Temporomandibular Disorders (TMD) are a group of conditions that cause pain and movement issues in the jaw and the facial muscles that control it. Symptoms include pain in the jaw, face, neck and ears when chewing, speaking or opening the mouth wide. Sufferers may also experience facial tiredness, trouble chewing, or clicking, popping or grating sounds in the jaw.
A recent study has found that patients with postural orthostatic tachycardia syndrome (PoTS) commonly suffer from painful TMD, with approximately half of the study sample reporting symptoms that led to them screening positive for the disorder. ME/CFS was diagnosed in 44% of the sample, and 56% of those with TMD. The study also found that TMD caused a significant decrease in quality of life.
You can read more about PoTS here.