Parkinson's Disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms. Pain is a significant symptom in PD, affecting a large percentage of patients and impacting their quality of life. The mechanisms of pain in PD involve complex changes in pain-modulating pathways, including dopaminergic and non-dopaminergic systems.

To address the lack of pain management strategies, the investigators propose exploring non-pharmacological therapies like transcranial direct current stimulation (tDCS). tDCS is a safe and non-invasive technique that modulates neuronal activity. It has shown positive effects on pain processing in healthy individuals and chronic pain patients, but its potential for PD-associated pain remains largely unexplored.

The primary motor cortex (M1) is a target for tDCS as it is believed to influence pain processing in other brain regions involved in sensory and emotional aspects. Initial studies suggest the benefits of tDCS in PD, including enhanced motor potentials and potential modulation of dopaminergic pathways. However, there are currently no published studies specifically investigating the effects of tDCS on PD-related pain, highlighting the need for further research.

A proof-of-concept trial is proposed to examine the effects of a single tDCS session on M1 in PD patients during the OFF state (without medication) and after taking dopaminergic medication. The study aims to assess the pain-relieving effects of tDCS in PD and explore potential synergies between tDCS and dopaminergic medication. By better understanding the impact of tDCS on pain relief in PD, this research may offer insights into alternative non-pharmacological approaches for managing pain in PD.

Parkinson's Disease (PD) is a complex progressive neurodegenerative disorder. It is the second most common neurodegenerative disease, with a prevalence of 4.1 to 4.6 million people over the age of 50. It is estimated that these numbers will increase to 8.7 to 9.3 million by the year 2030.

PD is multisystemic and include both motor and non-motor symptoms. These symptoms can fluctuate throughout the day, leading PD patients to frequently experience two distinct states: the "on" state, when dopaminergic medication is active in their system, and the "off" state, which occurs when there is no medication concentration in the body, resulting in more severe symptoms.

Pain in PD is a highly relevant symptom that significantly impacts patients' quality of life, with a prevalence of up to 85% and moderate to severe intensity reported in 42% of individuals. Additionally, in Spain, the prevalence of PD-associated pain has significantly increased following the COVID-19 pandemic, with up to 50% of PD patients reporting daily pain. Moreover, pain in PD is often underreported, emphasizing the need to investigate its mechanisms and treatment.

The pathophysiological mechanisms of pain in PD are complex. In general, PD can lead to alterations in peripheral transmission, sensory-discriminative processing, pain perception, and interpretation at multiple levels, due to neurodegenerative changes in both dopaminergic and non-dopaminergic pain-modulating pathways. This dysregulation of the dopaminergic system can impact the experience of pain directly by enhancing nociceptive signals and indirectly by influencing expectations and the interpretation of such signals. However, it is considered that there is no specific pain center in PD patients but rather a pain matrix involving various brain structures.

Given the multiple central structures affected and their consequent pathophysiological mechanisms in PD-associated pain, there is a wide variety of clinical manifestations. These include musculoskeletal pain, generalized central chronic pain, visceral pain, pain related to "on-off" fluctuations, dystonic-dyskinetic pain, nocturnal immobility-related pain or restless legs syndrome, orofacial pain, inflammation or edema-related pain, and radicular pain.

Regarding the assessment of pain in PD, there is only one specific scale that evaluates all pain phenotypes of the disease in terms of intensity and frequency, namely the King's Parkinson's Disease Pain Scale (KPPS). Therefore, due to the scarcity of objective pain assessment tools and the crucial role of the central nervous system and endogenous modulation systems in the origin of PD-associated pain, it is important to assess pain processing characteristics such as Conditioned Pain Modulation (CPM), and Pressure Pain Thresholds (PPTs). CPM assesses the descending inhibitory pain system, and PPTs assess the intensity of pressure required to elicit initial pain in a specific body area.

Dopaminergic medication is the primary treatment for PD symptoms; however, its effects on pain management have shown contradictory results according to recent systematic reviews. Moreover, there is currently a lack of robust evidence to establish guidelines for pain management in PD, which is why non-pharmacological therapies have been proposed as a safe and useful alternative with fewer side effects.

