The present study is particularly aimed at a population combining mental retardation and psychiatric co-morbidity, with relatively low intelligence quotient (1, 2) and maladaptive behavioral reactions (3). Indeed, such characteristics can lead to inadequate responses to changes in the individual’s environment. The frequency of aggression and abnormal episodes (e.g., self-mutilation, anger, verbal or physical aggression toward others, or property damage) can be a challenge with this population, with a prevalence of 4–10% depending on the country and the duration of observations (4–7). The unpredictability of these behaviors and the deficiencies in individuals’ abilities to communicate their feelings before these episodes take place make them particularly dangerous to themselves and the others.

A person’s ability to deal with situational changes is made possible through the dynamics of his autonomic nervous system (ANS). The ANS either activates or inhibits the body’s system of vigilance through the innervations of the myocardium. This occurs via the sympathetic nervous system (SNS) when activating and via the parasympathetic nervous system (PNS) when inhibiting. Depending on the psychiatric, psychological, or emotional conditions that an individual is facing, the ANS responses to the demands of their environment could be expressed by in an increase in SNS activity, a decrease of PNS control, or a combination of both (8–11). These autonomic functional differences are described as representative of different emotional state and social position, anxious, and submissive on the one hand with a key behavioral inhibition through the parasympathetic branch (12–16), and hostile and dominant on the other hand, requiring a rapid influx of blood to muscle and the brain through sympathetic-adrenaline activity, which may of may not occur with a lowering of parasympathetic inhibition (17, 18). The ability to recognize or predict of such patterns by examining a patient’s cardiovagal balance, especially in patients showing unpredictable reactions and language deficiencies, might be a major step forward in clinical research to help them. In addition, this type of data might help to clarify the concept of intentionality in the unexpected explosive behavior exhibited by this population.

Educational monitoring such as operant conditioning or applied behavior analysis (19, 20) and breathing control (21, 22) have shown encouraging results in attempts to defuse antisocial behavior and mood fluctuations in long term therapies. A means of recognizing the imminence of aggressive reactions and exploiting that recognition in order to avoid such unpleasant interventions as the seclusion, restraint, or antipsychotic medication of people with an intellectual disability, appears not only essential but also rational and ethical (23–26). Indeed, some heart rate and heart rate variability (HRV) fluctuations have been described to be predictive of reactive aggression (18, 27, 28). This study’s primary goal was to identify markers of increasing physiological agitation—the indicators of imminent aggressive behavior.

Seven individuals reached at least a level 3 of overt excitation (moderately tense) or more (very to extremely tense, levels 4 and 5). Among all the marked time periods picked up from the ECG signals, a total of 197 was retained after removing noisy or sleep episodes. In sum, the data of overt tension levels are made up of 46 periods of slight excitation (level 2), 18 of moderate excitation (level 3), 10 of high excitation (level 4), and 5 of extreme agitation (level 5). The detailed contribution of each subject for every tension level is provided in Figure 2.

The physiological values in the calm state (over the seven participants) were measured at 83 b.p.m. (±2 SE) for heart rate frequency, 2.5 (±0.2 SE) for LF/HF ratio, and 68 (±14 SE) for RMSSD index. These estimations are normal to above the expected standard (32, 44).

This study’s primary goal was to identify markers of increasing physiological agitation that might reveal imminent outburst, particularly in a group of patients with intellectual disabilities and associated behavioral disturbances. By recording continuous ECGs over long periods of time and evaluating the patient’s level of excitation every 30 min, it seems that we identify fluctuations in HRV that might be specific to the moments prior to an increase in the level of excitation. The results of this small pilot study require additional, more in-depth study before they can be confirmed. Nevertheless, we were able to show that the LF/HF ratio did not increase proportionally with excitation. Instead, the index of parasympathetic activity, RMSSD, decreased as the patient’s state changed from slightly tense (level 2) and very tense (level 4), and increased just when reaching extreme excitation (level 5). The increase of RMSSD at level 5 could be explained by the fact that level 5 was utilized when outburst occurred for the three participants concerned. The resolution phase of the outburst might lead to this increase. Thus, this pilot study demonstrated that fluctuations in HRV do not necessarily follow a linear relationship with the observed level of excitation. It also showed that this simple, non-invasive method of collecting ECG data may offer many advantages in efforts to identify changes in physiological arousal prior to potentially aggressive behavior.

Our results show that the RMSSD index measured for the tested population is normal to above the expected standard (32, 44). This normal-to-over-activation of parasympathetic activity discards suggestions that such population exhibit functional parasympathetic failure (12, 13, 45, 46). The deficiencies in behavioral adaptation observed in intellectually impaired subjects could thus be the result of a heightened general state of excitation, with a final breakdown in control when stress reaches an extreme level (30).

Overall, our results seem to suggest that the dominance of the parasympathetic branch of the ANS might demonstrate a position of subordination and a fear of losing control (16, 47, 48). This is similar to recent descriptions related to aggression in fragile X syndrome (49), or brain oscillations of this intellectually disabled population when confronted with emotionally negative images (50). The inhibitory parasympathetic branch appeared late in mammalian evolution in order to suppress the strong fight-or-flight reactions resulting from SNS accelerator inputs (9). Controlling the inhibitory parasympathetic branch is described as promoting interpersonal exchanges and developing social connections (51). According to this pilot study’s results, this nervous inhibitory pathway is mainly activated in individuals with intellectual disabilities and psychiatric conditions, offering new perspectives for therapies that put the emphasis on dealing with anxiety rather than aggressiveness (52). Specific intervention aiming at improving heart rate coherence might be useful. A pilot study using training in heart coherence was tested with mild intellectual disability volunteers working in sheltered workshops. Training in cardiac coherence was implemented during two consecutive weeks. Results indicated that participants could benefit physiologically, psychologically of both (53). However, these results are preliminary and need more controlled validation.

The main limitations of this study are the small sample of participants and the loss of participants due to failure in signal acquisition. The small number of participants prevent from finding a statistically significant result. The ideal design would require a continuous video recording of the behavior of participants to segment signals in realistic fitting periods, but this is difficult to achieve in natural conditions without limiting the freedom of the participants.

In conclusion, the data gathered in this study of a group of patients with intellectual disabilities and psychiatric co-morbidity showing overt aggressive behavior suggests that anxiety-based reactions predominate above hostile aggressive behavior. Furthermore, a decreased in parasympathetic activity during tense condition emerge to be a marker to imminence of agitation.

Commission cantonale d’éthique de la recherche sur l’être humain. Secrétariat administrative, Avenue de Chailly 23, 1012 Lausanne. This study was carried out in accordance with the recommendations of the Human Research Ethics Committee of the Canton de Vaud, Switzerland, and the Declaration of Helsinki. All subjects gave informed written consent to their participation in the study, with the agreement of their legal representative and their family.

JF, CC, FG, and JP contributed substantially to the conception and design of the study. JF and FG contributed substantially to data acquisition; JF, MA, and JP contributed substantially to the analysis and interpretation of data. JF and JP drafted the article. MA, CC, and FG revised it critically for important intellectual content. All authors approved the version to be published and agree to be held accountable for all aspects of the work, ensuring that questions related to the accuracy or integrity of any part of it are appropriately investigated and resolved.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer SO and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.