Intramuscular Injection Of Hypertonic Saline Research Articles

Nociceptive Craniofacial Muscle Primary Afferent Neurons Synapse in Both the Rostral and Caudal Brain Stem

Limited information is available on muscle afferent neurons with fine fibers despite their presumed participation in musculoskeletal disorders, including temporomandibular disorders. To study these neurons, intracellular recordings were made from the central axons of slowly conducting muscle afferent neurons in anesthetized rats. After intraaxonal impalement, axons were characterized by masseter nerve stimulation, receptive field testing, muscle stretching and intramuscular injection of hypertonic saline. Intracellular recordings were made from 310 axons (conduction velocity: 6.5-60(M)/s, mean = 27.3(M)/s; following frequency: 27-250 Hz, mean = 110Hz). No neurons responded to cutaneous palpation or muscle stretching. Some axons (n = 34) were intracellularly stained with biotinamide. These neurons were classified as group II/III noxious mechanoreceptors because their mechanical threshold exceeded 15 mN, and conduction velocities ranged from 12 to 40.2(M)/s (mean = 25.3(M)/s). Two morphological types were recognized by using an object-based, three-dimensional colocalization methodology to locate synapses. One type (IIIHTM(Vp-Vc)) possessed axon collaterals that emerged along the entire main axon and synapsed in the trigeminal principal sensory nucleus and spinal trigeminal subnuclei oralis (Vo), interpolaris (Vi), and caudalis (Vc). A second type (IIIHTM(Vo-Vc)) possessed axon collaterals that synapsed only in caudal Vo, Vi, and Vc. Our previous studies show that muscle spindle afferent neurons are activated by innocuous stimuli and synapse in the rostral and caudal brain stem; here we demonstrate that nociceptive muscle mechanoreceptor afferent axons also synapse in rostral and caudal brain stem regions. Traditional dogma asserts that the most rostral trigeminal sensory complex exclusively processes innocuous somatosensory information, whereas caudal portions receive nociceptive sensory input; the data reported here do not support this paradigm.

Experimental quadriceps muscle pain impairs knee joint control during walking

Pain is a cardinal symptom in musculoskeletal diseases involving the knee joint, and aberrant movement patterns and motor control strategies are often present in these patients. However, the underlying neuromuscular mechanisms linking pain to movement and motor control are unclear. To investigate the functional significance of muscle pain on knee joint control during walking, three-dimensional gait analyses were performed before, during, and after experimentally induced muscle pain by means of intramuscular injections of hypertonic saline (5.8%) into vastus medialis (VM) muscle of 20 healthy subjects. Isotonic saline (0.9%) was used as control. Surface electromyography (EMG) recordings of VM, vastus lateralis (VL), biceps femoris, and semitendinosus muscles were synchronized with the gait analyses. During experimental muscle pain, the loading response phase peak knee extensor moments were attenuated, and EMG activity in the VM and VL muscles was reduced. Compressive forces, adduction moments, knee joint kinematics, and hamstring EMG activity were unaffected by pain. Interestingly, the observed changes persisted when the pain had vanished. The results demonstrate that muscle pain modulated the function of the quadriceps muscle, resulting in impaired knee joint control and joint instability during walking. The changes are similar to those observed in patients with knee pain. The loss of joint control during and after pain may leave the knee joint prone to injury and potentially participate in the chronicity of musculoskeletal problems, and it may have clinically important implications for rehabilitation and training of patients with knee pain of musculoskeletal origin.

Intramuscular Injection of Hypertonic Saline: In vitro and in vivo Muscle Tissue Toxicity and Spinal Neurone c‐fos Expression