Transcranial Direct Current Stimulation (tDCS) is a non-invasive form of brain stimulation in which a relatively weak continuous current is delivered to the cortical areas through small electrodes placed on the scalp. It is a neuromodulatory intervention, and based on the assumption that the exposed tissue is polarized, tDCS would modulate neuronal excitability by depolarizing or hyperpolarizing the resting membrane potential of neurons. This modulation would result in increases or decreases in cortical excitability, depending on whether the active electrode is the anode or the cathode, respectively.

tDCS has been shown to have an effect on certain psychophysical variables of pain processing in healthy subjects. It has primarily been shown to enhance CPM and improvements have been reported in PPTs, cold and heat pain thresholds, acute pain induced by laser (26), and pain threshold after peripheral electrical stimulation. Importantly, tDCS has also demonstrated beneficial effects in patients with chronic pain. Specifically, it has shown improvements in pain processing characteristics, resulting in clinical pain improvements in other populations with chronic neuropathic pain. It is hypothesized that tDCS could be an effective alternative in the treatment of pain with central sensitization components, a process common in PD-associated pain.

The majority of the hypoalgesic effects generated by tDCS are due to its modulation of the primary motor cortex (M1). It is suggested that tDCS over M1 could reduce pain by increasing excitability not only in the stimulated area but also in other structures involved in pain processing, both sensory and emotional, through cortico-subcortical connectivity.

However, there are currently no published studies applying tDCS for the treatment of PD-associated pain, except for unpublished studies from our group confirming a favorable effect in this regard. Nonetheless, disregarding our results, its use can be considered safe as it has already been tested in PD patients for other purposes. tDCS has demonstrated an influence on Motor Evoked Potentials (MEPs) in PD patients, increasing MEPs after stimulating M1 with an active anode and decreasing MEPs with an active cathode. Furthermore, it has been observed that bilateral tDCS with an active anode leads to increases in BDNF, thus slowing down the degeneration of dopaminergic neurons. This last piece of evidence, along with other findings that demonstrate the capacity of tDCS to modulate dopaminergic transmission, is relevant for the present project, considering that part of the descending inhibitory pain system involves these pathways. Similarly, it has been demonstrated that pain causes reductions in corticomotor excitability, governed by M1, and tDCS applied to M1 has shown the ability to increase cortical excitability, which has abnormal activity in PD patients and could influence their pain.

Given the overlap between many influential areas in PD-associated pain and the areas excited by tDCS over M1, with these increases in excitability correlating with pain reduction, as well as the potential effect of tDCS on dopaminergic pathways, a hypoalgesic effect of tDCS in PD patients in their OFF state is hypothesized. Additionally, if the action is based on the activation of dopaminergic pathways, a possible enhancement of dopaminergic medication's action with better pain control during the ON period is also expected. However, there is currently no study that has specifically evaluated this effect in PD patients in the OFF state, making it pertinent to conduct a proof-of-concept trial with this objective. The objective is to determine the effect of a single session of treatment using transcranial direct current stimulation (tDCS) on the primary motor cortex (M1) on perceived clinical pain and pain processing characteristics in patients with Parkinson's disease (PD) in the OFF state (without dopaminergic medication) and after taking dopaminergic medication.

Inclusion Criteria:

• Diagnosis of Idiopathic Parkinson´s Disease.
• Presence of Parkinson´s Disease-related pain in the off-state.
• Neuroimaging study without previous pathologies.
• Score > 5 in transfers (bed to chair and back) item in Barthel Index.
• Score = or > 26 in Montreal Cognitive Assessment (MoCA).
• Tolerability for the application of electrotherapy.
• Able to provide informed consent to participate in the study
• Pain intensity >= 3 in Visual Analogue Scale or equivalent.

Exclusion Criteria:

• Neurologic disease different from PD.
• Pain non-related to PD.
• Dermatologic problems, wounds, or ulcers in the electrode's application area.
• Presence of implants or metal pieces in the head.
• Presence of cardiac pacemaker, vagal, brain or transcutaneous stimulators, medication pumps, ventriculoperitoneal shunts or aneurysm clips.
• Significative difficulties in language.
• History of alcohol or drugs abuse.
• Non-controlled medical problems.
• Pregnancy.
• Epilepsy

Individual anonymized participant data will be available to other researchers under request.

Four months at the end of the study

Individual anonymized participant data will be available to other researchers under request.

Triple-blind criteria will be achieved by identic collocation of the electrodes in both conditions and by activating the "double-blind" option in the Starstim tDCS® Software (Neuroelectrics Inc, Barcelona, Spain) that allows concealing the protocol by writing a neutral number. The evaluator, not allowed to stay in the same room while the interventions, will conceal the protocols with the neutral number and the therapist will read it in the envelope, ignoring which number coincides with each intervention. At the end of the second session, patients will be asked whether they received the order 1.active-2.sham or 1.sham-2.active, to assess the blinding success. The statistician will also be blinded through the mentioned neutral numbers. Unblinding will be permissible when any event could suppose a risk to the patient's health.