Cortical and spinal assessment - a comparative study using encephalography and the nociceptive withdrawal reflex

Spinal Excitability in Patients with Painful Chronic Pancreatitis

Purpose

Abdominal pain is common in patients with chronic pancreatitis (CP), but management is challenging - possibly due to altered pain processing within the central nervous system rendering conventional treatments ineffective. We hypothesized that many patients with painful CP have generalized hyperalgesia correlating with central neuronal hyperexcitability.

Patients and Methods

Seventeen CP patients with pain and 20 matched healthy controls underwent experimental pain testing, including repeated pain stimuli (temporal summation), pressure algometry performed in dermatomes with same spinal innervation as the pancreatic gland (pancreatic areas) and remote dermatomes (control areas), a cold pressor test and a conditioned pain modulation paradigm. To probe central neuronal excitability, the nociceptive withdrawal reflex was elicited by electrical stimulation of the plantar skin, and electromyography was obtained from the ipsilateral anterior tibial muscle together with somatosensory evoked brain potentials.

Results

Compared to healthy controls, patients with painful CP had generalized hyperalgesia as evidenced by 45% lower pressure pain detection thresholds (P<0.05) and decreased cold pressor endurance time (120 vs 180 seconds, P<0.001). In patients, reflex thresholds were lower (14 vs 23 mA, P=0.02), and electromyographic responses were increased (16.4 vs 9.7, P=0.04) during the withdrawal reflex, reflecting predominantly spinal hyperexcitability. Evoked brain potentials did not differ between groups. A positive correlation was found between reflex thresholds and cold pressor endurance time (ρ=0.71, P=0.004).

Conclusion

We demonstrated somatic hyperalgesia in patients with painful CP associated with spinal hyperexcitability. This highlights that management should be directed at central mechanisms using, eg, gabapentinoids or serotonin-noradrenaline reuptake inhibitors.

Opioid Specific Effects on Central Processing of Sensation and Pain: A Randomized, Cross-Over, Placebo-Controlled Study

Moderate to severe pain is often treated with opioids, but central mechanisms underlying opioid analgesia are poorly understood. Findings thus far have been contradictory and none could infer opioid specific effects. This placebo-controlled, randomized, 2-way cross-over, double-blinded study aimed to explore opioid specific effects on central processing of external stimuli. Twenty healthy male volunteers were included and 3 sets of assessments were done at each of the 2 visits: 1) baseline, 2) during continuous morphine or placebo intravenous infusion and 3) during simultaneous morphine + naloxone or placebo infusion. Opioid antagonist naloxone was introduced in order to investigate opioid specific effects by observing which morphine effects are reversed by this intervention. Quantitative sensory testing, spinal nociceptive withdrawal reflexes (NWR), spinal electroencephalography (EEG), cortical EEG responses to external stimuli and resting EEG were measured and analyzed. Longer lasting pain (cold-pressor test - hand in 2° water for 2 minutes, tetanic electrical), deeper structure pain (bone pressure) and strong nociceptive (NWR) stimulations were the most sensitive quantitative sensory testing measures of opioid analgesia. In line with this, the principal opioid specific central changes were seen in NWRs, EEG responses to NWRs and cold-pressor EEG. The magnitude of NWRs together with amplitudes and insular source strengths of the corresponding EEG responses were attenuated. The decreases in EEG activity were correlated to subjective unpleasantness scores. Brain activity underlying slow cold-pressor EEG (1-4Hz) was decreased, whereas the brain activity underlying faster EEG (8-12Hz) was increased. These changes were strongly correlated to subjective pain relief. This study points to evidence of opioid specific effects on perception of external stimuli and the underlying central responses. The analgesic response to opioids is likely a synergy of opioids acting at both spinal and supra-spinal levels of the central nervous system. Due to the strong correlations with pain relief, the changes in EEG signals during cold-pressor test have the potential to serve as biomarkers of opioid analgesia. PERSPECTIVE: This exploratory study presents evidence of opioid specific effects on the pain system at peripheral and central levels. The findings give insights into which measures are the most sensitive for assessing opioid-specific effects.

Inhibition and facilitation of nociceptively evoked muscular activity by fentanyl or dexmedetomidine in isoflurane-anaesthetized pigs

OBJECTIVE

To investigate motor and cardiovascular responses to dexmedetomidine or fentanyl in isoflurane-anaesthetized pigs.

STUDY DESIGN

Experimental, balanced, block randomized, two-group design.

ANIMALS

A group of 16 crossbred pigs, 55 ± 8 days (mean ± standard deviation) old.

METHODS

Deltoid electromyography (EMG) was recorded during isoflurane anaesthesia. Electrical stimulation using 5, 10, 20 and 40 mA of the distal right thoracic limb elicited a nociceptive withdrawal reflex (NWR), quantified by the area under the curve (AUC) for the simulation intensity versus EMG amplitude response curve. Latency to movement evoked by clamping a claw for maximum 60 seconds was noted. Arterial blood pressure and pulse rate were recorded. Data were sampled at baseline and during dexmedetomidine 0.25, 0.5, 1.0, 2.0, 4.0 and 8.0 μg kg^-1 hour^-1 or fentanyl 5, 10, 20, 40, 80 and 160 μg kg^-1 hour^-1 infusions. The influence of infusion rate on NWR AUC and spontaneous EMG was analysed using a mixed model, with p < 5%.

RESULTS

NWR AUC increased at fentanyl 5 μg kg^-1 hour^-1 but decreased at fentanyl 40, 80 and 160 μg kg^-1 hour^-1 and dexmedetomidine 4.0 and 8.0 μg kg^-1 hour^-1. All pigs at fentanyl 80 μg kg^-1 hour^-1, and three pigs at dexmedetomidine 8.0 μg kg^-1 hour^-1 had mechanical latencies greater than 60 seconds. Spontaneous EMG activity increased accompanied by visually evident 'shivering' at fentanyl 5, 10 and 20 μg kg^-1 hour^-1 but decreased at dexmedetomidine 2, 4 and 8 μg kg^-1 hour^-1. Clinically relevant effects of increasing infusion rates on blood pressure or pulse rate were not observed.

CONCLUSION AND CLINICAL RELEVANCE

If anaesthetic plane or antinociception is evaluated in pigs, response to claw clamping and NWR will not necessarily give uniform results when comparing drugs. If only one method is used, results should be interpreted cautiously.