Targeting pain mechanisms from the 'top-down' and the 'bottom-up' in chronic tension-type headache

INTERVENTION: Participants will receive 30 minutes of combined active transcranial direct current stimulation (tDCS) and active transspinal direct current stimulation (tsDCS) for five consecutive daily sessions. Transcranial direct current stimulation (tDCS): will be delivered using a direct current stimulator via two 35 cm2 surface sponge electrodes. The active electrode (anode) will be placed over the primary motor cortex and the reference electrode (cathode) over the contralateral supraorbital region. Current intensity will be ramped up (0 mA to 1 mA) and down (1 mA to 0 mA) over 10 s at the beginning and end of the stimulation period. Transspinal direct current stimulation (tsDCS): will be delivered using a direct current stimulator via two 35 cm2 surface sponge electrodes. The active electrode (anode) will be positioned over the thoracic spinal cord at the tenth thoracic vertebra and the reference electrode (cathode) above the right shoulder. Current intensity will be ramped up (0 mA to 1 mA) and down (1 mA to 0 mA) over 10 s at the beginning and end of the stimulation period. CONDITION: Chronic tension‐type headache PRIMARY OUTCOME: Headache intensity, frequency, duration and use of analgesics: Headache intensity will be recorded on a 10 cm visual analogue scale (VAS) with 'no pain' at zero and 'worst pain imaginable' at 10, headache duration as hours per day, frequency as the number of days with headache per month, and use of analgesics as the number of doses. Perceived response to therapy will be assessed using a 7 point Likert scale ranging from completely recovered to vastly worsened (Global Perceived Effect Scale (GPES)). Safety and adverse events: data on any side effects or adverse events (such as headache, nausea, dizziness) will be recorded. SECONDARY OUTCOME: Central pain modulation (CPM): CPM is tested as a change in the pain perceived in one body region [electrical stimulation, 4 ms pulse duration, applied to the left upper arm] as a result of pain induced in another [heat pain, 30 pulses/minute to 1 degree C above pain threshold via thermode, applied to the right forearm]. Twenty heat pain stimuli will be delivered to the left arm alone followed by 20 combined with electrical pain stimulation to the right arm. Nociceptive flexor withdrawal reflex (NFR): Surface stimulating electrodes will be positioned at a retromalleolar location along the right sural nerve. Recording electrodes will be positioned over the belly of the biceps femoris muscle. Stimulation will consist of pulse trains of 20 ms duration at an interval frequency of 300 Hz. The intensity needed to evoke a response in the biceps femoris muscles, indicating activation of the NFR, and the subjective pain threshold will be recorded. Pressure (PPT) and cold pain thresholds (CPT): PPT will be measured using a pressure algometer [probe size of 1 cm2, application rate 40 kPa/s]. CPT will be measured using the Thermotest system [preset to 30 degree C, temperature change rate of 1.8 degree C/s]. Participants will push a button when the pressure or cold sensation first becomes painful. The average of three recordings will be analysed at each of six sites: bilateral temporal muscle, bilateral upper trapezius muscle and bilateral thumbnail. INCLUSION CRITERIA: Participants with a diagnosis of chronic tension‐type headache (CCTH) according to the international Classification of Headache Disorders, 2nd edition (ICHD‐II) criteria will be recruited. Additional inclusion criteria will comprise: i) diagnosis of CTTH for greater than 12 months prior to commencing the trial, ii) experience painful episodes on 15 or more days per month, iii) experience painful episodes that last more than 4 hours when untreated, and iv) experienced symptom onset before age 50 years.