The autonomic nervous system (ANS) is responsible for the precise regulation of tissue functions and organs and, thus, is crucial for optimal stress reactivity, adaptive responses and health in basic and challenged states (survival). The fine-tuning of central ANS activity relies on the internal central autonomic regulation system of the central autonomic network (CAN), while the peripheral activity relies mainly on the two main and interdependent peripheral ANS tracts, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). In disease, autonomic imbalance is associated with decreased dynamic adaptability and increased morbidity and mortality. Acute or prolonged autonomic dysregulation, as observed in stress-related disorders, affects CAN core centers, thereby altering downstream peripheral ANS function. One of the best established and most widely used non-invasive methods for the quantitative assessment of ANS activity is the computerized analysis of heart rate variability (HRV). HRV, which is determined by different methods from those used to determine the fluctuation of instantaneous heart rate (HR), has been used in many studies as a powerful index of autonomic (re)activity and an indicator of cardiac risk and ageing. Psychiatric patients regularly show altered autonomic function with increased HR, reduced HRV and blunted diurnal/circadian changes compared to the healthy state. The aim of this article is to provide basic knowledge on ANS function and (re)activity assessment and, thus, to support a much broader use of HRV as a valid, transdiagnostic and fully translational dynamic biomarker of stress system sensitivity and vulnerability to stress-related disorders in neuroscience research and clinical psychiatric practice. In particular, we review the functional levels of central and peripheral ANS control, the main neurobiophysiologic theoretical models (e.g., polyvagal theory, neurovisceral integration model), the precise autonomic influence on cardiac function and the definition and main aspects of HRV and its different measures (i.e., time, frequency and nonlinear domains). We also provide recommendations for the proper use of electrocardiogram recordings for HRV assessment in clinical and research settings and highlight pathophysiological, clinical and research implications for a better functional understanding of the neural and molecular mechanisms underlying healthy and malfunctioning brain–heart interactions in individual stress reactivity and psychiatric disorders.

Polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, skin changes (POEMS) is a rare paraneoplastic syndrome, and its diagnosis is based on a series of clinical and laboratory findings. We present the case of a 46-year-old woman who was previously diagnosed with essential thrombocythemia. The patient complained about dyspnea on exertion, nausea, burning of the lower limbs, weight loss, recurrent episodes of lower back pain and polymenorrhea. Physical examination revealed hyperpigmentation, livedo reticularis of the lower limbs, sclerodermoid changes and plectrodactyly. A computed tomography-guided bone biopsy revealed the presence of plasmacytoma, and based on a combination of clinical features such as polyneuropathy, a diagnosis of POEMS syndrome has been established. The diagnosis of POEMS syndrome demands a high index of suspicion, especially in cases of peripheral neuropathy, peripheral edema or organomegaly of unknown origin. Since the syndrome can be fatal, early diagnosis is pivotal for patients’ survival and quality of life.