Intramuscular injection of hypertonic saline (4-6% NaCl) is widely used to induce muscle pain in volunteers. The quality of the pain is comparable to clinical muscle pain with localised and referred pain. The objective was to evaluate the muscle toxicity of hypertonic saline by characterisation of 1) cytotoxicity in vitro, 2) local muscle toxicity in rabbits and 3) number of spinal dorsal horn neurones expressing c-fos after intramuscular injection in pigs as an indicator of nociception. Rat myocyte cultures and erythrocyte suspensions were treated with hypertonic NaCl solutions. The creatine kinase activity remaining in the myocytes and haemolysis were measured. Groups of six rabbits were given an intramuscular injection of 0.5 ml of 0.9, 3 or 6% NaCl. Three days later, creatine kinase activity was determined in injection site muscle tissue and normal contralateral muscle. The amount of injection site muscle tissue totally depleted of creatine kinase was calculated. Groups of two pigs were given an intramuscular injection of 3.0 ml of 6% NaCl. The spinal cord was sampled 1, 2 or 3 hr later and processed for stereological quantification of the number of dorsal horn neurones expressing c-fos. Saline was not toxic in vitro at 0.9-6%, but toxic to erythrocytes at 7% or higher and rat myocytes at 15% or higher. No muscle toxicity was seen in rabbits. The number of dorsal horn neurones expressing c-fos was not above basal level. In conclusion, 6% saline caused no in vitro or in vivo toxicity in sensitive models. Consequently, the pain caused by intramuscular injection of hypertonic saline is most likely not related to tissue damage. Consistently, intramuscular injection of 6% NaCl did not activate dorsal horn neurones in pigs to express c-fos beyond basal level.

Experimental hand pain delays recognition of the contralateral hand—Evidence that acute and chronic pain have opposite effects on information processing?

Recognising the laterality of a pictured hand involves making an initial decision and confirming that choice by mentally moving one's own hand to match the picture. This depends on an intact body schema. Because patients with complex regional pain syndrome type 1 (CRPS1) take longer to recognise a hand's laterality when it corresponds to their affected hand, it has been proposed that nociceptive input disrupts the body schema. However, chronic pain is associated with physiological and psychosocial complexities that may also explain the results. In three studies, we investigated whether the effect is simply due to nociceptive input. Study one evaluated the temporal and perceptual characteristics of acute hand pain elicited by intramuscular injection of hypertonic saline into the thenar eminence. In studies two and three, subjects performed a hand laterality recognition task before, during, and after acute experimental hand pain, and experimental elbow pain, respectively. During hand pain and during elbow pain, when the laterality of the pictured hand corresponded to the painful side, there was no effect on response time (RT). That suggests that nociceptive input alone is not sufficient to disrupt the working body schema. Conversely to patients with CRPS1, when the laterality of the pictured hand corresponded to the non-painful hand, RT increased ∼380 ms (95% confidence interval 190 ms–590 ms). The results highlight the differences between acute and chronic pain and may reflect a bias in information processing in acute pain toward the affected part.

Expectation of pain replicates the effect of pain in a hand laterality recognition task: Bias in information processing toward the painful side?

Background People in pain, or expecting pain, sometimes bias their attention towards pain-relevant cues. Perhaps they also bias their attention towards the body part in question. Aim To determine if experimentally induced pain, and the expectation of pain, involve an information processing bias towards the hand in question. Methods Seventeen asymptomatic subjects performed a hand laterality recognition task during three conditions: control, during hand pain induced by intramuscular injection of hypertonic saline (pain), and during expectation of hand pain, induced by isotonic saline injection (expectation). Mean response time (RT) was determined for three 45 s epochs within each condition and RT was compared between hands, conditions and epochs using a 2 × 3 × 3 repeated measures multivariate analysis of variance. Results There was a hand × condition interaction and a hand × condition × epoch interaction ( p < 0.05 for both). RT to recognise the opposite hand was ∼600 ms longer during epochs when subjects were in pain or expected pain than during control trials. During those epochs, RT to recognise the opposite hand was ∼600 ms longer than RT to recognise the injected hand, which was consistent across conditions and across epochs. Conclusions Both pain and the expectation of pain increased RT to recognise the opposite hand. The findings are consistent with a bias in information processing toward the painful or impending painful hand.

