Andre P. Mauderli

Abnormal sensitization and temporal summation of second pain (wind-up) in patients with fibromyalgia syndrome

&NA; Although individuals with fibromyalgia syndrome (FMS) consistently report wide‐spread pain, clear evidence of structural abnormalities or other sources of chronic stimulation of pain afferents in the involved body areas is lacking. Without convincing evidence for peripheral tissue abnormalities in FMS patients, it seems likely that a central pathophysiological process is at least partly responsible for FMS, as is the case for many chronic pain conditions. Therefore, the present study sought to obtain psychophysical evidence for the possibility that input to central nociceptive pathways is abnormally processed in individuals with long standing FMS. In particular, temporal summation of pain (wind‐up) was assessed, using series of repetitive thermal stimulation of the glabrous skin of the hands. Although wind‐up was evoked both in control and FMS subjects, clear differences were observed. The perceived magnitude of the sensory response to the first stimulus within a series was greater for FMS subjects compared to controls, as was the amount of temporal summation within a series. Within series of stimuli, FMS subjects reported increases in sensory magnitude to painful levels for interstimulus intervals of 2–5 s, but pain was evoked infrequently at intervals greater than 2 s for control subjects. Following the last stimulus in a series, after‐sensations were greater in magnitude, lasted longer and were more frequently painful in FMS subjects. These results have multiple implications for the general characterization of pain in FMS and for an understanding of the underlying pathophysiological basis.

Temporal summation of pain from mechanical stimulation of muscle tissue in normal controls and subjects with fibromyalgia syndrome

&NA; Individuals diagnosed with fibromyalgia syndrome (FMS) report chronic pain that is frequently worsened by physical activity and improved by rest. Palpation of muscle and tendinous structures suggests that nociceptors in deep tissues are abnormally sensitive in FMS, but methods of controlled mechanical stimulation of muscles are needed to better characterize the sensitivity of deep tissues. Accordingly, force‐controlled mechanical stimulation was applied to the flexor digitorum muscle of the forearm in a series of brief contacts (15 stimuli, each of 1 s duration, at 3 or 5 s interstimulus intervals). Repetitive stimulation was utilized to determine whether temporal summation of deep muscular pain would occur for normal subjects and would be enhanced for FMS subjects. Moderate temporal summation of deep pain was observed for normal controls (NC), and temporal summation was greatly exaggerated for FMS subjects. Temporal summation for FMS subjects occurred at substantially lower forces and at a lower frequency of stimulation. Furthermore, painful after‐sensations were greater in amplitude and more prolonged for FMS subjects. These observations complement a previous demonstration that temporal summation of pain and after‐sensations elicited by thermal stimulation of the skin are moderately enhanced for FMS subjects. Abnormal input from muscle nociceptors appears to underlie production of central sensitization in FMS that generalizes to input from cutaneous nociceptors.

Enhanced temporal summation of second pain and its central modulation in fibromyalgia patients

&NA; We have previously shown that fibromyalgia (FMS) patients have enhanced temporal summation (windup) and prolonged decay of heat‐induced second pain in comparison to control subjects, consistent with central sensitization. It has been hypothesized that sensory abnormalities of FMS patients are related to deficient pain modulatory mechanisms. Therefore, we conducted several analyses to further characterize enhanced windup in FMS patients and to determine whether it can be centrally modulated by placebo, naloxone, or fentanyl. Pre‐drug baseline ratings of FMS and normal control (NC) groups were compared with determine whether FMS had higher pain sensitivity in response to several types of thermal tests used to predominantly activate A‐delta heat, C heat, or cold nociceptors. Our results confirmed and extended our earlier study in showing that FMS patients had larger magnitudes of heat tap as well as cold tap‐induced windup when compared with age‐ and sex‐matched NC subjects. The groups differed less in their ratings of sensory tests that rely predominantly on A‐delta‐nociceptive afferent input. Heat and cold‐induced windup were attenuated by saline placebo injections and by fentanyl (0.75 and 1.5 &mgr;g/kg). However, naloxone injection had the same magnitudes of effect on first or second pain as that produced by placebo injection. Hypoalgesic effects of saline placebo and fentanyl on windup were at least as large in FMS as compared to NC subjects and therefore do not support the hypothesis that pain modulatory mechanisms are deficient in FMS. To the extent that temporal summation of second pain (windup) contributes to processes underlying hyperalgesia and persistent pain states, these results indirectly suggest that these processes can be centrally modulated in FMS patients by endogenous and exogenous analgesic manipulations.

