Pain Modulation Mechanisms in ASD Adults

Indifference or hypersensitivity? Solving the riddle of the pain profile in individuals with autism

ABSTRACT

Excitatory-inhibitory (E/I) imbalance is a mechanism that underlies autism spectrum disorder, but it is not systematically tested for pain processing. We hypothesized that the pain modulation profile (PMP) in autistic individuals is characterized by less efficient inhibitory processes together with a facilitative state, indicative of a pronociceptive PMP. Fifty-two adults diagnosed with autism and 52 healthy subjects, age matched and sex matched, underwent quantitative sensory testing to assess the function of the (1) pain facilitatory responses to phasic, repetitive, and tonic heat pain stimuli and (2) pain inhibitory processes of habituation and conditioned pain modulation. Anxiety, pain catastrophizing, sensory, and pain sensitivity were self-reported. The autistic group reported significantly higher pain ratings of suprathreshold single ( P = 0.001), repetitive (46°C- P = 0.018; 49°C- P = 0.003; 52°C- P < 0.001), and tonic ( P = 0.013) heat stimuli that were cross correlated ( r = 0.48-0.83; P < 0.001) and associated with sensitivity to daily life pain situations ( r = 0.39-0.45; P < 0.005) but not with psychological distress levels. Hypersensitivity to experimental pain was attributed to greater autism severity and sensory hypersensitivity to daily stimuli. Subjects with autism efficiently inhibited phasic but not tonic heat stimuli during conditioned pain modulation. In conclusion, in line with the E/I imbalance mechanism, autism is associated with a pronociceptive PMP expressed by hypersensitivity to daily stimuli and experimental pain and less-efficient inhibition of tonic pain. The latter is an experimental pain model resembling clinical pain. These results challenge the widely held belief that individuals with autism are indifferent to pain and should raise caregivers' awareness of pain sensitivity in autism.

The Current View on the Paradox of Pain in Autism Spectrum Disorders

Autism spectrum disorder (ASD) is a neurodevelopmental disorder, which affects 1 in 44 children and may cause severe disabilities. Besides socio-communicational difficulties and repetitive behaviors, ASD also presents as atypical sensorimotor function and pain reactivity. While chronic pain is a frequent co-morbidity in autism, pain management in this population is often insufficient because of difficulties in pain evaluation, worsening their prognosis and perhaps driving higher mortality rates. Previous observations have tended to oversimplify the experience of pain in autism as being insensitive to painful stimuli. Various findings in the past 15 years have challenged and complicated this dogma. However, a relatively small number of studies investigates the physiological correlates of pain reactivity in ASD. We explore the possibility that atypical pain perception in people with ASD is mediated by alterations in pain perception, transmission, expression and modulation, and through interactions between these processes. These complex interactions may account for the great variability and sometimes contradictory findings from the studies. A growing body of evidence is challenging the idea of alterations in pain processing in ASD due to a single factor, and calls for an integrative view. We propose a model of the pain cycle that includes the interplay between the molecular and neurophysiological pathways of pain processing and it conscious appraisal that may interfere with pain reactivity and coping in autism. The role of social factors in pain-induced response is also discussed. Pain assessment in clinical care is mostly based on subjective rather than objective measures. This review clarifies the strong need for a consistent methodology, and describes innovative tools to cope with the heterogeneity of pain expression in ASD, enabling individualized assessment. Multiple measures, including self-reporting, informant reporting, clinician-assessed, and purely physiological metrics may provide more consistent results. An integrative view on the regulation of the pain cycle offers a more robust framework to characterize the experience of pain in autism.

