Neurophysiology of gastrointestinal pain

Colonoscopy as an adjunctive method for the diagnosis of irritable bowel syndrome: focus on pain perception

BACKGROUND AND AIM

Visceral hypersensitivity is an important component of the pathophysiology of irritable bowel syndrome (IBS). In the present study, we investigated differences in pain perception during colonoscopy between IBS patients and non-IBS patients. We further assessed the sensitivity, specificity, and predictive values of pain scores to diagnose IBS.

METHODS

Patients who underwent colonoscopy for the evaluation of gastrointestinal symptoms or for screening purposes were included. All patients completed Rome III criteria questionnaires and reported pain scores on 0-100-mm visual analog scales after colonoscopy. The patients were divided into three groups: (i) IBS; (ii) other functional gastrointestinal disorders (FGID), including functional bloating, functional diarrhea, and functional constipation; and (iii) healthy controls.

RESULTS

A total of 217 patients were included. The pain scores (median, interquartile range) of IBS patients (52, 34-71) were higher than those of the healthy controls (22, 12-35) or other FGID patients (18, 10-29) (P < 0.001). Upper gastrointestinal symptoms were observed more often in the IBS group than in the non-IBS group (83.2% vs 34.5%, P < 0.001). At the pain score level of 31, the sensitivity, specificity, positive predictive value, and negative predictive value for IBS diagnosis were 86.1%, 75.9%, 75.7%, and 86.3%, respectively.

CONCLUSIONS

The degree of pain perception during colonoscopy was higher in IBS patients than in non-IBS patients. We concluded that colonoscopy can be useful in identifying IBS patients, with the additional benefit of excluding organic disorders of the lower gastrointestinal tract.

Cystitis increases colorectal afferent sensitivity in the mouse

Studies in humans and rodents suggest that colon inflammation promotes urinary bladder hypersensitivity and, conversely, that cystitis contributes to colon hypersensitivity, events referred to as cross-organ sensitization. To investigate a potential peripheral mechanism, we examined whether cystitis alters the sensitivity of pelvic nerve colorectal afferents. Male C57BL/6 mice were treated with cyclophosphamide (CYP) or saline, and the mechanosensitive properties of single afferent fibers innervating the colorectum were studied with an in vitro preparation. In addition, mechanosensitive receptive endings were exposed to an inflammatory soup (IS) to study sensitization. Urinary bladder mechanosensitive afferents were also tested. We found that baseline responses of stretch-sensitive colorectal afferents did not differ between treatment groups. Whereas IS excited a proportion of colorectal afferents CYP treatment did not alter the magnitude of this response. However, the number of stretch-sensitive fibers excited by IS was increased relative to saline-treated mice. Responses to IS were not altered by CYP treatment, but the proportion of IS-responsive fibers was increased relative to saline-treated mice. In bladder, IS application increased responses of muscular afferents to stretch, although no differences were detected between saline- and CYP-treated mice. In contrast, their chemosensitivity to IS was decreased in the CYP-treated group. Histological examination revealed no changes in colorectum and modest edema and infiltration in the urinary bladder of CYP-treated mice. In conclusion, CYP treatment increased mechanical sensitivity of colorectal muscular afferents and increased the proportion of chemosensitive colorectal afferents. These data support a peripheral contribution to cross-organ sensitization of pelvic organs.

Thyrotoxicosis and acute abdomen--still as defying and misunderstood today? Brief observations over the recent decade

BACKGROUND

Clinicians managing thyrotoxic patients with acute abdomen face challenging diagnostic and risky therapeutic dilemmas.

AIM

To analyse the frequency of medical vs. surgical acute abdomen, and to characterize the poorly understood thyrotoxic medical acute abdomen phenomenon.

DESIGN

Retrospective review of case notes.

METHODS

All case files with a simultaneous diagnosis of thyrotoxicosis and acute abdomen admitted between 1994 and 2004 were traced and audited.

RESULTS

Thirteen had a history of thyrotoxicosis while 12 were newly diagnosed. The commonest cause was Graves' disease. Twenty-three (92%) cases were thyrotoxic, of whom six (24%) had thyroid crisis, while two (8%) had subclinical thyrotoxicosis. The provisional diagnosis of acute abdomen was correct in 14 cases (56%), but discordant with the final diagnosis in 11 cases (44%). Eight cases (32%) without any demonstrable pathology were medical, vs. four (16%) with surgical acute abdomen, while 11(44%) had gastritis, hepatobiliary-pancreatic disorders or diverticulitis conservatively managed. The epigastrium and/or central abdomen (72.7%) were the commonest affected regions in medical acute abdomen.