Psychophysical and EEG responses to repeated experimental muscle pain in humans: Pain intensity encodes EEG activity

Clinical pain is often characterized by repetitive and persistent occurrence in deep structures, but few studies investigated repetitive tonic pain in humans. To determine cerebral responses to repetitive tonic pain, psychophysical responses, and electroencephalographic (EEG) activation to five trials of repeated tonic muscle pain induced by hypertonic saline were examined and analyzed in 13 male subjects. The study was composed of two experimental sessions performed in separate days. Five sequential injections of hypertonic saline (5.8%) were used to induce repeated muscle pain in the left forearm, and five sequential injections of isotonic saline (0.9%) acted as control. Visual analogue scales (VAS) for pain intensity and 32-channels EEG activities were recorded simultaneously. Five trials of relatively stable muscle pain were induced by intramuscular injections of hypertonic saline, but no evident pain was induced by the injections of isotonic saline. Significant decreases in alpha-1 and -2 activities in posterior part of the head were found during repeated muscle pain in comparison with non-pain. In comparison with baseline, alpha-1 and -2 activities reduced significantly during the first two trials, and gradually resumed in the following three trials of muscle pain. However, beta-2 activity increased consistently throughout the five trials of muscle pain compared to baseline. Alpha-1 activity was negatively, but beta-2 activity was positively correlated to the pain intensity and pain area on the skin. Throughout five injections, the reduction of alpha-1 activity was contrary to the changes of pain intensity. These results indicates that pain-related EEG activities were encoded by the pain intensity. The thalamo-cortical system and descending inhibitory neuronal networks may be involved in the regulation of pain intensity.

Changes in human regional cerebral blood flow following hypertonic saline induced experimental muscle pain: a positron emission tomography study

A positron emission tomography imaging study was performed on 16 healthy volunteers to reveal changes in cortical activation during acute muscle pain induced by intra-muscular injection of hypertonic saline into the left triceps brachii muscle. Changes in regional cerebral blood flow (rCBF) were measured with the use of [ 15O] labelled water during ‘Rest1’, ‘Needle’ (insertion of a needle without injection), ‘Rest2’ and ‘Pain’ conditions. Differences in rCBF were found in the comparison of Pain and Needle, and Pain and Rest2 conditions, revealing activation of the contralateral insula and putamen. The results are discussed with respect to possible differences in brain processing of muscle and cutaneous noxious inputs.

Effects on the fusimotor-muscle spindle system induced by intramuscular injections of hypertonic saline.

Intramuscular injection of hypertonic saline (HS) is a procedure widely adopted to experimentally induce deep muscle pain in humans. This study was undertaken to test whether intramuscular injections of HS (5%) influence the activity of primary and secondary muscle spindle afferents (MSAs) from homonymous as well as heteronymous muscles. The experiments were performed on six cats anaesthetised with alpha-chloralose. Usually responses of two to nine MSAs from gastrocnemius medialis (GM) and/or gastrocnemius lateralis (GL) muscles were recorded simultaneously, while HS was injected either into the receptor-bearing muscle (homonymous responses) or into a close (GM/GL) or remote synergistic muscle (posterior biceps, PB, heteronymous responses). The mean rate of discharge and the depth of modulation of the MSA responses to sinusoidal stretching of the receptor-bearing muscle were calculated. Out of the 42 afferents tested (7 from GM and 35 from GL), 38 (90%) exhibited statistically significant responses to injections of HS into homonymous and/or heteronymous muscles. With injections into the homonymous muscle, the average maximal increase in mean rate of discharge was 74% and the average decrease in depth of modulation was --18%. The mean duration of the effects was 2.1 min. The corresponding values for heteronymous injections into a close synergist were 87%, -17% and 2.1 min (GM or GL), and for injections into PB 52%, -11%, and 1.8 min. The majority of the responses (72%) were compatible with reflex action on static fusimotor neurones, whereas 20% of the responses could be attributed to mixed static and dynamic fusimotor action. The remaining 8% of the responses were attributed to inhibition of fusimotor activity. There were no statistically significant differences between the responses following injections into homonymous or heteronymous muscles. Injections of Tyrode's solution did not induce any significant alterations in MSA responses, implying that they were not induced by direct and/or injury effects of the injections. HS-related changes in MSA activity were completely abolished after the nerves to corresponding muscles were cut, confirming the reflex nature of the effects. Thus, intramuscular injections of HS induce reflex changes in MSA activity from both homonymous and heteronymous muscles, most likely via fusimotor reflexes. Predominantly static fusimotor neurones were activated. The possible role of the fusimotor-muscle spindle system in altered motor control during experimentally induced muscle pain is discussed.

