Pilot Trial on the Effects of Propranolol on Gastrointestinal Symptoms in Autism Spectrum Disorder and Heart Rate Variability as a Treatment Response Biomarker

Abstract

Background:

Many individuals with autism spectrum disorder (ASD) experience gastrointestinal (GI) symptoms, which can impact social interactions, exacerbate social communication deficits, and decrease the quality of life. GI symptoms have been shown to be correlated with the autonomic nervous system and endocrine response to stress in some people with ASD. Furthermore, propranolol, a central and peripheral beta-adrenergic antagonist that inhibits the stress response, has been shown to provide GI relief for some individuals with ASD, but not others. This pilot study examined whether baseline (i.e., resting) heart rate variability (HRV), a biomarker that is sensitive to the stress response, predicted the response to propranolol in decreasing GI symptoms.

Methods:

In this pilot study, a sample of 37 individuals with ASD participated in a 12-week open-label trial of propranolol. The Gastrointestinal Severity Index and HRV were collected at baseline (i.e., prior to taking propranolol) and again at the end of the 12-week trial period.

Results:

Higher HRV was associated with the greatest reduction in GI symptoms, with a strong effect size, but only for adolescents and young adults (15–24 years old). Baseline HRV and GI change scores were not significantly correlated for younger children (7–14 years old).

Conclusions:

The results from this open-label pilot trial suggest that autistic adolescents and young adults with higher baseline HRV, indicating greater parasympathetic tone, may respond better to propranolol and show the greatest reduction in GI symptoms. The data from this pilot should be interpreted with caution until larger, randomized, double-blinded, placebo-controlled trials of propranolol for GI symptoms in ASD are completed.

Introduction

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairments in both social communication and interaction, as well as restricted, repetitive patterns of behavior, interests, or activities, that begin early in life (American Psychiatric Association, 2013). Numerous individuals diagnosed with ASD contend with gastrointestinal (GI) symptoms, exerting notable effects on their social interactions and exacerbating deficits in social communication. Studies have demonstrated a higher prevalence of GI symptoms in children with ASD compared with typically developing children (Chaidez et al. 2014; Lai et al. 2020). Prevalence rates for GI symptoms among individuals with ASD range from 9% to 70% or more, while typically developing individuals experience GI problems at rates of 15%−18% (Buie et al. 2010; Fulceri et al. 2016; Gondalia et al. 2012; Restrepo et al. 2020). As such, GI symptoms in children with ASD are a significant health issue that reduce the quality of life and are in need of additional study.

In children with ASD, GI symptoms are correlated with heightened internalizing problems such as irritability, anxiety, stereotypy, and social withdrawal (Ferguson et al. 2019; Fulceri et al. 2016; Horvath and Perman, 2002; Mazefsky et al. 2014; Prosperi et al. 2019). Prevalence estimates of anxiety in ASD vary widely, likely due to the large amount of heterogeneity in ASD, which complicates treatment efforts (Bougeard et al. 2021). Additionally, GI issues in ASD are linked to externalizing symptoms such as aggression (Ferguson et al. 2019) and unexplained irritability, as well as disrupted sleep (Horvath and Perman, 2002). Some studies propose that heightened stress and/or anxiety may contribute to GI problems in ASD (Ferguson et al. 2016, 2017, 2019). To this point, a recent empirical study corroborates a unidirectional model wherein internalizing symptoms, notably anxious behaviors, serve as precursors to GI problems (Dovgan, Gynegrowski, and Ferguson, 2023). Interestingly, research has shown that autistic individuals with anxiety symptoms are at an increased risk of GI symptoms, particularly those with greater heart rate variability (HRV) (Ferguson et al. 2017). One possible explanation for this finding is that in the setting of chronic stress and anxiety, there is sustained systemic inflammation. This, in turn, activates the cholinergic anti-inflammatory pathway, which leads to a compensatory high baseline parasympathetic tone, which manifests as greater vagal tone and increased HRV (e.g., Pavlov et al. 2003; Tracey, 2007). This notion is supported by previous research, which found a positive correlation between GI symptoms and HRV, as well as a positive correlation between GI symptoms, and the pro-inflammatory cytokine tumor necrosis factor alpha (TNF-α) (Ferguson et al. 2016, 2017). As such, the treatment of GI symptoms in ASD also has the potential to alleviate a range of internalizing and externalizing symptoms, which will ultimately increase the quality of life.

