Effects of Transcranial Direct Current Stimulation (tDCS) on Pain Associated With Diabetic Neuropathy

Background: Worldwide, the prevalence of diabetes mellitus (DM) and its chronic complications are reaching epidemic proportions. By 2045 DM expected to affect 629 million persons. Diabetic neuropathy is one of the most common complications of DM and is actually the set of heterogeneous clinical forms. Among diabetic neuropathies, generalized, focal or multifocal forms are distinguished. The most typical and most common form (up to 80% of cases is sensory-motor polyneuropathy. It has recently been redefined as a length-dependent symmetric sensory-motor polyneuropathy attributable to metabolic and microvascular alterations following exposure to chronic hyperglycemia and cardiovascular risk cofactors. Painful diabetic peripheral neuropathy (PDPN) may be considered a variant; about 15-25% of patients have painful diabetic polyneuropathy.

According to the definition of neuropathic pain of the International Association for the Study of Pain (IASP), PDPN is considered as asymmetric sensory-motor polyneuropathy chronic in which neuropathic pain has been present, for at least 3 months, as direct consequence abnormalities of the peripheral somatic-sensitive system following diabetic pathology. Sensory symptoms are more evident than motor symptoms; which is appear in the most distal portions of the limbs and progress proximally according to a "glove" or "sock" distribution associated with paresthesias and dysesthesias. The clinical symptoms are related to the type of nerve fiber involved and typically there are "positive" (tingling, burning, pain, allodynia) or "negative" sensory symptoms (loss of sensitivity, numbness). The involvement of small sensitive fibers causes a reduction in thermal sensitivity and pain and this, in turn, contributes to the deficits in tactile and proprioceptive sensitivity to increase the risk of a foot injury.

The PDPN is associated with negative repercussions on the quality of life (QoL), activities of daily livings, sleep disorders, and is the major determinant of depressive symptoms in patients with PDNP. In fact, depression and sleep disturbances play a negative prognostic role in the general and diabetic population, and sleep disturbances are associated with glycated hemoglobin levels higher than 0.35% in type 2 diabetes. The presence of PDPN determines an increase in the use of medical care and social costs, both directly related to treatments, and indirect secondary to the reduction of working capacity. The presence of chronic pain of any nature increases the risk of difficulties in the self-management of diabetes, in relation to taking therapy, physical activity, adherence to the diet, foot care and even glycemic self-monitoring.

Transcranial Direct Current Stimulation (tDCS) has shown promising results for the treatment of chronic pain in several types of diseases, being a good and non-invasive alternative to treat pain in PDPN patients. Also, based on studies that showed changes in the brain activity of chronic pain patients and on the difficulties faced to treat PDPN, there is a need to conduct clinical trials using new techniques, such as neuromodulation, to help these patients to control their pain. Therefore, tDCS emerges as a new tool to be coupled with other effective treatments already used for these patients.

The dorsolateral prefrontal cortex (DLPFC) is a functionally and structurally heterogeneous region as well as a key area of numerous brain networks involved in the processing of cognition effect. Numerous studies have shown that some chronic pain syndromes are associated with a reduction in the volume of the gray matter of DLPFC. Some studies show that pain relief manages to reverse these structural changes PDPN. Moreover, previous studies reported that DLPFC stimulation induces a significant analgesic and antidepressant effect.