ANALGESIC EFFECT OF CATHODAL TRANSCRANIAL CURRENT STIMULATION OVER RIGHT DORSOLATERAL PREFRONTAL CORTEX IN SUBJECTS WITH MUSCULAR TEMPOROMANDIBULAR DISORDERS: A Double Blind Crossover Randomized Clinical Trial

1. INTRODUCTION Pain is among the main complaints of most patients that seek care in a hospital or a primary care unit. Temporomandibular Disorders (TMD) patients have become part of the daily routine of all the health care professionals. Temporomandibular and cranial-facial disorders are so prevalent that, since 1987, the International Headache Society holds meetings to classify different types of pain associated with head. This resulted in the publication of the Manual of Classification of Headaches - The International Classification of Disorders Headache was revised in 2013.

There are different strategies to treat TMD patients, and the best is chosen depending on the type of problem diagnosed or the most suitable strategy according to professional experience. Strategies range from advice on lifestyle, psychotherapy, containment of the jaw, kinesitherapy, ultrasound, TENS, muscle relaxants plates, allopathic and homeopathic therapies and surgical interventions to the temporomandibular joint (TMJ). Some of these techniques aim to treat the muscles, others treat the dental occlusion or joint structures and there are those whose primary focus is the psycho-emotional factors. As The TMDs are due to various factors of muscular, joint, emotional, inflammatory, autoimmune or even infectious reasons, the treatment should include different therapies during the rehabilitation process, considering the best strategies for each patient. Literature has shown that the causes of muscular origin are more prevalent in patients with this type of disorder). A systematic review using studies with functional brain imaging, showed that, although the majority of disorders are associated with muscle pain, this would be secondary to a process triggered by the Central Nervous System (CNS). Following this theory, some behavioral studies have shown that the TMD, almost always, are related to preexisting psychopathology.

Chronic pain is involved with neuronal excitability. a study using transcranial magnetic stimulation (TMS) found higher threshold in both sides of resting motor cortex with lower up regulation responses. Furthermore they found lower responses in the intracortical facilitation and inhibition.

The transcranial direct current stimulation (tDCS) allows the neuronal membranes to neuromodulate and it can enhance or inhibit the potential actions on the cortex. Studies with animals has shown that anodal stimulation modulate the membrane in the way to depolarize which results in a long term potential in the stimulated area. Some studies have shown improvement in subjects with fibromyalgia and chronic pain using neuromodulation . Some authors suggest anodal tDCS over motor cortex to decrease pain while modulating the activity in the neuronal networks responsible for pain, an example of that is the thalamus. It also facilitates the descending inhibition of pain. However few evidences support the efficacy of tDCS.

Another montage is tDCS over dorsolateral prefrontal cortex (DLPFC). This area is involved with anxiety and depression without direct analgesic effect. A study showed a pain level decrease after tDCS over DLPFC in patients with emotional and affective troubles. The authors suggested it based on emotional modulation by pain independent networks by somatosensory pain perception. A study found anxiety modulation after tDCS application over DLPFC using cathodal current on the right side and anodal on the left side. This result is just a weak evidence that the tDCS anxiety effect, there is a demand to investigate that approach.