Definition and Overview
Stress is a biological defense response activated by the body to maintain homeostasis against external environmental threats or internal pressure. Since Hans Selye first proposed the concept of the general adaptation syndrome in 1936, the stress response has been a central topic of research in neuroendocrinology and autonomic medicine [1].
There are two key pathways of the stress response. One is the hypothalamic-pituitary-adrenal (HPA) axis, which regulates cortisol secretion, and the other is the sympathetic-adrenomedullary (SAM) axis, which is responsible for epinephrine and norepinephrine secretion. These two axes are closely interlinked and directly influence autonomic nervous system balance [1][2].
Autonomic nervous system activation during acute stress is a normal response essential for survival. However, when stress becomes chronic, sympathetic hyperactivity becomes entrenched and parasympathetic (vagal) function is suppressed, resulting in autonomic imbalance. This imbalance is directly linked to cardiovascular disease, metabolic abnormalities, immune dysfunction, and mental health problems [2][3].
Physiology of the Stress Response
HPA Axis (Hypothalamic-Pituitary-Adrenal Axis)
When a stress stimulus reaches the brain, corticotropin-releasing hormone (CRH) is secreted from the paraventricular nucleus of the hypothalamus. CRH stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH) into the bloodstream, and ACTH promotes cortisol synthesis in the adrenal cortex [1].
Cortisol raises blood glucose, promotes protein and fat breakdown, and modulates immune responses. Under normal conditions, elevated blood cortisol levels send feedback signals to the hypothalamus and pituitary to suppress HPA axis activity. This negative feedback must function properly for the stress response to terminate appropriately [1][2].
In chronic stress states, this feedback mechanism becomes blunted, causing cortisol to remain persistently elevated. According to research by Chrousos (2009), flattening of the diurnal cortisol rhythm is observed in approximately 40–60% of chronic stress patients [1].
SAM Axis (Sympathetic-Adrenomedullary Axis)
The SAM axis is a faster-responding pathway than the HPA axis. When a stress stimulus reaches the hypothalamus, sympathetic activation occurs within seconds and the adrenal medulla releases epinephrine and norepinephrine into the bloodstream [2].
Epinephrine triggers the classic fight-or-flight response: increased heart rate, elevated blood pressure, bronchodilation, pupil dilation, and elevated blood glucose through hepatic glycogenolysis. Norepinephrine primarily constricts peripheral blood vessels and maintains blood pressure [1][2].
SAM axis activation simultaneously suppresses parasympathetic (vagal) activity. This sympathovagal switch is the direct cause of reduced heart rate variability (HRV) during the stress response [3].
Interaction Between Cortisol and Epinephrine
The HPA and SAM axes do not operate independently but synergize with each other. Cortisol increases the sensitivity of catecholamine receptors at sympathetic nerve terminals, amplifying the effects of epinephrine and norepinephrine. Conversely, sympathetic activation stimulates CRH secretion, forming a positive feedback loop that further activates the HPA axis [1][2].
This synergistic interaction enables efficient coping during acute stress, but in chronic states becomes a vicious cycle mechanism that further exacerbates autonomic imbalance [2].
Acute Stress and the Autonomic Nervous System: Normal Response
The acute stress response is an adaptive mechanism essential for survival. When confronted with sudden danger, the hypothalamus activates the sympathetic nervous system within seconds, followed by activation of the HPA axis. The resulting physiological changes include the following [1][2].
Heart rate increases from 60–80 beats per minute to over 100–120 beats per minute, and systolic blood pressure rises by 20–30 mmHg. Respiration quickens and bronchi dilate to increase oxygen supply, while blood flow is redirected to skeletal muscles. Simultaneously, digestive function and immune responses are temporarily suppressed [2].
These changes reverse within minutes to tens of minutes once the threat subsides, as the parasympathetic (vagal) system regains dominance and the body returns to its baseline state. A healthy autonomic nervous system performs this transition flexibly, and this flexibility is reflected in HRV values [3].
Thayer and Lane (2009) proposed in the neurovisceral integration model that autonomic flexibility is directly linked to top-down regulatory function of the prefrontal cortex. Individuals with higher HRV demonstrate superior prefrontal function and better emotional regulation capacity [3].
Chronic Stress and Autonomic Damage
Allostatic Load
The concept of allostatic load proposed by McEwen (2007) explains the process by which chronic stress inflicts cumulative damage on the body. Allostasis is the adaptive process by which the body adjusts physiological parameters in response to stress. However, when this adjustment persists long-term, the regulatory systems themselves become worn, and allostatic load accumulates [2].
Key indicators of allostatic load include flattened diurnal cortisol rhythm, reduced HRV, elevated resting heart rate, increased blood pressure, abdominal obesity, and increased insulin resistance. According to McEwen's research, the group with allostatic load scores in the top 25% had a 2–3 times higher risk of cardiovascular events [2].