Alterations in the intrinsic discharge activity of CA1 pyramidal neurons associated with possible changes in the NADPH diaphorase activity in a rat model of autism induced by prenatal exposure to valproic acid

Autism spectrum disorder is a neurodevelopmental disorder characterized by sensory abnormalities, social skills impairment and cognitive deficits. Although recent evidence indicated that induction of autism-like behavior in animal models causes abnormal neuronal excitability, the impact of autism on neuronal properties is still an important issue. Thus, new findings at the cellular level may shed light on the pathophysiology of autism and may help to find effective treatment strategies. Here, we investigated the behavioral, electrophysiological and histochemical impacts of prenatal exposure to valproic acid (VPA) in rats. Findings revealed that VPA exposure caused a significant increase in the hot plate response latency. The novel object recognition ability was also impaired in VPA-exposed rats. Along with these behavioral alterations, neurons from VPA-exposed animals exhibited altered excitability features in response to depolarizing current injections relative to control neurons. In the VPA-exposed group, these changes consisted of a significant increase in the amplitude, evoked firing frequency and the steady-state standard deviation of spike timing of action potentials (APs). Moreover, the half-width, the AHP amplitude and the decay time constant of APs were significantly decreased in this group. These changes in the evoked electrophysiological properties were accompanied by intrinsic hyperexcitability and lower spike-frequency adaptation and also a significant increase in the number of NADPH-diaphorase stained neurons in the hippocampal CA1 area of the VPA-exposed rats. Taken together, findings demonstrate that abnormal nociception and recognition memory is associated with alterations in the neuronal responsiveness and nitrergic system in a rat model of autism-like.

Neuroinflammation as a possible link between attention-deficit/hyperactivity disorder (ADHD) and pain

Attention-deficit/hyperactivity disorder (ADHD) and pathological pain are two complex syndromes of multifactorial origin. Despite their prevalence and broad impacts, these conditions are seldom recognized and managed simultaneously. The co-existence of neuropsychiatric conditions (such as ADHD) and altered pain perception and chronic pain has been noted in children, and the comorbidity of ADHD and chronic pain is well documented in adults. Pathophysiological studies have suggested dysfunction of the dopaminergic system as a common neurochemical basis for comorbid ADHD and pain. Considerable evidence supports the role of neuroinflammation in the pathophysiology of both. We suggest that central neuroinflammation underlies altered pain perception and pain sensitization in persons with ADHD. Based on our hypothesis, targeting neuroinflammation may serve as a potential new therapeutic intervention to treat ADHD and comorbid pain in children and adolescents and a preventive strategy for the development of chronic pain in adults with ADHD.

Increased pain sensitivity and pain-related anxiety in individuals with autism

INTRODUCTION

Individuals with autism spectrum disorder (ASD) often exhibit differences in pain responsivity. This altered responsivity could be related to ASD-related social communication difficulties, sensory differences, or altered processing of pain stimuli. Previous neuroimaging work suggests altered pain evaluation could contribute to pain-related anxiety in ASD.

OBJECTIVES

We hypothesized that individuals with ASD would report increased pain sensitivity and endorse more pain-related anxiety, compared to typically developing controls.

METHODS

We recruited 43 adults (ASD, n = 24; typically developing, n = 19) for 3 heat pain tasks (applied to the calf). We measured heat pain thresholds using a method of limits approach, a pain-rating curve (7 temperatures between 40 and 48°C, 5 seconds, 5 trials each), and a sustained heat pain task with alternating low (42°C) and high (46°C) temperatures (21 seconds, 6 trials each). Individual differences in pain-related anxiety, fear of pain, situational pain catastrophizing, depressive symptoms, and autism-related social communication were assessed by self-report.

RESULTS

There were no group differences in pain thresholds. For suprathreshold tasks, mean pain ratings were higher in ASD across both the pain-rating curve and the sustained heat pain tasks, but responses in the ASD group were more varied. Pain anxiety (PASS-Total) and pain-related fear (FOP-III-Total) were higher in the ASD group and were positively associated with pain ratings.

CONCLUSIONS

Our results suggest that both sensory and cognitive experiences of pain are heightened and interact reciprocally in adults with ASD. Future studies are needed to evaluate the impact of pain-related anxiety on treatment-seeking and pain behaviors, given higher levels of pain-related anxiety in ASD.