Deficiency in endogenous modulation of prolonged heat pain in patients with Irritable Bowel Syndrome and Temporomandibular Disorder

ABSTRACT Females with Irritable Bowel Syndrome (IBS) and Temporomandibular Disorder (TMD) are characterized by enhanced sensitivity to experimental pain. One possible explanation for this observation is deficiencies in pain modulation systems such as Diffuse Noxious Inhibitory Control (DNIC). In a few studies that used brief stimuli, chronic pain patients demonstrate reduced DNIC. The purpose of this study was to compare sensitivity to prolonged heat pain and the efficacy of DNIC in controls to IBS and TMD patients. Heat pain (experimental stimulus; 44.0–49.0 °C), which was applied to left palm, was continuously rated during three 30‐s trials across three separate testing sessions under the following conditions: without a conditioning stimulus; during concurrent immersion of the right foot in a 23.0 °C (control); and during noxious cold immersion in a (DNIC; 8.0–16.0 °C) water bath. Compared to controls, IBS and TMD patients reported an increased sensitivity to heat pain and failed to demonstrate pain inhibition due to DNIC. Controls showed a significant reduction in pain during the DNIC session. These findings support the idea that chronic pain patients are not only more pain sensitive but also demonstrate reduced pain inhibition by pain, possibly because of dysfunction of endogenous pain inhibition systems.

Limits of human bite strength

lhe greatest human bite strength in the early literature was reported more than 300 years ago by Borelli of Rome, Italy, in 1681.’ He attached weights to a cord, which passed over the molar teeth of the open mandible, and with closing of the jaw, up to 440 lbs (200 kg) were raised.’ In recent times, the greatest reported bite strength was 348 lbs (158 kg) in the Alaskan Eskimo.2 Bite strength records have been limited by instrumentation. Black,3 for example, reported that at least one of his subjects could have exceeded the 275 lb (125 kg) limit of his gnathodynamometer. Furthermore, his subjects were biting unilaterally and, as in many other studies, did not have the advantage of bilateral support. An improved gnathodynamometer was needed if increased bite strengths were to be measured. Today’s soft diet cannot compare with the hard, frozen diet of the Eskimos for strengthening the mandibular muscles.4 However, many people today undergo jaw muscle-strengthening through clenching and bruxing habits that may be considerably more rigorous than even the chewing demands of the Eskimo. Our hypothesis was that human bite strength of the bruxer-clencher has been underestimated, and some individuals can exceed the bite strength of the Eskimo.

Superior and inferior bellies of the lateral pterygoid muscle EMG activity at basic jaw positions

I n 1961 Kamiyama’ reported reciprocal activity of the superior (SLP) and inferior (ILP) bellies of the lateral pterygoid muscle in 12 human subjects. He found that the SLP was active during closing, retraction, and lateral movement in the ipsilateral direction while the ILP was active during opening, protrusion, and lateral movement in the contralateral direction. McNamara2 demonstrated in monkeys that the ILP acts synergistically with the suprahyoid muscles in mouth opening while the SLP is active during mouth closing. Lipke et a1.3 reported in 1977 that electromyographic (EMG) studies in 10 human subjects revealed independent activity of the two bellies of the lateral pterygoid muscle. However, recent reports by Lehr and Owens4 and Auf der Mau? have claimed that separate roles for the ILP and SLP in humans could not be supported electromyographically. Miller and Vargervik” and Mahan et al.’ reported three different EMG patterns from the right lateral pterygoid muscle. The objective of the present study was to record simultaneous EMG activity in the right SLP and ILP and to determine the response of each belly of the muscle during clenching of the teeth and at basic mandibular positions. Fig. 1. Parasagittal section through left TMJ bisecting superior belly of lateral pterygoid muscle. A = Mandibular condyle; B = articular eminence; C = inferior belly of lateral pterygoid muscle (ILP); D = superior belly of lateral pterygoid muscle (SLP); E = fascia and fat lateral to ILP; F = temporal muscle fibers; G = buccal fat pad; and H = maxillary antrum.

Ratings of experimental pain and pain-related negative affect predict clinical pain in patients with fibromyalgia syndrome

Patients with fibromyalgia syndrome (FMS) report chronic pain related to abnormal sensitivity of muscles that is reflected by so‐called tender points (TP). TP represent areas of abnormal mechanical pain thresholds that have only shown a minor correlation with clinical pain of FMS patients and seem to be better suited for predicting distress. Pain‐related negative affect (PRNA), abnormal temporal summation of second pain (termed wind‐up or WU), and abnormal WU decay are frequently present in FMS patients. WU and WU decay can provide measures of central sensitization, which may contribute to clinical FMS pain. We therefore investigated the role of WU, WU decay, TP count, and PRNA as predictors of clinical pain in FMS subjects. Fifty‐five FMS subjects rated their clinical pain at entry into the study using a visual analogue scale (VAS). After a TP evaluation, all subjects received two trials of thermal WU and WU decay testing. Hierarchical regression analysis demonstrated that the combination of PRNA ratings, TP count, and WU decay ratings predicted 49.7% of the variance of clinical pain in FMS. This model demonstrates independent relationships of biological and psychological factors to clinical pain and underscores the important role of abnormal peripheral and central pain mechanisms for FMS. Therefore, the combination of PRNA, TP count, and WU decay may provide an excellent measure for future clinical studies of FMS patients.