HRV Reduction
The autonomic indicator most sensitively altered by chronic stress is heart rate variability (HRV). HRV measures subtle variations in the time intervals between heartbeats, reflecting autonomic flexibility and adaptive capacity [3].
In a study by Lucini et al. (2002) analyzing healthy adults exposed to chronic workplace stress, the LF/HF ratio in the stress group was significantly higher than in controls, indicating sympathetic dominance and parasympathetic suppression. The HF power (parasympathetic component) of the chronic stress group was approximately 30% lower than controls [5].
Thayer and Sternberg (2006) reported that low HRV is not only a consequence of chronic stress but also an independent predictor of future cardiovascular disease, depression, and increased mortality [6].
Entrenchment of Sympathetic Hyperactivity
When chronic stress persists, the sympathetic nervous system becomes entrenched in a hyperactive state. In this process, norepinephrine release at sympathetic nerve terminals increases, and epinephrine secretion from the adrenal medulla remains above baseline levels [1][2].
When sympathetic hyperactivity becomes entrenched, heart rate and blood pressure remain elevated even at rest, and peripheral vascular resistance increases. Brosschot et al. (2010) reported that this phenomenon is maintained not only by conscious worry but also by unconscious perseverative cognition -- meaning that even after a stressful situation has ended, the brain continues to unconsciously process threat-related information, sustaining sympathetic activation [4].
According to this research, heart rate and blood pressure remained significantly above baseline even during times when subjects did not subjectively perceive stress, and this was observed throughout a substantial portion of the day (including sleep time in some subjects) [4].
Stress-Related Symptoms
Autonomic imbalance caused by chronic stress produces symptoms across multiple organ systems throughout the body. This is because the autonomic nervous system regulates virtually all bodily functions, including cardiovascular, respiratory, digestive, and immune function [1][2].
Cardiovascular Symptoms
Palpitations due to sympathetic hyperactivity are common. Resting heart rate may persistently exceed 90 beats per minute, or sudden heart rate fluctuations may recur. Blood pressure also becomes unstable, and some patients experience dizziness due to impaired blood pressure regulation upon standing [5].
Gastrointestinal Symptoms
When sympathetic dominance prevails during stress, gastrointestinal motility and secretion are suppressed. Gastrointestinal blood flow decreases and gastric acid regulation becomes unstable, resulting in heartburn, abdominal bloating, decreased appetite, and irritable bowel syndrome (IBS)-like symptoms. Functional digestive symptoms are reportedly present in approximately 40–60% of chronic stress patients [1][2].
Sleep Disturbances
Sympathetic hyperactivity degrades sleep quality. When the diurnal cortisol rhythm is disrupted, cortisol remains elevated at night, making sleep onset difficult and increasing nocturnal arousals. Polysomnography in chronic stress patients shows a decreased proportion of deep sleep (slow-wave sleep) and elevated sympathetic activation markers during sleep compared with healthy controls [2][4].
Headache and Musculoskeletal Symptoms
Sustained chronic tension increases muscle tone in the cervical and shoulder regions, leading to recurrent tension-type headaches. Vasoconstriction of scalp and cervical blood vessels due to sympathetic hyperactivity also contributes to headache development [1].
Immune Function Changes
Cortisol has immunosuppressive effects. Persistently elevated cortisol during chronic stress reduces natural killer cell activity and disrupts inflammatory cytokine regulation. This creates a paradoxical situation of increased vulnerability to infection while simultaneously maintaining a chronic inflammatory state [1][2].
Diagnosis and Assessment
Autonomic imbalance due to stress is assessed by integrating multiple indicators rather than relying on a single test [3][5].
Heart Rate Variability (HRV) Analysis
HRV analysis is the core tool for autonomic balance assessment. R-R interval variability on electrocardiography is interpreted through time-domain analysis (SDNN, RMSSD) and frequency-domain analysis (LF, HF, LF/HF ratio). RMSSD and HF components reflect parasympathetic (vagal) activity, while the LF/HF ratio reflects sympathovagal balance [3].
Typical findings in chronic stress patients include decreased SDNN, decreased RMSSD, decreased HF power, and elevated LF/HF ratio. According to Thayer (2009), the group with low HRV had approximately 32–45% higher all-cause mortality compared with the high HRV group [3][6].
Cortisol Testing
Salivary cortisol testing is performed four times daily (upon waking, morning, afternoon, and before bedtime) to confirm the diurnal cortisol rhythm. Normally, cortisol peaks 30–45 minutes after waking and reaches its lowest point at night; in chronic stress, this variability decreases and the overall pattern flattens [1].
