Assessment of Nociceptive Processing Among Patients With Temporo-mandibular Disorders

This case control study will investigate the physiological effect of nociceptive input in individuals with temporomandibular disorders. To do so, the investigators will compare the development of secondary hyperalgesia following high frequency electrical stimulation (HFS) of skin nociceptors in the forearms for up to 48 hours in individuals with chronic TMD (as a main complain) and in healthy controls. Furthermore, the investigators will evaluate the association between the response to HFS and various factors, such as demographic, psychosocial and pain-related clinical factors.

This control study is designed to compare the development of secondary hyperalgesia following high frequency electrical stimulation (HFS) of skin nociceptors in the forearms in individuals with chronic TMD (as a main complain) and in healthy controls.

During the first part of the experiment (time1, day 1), all potential participants will fill a basic demographic questionnaire concerning their age, weight, height, and current medications (appendix 2). Participants with TMD will be diagnosed according to the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD). The DC/TMD includes the following questionnaires: the Graded Chronic Pain Scale, the Jaw Functional Limitation scale, the Generalized Anxiety Disorder-7, the Patient Health Questionnaire-9 and -15 and the Oral Behaviors Checklist. In addition, participants will be asked to fill the Central sensitization inventory questionnaire, as well as the symptom Severity Scale. Lastly, the investigators will evaluate the distribution of pain by asking participants to draw the pain that they felt during the past 7 days on a digital tablet. This will allow us to determine the total area of pain of each participant, as well as the Widespread Pain Index. Subjects will also be asked to provide a rating of their average pain for the last 24h

The psychophysical evaluation will be conducted later on the same day (day1). Subjects will be asked to provide a rating of their average pain at present time (time 2). Then, participants will be familiarized with the different procedures involved in the experiment.

Pressure pain thresholds will be assessed on the temporalis muscle, the trapezius muscle, and tibialis anterior (time 3).

The intensity of pain resulting from pinprick stimulation and allodynia in the area surrounding the electrode ("test area") will be assessed, as well as on the homologous site of the contralateral control arm before HFS (time 4).

Then HFS will be applied (time 5) on the left or right volar forearm (10 cm distal to the cubital fossa). HFS will consist in trains of transcutaneous electrical stimuli consisting of 42 Hz trains (pulse width: 2 ms) lasting 1 s each. The trains will be repeated 12 times. Each train will be separated by several 10 seconds of rest. The intensity of each stimulus will be set to 5 milliampere.

The of pain resulting from pinprick stimulation and allodynia in the area surrounding the electrode ("test area") will be assessed 30 min after HFS (in a counterbalanced order) on the homologous site of the contralateral control arm (time 6).

The extent of secondary hyperalgesia will be assessed approximately 45 min following HFS (time 7).

Then, they will be asked assess the presence of secondary hyperalgesia in the test-site 24 (time 8, day 2) and 48 hours (time 9, day 3) following HFS with with calibrated neurological examination pins (Neuropen, Owen-Mumford Ltd, Oxford, UK).

6 months after their initial assessment (time 10, day 180), if they gave their consent for long-term follow-up, patients will be asked to fill the following questionnaires again

Eligibility Criteria:
Inclusion Criteria for healthy controls:

* Females aged between 18 and 50 years.
* BMI between 17 and 30.
* Ability to provide written informed consent.

Exclusion Criteria for healthy controls:

* Evidence of psychiatric/inflammatory/neurological or metabolic comorbidities on direct questioning as this study is meant to focus on musculoskeletal disorders.
* Evidence of skin alteration on the volar forearms that may interfere with HFS.
* Any substance abuse, or the use of antidepressant or neuroleptic medication as these may interfere with the psychophysical examination.
* Any painful musculoskeletal disorder.
* Lack of dental check-ups during the last 12 months.

Inclusion Criteria for the TMD group:

* Females aged between 18 and 50 years.
* BMI between 17 and 30.
* Ability to provide written informed consent.
* Chronic TMD (for at least 3 months) as a main complain (according to the DC/TMD), with an average of at least 3/10 on a pain numerical rating scale during the past month in the orofacial region.

Exclusion Criteria for the TMD group:

* Evidence of psychiatric/inflammatory/neurological or metabolic comorbidities on direct questioning as this study is meant to focus on musculoskeletal disorders.
* Evidence of skin alteration on the volar forearms that may interfere with HFS.
* Any substance abuse, or the use of antidepressant or neuroleptic medication as these may interfere with the psychophysical examination.
* Lack of dental check-ups during the last 12 months