CONCLUSION

Although the majority of acute abdomen in thyrotoxicosis was medical in nature, our experience indicates that surgical conditions were not uncommon. Thus, serious causes requiring life-saving surgery should be excluded before attributing it to medical acute abdomen.

Managing nonmalignant chronic abdominal pain and malignant bowel obstruction

Evaluation of abdominal pain requires an understanding of the possible causes(benign or malignant) and recognition of typical patterns and clinical presentation. Abdominal pain has multiple causes; associated signs and symptoms may aid in the diagnosis. Remember that some patients will not have a textbook presentation, and unusual causes for pain must be considered. Those with chronic pancreatitis with structural complications should be operated on early, whereas those with other types of chronic pancreatitis should receive medical therapy focusing on alleviating symptoms. Control of the most troublesome symptoms will provide the best management for IBS. Pharmacologic success in bowel obstruction depends on the level and degree of obstruction. Decision making is based on reasonable expectations of survival, treatment-related success, performance status, and goals of care. Quality of life will be enhanced by appropriate symptom management.

Antinociceptive and behavioral effects of ribavirin in mice

The antinociceptive effect of ribavirin, an antiviral drug, was studied after systemic injection using several pain tests in mice. In the hot-plate test of thermal pain, capsaicin-induced chemogenic pain, formalin test and abdominal stretching assay induced by the i.p. injection of 0.6% acetic acid, ribavirin produced a dose-related reduction in nociceptive responses. The visceral antinociceptive effect of ribavirin was unaffected by co-treatment with yohimbine, atropine or theophylline, but partially reversed by naloxone. Antinociception by ribavirin was augmented by treatment with prazosin, doxazosin, propranolol, guanethidine, glibenclamide, baclofen, indomethacin or cysteamine. Further, the ribavirin induced antinociception was enhanced by D2 receptor antagonists haloperidol, sulpiride, clozapine or domperidone and by the dopamine D2 receptor agonist bromocryptine. Ribavirin did not exhibit depression-like effect, nor it influenced the effect of amitriptyline in the forced swimming test. It did not impair cognitive performance in the Morris water Maze test. The present data demonstrate that ribavirin administered via systemic route possesses visceral and thermal anti-nociceptive properties. The ribavirin analgesic effect was partially reversed by naloxone, an opioid antagonist.

Temporal progression of viral replication and gross and histological lesions in Balb/c mice inoculated epidermally with Saimiriine herpesvirus 1 (SaHV-1)

Saimiriine herpesvirus 1 (SaHV-1), an alphaherpesvirus enzootic in squirrel monkeys, is genetically related to monkey B virus and human herpes simplex virus (HSV). To study the temporal progression of viral spread and associated lesions, Balb/c mice were inoculated epidermally by scarification with a green fluorescent protein (GFP)-expressing recombinant strain of SaHV-1 and killed sequentially. Pinpoint ulcerative lesions in the inoculated epidermis progressed over a few days to unilateral or bilateral hindlimb paresis or paralysis, urinary and faecal incontinence, abdominal distension, hunched posture and eventual depression warranting euthanasia. Viral replication was present within epidermal keratinocytes, neurons of the dorsal root ganglia and thoracolumbar spinal cord, regional autonomic ganglia, lower urinary tract epithelium and colonic myenteric plexuses, as indicated by histological lesions and GFP expression. Almost all mice inoculated with 10(5) or 10(6) plaque-forming units (PFU) of SaHV-1 developed rapidly progressive disease. Two of eight mice given 10(4)PFU developed disease, but no mice receiving less than 10(4)PFU gave evidence of infection. Mice that showed no clinical signs also failed to develop an antiviral IgG response, indicating absence of active viral infection. For SaHV-1 inoculated epidermally, the ID(50), CNSD(50) and LD(50) values were identical (10(4.38)), indicating that successful infection by this route invariably resulted in lethal CNS (central nervous system) disease. Consistently severe disease in all infected animals, with regionally extensive distribution of viral replication, constituted a marked difference from the disease produced by intramuscular inoculation.

Abdominaler Schmerz

Abdominal pain can result from a variety of different intra- and extra-abdominal disorders. Given the wide variety of etiological triggers for this pain, the primary task during the first stage of the diagnostic work-up is to determine as soon as possible the underlying cause and the degree of emergency. The aim of this evaluation is to adapt the therapeutic measures which are necessary for a causal treatment to the individual situation. Contrary to somatic causes of abdominal pain, the availability of such a causal therapy for functional bowel disorders is still very limited. Given this dilemma, the therapeutic focus of abdominal pain associated with these functional syndromes has to be placed on symptom-oriented treatment.

