Shifting Pain Modulation From Pro-to Anti-nociceptive

Background

Pro-nociceptive pain modulation profile (PMP) is characterized by either:

(i) decreased inhibition of pain, i.e.inhibitory pro-nociceptive PMP, (ii) increased facilitation, i.e. facilitatory pro-nociceptive PMP, or (iii) both, i.e. dual pro-nociceptive PMP.

The counterpart of pro-nociceptive, i.e., anti-nociceptive PMP, has not yet been explored. The investigators expect individuals harboring this profile to be at lower risk for pain acquisition, and experience less intense pain. Pain modulation is commonly altered in pain patients toward a pro-nociceptive pain modulation profile (PMP), expressed, in the lab, by increased facilitation and/or decreased inhibition of experimental pain, and clinically by high pain phenotype.

The proposed study seeks to explore the yet to be characterized mirror image of pro-nociception, i.e., the situation where individuals exhibit reduced pain facilitation and more efficient pain inhibition. This domain of 'anti-nociceptive' PMP is a potential platform for improving pain therapy and prevention. Extrapolating from the clinical picture of pro-nociception, it is likely that 'antinociceptive' individuals will express a lower pain phenotype, with less frequent and less intense pain experiences, lower risk of acquiring pain after surgery or trauma, and, possibly, better response to analgesics. The investigators would like to explore whether it is possible to shift pain modulation towards anti-nociception, in order to obtain the possible benefits of this modulation profile in pain-prone situations.

The clinical model the investigators propose for addressing the hypotheses is preemptive drug treatment for reduction of postoperative pain. Our specific choice of surgery is inguinal herniorraphy. In addition to acute post-operative pain, this operation carries a relatively high rate of chronic post-operative pain (ranging from 10 to 54% of patients, with most reports indicating towards the lower end of this range).

Our specific aims are to explore, in the setup of post operative pain after inguinal herniorraphy, the shift of PMP from pro- towards anti-nociceptive based on each of the following testing domains:

(i) Psychophysical and neurophysiological data describing facilitatory and inhibitory modulation capacity of the patient.

(ii) Psychological data describing patient's behavior such as catastrophizing, anxiety and depression, pain sensitivity and life orientation.

(iii) Blood tests to evaluate Micro RNA which regulate downstream transcription or shutting down gene expression, in order to evaluate their role as predictor for evolving chronic pain, and (v) individual case data related to age, gender, education, socioeconomic parameters and personal medical history.

Subjects

• Study A: One hundred and forty healthy subjects (range 20-79; 20 subjects per age decade, 10 M, and 10 F) will participate in the first phase of the study aimed to collect normative data from healthy population.
• Study B: Two hundred and twenty patients (range 18-75) scheduled for inguinal herniorraphy will be enrolled.

Study design Study I - Normative data collection for the inhibitory and excitatory pain modulation responses, a study on healthy subjects

The study will include one session. In this session, after signing informed consent, subjects will:

(i) fill out state of health questionnaire, (ii) record 5 minutes of resting state EEG, and (iii) undergo a short familiarization with the various stimulation modalities.

Thereafter, and along the session subjects will fill the pain related personality questionnaires (anxiety and depression, catastrophizing, pain sensitivity, life orientation test and ten item personality measure) followed by psychophysical assessments, which will include assessment of conditioned pain modulation (CPM) and temporal pain summation (TS) as described later on. A sample of 20 ml of blood will be drawn from all subjects at the end of the session.

Study II - Herniorraphy surgery patients This study is a randomized, placebo-controlled, double-blind, three parallel arms non-crossover longitudinal study in four groups of patients identified by their PMPs. The study will consist of two assessment sessions: the first between 1 to 2 weeks before surgery and the second 3 month after surgery. The sessions will be conducted by experienced experimenter, and will include the same psychophysical and psychological assessments as in Study I; after signing informed consent, patients will undergo a short familiarization with the various stimulation modalities and then undergo the various stimuli.

At the end of this session patients will be assigned into one of four groups:

(i) dual pro-nociceptive, (ii) inhibitory pro-nociceptive, (iii) facilitatory pro-nociceptive, and (iv) anti-nociceptive.

45 patients will be assigned into each group, by order of their recruitment. Third of the patients in each group will be randomized to Duloxetine (DUL), third to Pregabalin (PGB) and the other third to Placebo (PLA). Each group will receive two treatment doses of the drug before surgery, i.e. one in the evening before and one at the morning of the surgery. Patients will then undergo surgery, to be performed by one team of surgeons, under a standardized protocol of general anesthesia. After surgery patients will be treated by a standardized analgesic protocol until discharge, normally the morning after surgery. Acute pain intensity and analgesics consumption during the first 24 hours will be recorded. On post-operative day (POD) 1 patients will fill a "patient outcome questionnaire" that will include information about their perceived post-operative pain intensity and the extent of pain interference with activities and effects on mood and anxiety. A periodic phone call for chronic postoperative pain will be pursued at 1, 2, 4 weeks and 6 months after surgery. An experimenter blinded to the psycho-physical group assignment, will follow the pain state after surgery.

Two samples of 20 ml of blood will be drawn from all patients; first blood withdrawal will be done before surgery, and the second at a clinic visit between 3 and 6 month after surgery.