Precision medicine, or personalized medicine, uses biomarkers, or unique biological signatures from the body, to determine which treatments are effective in which patients. Precision medicine efforts are critical for the development of effective treatments, given the large amount of heterogeneity in ASD (Beversdorf and Missouri Autism Summit Consortium, 2016). Anxiety disorders commonly co-occur with ASD, so pharmacological treatments targeting the stress response may provide benefit to this subgroup within ASD. One promising pharmacological agent in the treatment of anxiety-related conditions in ASD is propranolol, a central and peripheral nervous system beta-adrenergic antagonist that downregulates the activity of the sympathetic branch of the autonomic nervous system (ANS). Some individuals with ASD exhibit heightened physiological stress responses to their environment, a phenomenon intertwined with their GI problems (Ferguson et al. 2017). The vagus nerve is involved in modulating both stress responses and digestive functions, including GI motility. Interestingly, constipation is typically the most often reported GI symptom by people with ASD (Ferguson et al. 2017), which is associated with reduced GI motility. However, it is not known if propranolol has an effect on GI symptoms in ASD, and if HRV can serve as a biomarker for which patients might receive the most benefit, if any, from propranolol. Both GI symptoms (Ferguson et al. 2019) and ANS changes (Harteveld et al. 2021) have been shown to differ by age, but the effects of propranolol on these variables are not known in this ASD population.

This pilot study aimed to examine whether HRV, a measure that is sensitive to cardiac vagal tone, predicted the response to a 12-week open-label trial of propranolol in regard to the severity of GI symptoms in children and adolescents with ASD.

Methods

Participants

The original sample for this study included children and adolescents with an ASD diagnosis who were enrolled in a randomized controlled trial of propranolol, which suggested improvements in some measures of anxiety (ClinicalTrials.gov Identifier: NCT02871349) at the University of Missouri Thompson Center for Autism & Neurodevelopment (Beversdorf et al. 2024). An open-label trial examining the effects of propranolol on GI symptoms was conducted following the blinded trial after at least a 2-week washout period. The study was approved by the University of Missouri Health Sciences Institutional Review Board and was carried out by the Declaration of Helsinki. Participants over the age of 18 provided informed consent, and assent was obtained from those under the age of 18. A sample of 46 individuals with ASD aged 7–24 participated in a 12-week open-label trial of propranolol. The GI Severity Index (GSI) and 5-minute electrocardiogram (ECG) recordings were collected at baseline and at the end of the trial period. Twenty-seven individuals with complete data (baseline and posttreatment) were included in the analyses using GI change scores.

Medication dosage

Participants received propranolol in an open-label fashion as previously described (Beversdorf et al. 2024). For participants weighing 50 kg or greater, propranolol was initiated at a dose of 40 mg nightly for 1 week, which was in accordance with the same dose titration of propranolol in previous research (Ratey et al. 1987), then increased to 40 mg twice a day for 1 week. This dose was maintained at 100 mg a day in divided doses (40/20/40), based on a titration schedule that was previously shown to be well tolerated in a neuropsychiatric population (Kampman et al. 2006). The dose was decreased back to the previous level if the participant’s heart rate dropped below 55 (i.e., bradycardia), or systolic blood pressure dropped below 90 after the increase, which was assessed with an arm cuff given to the participant within the first 48 hours after each change in dose, and a call to confirm no further problems before each increase. For participants experiencing side effects with an increase, the dose was decreased to the previous level. For participants under the age of 15, the minimum heart rate allowed was 60, and the minimum systolic blood pressure allowed was 80, and doses were adjusted for weight. Propranolol was continued for 12 weeks. Starting at week 13, participants were weaned off medication by decreasing the daily dose to 60 mg (i.e., three doses daily), and during week 14 the daily dose was decreased to 20 mg (one dose daily). For participants under 50 kg, the procedure was identical to the procedure above except for minimum heart rate and systolic blood pressure readings, as noted above. For participants 40–50 kg, each participant received 75% of the dose at each stage, with the maximum dosage at 75 mg per day (30/15/30). For participants 30–40 kg, each participant received 50% of the dose, with the maximum dosage at 50 mg per day (20/10/20). For participants 20–30 kg, each participant received 25% of the dose, with the maximum dosage at 25 mg per day (10/5/10). Participants under 20 kg were excluded from the study for safety reasons.

