Pain perception following a standard bolus injection of hypertonic saline into the masseter muscle and associations with psychological variables.
Supervisors: Prof Greg M Murray (University of Sydney), Dr Rahena Akhter (Charles Sturt University)
BACKGROUND
Chronic orofacial muscle pain imposes significant personal and economic burdens on at least 5% of the general population.1, 8, 16 It is characterised by regional pain and limitations of jaw movement and is a major symptom of Temporomandibular Disorders (TMD). Current treatments are based on little scientific evidence, and there is a significant placebo effect.16, 17
Experimental models of muscle pain have been applied extensively in both humans and experimental animals in both spinal and trigeminal systems in an attempt to understand the pathophysiological mechanisms involved in muscle pain.5, 11, 14, 15, 17, 20-22 One model that has been extensively used is the model where sterile hypertonic saline (~5%) is injected into the belly of a jaw muscle, typically the masseter muscle.11, 14, 15, 17, 20, 21
In a previous study in our research unit, we have injected hypertonic saline into the middle region of the masseter muscle14, 15 to evoke pain intensity of 30-50 mm on a 100-mm visual analogue scale (VAS). In this study, the mean (standard deviation) volume of infused solution, in 17 asymptomatic healthy subjects, was 2.3 (0.9 ml). There was a high variability in the volume of infused solution between individuals to achieve approximately the same intensity of pain. Thus, in one subject a very high infusion volume was required (e.g., 4.2 ml), and in another, only a very low volume (e.g., 0.4 ml) was required to evoke pain intensity of 30-50 mm on a 100-mm visual analogue scale.
One reason for this variability might relate to the concentration of nociceptors in the vicinity of the injected solution. It is also known that there are gender differences in pain perception and nociceptor sensitivity between males and females (e.g.2). There is also evidence that measures of pain perception can vary with psychological variables, such as catastrophizing. For example, lowered thresholds for pain perception, increased numbers of McGill Pain Questionnaire (MPQ) pain descriptor words chosen, enlarged pain areas, and enhanced pain intensity have all been demonstrated in high catastrophizing individuals in the spinal system,4, 6, 7, 12, 13, 19 and we have preliminary data that these features may also correlate with catastrophizing in the trigeminal system (Akhter et al, unpublished observations).
HYPOTHESIS
A standard bolus injection of hypertonic saline into the masseter muscle will generate pain intensity levels and pain maps that will vary with
(a) the location of the injection,
(b) the gender of the subject, and
(c) psychological variables, e.g. the level of catastrophizing of the individual.
AIM
To inject a standard bolus of hypertonic saline into the anterior or posterior region of the masseter muscle and to determine if there is an association between pain intensity levels and pain maps and
(a) the location of the injection,
(b) the gender of the subject, and
(c) psychological variables, e.g. the level of catastrophizing of the individual.
METHODS
The study will be carried out in 40 volunteers, 20 females and 20 males.
All subjects will give informed consent. Experimental procedures will be approved by the Western Sydney Local Health District Human Ethics Committee and the Human Ethics Committee of the University of Sydney. The Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD)3 will be used to verify the absence of signs and symptoms of TMD.
Each participant will then complete the following questionnaires:
The Pain Catastrophizing Scale (PCS)18 which assesses negative cognitive and affective reactions to pain. Three sub-scales (magnification, rumination and helplessness) capture a person’s orientation towards noxious stimuli and/or previous memories of pain. Each of the 13 questions is rated on a 5-point Likert scale ranging from 0 (not at all) to 4 (all the time), and the total score ranges from 0-52.
The Depression, Anxiety and Stress Scales (DASS) which is a reliable and well-validated scale9 that measures the cognitive and affective dimensions of psychological distress. It has three scales (depression, anxiety and stress) and 42 items. Symptoms in the past week are rated from 0 (“not at all”) to 3 (“most of the time”). The total scores for each scale consist of the sum of the items.
In each participant, experimental jaw muscle pain will be induced by a bolus infusion of 5% hypertonic saline into the anterior or posterior region of the right masseter muscle. The outline of masseter will be established during clenching and will be divided into an anterior and a posterior half. A disposable 22 gauge needle will be inserted into the right masseter, midway between origin and insertion and into the middle of the anterior or the posterior half of the masseter and to a depth of the whole length of the needle. Injection location will be randomized between subjects so that approximately 10 females and 10 males will receive injections into the posterior masseter and the same number into the anterior masseter.
The needle will be attached to a 1 ml syringe (Becton Dickinson, Singapore) containing 5% hypertonic saline. Negative aspiration will be confirmed prior to an initial bolus injection of 0.2 ml hypertonic saline which will be infused over 20 s. Pain intensity will be quantified with a 100-mm visual analogue scale (VAS) before needle insertion, immediately after needle insertion, immediately after completion of injection, and then every 30 s after infusion for the next 15 minutes. Participants will be informed that zero on the VAS denotes “no pain at all” while 100 mm represents “the worst imaginable pain”. Finally, each subject will map the pain location and perceived distribution of pain on lateral-profile outline pictures of the head and neck. Pain affect will be quantified with the McGill pain questionnaire (MPQ) after the infusion is terminated and pain returns to zero.
The McGill Pain Questionnaire (MPQ) 10 consists primarily of 3 major classes of word descriptors — sensory, affective and evaluative — that is used by patients to specify subjective pain experience. The questionnaire was designed to provide quantitative measures of clinical pain that can be treated statistically. The 3 major measures are: (1) the pain rating index, based on two types of numerical values that can be assigned to each word descriptor, (2) the number of words chosen; and (3) the present pain intensity based on a 1–5 intensity scale. After the pain had declined to zero following cessation of the hypertonic saline infusion, volunteers completed the MPQ.
The areas of pain spread will be converted to square mm using a 2mm x 2mm box grid. Referral areas will be counted.
Data Analysis
Independent variables: anterior masseter, posterior masseter; male, female; PCS score; DASS scores,
Dependent variables: VAS, pain areas, referral sites.
Statistical tests: independent T-tests for binary variables; linear regression for discrete variables.
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