Injection of hypertonic saline into musculus infraspinatus resulted in referred pain and sensory disturbances in the ipsilateral upper arm

A confounding factor in the analysis of chronic pain patients is the finding of somatosensory disturbances not only in neuropathic pain patients, but also in a subgroup of patients with musculoskeletal pain. The purpose of the study was to examine if referred pain, induced by intramuscular injections of hypertonic saline (5% NaCl) into the left musculus infraspinatus, resulted in somatosensory alterations. Thermal sensitivity, pressure pain sensitivity, as well as low threshold mechanoreceptive function, were assessed in the referred pain area and the homologous contralateral site before, during and following the injections. In 10 out of 12 subjects the procedure induced only referred pain localized in the dorsolateral part of the ipsilateral proximal upper arm. In this referred pain area there was a significantly decreased sensitivity to light touch, as tested with von Frey filaments, during the pain period and the post-injection period compared to the contralateral side (p< 0.004 and p< 0.009, respectively). A trend for thermal hypoaesthesia, which was only demonstrable in the sum of warm and cold thresholds, was found in the referred pain area, but not contralaterally, during the pain period compared to the pre-injection period. Significantly increased sensitivity to threshold and suprathreshold heat pain was found bilaterally during post-injection assessments (p< 0.02 and p< 0.006, respectively). There were no statistically significant changes in sensitivity to innocuous thermal stimuli when assessing the two percepts separately, or to pressure pain or brush-evoked touch. In conclusion, intramuscular injections of hypertonic saline resulted in referred pain and tactile hypoaesthesia in the referred pain area. Copyright 2000 European Federation of Chapters of the International Association for the Study of Pain

Interaction between cutaneous and muscle afferent activity in polysynaptic reflex pathways: a human experimental study

Interactions between the input from cutaneous and nociceptive muscle afferents in polysynaptic reflex pathways were investigated in man. Interaction was tested by evoking reflexes before, during, and after a period of muscle pain induced by intramuscular injection of hypertonic saline. Muscle pain was induced either in the ankle flexor (tibialis anterior, TA) or in the extensor (soleus, SOL) muscles by injection of 1 ml hypertonic saline. Electrical skin stimulation (1.1×initial reflex threshold) at the dorsum of the foot over the tarsal joint was used to elicit cutaneo-muscular polysynaptic reflexes in the knee flexor semitendinosus (ST). The injected hypertonic saline evoked a robust muscle pain (the subjects made a continuous score of the muscle pain on a 0–10 cm VAS scale, and the mean VAS area was 1229±251 cm×s and lasting 390±30 s). In five of 12 subjects, the infusion of hypertonic saline into TA evoked referred pain to the dorsal aspect of the ankle. A significant inhibition (17±8.2%, P<0.05) of the ST-reflex by pain in SOL was observed. Pain in TA facilitated (92±36%, P<0.05) the short-latency part (50–70 ms post stimulation) of the reflex. The muscle pain did not modulate the perceived sensory intensity of the electrical stimuli. The findings indicate an interaction of input from thin muscle afferents and cutaneous group A-fibre afferents in polysynaptic segmental reflex pathways, which seems to depend on the location of the muscle pain.

