Note: This post is research-oriented, rather than personal, and is a review for those interested in learning more about the nervous system and about dysautonomia (autonomic nervous system dysfunction), specifically Postural Orthostatic Tachycardia Syndrome (POTS), including the history of POTS.
Peripheral Nervous System and Autonomic Dysfunction
The nervous system is an intricate network, made of the brain, spinal cord, sensory organs, and nerves, that connects to the body’s organs and allows for functioning in three primary roles, including sensory, integration, and response (OpenStax College, 2018). The nervous system can be broken down into the central nervous system (CNS), containing the brain and spinal cord, and the peripheral nervous system (PNS), which includes everything outside of the brain and spinal cord and is formed from neurons that connect throughout the body and link back to the CNS (Goldman, 2018; Gray, 1918; OpenStax College, 2013). The PNS includes the cranial nerves, which are the nerves connecting to the face and head, eyes, nose, ears, and muscles, the thirty-one spinal nerves, which connect the spinal cord to the rest of the body, and over 100 billion bodily nerve cells (Rubin, 2018).
Without the nervous system, the human body could not survive. Individually, the CNS and PNS contribute imperative functions, but together, they allow the human body optimal conditions in which to integrate information and produce responses, both voluntarily (somatic nervous system) and automatically [autonomic nervous system (ANS)] (OpenStax College, 2018). If homeostatic control is lost, wide-ranging dysfunction can result, including the most common form of ANS dysfunction, POTS, which is Postural Orthostatic Tachycardia Syndrome (Grubb, Kanjwal, & Kosinski, 2006; Raj, 2006; Schmidt, Karabin, & Malone, 2017). While the scientific community has made great strides in understanding and diagnosing this disease, POTS still remains under-recognized, under-diagnosed, and poorly treated (Collins, 2018). To add to these problems, the ANS’s role in health and disease has been “neglected in clinical research and training programs” (Pertab et al., 2018; Stiles, Cinnamon, & Balan, 2018; Zahedivash & Lee, 2018).
Functions of the Nervous System
In contributing to the functions of sensation, integration, and response, both the CNS and PNS contain neurons and glial cells, and both systems have white matter and gray matter (OpenStax College, 2018) – [see presentation, after this post, for more information on nervous system cells and tissues]. While the CNS and PNS contribute to the same functions, these functions are ascribed to different brain regions or areas of the PNS (OpenStax College, 2018). To create a response, nervous system regions participate in integration, where sensory input, from sensory organs and nerves, is combined with higher level brain functions, including memories, learning, and emotion (OpenStax College, 2018). Therefore, in order for the body to properly function and be capable of response, the CNS, PNS, and sensory organs are dependent upon each other and must work together (OpenStax College, 2018).
The Nervous System’s Bodily Control and Maintenance of Homeostasis
The nervous system can also be divided, based upon control of bodily functions, into the somatic nervous system, which is under voluntary control, the autonomic nervous system (ANS), which is involuntary and can affect cardiac muscle tissue and glands, as well as controlling processes such as heart rate, blood pressure, breathing rate, pupillary response, and the enteric nervous system, which overlaps with the ANS, is dedicated to the digestive system (OpenStax College, 2018). The ANS can be subdivided into the sympathetic and parasympathetic subsystems (Ernsberger & Rohrer, 2018; Jensen, 2014). These two subsystems serve to maintain balance, and therefore homeostasis, within the body. While the sympathetic nervous system is the body’s “fight or flight” response, the parasympathetic nervous system allows for resting and digesting (OpenStax, 2013; Teff, 2008, p. 569). Homeostasis is achieved when there is a balance between sympathetic and parasympathetic activity (OpenStax, 2013).
As the ANS controls all automatic bodily functions, that are not under conscious control, when there is a change, the ANS senses this imbalance and produces a reaction, such as triggering the sweat glands, in response to hot weather, and the pupillary response to light, as depicted in Figure 1 (OpenStax, 2013; OpenStax College, 2018). When light activates the ganglion cells of the retina, the signal is sent along the optic nerve to the brain (OpenStax, 2013). If dim light is perceived, the output results through the sympathetic nervous system, passes through the cervical ganglion, causing norepinephrine release, which stimulates the iris muscles, resulting in pupillary dilation (OpenStax, 2013). Alternatively, when bright light is perceived, the parasympathetic output arises in the midbrain, travels to the prevertebral ganglia, and acetylcholine is triggered and released, which activates the iris fibers, causing the pupils to constrict (OpenStax, 2013). This homeostatic balance mechanism is also activated by bodily positional changes, such as standing up, after lying down or sitting (Grubb et al., 2006; Raj, 2006; Schmidt et al., 2017). Dysfunction, within this response to standing, exemplifies disorders of orthostatic intolerance, a form of dysautonomia, which is ANS dysfunction; the most prevalent autonomic dysfunction disease, as previously mentioned, is POTS (Grubb et al., 2006; Raj, 2006; Schmidt et al., 2017).
