The effect of injections with botulinum toxin into jaw elevators

Objective: Botulinum toxin (BTX) injections into masticatory muscles are reported to be increasingly used for dystonia and hypertrophy. However, the effect of BTX on normal biting functions is poorly documented. Methods: In 8 patients (2 women and 6 men, aged 29-72 yr) 15-25 units Botox were injected bilaterally with electromyographic (EMG) guidance into each masseter muscle in half of the patients, and into each masseter and temporalis muscle in the other half of the patients, as determined from their medical condition. The diagnoses were oromandibular dystonia in 6 patients and painful masseter hypertrophy in 2 patients. Ongoing pharmacological medication, e.g. clonazepam, was kept constant. Bite-force measurements and surface EMG according to our standard procedures were performed before and about 1 mo after first treatment series. In addition, 100 mm-visual analog scales (VAS) were used for subjective evaluation of the condition. Results: The mean value of the VAS ratings was significantly (P<.006) reduced from 63 (SD 19) before to 34 (SD 19) after the injections. Also the amplitude of maximal masseter activity was significantly lower (43%, P<.006) with a mean pretreatment value of 189 µV (SD 84) and 94 µV (SD 73) after treatment. Neither the maximal bite force, decreasing 19% from 466 N (SD 305) to 341 N (SD 246), or the maximal temporalis activity for all patients (32%; from 178 µV SD 71, to 115 µV SD 79) changed significantly. However, the change of temporalis activity varied significantly (P=.01) depending on whether temporalis injections were included in the BTX treatment or not (average reductions of respectively 62% and 1%). Conclusion: The chemical denervation from BTX was clearly reflected in the injected muscles. Even if muscle activity is supposed to recover in 3-5 mo, it seems advisable that patients are followed closely when treated with repeated series of BTX injections.