The current study is aimed at finding the differences in HR variability among children and adolescents with anxiety disorders as compared to healthy controls. Additionally, the study explores the correlation between clinical parameters and autonomic dysfunction among children and adolescents with anxiety disorders. We have included an age- and sex-matched control group.
Both study groups were comparable on different socio-demographic parameters. A majority of the subjects [73.6%] had one or two siblings and were first in the birth order [61.8%]. It was found that a majority of the study subjects [85.3% in the disease group and 100% in control group] were from an urban setting, which could be due to the fact that the study was conducted in an urban setting. Another reason might be that patients in the urban settings had more awareness and accessibility to treatment centers and thus sought treatment. Moreover, the available epidemiological studies from India have reported comparable rates of psychiatric disorders among children and adolescents from the rural and urban settings.[6
Generalized anxiety disorder [GAD] was the most common diagnosis [44.1%] followed by social phobia [SP] [26.5%], and separation anxiety disorder [SAD] [5.9%]. In the present study, the gender differences with regard to the type of anxiety disorder were not significant. Comparable rates of anxiety disorders have been reported among male and female children and adolescents.[19
Literature reveals that anxiety disorders in the youth are receiving increasing attention. Such attention is understandable considering that an estimated 10 – 20% of the youngsters suffer from anxiety and anxiety-related symptoms. Anxiety disorders comprise the most prevalent set of psychiatric disorders in children and adolescents.[1
] This can affect the developmental process, school performance, and result in a strained relationship with peers and family members.
Anxiety is often accompanied by somatic manifestations that suggest morbid changes in the autonomic nervous system [ANS] activity, such as rapid heart rate [HR], shortness of breath, and sweating. The autonomic characteristics of depression and panic disorder have been studied extensively.[19
] However, relatively few studies have examined the autonomic characteristics of anxiety disorder, especially in the case of children and adolescents.
Heart Rate [HR] variability, a measure of both sympathetic and parasympathetic activity exhibited significant differences between the disorder group and control group for both the time and frequency domains in the current study.
Although heart rate variability has been addressed among adult populations it has been given little attention among children and adolescents with anxiety disorders. A recent study done among children with ADHD has found that these children have lower HR reactivity immediately after a stress task.[20
] Dietrich et al
., found that while externalizing problems (e.g, conduct disorder, oppositional defiant disorder) were associated with lower HR, internalizing problems (e.g. depression) were associated with higher HR among pre-adolescents.[14
The HR variability has been postulated to have a heritable effect. Srinivasan et al
., compared children of parents with panic disorders with healthy controls. Children of patients with panic disorder had a significantly lower Lyapunov exponent [LLE] of heart rate time series in the supine posture. It suggested a relative decrease of cardiac vagal function in this group of children.[15
The finding of decreased sympathetic activity in the present study are in contrast to a previous study by Thayer et al
] A similar spectral analysis of HR variability has been used in this study. This method has been widely recommended and employed in deriving indices that reflect sympathetic and parasympathetic cardiovascular influences. Shorter interbeat intervals and lower high-frequency spectral power has been observed among adult patients of GAD as compared to controls. These findings indicate a tonically weak vagal autonomic control, which is in-keeping with our reported findings. However, strong sympathetic autonomic control has been observed in this study, which is in contrast to our findings of decreased sympathetic activity. This could be explained due to the age difference between the two patient groups or by the difference in the socio-cultural background and ethnicity. Cultures instill beliefs about how emotions should be experienced and expressed, which can translate into cultural group differences in actual emotional responses.[21
The sympathetic inhibition found in the anxiety disorder group was similar to that of a previous study on GAD patients in the adult population, which also reported sympathetic inhibition.[22
] The study assessed skin conductance and interbeat intervals in 40 GAD patients and 20 controls.
Our findings show a pattern of decreased parasympathetic activity in the disorder group. This was similar to a previous study done on adults aged 23 – 40 years.[21
] This study postulated that anxiety was related to a reduced vagal control [parasympathetic activity] of the heart. Furthermore, a stepwise increase in trait anxiety scores were associated with reduction in vagal tone. BRC [Baroreflex control of heart rate] and RSA [Respiratory sinus arrhythmia] were used to assess the vagal tone in this study. Licht et al
., reported a lower HR variability among adults with anxiety disorders [including GAD and social phobia]. However, after controlling for potential confounders it was found to be related to the use of tricyclic antidepressants.[23
] The current study had recruited drug-free subjects, and hence, the findings were free of likely confounding by their effect.
In the current study, there were significant differences in the parasympathetic reactivity between the control and anxiety groups, thus implying that children and adolescents with anxiety disorder had their parasympathetic reactivity [heart rate response to given perturbation] outside the normal range. Two other studies with a similar methodology had findings, which were in contrast to the findings of the present study.[25
] This disparity could be explained on the basis of methodological differences. In the current study, a detailed analysis of the interbeat intervals could only be done after plotting Fast Fourier Transform [FFT], in contrast to the aforementioned studies. However, the findings from a similar study among adults with GAD showed that no differences were observed in vagal tone (parasympathetic reactivity).[19
Our findings are suggestive of autonomic rigidity or diminished physiological flexibility in children and adolescents with anxiety disorder. This loss of variability in the autonomic control system, in general, has been linked with a number of diseases and dysfunctions. Reduced heart rate variability is a prognostic factor for cardiac mortality.[27
] A decrease in cardiac vagal function, as suggested by a decrease in heart rate [HR] variability, has been linked to sudden death.[28
] HR variability measurement is useful in investigating the pathophysiology of various psychiatric disorders.[29
Anxiety disorders during childhood and adolescence are associated with significant morbidity. Findings of studies exploring the role of childhood anxiety disorders as future predictors of adult psychiatric disorders have come up variable findings.[30
] The current study has shown reduced variability of the heart rate in both the time and frequency domains, as compared to the control population. These findings signify decreases in sympathetic and parasympathetic activity in the disorder group, thus representing diminished physiological variability at rest. The presence of a high vagal tone seems to be a marker of physiological and psychological flexibility. Alternatively, reductions in the complexity of responding in a wide range of physiological channels are associated with poor health outcomes and a lack of adaptive variability in behavioral and cognitive functioning.
The notion of autonomic inflexibility has important implications for recent conceptualizations of stability and instability in biological systems. Contemporary models of nonlinear dynamical systems view flexibility in response to changing environmental demands as a marker of system integrity, and interpret reduced high frequency variability as being pathological.[16
Reduced HR power reflects a shift away from the parasympathetic control of the heart. The loss of complexity and variability in the physiological systems in general, and in the cardiovascular system in particular, has recently been linked with a number of diseases and dysfunctions. Several behavioral and psychological states, such as, acute and chronic smoking, acute and chronic alcohol ingestion, sedentary lifestyle, depression, panic disorder, and aging have all been associated with a loss of heart rate variability and complexity.[32
] Additionally physiological disorders such as fetal distress syndrome, sudden cardiac death, ventricular fibrillation, hypertension, diabetes mellitus, and coronary atherosclerosis have been associated with reduced HR variability.[37
The current study is one of the limited studies, exploring the role of the parasympathetic system on HR variability among childhood anxiety disorders. Although studies exist on adult anxiety disorders, literature is limited on children and adolescents. We have made use of a widely accepted technique to study HR variability and followed the standardized protocol.[8
We made only cross-sectional observations. A prospective cohort study of these subjects, following remission of clinical symptoms, would further add to the understanding on the issue.