The sequential analysis of local inflammatory cells during abomasal nematode infection in periparturient sheep

A technique to take sequential tissue biopsy samples in multiparous, periparturient ewes from the abomasal mucosa is described, developed in parallel in Scotland and New Zealand. Samples were extracted via abomasal cannulae inserted into the wall of the abomasum and exteriorised through dorso-ventral laparotomy. Animals recovered quickly post-surgery, and tolerated the cannula and sampling without any adverse signs of pain or discomfort. The technique was deployed in two pilot studies to investigate the sequential mucosal inflammatory cell responses in well-defined parasitological models, during the periparturient relaxation of immunity in ewes infected with gastrointestinal nematodes and subjected to different feeding treatments. One experiment (Moredun Research Institute, Scotland) involved the infection of twin-bearing ewes with Teladorsagia circumcincta L3 either before, or after lambing. By feeding ewes with different levels of protein supplementation, preliminary data on the impact of nutrition on the eosinophil, mucosal mast cell and globule leucocyte responses during this period were investigated. A similar study was also performed at Lincoln University, New Zealand, to investigate these cell responses in sheep fed relatively high or low protein diets during pregnancy, and infected with a combined immunisation regime of T. circumcincta and Trichostrongylus colubriformis L3. These studies confirmed the phenomenon termed the periparturient relaxation in immunity (PPRI) where a transitory increase in faecal egg counts is observed during late pregnancy and lactation, and this effect was exacerbated during protein undernutrition. Although the number of animals was low in each experiment and the cell responses variable, the results together suggest a reduction in the number of mucosal mast cells and globule leucocyte during the PPRI when protein supply was restricted. The present paper thus describes a successful technique to monitor ovine mucosal cell populations during local immune responses in normal and pregnant sheep. It is envisaged that this technique will be a powerful adjunct to investigations into mucosal immune mechanisms and disease pathogenesis, and will be employed to confirm the influence of dietary protein on the local inflammatory cell responses during the PPRI.

Distribution of the vanilloid receptor (VR1) in the gastrointestinal tract

The gastrointestinal (GI) tract responds to a variety of stimuli through local and centrally mediated pathways. Changes in the intestinal microenvironment are sensed by vagal, spinal, and intrinsic primary afferent fibers. Sensory nerve endings located close to the lumen of the GI tract respond to pH, chemical composition of lumenal contents, or distortion of the mucosa. Afferents within the muscle layers are thought to be tension sensitive, whereas those located within the myenteric plexus are also thought to respond to changes in chemical composition and humoral substances. Subpopulations of these afferent fibers are activated by capsaicin. However, the exact location of these nerves is currently not known. The vanilloid receptor (VR1) is a nonselective cation channel that is activated by capsaicin, acid, and temperature. Antibodies to VR1 make it possible to determine the location of these afferents, their morphology, and their relationships with enteric nerves and other cell types in the GI tract. VR1-like immunoreactivity was observed on nerves within myenteric ganglia and interganglionic fiber tracts throughout the GI tract. VR1 nerves were also observed within the muscle layers and had an irregular profile, with varicose-like swellings along their lengths. Blood vessels within the GI wall had VR1-immunoreactive nerve fibers associated with them. VR1-like nerves and other immunopositive cells were also observed within the mucosa. In summary, VR1-like immunoreactivity was found in several locations within the GI tract and may provide sensory integration of chemical, physical, or inflammatory stimuli. VR1-like fibers appear to be predominantly spinal in origin, but a few vagal VR1-like fibers exist in the stomach.

Abdominal pain during pregnancy

Numerous medical, surgical, psychiatric, gynecologic, and obstetric disorders can cause abdominal pain during pregnancy. The patient history, physical examination, laboratory data, and radiologic findings usually provide the diagnosis. The pregnant woman has physiologic alterations that affect the clinical presentation, including atypical normative laboratory values. Abdominal ultrasound is generally the recommended radiologic imaging modality; roentgenograms are generally contraindicated during pregnancy because of radiation teratogenicity. Concerns about the fetus limit the pharmacotherapy. Maternal and fetal survival have recently increased in many life-threatening conditions, such as ectopic pregnancy, appendicitis, and eclampsia, because of improved diagnostic technology, better maternal and fetal monitoring, improved laparoscopic technology, and earlier therapy.

