Study population~The population considered for inclusion in the study is patients with CM treated with botulinum toxin A injections every third months at DHC. This is an observational study, and inclusion will not affect treatment. Patients will be included consecutively.~Inclusion criteria~Patients being treated (or are starting treatment) with botulinum toxin A injections in the clinical setting at the DHC. Treatment is based on confirmed CM-diagnosis according to the ICHD-III (4), diagnosed by headache specialists at DHC~Aged 18 - 65 years~Able to sign an informed consent~Exclusion criteria~Medication overuse headache~Known cervical disorders, e.g. cervical spine disc prolapses, whiplash or spinal stenosis~Change in other pharmacological preventive medication during the study period. Stable pharmacological preventive medication is allowed.~Treatment with botulinum toxin A injections~Preventive treatment with botulinum toxin A injections is standard care for chronic migraine and follows the PREEMPT protocol. Patients are injected at 31 locations in the frontal, corrugator and procerus muscle, the temporalis muscle, the occipitalis muscle, the cervical paraspinal muscle group and the trapezius muscles (11). The treatment is repeated every 12 weeks, performed by specially trained medical doctors or nurses.~Baseline characteristics~The following information is gathered for research purpose after the patients written consent is obtained.~Information collected from the patient's record or baseline questionnaire at inclusion time~Age, gender, years of migraine and chronic migraine, current use of pharmacological preventive treatment, co-morbidities, number of previous treatments with botulinum toxin A injections, level of physical activity, height (cm) and weight (kg). Any MRI or CT scans of head and neck to verify diagnosis~Information collected from baseline headache and neck pain calendar during 3 months run-in period~Headache days/month, migraine days/month, types of acute medication, days with use of acute medication, days with neck pain and menstrual cycle.~Ultrasound shear-wave elastography~For measuring the muscle stiffness, we use a linear 9 MHz ultrasound probe (9L) and the Logic E9 scanner, both from GE Healthcare (Chalfont St Giles, UK). All measurements are performed with the probe parallel to the muscle fibers. We will measure the stiffness of the trapezius muscle in the middle botulinum toxin A injection side (G1) according to the PREEMPT protocol. All data are saved as images by the observer and subsequently analyzed by a blinded observer.~Total Tenderness Score~Eight paired pericranial muscles and tendons are palpated in a standardized manner with a standardized pressure. small rotating movements are used while palpating. Subjects are asked to rate their tenderness on a 4-point scale (0-3). Scores from each location are added to get a total tenderness score (TTS) ranging from 0 to 48. The eight examined areas are the frontal muscle, temporal muscle, coronoid process, masseter muscle, mastoid process, sternocleidomastoid, trapezius muscle and neck muscle insertions.~Pressure Pain Threshold~Pressure pain threshold will be assessed bilaterally over the temporalis muscle (over injection side G1), the temporalis, and index finger. The pain threshold is defined as the minimal pressure where the sensation of pressure changes to pain. The electronic pressure algometer (Somedic Algometer type 2, Sollentuna, Sweden) will be used. Three PPT measurements are measured over each area with a 30-sec interval between two consecutive measurements at the same area. Pressure will be applied at a rate of 50 kPa /second with a 1 cm2 probe.
Preventive treatment with botulinum toxin A injections is standard care for chronic migraine and follows the PREEMPT protocol. Patients are injected at 31 locations in the frontal, corrugator and procerus muscle, the temporalis muscle, the occipitalis muscle, the cervical paraspinal muscle group and the trapezius muscles (11). The treatment is repeated every 12 weeks, performed by specially trained medical doctors or nurses.~We will measure elastography of the trapezius muscle before and 5-6 weeks after botox injection. In addition measure with QST