Measures

GI symptoms

The GI Severity Index (Schneider et al. 2006) is an eight-item questionnaire about the participant’s GI functioning over the past week including constipation, diarrhea, stool consistency, stool smell, flatulence, and abdominal pain. The GSI also assesses unexplained daytime irritability and nighttime awakening, which are associated with GI symptoms. Total severity scores range from 0 to 16, wherein higher scores reflect greater or more severe GI-related symptoms. Three groups were created to examine relationships between HRV and response to propranolol on different types of GI symptoms: upper GI (abdominal pain or flatulence), lower GI (constipation, diarrhea, stool consistency, stool smell), and both types of GI issues.

Heart rate variability

Participants were brought into the psychophysiology laboratory at the University of Missouri Thompson Center for Autism & Neurodevelopment where they were seated in a comfortable chair. Participants were outfitted with a three-lead ECG setup consisting of BIOPAC EL503 Ag/AgCl disposable electrodes with moderate adhesive backing for contact with the skin. After placing electrodes on the participant, the ECG signal was verified by observing a QRS complex, or the series of electrical waves observed on an ECG that represents the depolarization (contraction) of the heart's ventricles. Electrodes were replaced on the participant if the initial ECG signal was not suitable for analysis. Participants were asked to remain still, refrain from talking, and breathe normally during ECG data acquisition. An 8-minute resting state ECG dataset was collected from each participant using a BIOPAC MP160 data acquisition system with an ECG100 amplifier (BIOPAC Systems, Inc., Goleta, CA) attached to a laptop computer. The ECG recording produced a measure of HRV, which is the variation in the time interval between heartbeats, measured by the variation in the beat-to-beat interval.

Kubios HRV Standard analysis software was used to derive HRV from the ECG data as previously described (Ferguson et al. 2017). Kubios software was used to compute HRV from inter-beat-intervals as with previous work (Ferguson et al. 2017). This metric plays a pivotal role in the evaluation of ANS functionality. Elevated HRV, characterized by increased variability between heartbeats, signifies a robust capacity of the body to withstand stress or recover from preceding stressors. That is, high HRV is associated with better parasympathetic control and implies an individual’s ability to autonomously regulate and achieve emotional calming following arousing events. Conversely, low HRV is associated with poor parasympathetic control and implies suboptimal self-regulation and prolonged periods of arousal following a stressor.

While there are many measures of HRV, the present study focused on the standard deviation of instantaneous heart rate values (standard deviation of instantaneous heartbeat [STD HR]; 1 per minute), which can easily be derived from a wide variety of wearables (Taj-Eldin et al. 2018) that can be utilized in future trials examining ANS biomarkers associated with the treatment response in ASD. Relatively higher STD HR variables are indicative of higher HRV, and hence, greater parasympathetic tone (i.e., “rest-and-digest” or parasympathetic tone is predominant), and relatively lower STD HR variables are indicative of lower HRV, and hence, lower parasympathetic tone (i.e., the “fight-or-flight” or sympathetic nervous system is predominant).

Statistical analyses

Participants were divided into two age groups: younger children (aged 7–14) and adolescents (aged 15–24) for the purposes of propranolol dosing and to account for age differences in GI symptoms and ANS functioning in ASD. Descriptive statistics were used to determine the extent of improvement in GI symptoms, and independent samples t-tests and chi-square analyses were used to assess the influence of the age group on both the level of change and type of change (improvement, stability, or worsening) in GI symptoms. Pearson correlations were used to examine the relationship between baseline STD HR and GI change score by age group.

