Dyspnea-pain counterirritation induced by inspiratory threshold loading: a laser-evoked potentials study

Clinical and preclinical evidence of somatosensory involvement in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron neurodegenerative disease. Although it has been classically considered as a disease limited to the motor system, there is increasing evidence for the involvement of other neural and non-neuronal systems. In this review, we will discuss currently existing literature regarding the involvement of the sensory system in ALS. Human studies have reported intradermic small fibre loss, sensory axonal predominant neuropathy, as well as somatosensory cortex hyperexcitability. In line with this, ALS animal studies have demonstrated the involvement of several sensory components. Specifically, they have highlighted the impairment of sensory-motor networks as a potential mechanism for the disease. The elucidation of these "non-motor" systems involvement, which might also be part of the degeneration process, should prompt the scientific community to re-consider ALS as a pure motor neuron disease, which may in turn result in more holistic research approaches. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.6/issuetoc.

Interferences between breathing, experimental dyspnoea and bodily self-consciousness

Dyspnoea, a subjective experience of breathing discomfort, is a most distressing symptom. It implicates complex cortical networks that partially overlap with those underlying bodily self-consciousness, the experience that the body is one's own within a given location (self-identification and self-location, respectively). Breathing as an interoceptive signal contributes to bodily self-consciousness: we predicted that inducing experimental dyspnoea would modify or disrupt this contribution. We also predicted that manipulating bodily self-consciousness with respiratory-visual stimulation would possibly attenuate dyspnoea. Twenty-five healthy volunteers were exposed to synchronous and asynchronous respiratory-visual illumination of an avatar during normal breathing and mechanically loaded breathing that elicited dyspnoea. During normal breathing, synchronous respiratory-visual stimulation induced illusory self-identification with the avatar and an illusory location of the subjects' breathing towards the avatar. This did not occur when respiratory-visual stimulation was performed during dyspnoea-inducing loaded breathing. In this condition, the affective impact of dyspnoea was attenuated by respiratory-visual stimulation, particularly when asynchronous. This study replicates and reinforces previous studies about the integration of interoceptive and exteroceptive signals in the construction of bodily self-consciousness. It confirms the existence of interferences between experimental dyspnoea and cognitive functions. It suggests that respiratory-visual stimulation should be tested as a non-pharmacological approach of dyspnoea treatment.

Experimental dyspnea as a stressor: differential cardiovegetative responses to inspiratory threshold loading in healthy men and women

Dyspnea is associated with an emotional reaction that involves limbic activation. The inspiratory threshold load (ITL) is known to elicit a dyspneic response in healthy subjects. Laboratory-induced stress conditions have been shown to elicit sex-related differences in cardiovascular responses. The aim of this study was to evaluate how healthy men ( n = 8) and women ( n = 9) react and adapt to 5-min periods of ITL at three levels (low, medium, and high) in terms of heart rate (HR), temporal (RMSSD) and spectral (LF, HF, LF/HF ratio) HRV indexes, and rating of breathing discomfort. HR increased with low, medium, and high ITL in men, whereas it increased only with high ITL in women. LF/HF ratio increased at low ITL in both men and women. Modifications appear to depend essentially on increased LF in men and on reduced HF in women. In addition, HRV modifications differ between men and women, following the order of presentation of ITLs. Our results show a continuous and sustained stress in men (increased HR, LF, and LF/HF ratio across ITL presentation) and a stress adaptation in women. Subjective responses of breathing discomfort were not correlated with sympatho-vagal balance modifications for a subgroup of subjects ( n = 10). Breathing against the ITL induced autonomic modifications that are different between men and women, i.e., driven by sympathetic mediated responses in men, whereas women showed a greater parasympathetic modulation of cardiovascular activity. These results highlight the role of the mechanical inspiratory load in the heart rate variability seen in chronic obstructive pulmonary disease.

NEW & NOTEWORTHY Breathing against the ITL induced autonomic modifications driven by sympathetic mediated responses in men, whereas women showed a greater parasympathetic modulation of cardiovascular activity, even for low load. A stress circuit could be at the origin of autonomic modifications induced by ITL. Our results would underline the role of the mechanic inspiratory load in the abnormalities in heart rate variability seen in COPD patients.

Relieving dyspnoea by non-invasive ventilation decreases pain thresholds in amyotrophic lateral sclerosis

BACKGROUND

Dyspnoea is a threatening sensation of respiratory discomfort that presents many similarities with pain. Experimental dyspnoea in healthy subjects induces analgesia. This 'dyspnoea-pain counter-irritation' could, in reverse, imply that relieving dyspnoea in patients with chronic respiratory diseases would lower their pain thresholds.

