatory spasm unless it is of the central type. This conclusion, however, is displaced by the observation of the regression of the spasm after pregnancy which generates further doubts on the pharmacological effect of carbamazepine.

In a nutshell we can only confirm the rarity and diagnostic complexity of the disease.

«Why is early diagnosis so important?»

Because the disease is so complex that it can hide much more serious pathologies such as tumors of the parodi as well described in the chapter ' Intermittent facial spasms as the presenting sign of a recurrent pleomorphic adenoma' in which the authors confirm that to date the development of facial spasms it has not been reported in parotid neoplasms. The most common etiologies for hemifacial spasm are vascular compression of the facial nerve ipsilateral to the cerebellopontine angle (defined as primary or idiopathic) (62%), hereditary (2%), secondary to Bell's palsy or facial nerve injury (17 %) and imitators of hemifacial spasms (psychogenic, tics, dystonia, myoclonus, myochemia, myorhythmias and hemimasticatory spasm) (17%).

All this partly justifies the diagnostic delay but requires a new model of clinical interpretation of the patient and above all the search for the machine language code to be decrypted to speed up the definitive diagnosis.

Bibliography & references