ANS Dysfunction: Postural Orthostatic Tachycardia Syndrome (POTS)
POTS is a common disorder of orthostatic intolerance, estimated to affect approximately 3 million people in the United States and 11 million individuals worldwide (Schmidt et al., 2017). Normally, when a healthy person stands, gravity draws blood downward into the legs, and the cardiovascular system automatically adjusts, in order to keep blood pumping to the brain (OpenStax, 2013). To perform this function, sensory neurons identify that the body is changing position, and this signal is sent to the CNS, which then communicates to the sympathetic upper thoracic spinal cord neurons, which increase the heart rate and tell the blood vessels to constrict (OpenStax, 2013). When this process is dysfunctional, orthostatic intolerance can occur.
POTS Diagnostic Criteria
Specifically, POTS is diagnosed when the heart rate rises to greater than or equal to 30 beats/minute (bpm), or exceeds 120 bpm, within the first ten minutes of standing, without a corresponding large drop in blood pressure (orthostatic hypotension), though these two disorders do sometimes coexist; other patients, too, can be found to have comorbid POTS and orthostatic hypertension (Abed, Ball, & Wang, 2012; Cardet, Castells, & Hamilton, 2013; Chhabra & Spodick, 2013; Fessel & Robertson, 2006; Grubb et al., 2006; Lee & Kim, 2016; Raj, 2006; Schmidt et al., 2017; Wen et al., 2017; Zhao et al., 2015). To complicate diagnostic matters, POTS is more likely to reveal itself when testing is done in the morning hours; testing can commonly produce false negatives, and clinical suspicion of the disease must be high (Brewster et al., 2012; Moon et al., 2016).
POTS Symptoms and Statistics
POTS symptoms are wide-ranging and can include any or all of the following: dizziness, lightheadedness, syncope (fainting), fatigue, weakness, urinary problems, sexual dysfunction, headaches, migraines, breathing too fast, tachycardia, bradycardia, sweating too much or too little, digestive disturbances, sleep abnormalities, orthostatic hypotension, orthostatic hypertension, and more (Collins, 2018; Dysautonomia International, n.d.; Dysautonomia International, 2016; Raj, 2006; Schmidt et al., 2017; Vykoupil, 2016). POTS can be highly disabling, with functional impairment to the equivalent degree as that seen in chronic obstructive pulmonary disease (COPD) and congestive heart failure (Benrud-Larson et al., 2002; McDonald et al., 2014; Raj, 2006). However, despite this reality, patients are commonly young, female, and appear healthy; diagnosis is usually delayed, on average by eight to ten years, and the majority of patients, 76%, are first misdiagnosed, most commonly as having a psychological illness, and the disease course continues unrecognized, invalidated, and untreated (Anjum et al., 2018; Collins, 2018; Schmidt et al., 2017; Stiles et al., 2018). [2018 research suggests the average diagnostic delay has now been lowered to “over four years” (Stiles et al., 2018), but personally, I am skeptical of this number, as there are still too many people without diagnoses and treatment and who are lost and seeking answers that they are not receiving.]
History of POTS
Although ANS dysfunction is an increasingly recognized problem, across all fields of medicine, the complexity of the nervous system, lack of training provided to medical students, and neglect of the subject, by the research community, has led to today’s frequent conundrum of delayed diagnosis, prolonged time to treatment, and poor treatment options and efficacy (Collins, 2018; Palma et al., 2015; Stiles et al., 2018; Zahedivash & Lee, 2018). Further still, not even a single therapy has been Food and Drug Administration (FDA) approved for the treatment of POTS (Miller & Raj, 2018; Raj, 2006). While orthostatic hypotension was first described in 1925 and POTS, allegedly, in 1940, the American Autonomic Society was not founded until 1990, and POTS was first named only recently, at the Mayo Clinic, in 1993 (Agarwal et al., 2007; Sidhu et al., 2013). Despite POTS only being recently recognized, and many doctors still being unfamiliar with the syndrome, it has a long history; Dr. Jacob Mendez Da Costa first described, in 1871, what would later become known as POTS, which he called “irritable heart,” which came to be called Da Costa’s syndrome, but was also known throughout history by other names, such as: the effort syndrome, neurocirculatory asthenia, anxiety neurosis, mitral valve prolapse syndrome, and soldier’s heart, as Da Costa originally described the syndrome, within the context of soldiers, during the Civil War, which he stated was a common “cardiac malady” finding (Da Costa, 1871; Wooley, 1982). Given the lengthy passage of time, since the original description of POTS in 1871, the medical community has made shockingly little progress in understanding, diagnosing, and treating POTS and other disorders of autonomic dysfunction.
Overall, the nervous system is a complicated entity, within which proper functioning is vital, in order to carry out the three primary roles of sensation, integration, and response (OpenStax College, 2018). Given the complexity of this system, it is all too easy for abnormalities to arise and for dysfunction to occur, which can wreak havoc on quality of life and contribute to profound disability, such as in the case of POTS, despite individuals often continuing to appear seemingly normal (Stiles et al., 2018). While the medical community still has a long way to go, in terms of understanding, diagnosing, and treating disorders of ANS dysfunction, it is promising that each passing year produces an increasing wealth of knowledge, regarding normal and abnormal nervous system functioning.
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