An experimental test of high frequency electrical stimulation of peripheral nerves for treating chronic pain

INTERVENTION: Clinical audit data suggests that high frequency electrical stimulation of peripheral nerves is an effective treatment for occipital headaches and chronic pain that develops after peripheral nerve injury. We now wish to investigate effects of this stimulation on pain and sensitivity to painful stimulation in patients with a stimulator implanted along or adjacent to a peripheral nerve. The implanted stimulator consists of two components: a cylindrical multi‐contact electrode wire placed adjacent to a peripheral nerve and a rechargeable electrical pulse generator. We will compare pain and sensitivity to painful stimuli (pressure, sharpness, heat, cold) after the stimulator has been switched on or off for 4 hours. As stimulation itself produces no detectable sensations, we will use a double blind strategy to investigate effects of stimulation (i.e., neither the participant nor the investigator will know whether the stimulator is switched on or off). The pressure‐pain threshold will be identified by applying pressure at 100 grams/second to the skin with a pressure sensor (a Wagner Instruments algometer) until the patient reports pain or to a maximum of 5 kg. Patients will rate sharpness between 0 and 10 to the application of a calibrated pin (40 grams force, Neuropen device). Temperature ramps of 1 degree/second will be delivered by a purpose‐built Peltier device until the patient reports pain or to limits of 0 and 50 degrees C. These stimuli will be administered by Professor Drummond within the region of pain and at a similar contralateral site while the stimulator is switched off at baseline and four hours after the stimulator has either been switched on or switched off. The washout period between treatments will be at least one week. CONDITION: neuropathic pain occipital headache peripheral nerve injury PRIMARY OUTCOME: numerical pain rating between 0 (no pain) and 10 (extreme pain). SECONDARY OUTCOME: A graded decrease in temperature at 1 degree C/second will be applied from a starting temperature of 32 degrees C in the distribution of pain and at a similar site on the other side of the body until the participant reports pain or to a minimum of 0 degrees C. The temperature ramp will be applied using a purpose‐built Peltier device. A graded increase in pressure at 100 grams/second will be applied using a calibrated pressure sensor (algometer, Wagner Instruments) in the distribution of pain and at a similar site on the other side of the body until the patient reports pain or to a maximum of 5 kg. A graded increase in temperature at 1 degree C/second will be applied from a starting temperature of 32 degrees C in the distribution of pain and at a similar site on the other side of the body until the participant reports pain or to a maximum of 50 degrees C. The temperature ramp will be applied using a purpose‐built Peltier device. Patients will rate sharpness (between 0 and 10) to a calibrated pin (Neuropen device) applied at 40 grams of pressure in the distribution of pain and at a similar site on the other side of the body Skin temperature will be measured with a non‐contact infrared device (Cool‐Tech) in the distribution of pain and at a similar site on the other side of the body. The tactile threshold to graded nylon monofilaments (Marstock Instruments) will be assessed in the distribution of pain and at a similar site on the other side of the body. INCLUSION CRITERIA: Patients who have been implanted with a high frequency electrical nerve stimulator (Nevro stimulator, ARTG Identifier 186043) approximately 2 cm from a peripheral nerve within the past two years will be invited to participate in this study. They will be informed that we wish to investigate the effect of the stimulator on pain.