Sensory processing is one of the aspects of Autism Spectrum Disorder (ASD) that has been under debate in the medical community.
Part of the new DSM-5 criteria for the diagnosis of ASD is “hypo- or hypersensitivity to sensory input”, with a given example: “apparent indifference to pain” (Centers for Disease Control and Prevention). This criterion was not present in the previous (DSM-IV) (Autism Society of Southern Arizona) version, and its addition underscores the ongoing discussion about sensory processing in ASD, and maybe in particular, pain perception.
In a newly published study by Hoffman et al. (Hoffman) out of Haifa, Israel, the relationship between pain, pain processing and Autism has been explored. Inspired by the excitatory/inhibitory (E/I) imbalance known to be associated with Autism, these researchers set out to psychophysically assess excitatory and inhibitory sensory pathways using appropriate laboratory tests.
Participants and questionnaires
Fifty-two adults diagnosed with ASD, and fifty-two age and sex-matched typically developed (TD) controls were recruited. All were void of chronic or acute pain, and scored higher than 80 on the Wechsler Abbreviated Scale of intelligence-II (IQ) scale. Questionnaires that were used were the Autism Spectrum Quotient questionnaire, the Pain Catastrophizing Scale (PCS), the Spielberg State-Trait Anxiety Inventory, The Pain Sensitivity Questionnaire (PSQ), and the Sensory Responsiveness Questionnaire-Intensity Scale (SRQ-IS).
QST Lab testing
Pain psychophysics consisted of thermal quantitative sensory testing using Medoc’s TSA-II. Cold detection, warm detection and heat pain thresholds were assessed using the method of Limits. Heat pain sensitivity was tested using Medoc’s Pathway CHEPS thermode in 3x20 semi-randomized high speed pulses series to 46°C, 49°C and 52°C which were rated verbally by the participants. Phasic temporal summation was assessed with 15 stimuli to 48°C using the CHEPS thermode. Pain habituation was tested by using two sets of 20 phasic stimuli with the CHEPS thermode at the calibrated phasic temperature of Pain 50/100. Each stimulus was individually rated.
Two Conditioned Pain Modulation (CPM) protocols were tested; one in which the Pain Habituation protocol was used as the test stimulus, and another one in which a calibrated tonic stimulus at the intensity of Pain50 was given for twenty seconds. The conditioning stimulus consisted of a hot bath (of personally calibrated temperatures between 46-47°C). Each test stimulus was run alone and then in parallel with the conditioning stimulus, to assess the CPM effect.
Commonalities and differences in pain between persons with Autism and Typical Development controls
No differences were found between the ASD and the TD groups in sensory or pain thresholds.
In supra-threshold pain ratings in the heat pain sensitivity protocol, ASD participants were more sensitive to all three supra-threshold stimulus intensities. In the Autism group supra-threshold ratings were significantly correlated to PSQ scores. Neither of the groups showed significant habituation to the stimuli.
For the temporal summation protocol neither of the groups on average showed temporal summation. The marked difference between the two groups was the higher pain ratings on average of the Autism group for both the first and the last stimulus as compared to the controls. There was no difference in temporal summation between the groups.
For pain habituation, neither of the two groups reached Pain50 even at the highest testing temperature. At this temperature the pain ratings of the Autism groups were significantly higher than of the control group. The comparison between the ratings of the two series in the habituation protocol showed that neither group had any significant habituation.
In both CPM protocols, the Autism group had a lower temperature of the conditioning stimulus, which was rated as more painful as compared to the controls. In the phasic protocol there was no difference in CPM effect between the two groups, in which both showed inhibition. Similar to the conditioning stimulus, in the tonic CPM protocol, the individually calibrated test stimulus was of lower temperature in the Autism group as compared to the controls. With respect to the CPM effect in the tonic protocol, the control group showed pain inhibition, while this time, the Autism group showed no CPM efficacy.
Are people with Autism more sensitive to pain?
This study demonstrates that persons with ASD seem to have normal peripheral nervous system functioning, evident by the lack of differences in sensory and pain thresholds. However, pain processing in ASD seems to diverge as reflected by a higher sensitivity to supra-threshold pain compared to controls. When presented with tonic stimuli, a lower efficacy in pain inhibition was found. Authors concluded that people with Autism showed a pronociceptive pain modulation profile.
The findings of this study supply important information regarding pain processing in autism and contradicts the belief that persons with ASD are less sensitive to pain. This knowledge may raise awareness among the caregivers and potentially lead to better treatment and improvement of the quality of patient life.