Results

GI severity at baseline, GI severity at follow-up, GI severity change score, and HRV were normally distributed, with skewness statistics within an absolute value of 1. See Table 1 for descriptive statistics of GI severity and HRV by age group and response to propranolol. The severity of GI symptoms was similar for both age groups at baseline before treatment [t(35) = 0.898, p = 0.375] and at the end of the 12-week propranolol trial [t(32) = 1.437, p = 0.080]. In addition, the type of GI symptoms experienced (upper-GI, lower-GI, or both) was similar for both age groups at baseline [χ ²(2) = 1.809, p = 0.405] and follow-up [χ ²(2) = 0.220, p = 0.896]. Ten participants (37%) experienced fewer GI problems after the trial, nine participants (33.3%) displayed more GI problems, and eight participants (29.6%) reported the same level of GI problems at the end of the 12-week propranolol trial. The age group alone did not influence the amount of change in GI symptoms [t(25) = −0.246, p = 0.808] or type of change (improvement, worsening, or stability) [χ ²(2) = 2.25, p = 0.324]. Similarly, the change in GI symptoms was not statistically different for participants who experienced upper, lower, or both types of GI symptoms [F(2, 24) = 0.227, p = 0.798].

Table 1.

Descriptive Statistics of Baseline (i.e., Before Treatment with Propranolol) GSI Severity and HRV by Age Group and Response to Propranolol

	7–14 year olds			15–24 year olds
	GI improved	GI worsened	GI stable	GI improved	GI worsened	GI stable
Number of Participants	n = 7	n = 6	n = 3	n = 3	n = 3	n = 5
Percentage of Participants	43.8%	37.5%	18.7%	27.3%	27.3%	45.4%
Baseline GSI Severity Scores (total)	M = 3.57	M = 3	M = 3	M = 4.33	M = 2.0	M = 2.0
Baseline GSI Severity Scores (total)	SD = 1.27	SD = 1.10	S = 2.65	SD = 1.53	SD = 1.73	SD = 1.22
Baseline STD HR (1 per minute)	M = 4.88	M = 5.57	M = 7.65	M = 5.03	M = 2.85	M = 2.92
Baseline STD HR (1 per minute)	SD = 1.35	SD = 2.82	SD = 1.13	SD = 3.53	SD = 0.92	SD = 0.63

GI, gastrointestinal; GSI, Gastrointestinal Severity Index; M, mean; n, number of participants in each group; SD, standard deviation; STD HR, standard deviation of instantaneous heartbeat (i.e., heart rate variability [HRV]).

In this sample, young children showed greater STD HR than adolescents both at baseline, t(25) = 2.71, p = 0.012, and at the 12-week follow-up, t(20) = 3.58, p = 0.002. However, for the adolescent group (15–24 years old), baseline STD HR was positively correlated with response to propranolol (r = 0.646, p = 0.044, r ² = 0.417), wherein higher HRV was associated with greater reduction of GI symptoms over the 12-week trial period of propranolol. The coefficient of determination showed a strong effect, wherein 41.7% of the variance in response to propranolol in adolescents was explained by baseline STD HR. STD HR was not significantly related to response to propranolol for young children (r = −0.022, p = 0.952). See Figure 1 for a visual of the relationship between baseline HRV and GI symptom change across age groups.

FIG. 1.

Total GI severity score for each participant in the (A) 7–14-year-old group and (B)15–24-year-old group at baseline and after 12 weeks of receiving propranolol. Lower total GI severity scores are indicative of better GI functioning. GI, gastrointestinal.

Discussion

The findings from this open-label pilot trial of propranolol suggest that some individuals with ASD and at least one GI symptom, who also have higher baseline HRV, may have a more favorable response to propranolol, resulting in the greatest reduction in GI symptoms, especially for adolescent and young adult participants aged 15–24. However, HRV did not predict the response to propranolol in children aged 7–14. Furthermore, 37% of participants had fewer GI symptoms after the trial, 33% had more GI symptoms, and 29.6% had no change in GI symptoms. These findings begin to suggest ANS involvement in GI symptoms in at least some individuals with ASD, and that future randomized controlled trials are needed in this area. Specifically, future trials should examine other stress and anxiety-related biomarkers that may better explain why some individuals experience a reduction in GI symptoms after taking propranolol.

