"Temporomandibular disorder" (TMD) encompasses all musculoskeletal disorders of the masticatory system and includes myalgia, arthralgia, temporomandibular joint (TMJ) disc displacements, and TMJ degenerative joint diseases. The prevalence of TMD ranges from 6% to 12% in the general population, with muscle dysfunction the most prevalent TMD diagnostic group. TMD is associated with substantial disability and suffering and negatively impacts quality of life. Jaw pain is the most common symptom that compels treatment seeking. In addition to facial pain, TMD patients frequently report comorbid pain conditions such as headache, low back pain, and fibromyalgia. New approaches to TMD therapy are urgently needed to improve clinical outcomes and reduce economic impact of this disorder.

There is currently no FDA-approved product labeled specifically to manage/treat TMD; however, classes of drugs are used to relieve TMD-associated pain, such as non-steroidal anti-inflammatory drugs (NSAIDs), anti-inflammatory drugs, corticosteroids, benzodiazepines, sedative hypnotics, muscle relaxants, opioids, antidepressants, and anticonvulsants - although evidence to establish their efficacy and safety in this population is scarce. Practitioners' justification for their use may be based on poorly controlled clinical trials or clinical trials in other pain disorders such as acute postsurgical dental pain, arthritic pain, chronic lower back pain, and neuropathic pain. Thus, there is a need for controlled clinical trials to better understand the physiological mechanisms responsible for TMD symptoms.

Evidence suggests that enhanced β-adrenergic drive contributes to the pathogenesis of TMD and other complex persistent pain conditions. For example, individuals with myofascial pain conditions have elevated catecholamine levels and augmented sympathetic responses to stressors. While increased β-adrenergic drive appears to heighten pain, β-adrenergic antagonists can reduce clinical pain and/or nociceptive sensitivity. A recent study of a single infusion of propranolol in TMD and fibromyalgia patients revealed short-term improvement in clinical pain ratings. The antagonist pindolol was similarly efficacious in alleviating cardinal symptoms of fibromyalgia pain. In addition, intramuscular injections of low-dose propranolol in rats reduced inflammatory pain associated with carrageen-induced inflammation of the gastrocnemius muscle.

The study hypothesis is that therapy with the nonselective β-adrenergic receptor antagonist propranolol extended-release capsules (FDA approved to treat many cardiac conditions, tremor, migraine, and pheochromocytoma) will provide efficacious and safe treatment for painful TMD. It has well-studied pharmacodynamic, pharmacokinetic, and side-effect profiles. Peak blood level occurs at approximately 6 hrs, and the plasma half-life is approximately 10 hrs. The primary objective is to investigate the efficacy of propranolol compared with placebo over 9 weeks to reduce pain in patients with TMD. Secondary objectives are to: investigate by treatment group whether reduction in pain varies according to polymorphisms in the COMT gene coding region; and investigate the effect of propranolol compared with placebo to affect pain sensitivity, physical and emotional function, adverse effects, and use of rescue medications. Exploratory objectives are to: investigate gene-by-treatment group interaction to determine the effect of propranolol on reduction in the pain index according to polymorphisms in the COMT, beta-2 adrenergic receptor (ADRβ2), and beta-3 adrenergic receptor (ADRβ3) genetic coding regions.