Sensory manifestations in experimental and work-related chronic neck-shoulder pain

Little is known about the mechanisms leading to chronic neck-shoulder musculo-skeletal disorders. The aim of the present study was to investigate and compare the sensory manifestations of experimental or chronic neck-shoulder pain under controlled, low load, repetitive work. Experimental and clinical experiments were carried out. In Experiment 1, experimental muscle pain was induced in healthy subjects by intramuscular injection of hypertonic saline in the trapezius ( n=10) and infraspinatus ( n =10) muscles. Experiment 2 was performed on 18 workers with ( n = 12) and without ( n = 6) chronic neck-shoulder pain. Sensory assessments were performed before and/or after a session of controlled, low load, repetitive work. The pain intensity was assessed by a visual analogue scale. The pain quality and location(s) were monitored together with pressure-pain thresholds. Moderate-to-strong deep pain intensity was experienced in the experimental and clinical part of the working session (six patients out of 12). The verbal descriptors used to describe the experimental or chronic pain qualities were partially similar, i.e. first of all verbal descriptors from the sensory class. The patterns of local and referred pain characteristics were similar (around the shoulder girdle), but it is difficult to evoke pain in extensive areas with hypertonic saline. Patients exhibited hypersensitivity or tenderness to pressure stimuli in both sides of the upper body, i.e. in the trapezius, supraspinatus, infraspinatus and pectoralis major muscles ( p<0.05). This hypersensitivity was only seen at the referred pain site in the healthy volunteers exposed to infraspinatus experimental muscle pain. In conclusion, the present study demonstrated similar sensory manifestations in the experimental and clinical neck-shoulder pain experiments. The presented experimental model can most likely be used to investigate the underlying pain mechanisms involved in work-related chronic musculoskeletal pain.

In vivo model of muscle pain: Quantification of intramuscular chemical, electrical, and pressure changes associated with saline-induced muscle pain in humans

Intramuscular injection of hypertonic saline is a good model to study human muscle pain (Kellgren 1938). The present study concerns the intramuscular (i.m.) pain mediators in saline-induced muscle pain. In experiment 1, the diffusion of infused hypertonic and isotonic saline (0.5 ml) in m. tibialis anterior was illustrated by magnetic resonance imaging (MRI) in one subject. In experiment 2, six volunteers received four sequential infusions (0.5 ml given at 5 min intervals) of isotonic saline and thereafter four sequential infusions (0.5 ml given at 5 min intervals) of hypertonic saline into m. tibialis anterior. The isotonic and hypertonic saline infusions were computer-controlled and separated by 20 min. The muscle pain intensity was assessed by continuous recordings on a visual analogue scale (VAS). One microdialysis probe was inserted 1 cm from the infusion needle in m. tibialis anterior and another probe in the other m. tibialis anterior. Concentrations of the i.m. sodium, potassium, magnesium, and prostaglandin E 2 (PGE 2) were assessed from the dialysates. Intramuscular electromyography (EMG) and pressure were assessed in the area of the infused saline. In experiment 1, the infusion of hypertonic and isotonic saline created a visible saline-pool on the MRI scans. These saline-pool volumes were stable and not correlated to the pain scores. In experiment 2, infusion of isotonic saline produced little pain compared to infusion of hypertonic saline. Maximal pain was reported after the first infusion of hypertonic saline and thereafter the pain gradually decreased with subsequent infusions of hypertonic saline. During infusion of hypertonic saline the i.m. sodium and potassium concentrations increased significantly, i.m. magnesium concentration tended to be increased, and the i.m. PGE 2 concentration tended to be decreased although these changes were not significant. The i.m. EMG was smaller during and after infusions of hypertonic saline compared with isotonic saline. The i.m. pressure was not different during the infusions of hypertonic and isotonic saline but was increased between the infusions of hypertonic saline. This study has shown that i.m. infusion of hypertonic saline produced a saline-pool, causing the i.m. pressure to increase. Possibly, pain activation and cessation are related to increased intramuscular sodium and potassium content respectively.

In vitro myogenesis of the mononucleate cells derived from regenerating muscles of adult mice

Regeneration was induced in the matured skeletal muscles of adult mice by intramuscular injection of hypertonic saline. After passing through the degeneration phase, the regeneration of muscle took place and proceeded with considerable synchrony. Regenerating myogenic cells were isolated from the saline-treated muscles by cold trypsinization with the aid of Yaffe's selective plating procedure [Yaffe, D. (1968) Proc. Nat. Acad. Sci. USA 61, 477–483]. The myogenic cells thus isolated readily developed into muscle fibers possessing cross striations and the ability to contract when cultured in vitro.