METHODS

We first determined pressure pain thresholds in 25 healthy volunteers (22-31 years; 13 men; handheld algometer), during unloaded breathing (BASELINE) and during inspiratory threshold loading (ITL). Two levels of loading were used, adjusted to induce dyspnoea self-rated at 60% or 80% of a 10 cm visual analogue scale (ITL6 and ITL8). 18 patients with chronic respiratory failure due to amyotrophic lateral sclerosis (ALS) were then studied during unassisted breathing and after 30 and 60 min of non-invasive ventilation-NIV30 and NIV60-(same dyspnoea evaluation).

RESULTS

In healthy volunteers, pressure pain thresholds increased significantly in the deltoid during ITL6 (p<0.05) and ITL8 (p<0.05) and in the trapezius during ITL8 (p<0.05), validating the use of pressure pain thresholds to study dyspnoea-pain counter-irritation. In patients with ALS, the pressure pain thresholds measured in the deltoid during unassisted breathing decreased by a median of 24.5%-33.0% of baseline during NIV30 and NIV60 (p<0.05).

CONCLUSION

Relieving dyspnoea by NIV in patients with ALS having respiratory failure is associated with decreased pressure pain thresholds. Clinical implications have yet to be determined, but this observation suggests that patients with ALS could become more susceptible to pain after the institution of NIV, hence the need for reinforced attention towards potentially painful diagnostic and therapeutic interventions.

Interactions Between Dyspnea and the Brain Processing of Nociceptive Stimuli: Experimental Air Hunger Attenuates Laser-Evoked Brain Potentials in Humans

Dyspnea and pain share several characteristics and certain neural networks and interact with each other. Dyspnea-pain counter-irritation consists of attenuation of preexisting pain by intercurrent dyspnea and has been shown to have neurophysiological correlates in the form of inhibition of the nociceptive spinal reflex RIII and laser-evoked potentials (LEPs). Experimentally induced exertional dyspnea inhibits RIII and LEPs, while “air hunger” dyspnea does not inhibit RIII despite its documented analgesic effects. We hypothesized that air hunger may act centrally and inhibit LEPs. LEPs were obtained in 12 healthy volunteers (age: 21–29) during spontaneous breathing (FB), ventilator-controlled breathing (VC) tailored to FB, after inducing air hunger by increasing the inspired fraction of carbon dioxide -FiCO₂- (VCCO₂), and during ventilator-controlled breathing recovery (VCR). VCCO₂ induced intense dyspnea (visual analog scale = 63% ± 6% of full scale, p < 0.001 vs. VC), predominantly of the air hunger type. VC alone reduced the amplitude of the N2-P2 component of LEPs (Δ = 24.0% ± 21.1%, p < 0.05, effect-size = 0.74) predominantly through a reduction in P2, and the amplitude of this inhibition was further reduced by inducting air hunger (Δ = 22.6% ± 17.9%, p < 0.05, effect-size = 0.53), predominantly through a reduction in N2. Somatosensory-evoked potentials (SEPs) were not affected by VC or VCCO₂, suggesting that the observed effects are specific to pain transmission. We conclude that air hunger interferes with the cortical mechanisms responsible for the cortical response to painful laser skin stimulation, which provides a neurophysiological substrate to the central nature of its otherwise documented analgesic effects.

Gender differences in the effect of urge-to-cough and dyspnea on perception of pain in healthy adults

Previous studies have reported that respiratory sensations, such as urge‐to‐cough and dyspnea, have an inhibitory effect on pain. Considering the existence of gender differences in both urge‐to‐cough and pain, it is conceivable that a gender difference also exists in the analgesia induced by urge‐to‐cough. In this study, we evaluated gender differences in the pain perception response to urge‐to‐cough, as well as to dyspnea. Twenty‐seven male and 26 female healthy nonsmokers were originally enrolled. Citric acid challenge was used to induce the urge‐to‐cough sensation, and dyspnea was elicited by inspiratory loaded breathing. Before and during inductions of these two respiratory sensations, perception of pain was assessed by the thermal pain threshold, and differences between men and women were compared. The thermal pain threshold in women (43.83 ± 0.17°C) was significantly lower than that in men (44.75 ± 0.28°C; P < 0.05) during the baseline period. Accompanying increases in both citric acid concentration and inspiratory resistive load, thermal pain threshold values significantly increased in both men and women. The average thermal pain threshold changes for comparable increases in the urge‐to‐cough Borg score were parallel between men and women. Furthermore, the mean value of the thermal pain threshold plotted against the dyspnea Borg score also showed no significant gender difference. These results demonstrate that although gender differences exist in respiratory sensations, that is, urge‐to‐cough and dyspnea, the inhibitory effects of these respiratory sensations on the perception of pain are not significantly different between the sexes.

The relationships between citric acid dose and urge‐to‐cough rating or thermal pain threshold in each subject grouped by sex.