Stress Questionnaire Assessment
The Perceived Stress Scale (PSS), stress response scales, and burnout assessment tools are used to quantify subjective stress levels. These questionnaire results are interpreted alongside objective indicators such as HRV to determine the degree of autonomic imbalance and guide treatment direction [5].
Tilt Table Test
When stress-related autonomic dysregulation is suspected, a tilt table test may be performed. This test evaluates the autonomic nervous system's response to postural changes and helps differentiate conditions such as orthostatic hypotension and postural orthostatic tachycardia syndrome (POTS) [5].
Treatment and Management
Treatment of stress-induced autonomic imbalance aims to suppress sympathetic hyperactivity and restore parasympathetic (vagal) function. Non-pharmacological treatment is recommended as first-line, with pharmacotherapy and neuromodulation therapy added as needed [3][5].
Breathing Training
Slow breathing is the most accessible method for directly stimulating the vagus nerve and enhancing parasympathetic activity. Practicing 6 breaths per minute (inhale 4–5 seconds, exhale 5–6 seconds) for 10–15 minutes daily significantly increases the HF component of HRV. This breathing frequency corresponds to the cardiovascular system's resonance frequency, maximizing autonomic regulatory efficiency [3].
Regular Exercise
Moderate aerobic exercise has strong evidence for autonomic balance restoration. Sustaining aerobic exercise (brisk walking, jogging, swimming, cycling) for 30–60 minutes, 3–5 times per week for 12 weeks or more produces decreases in resting heart rate, increases in HRV, and normalization of diurnal cortisol rhythm [2].
However, excessive high-intensity exercise may actually worsen sympathetic hyperactivity, so it is recommended to start at 60–70% of maximum heart rate and gradually increase intensity [2].
Cognitive Behavioral Therapy (CBT)
Cognitive behavioral therapy (CBT) is a structured psychotherapy that identifies and corrects thought patterns and behavioral styles that trigger stress. It is effective in blocking the sustained sympathetic activation caused by unconscious perseverative cognition reported by Brosschot et al. (2010), and improvements in HRV and reduced cortisol levels have been reported after treatment [4].
Biofeedback
HRV biofeedback is a method that trains autonomic regulatory capacity by monitoring one's own heart rate variability in real time. Patients adjust their breathing and relaxation while viewing HRV data on a screen, thereby strengthening autonomic self-regulation capacity [3].
HRV biofeedback training combined with resonance frequency breathing shows significant HRV improvement after 10–20 sessions and is also reported to be effective in reducing stress-related symptoms [3][6].
Neuromodulation Therapy
Vagus nerve stimulation (VNS) is a neuromodulation therapy that directly enhances parasympathetic function. Transcutaneous VNS non-invasively stimulates vagal branches at the ear or neck to increase parasympathetic activity [3].
Additionally, brain stimulation techniques such as transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS) are being studied as methods to strengthen prefrontal-autonomic connections [3].
Lifestyle Guide
The following lifestyle guidelines are recommended for the prevention and management of stress-induced autonomic imbalance [1][2][5].
Sleep management is fundamental. Go to bed and wake up at the same time each day to maintain a consistent sleep-wake rhythm. Seven to eight hours of sleep is recommended, and reducing smartphone and computer screen exposure starting 2 hours before bedtime is helpful.
Dietary management also affects autonomic balance. Limit caffeine intake to 200 mg or less per day (2 cups of coffee or fewer), and reduce or eliminate alcohol as it disrupts autonomic regulation. A diet rich in omega-3 fatty acids and magnesium helps maintain vagal function.
Social connection also contributes to autonomic health. Conversation and emotional exchange with trusted individuals promotes oxytocin release and enhances vagal activity. Conversely, social isolation is a known risk factor for sympathetic hyperactivity [2].
Spending time in natural environments is also effective. Studies report that spending 20 minutes or more in forests or parks significantly reduces cortisol levels and increases parasympathetic activity [2].
Finally, it is important to develop the habit of regularly monitoring your own stress levels. If symptoms such as palpitations, indigestion, or insomnia persist for more than 2 weeks, it is advisable to undergo autonomic function testing to objectively assess your condition [5].
About OSANG Neurosurgery
OSANG Neurosurgery accurately diagnoses the degree and type of stress-induced autonomic imbalance through HRV analysis, tilt table testing, and comprehensive autonomic function evaluation. Dr. Seungjae Kim, a neurology specialist, provides expert care in autonomic nervous system dysfunction and develops individualized treatment plans.
Evidence-based treatments are offered including breathing training guidance, HRV biofeedback, and neuromodulation therapy, along with an integrated stress management program that includes cognitive behavioral therapy and exercise prescription when needed.
Inquiries: OSANG Neurosurgery 1599-5453 | osns.co.kr