OBJECTIVETo determine the classification of masticatory myospasm by analyzing characteristics of clinical appearances.

METHODSThirty-six cases of masticatory myospasm from 2000 to 2010 were included. The clinical data of these patients were analyzed, including patient information, patient history, clinical characteristics, severity and the frequency of myospasmodic movement, electromyogram (EMG), and the efficacy of botulinum toxin injection treatment.

RESULTSThere were 11 males and 25 females, aged from 15 to 71. According to the clinical manifestation and EMG findings, patients could be divided into two groups: 18 cases were classified as jaw closing type which involved masseter and/or temporalis muscles presenting as trismus and acute pain, the other 18 cases were jaw opening type which involved lateral pterygoid muscles complaining difficulty in jaw closing and teeth clenching. The jaw closing type was often seen in patients of 20 to 50 years old, the jaw opening was frequently seen in patients over 50 years old. Jaw closing type was attacked intermittently and unilaterally, but jaw opening was often attacked continually and bilaterally. The rating scale of the severity of spasmodic movement was not different between the two types, but the frequency of spasmodic attack was much higher for jaw opening type (P < 0.05). The EMG of jaw closing type was classified into persistent, rhythmic and irregular type. The EMG of jaw opening type was classified into spontaneous and exercise-induced type. Twelve cases were treated by botulinum toxin injection that could significantly relieve symptoms.

CONCLUSIONSMasticatory myospasm can be classified into jaw closing and jaw opening types. Jaw closing type involves masseter and/or temporalis muscles and jaw opening type involves lateral pterygoid muscles. Botulinum toxin injection was the most effective therapy for the masticatory myospasm.