Maintenance of windup of second pain requires less frequent stimulation in fibromyalgia patients compared to normal controls

&NA; Many chronic pain syndromes, including fibromyalgia (FM), show evidence of central nervous system hyperexcitability related to central sensitization. Windup (WU) of second pain reflects increased excitability of spinal cord neurons that is related to central sensitization. Psychophysical testing can help characterize this important central nervous system phenomenon because of the parallels between electrophysiological WU and WU of second pain. Animal experiments have shown that once WU has been established, only low frequency tonic nociceptive input is required to maintain the sensitized state of dorsal horn neurons (WU‐maintenance or WU‐M). The stimulus frequency necessary to maintain the hyperexcitability of spinal cord neurons can provide a measure of central sensitization. Because central sensitization plays an important role in many chronic pain syndromes including FM, we compared WU‐M in 72 normal controls (NC) and 104 FM subjects. WU of second pain was produced by a train of 0.7 s duration thermal pulses applied to the glabrous surface of the hands at a frequency of 0.3 Hz. Enhanced second pain associated with WU could, thereafter, be maintained in FM but not NC subjects for up to 120 s by stimuli delivered at 0.16 and 0.08 Hz (WU‐M stimuli). These two frequencies of stimulation do not produce WU when delivered alone. Thus, unlike NC subjects, FM subjects showed enhanced second pain during WU‐M stimuli at very low stimulus frequencies, indicating central sensitization. Increased WU sensitivity, enhanced WU‐M, and increased WU‐related aftersensations help account for persistent pain conditions in FM subjects. In addition to WU, WU‐M appears to be a useful tool to study mechanisms of pain in patients with characteristics of central sensitization.

EMG activity of the superior belly of the lateral pterygoid muscle in relation to other jaw muscles

A lthough there have been a number of studies that involved the function of jaw muscles, due to its deep placement, the action of the superior belly of the lateral pterygoid (SLP) muscle has not been studied widely. The electromyographic (EMG) activities of the superior and inferior bellies of the lateral pterygoid muscles have been shown to be reciprocal in humans’-‘,+ and rhesus monkeys. *-lo The SLP muscle is of particular interest because it attaches to the anterior border of the mandibular disk anterior to the point where it inserts at from the pterygoid fovea. Activity of this muscle in relation to the function of other jaw muscles seems to be important in mandibular condyle-disk discoordination and in temporomandibular joint (TMJ) and muscle pain. A previous article from our laboratory’ described the EMG activities of the superior and inferior bellies of the lateral pterygoid muscles during basic jaw positions and clenching the teeth. The present article describes the activities of SLP and the inferior belly of the lateral pterygoid (ILP) in relation to masseter, temporal, anterior belly of the digastric, and medial pterygoid muscles during some basic jaw positions and movements. Understanding the relationship of muscle functions during basic jaw movements is an important step in understanding muscle function during preconscious chewing, swallowing, and speech.

Lack of endogenous modulation and reduced decay of prolonged heat pain in older adults

&NA; This study supports the hypothesis that healthy older adults exhibit decreased endogenous pain inhibition compared to younger healthy controls. Twenty‐two older adults (56–77 years of age) and 27 controls aged 20–49 participated in five experimental sessions following a training session. Each experimental session consisted of five 60‐s trials in which the experimental heat stimulus was presented to the thenar eminence of the left palm with or without a conditioning stimulus (cold‐water immersion of the foot). The temperature for the palm (44–49 °C) and foot (8–16 °C) was customized for each subject. The intensity of experimental pain produced by the contact thermode was continuously measured during the 60‐s trial with an electronic visual analogue scale. No significant associations were found between subjects rating of concentration and the overall inhibitory effect. Older subjects failed to demonstrate conditioned pain modulation (CPM) and showed facilitation in the trials using painful concurrent immersion of the foot. A novel aspect of the study was that we recorded “pain offset” (i.e., after‐sensations) and found that ratings for the older sample decreased at a slower rate than observed for the group of younger adults suggesting increased central sensitization among the older sample. Decrements in CPM could contribute to the greater prevalence of pain in older age. Since a number of neurotransmitter systems are involved in pain modulation, it is possible age‐related differences in CPM are due to functional changes in these systems in a number of areas within the neuroaxis.