As previously mentioned, there is a large amount of heterogeneity in ASD, so it is unlikely that one biomarker will be able to differentiate between treatment responders and nonresponders. In this study, it appears that adolescents and young adults with greater baseline HRV, indicating greater parasympathetic control, were more likely to have reduced GI symptoms after taking propranolol for 12 weeks. This finding builds upon previous research where participants (N = 110) with more severe lower GI tract symptoms, especially constipation, were found to have greater parasympathetic tone at baseline (Ferguson et al. 2017). This paradoxical finding (i.e., those with greater parasympathetic tone at rest are more likely to have GI symptoms and more likely to respond favorably to propranolol) is a topic for future research. As previously mentioned, one possible explanation for this finding is that some autistic children with chronic stress and anxiety activate the cholinergic anti-inflammatory pathway, which leads to a compensatory high baseline parasympathetic tone, manifesting as increased HRV. The present pilot study did not aim to measure differences between individuals with acute versus chronic stress, so this may be one reason for the variability in treatment response among the participants. Regardless, it should be noted that propranolol is not Food and Drug Administration-approved for the treatment of GI symptoms in ASD, and its use of propranolol for this purpose is considered off-label.

The present pilot study has a number of limitations that should be considered when interpreting the data. First, the results are from an open-label trial, so the results may be due to the expectancy effect. Second, the GI assessment used was not robust, which forced us to examine total GI symptoms as a whole. Future studies should use more robust GI questionnaires with larger samples to examine the effects of propranolol on discrete GI symptoms (e.g., constipation vs. diarrhea). Furthermore, it can be argued that the distinction between upper and lower GI tract symptoms using the GSI is not robust, so future research wishing to make this distinction should use more robust GI questionnaires (e.g., Rome-IV) or conduct a thorough clinical evaluation by a gastroenterologist. Third, the only biomarker of interest examined in this study was one HRV variable, and it is likely that other variables such as the microbiome and/or the microbiota, for example, may interact with the ANS via the gut-brain axis to cause and maintain GI symptoms in ASD. However, the selected HRV variable lends itself well to scalability as it can be monitored in a variety of manners in future studies on ASD (Taj-Eldin et al. 2018). Furthermore, the laboratory environment in which the data were collected may have induced stress, which could confound the results. Future research in this area as well as research examining the stress response in general in ASD may benefit from ecological measurements of stress using wearables such as smartwatches utilizing photoplethysmography that noninvasively collects stress data over several days in a real-world environment. Lastly, it is possible that the younger group of participants did not experience GI improvements given that their dose of propranolol was based on the participants’ weight, whereas older participants received 40 mg. This decision was made for the safety of younger participants, but perhaps future trials could examine higher doses of propranolol under closer supervision in an inpatient setting, for example. Regardless, these pilot data suggest that some individuals with ASD and co-occurring GI symptoms may respond favorably, so future work in this area is warranted.

Conclusions

Preliminary evidence from this open-label pilot investigation suggests that HRV may differentiate between responders versus nonresponders for the treatment of GI symptoms with the beta blocker propranolol in autistic adolescents. The results should be interpreted with caution until randomized clinical trials are conducted in a larger sample of autistic people.

Clinical Significance

There is a large amount of heterogeneity in ASD which complicates treatment efforts. Hence, many clinical trials in ASD likely fail due to this heterogeneity, so biomarkers that differentiate between treatment responders and nonresponders are needed. Up to 90% of children and adolescents with ASD may have co-occurring GI symptoms, which have been shown to be associated with internalizing and externalizing behaviors. Further, propranolol has been shown to reduce anxiety and problem behaviors in ASD, so the fact that propranolol may also be used to treat GI symptoms suggests the potential to further improve the quality of life in this population.

Footnotes

Authors’ Contributions

B.J.F.: Conceptualization, investigation, methodology, writing, reviewing, and editing the original draft. K.D.: Formal Analysis, writing, reviewing, and editing the original draft. M. Hoffman: Data curation, reviewing the final draft. M. Hogg: Data curation and reviewing the final draft. C.R.: Data curation and reviewing the final draft. D.Q.B.: Funding acquisition, supervision, project administration, writing, reviewing, and editing the original draft.

Data Sharing Statement

The datasets generated during and/or analyzed during the current study are not publicly available but are available from the corresponding author upon reasonable request.

Author Disclosure Statement

D.Q.B. serves as a consultant for YAMO Pharma, Impel Pharma, Scioto Biosciences, and Stalicla Biosciences, not related to this work. For the remaining authors, none were declared.

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