Masseter Myofascial Pain Relief Following an Intra-muscular Injection With Botulinum Toxin Type A: A Randomized Double-blind Controlled Multicenter Pilot Study

Temporomandibular disorders (TMD) is the second most common musculoskeletal condition (after chronic low back pain) and refer to pain and dysfunction in the temporomandibular system and its muscles, joints, and associated structures. The prevalence of TMD is 4-15% among adults where myalgia is the most common form of TMD.

Diagnostic Criteria for temporomandibular disorders (DC/TMD) was introduced in 2014 and is used worldwide for classifying TMD. Myalgia is the second group of disorders of DC/TMD and is defined by pain in a masticatory structure that is modified by jaw movement, function, or parafunction and familiar pain in masticatory muscle(s) with either muscle palpation or maximum opening.

Myofacial pain is the most common form of TMD, affecting about half of the patients presenting in TMD clinics. Myofascial pain is usually treated with conservative treatment including occlusal, physical and pharmacologic therapies. However, there is no evidence on which conservative treatment is the most beneficial in treating myalgia pain, mainly due to lack of randomized controlled trials.

Botulinum toxin A (BTX) is a neurotoxin isolated from Clostridium botulinum. Injected into a muscle the effect occurs in the presynaptic junction by blocking the release of acetylcholine, resulting in weakening of the muscular activity by affecting alpha and gamma motor neurons. The clinical effect occurs after 3-7 days after administration with a duration of up to 4 months. The BTX-effect diminish when new formation of synaptic connections occur due to sprouting.

Regional myofascial pain is characterized by the presence of trigger points and BTX, as a potent neurotoxin, has been used for its treatment. However, a Cochrane review reported inconclusive evidence to support the use of BTX in the treatment of myofascial pain. In another Cochrane review on cervical dystonia significant improvements were found with odds ratio (OR) of 20% in Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) total score after four weeks (OR 4.69; 95% CI 2.06 to 10.69) and a weighted mean difference of -5.92 (95% CI -9.61 to -2.23). Studies investigating the effect of BTX on masticatory muscle myalgia have been summarized in systematic reviews and with inconclusive evidence to support the effect of BTX. Meta-analyses were inconclusive due to the heterogeneity of the studies and methodological flaws.