[Physiology of nociception]

Nociception is related to the mechanisms elicited by stimuli threatening the integrity of the organism. At the peripheral level, unmyelinated C fibres (C polymodal nociceptores) or fine myelinated A delta fibres are excited by noxious stimulation, directly or indirectly by inflammatory processes. Nociceptive afferent fibres terminate in the superficial laminae of the dorsal horn of the spinal cord where informations are integrated and controlled. These first synapses are modulated by excitatory amino acids (glutamate and aspartate) and many peptides (substance P, CGRP, CCK, endogenous opiods). The majority of ascending pathways involved in nociception are located in the ventrolateral controlateral quadrant of the cord (spinorelicular and spinothalamic tracts). Many supraspinal sites are activated following nociceptive stimuli, with relays in the reticular formation of the brain stem (including the subnucleus reticularis dorsalis), the ponto-mesencephalic regions (periaqueducal gray matter and parabrachial area) and thalamic sites. Amygdala and hypothamic targets could be involved in motivational reactions and neuroendocrine adaptations to a noxious event. The cingular, insular and somatosensory cortices also receive nociceptive informations. Nociceptive signals are modulated at all levels of their transmission; the more extensively studied controls are located at the spinal level. Segmental controls are inhibitory effects produced by non-noxious mechanical stimuli. Spinal signals can also be inhibited following activation of bulbopinal descending inhibitor pathways and release of serotonin, norepinephrine and, indirectly, endogenous opiods. Inhibitory controls triggered by noxious stimuli could facilitate the extraction of the nociceptive tone of informations having priority over other stimuli.

Sensitization to painful distention and abnormal sensory perception in the esophagus

BACKGROUND/AIMS

The mechanism of abnormal esophageal sensory perception in patients with unexplained chest pain is unknown. The aim of this study was to test the following two hypotheses: (1) similar to cutaneous nociceptors, esophageal stretch receptors can be sensitized by a noxious chemical stimulus, and (2) patients with esophageal chest pain have sensitized receptors.

METHODS

Twenty-five patients with noncardiac chest pain underwent esophageal manometry, provocation tests, and 24-hour pH monitoring. Eleven patients had positive and 14 had negative results of esophageal provocation tests. Esophageal perception and pain thresholds for balloon stretch (in milliliters) and electrical stimulation (in milliamperes) were determined in patients and 7 healthy controls performed before and after randomized, double-blind esophageal perfusion with normal saline or 0.1N HCl.

RESULTS

The basal balloon pain threshold was lower in patients with positive results of esophageal tests than in either those patients with negative results of esophageal tests or controls (P < 0.05). After acid perfusion, balloon perception and pain thresholds decreased in patients with negative results of esophageal tests (P < 0.05) and in controls (P < 0.05) but not in patients with positive results of esophageal tests. No change occurred after saline perfusion nor in electrical pain thresholds in any group after the perfusion of either fluid.

CONCLUSIONS

In patients with negative results of esophageal tests and in controls, the pain threshold to balloon distention is lowered by acid. The lack of such effect in patients with positive results of esophageal tests implies that pain receptors may already be sensitized.

Pain management: physiopathology, future research and endpoints

In this article, first, the different stages of acquisition and processing of nociceptive information from peripheral receptor to brain are reviewed and the plastic changes that accompany tissue injury are underlined. For instance, the subclassification of peripheral receptors in nociceptors and non-nociceptors (e.g., mechanoreceptors, thermoreceptors) must be understood in the light of peripheral sensitization. This phenomenon is the probable explanation for primary hyperalgesia, the decrease in pain threshold at the site of injury. The observation that substance P enhances N-methyl-D-aspartate (NMDA)-elicited responses suggests that these two receptors may operate in concert to prolong and amplify the afferent input generated by peripheral tissue injury. Such afferent barrage induces a state of central sensitization. Second, the major problems in the management of cancer pain, i.e. the development of tolerance to opioids and opioid-insensitive pain, are discussed. The loss of drug effect observed after chronic exposure of the opioid receptor (tolerance) may be the consequence of the down-regulation or desensitization phenomenon (where the total number of receptors coupled to the second messenger is reduced). The agonist dose-response begins to shift to the right. The dramatic analgesic improvement obtained with subanaesthetic doses of ketamine, an NMDA receptor antagonist, in those of our cancer patients who have become resistant to morphine is intriguing. As shown for tolerance, insensitivity to opioids may represent a rightward shift in the opioid dose-response curve and the analgesic effect of ketamine the reversal of that shift.