Gastrointestinal Symptoms in Postural Orthostatic Tachycardia Syndrome

Postural Tachycardia Syndrome (POTS) is a disabling condition that mostly affects young women in their reproductive age. It is characterized by chronic (>6 months) orthostatic intolerance symptoms (palpitation, lightheadedness, blurred vision and mental clouding) triggered by assuming an upright posture and that improved upon recumbency. These symptoms are associated with a rapid increase in heart rate (≥30 bpm) that occur within 10 minutes upon standing. POTS is estimated to affect up to 3 million persons in the United States and is considered a syndrome rather than a single disease.

The pathophysiology of POTS is complex, and are related to abnormal cardiovascular autonomic adaptation to postural changes. Under normal conditions, the assumption of upright posture does not result in major changes in blood pressure due to the integration of complex autonomic, circulatory and neurohumoral responses. Upright posture-induced a fluid shift of approximately 700 mL of blood from the upper thorax to the splanchnic circulation and lower extremities, which result in decrease in venous return to the heart, ventricular filling, and stroke volume. These changes cause unloading of the arterial baroreceptors and increase in sympathetic activity, vasoconstriction and restoration of stroke volume and cardiac output.

In POTS patients, multiple mechanisms have been proposed to explain the exaggerated increase in heart rate. The orthostatic tachycardia could be a compensatory phenomenon to hypovolemia, impaired sympathetic-mediated vasoconstriction or increased vascular compliance. The later could induce an exaggerated fluid shift upon standing from thorax to lower body. Depending on the mechanism involved different POTS phenotype has been described: (i) hypovolemic POTS; (ii) neuropathic POTS; and (iii) POTS associated with Ehlers-Danlos and joint hypermobility syndrome (EDS/JHS). Of note, there is overlapping in the pathophysiology of POTS with patients having more than one etiology.

In addition to the cardiovascular symptoms, patients with POTS experience significant gastrointestinal symptoms namely nausea, bloating, diarrhea or even severe constipation. Furthermore, large meals or high carbohydrate meals exacerbates the feelings of palpitations, weakness, and fatigue in these patients.

Multiple studies have reported the presence of alterations in the gastrointestinal motility. Pooled data from 352 patients recruited from 6 different studies, showed 21-80% prevalence of nausea, vomiting, and abdominal pain. In four of these studies that measured gastric motility, they found that 43% prevalence of rapid gastric emptying and 20% prevalence of delayed gastric emptying. Furthermore, Al-Shekhlee et al. reported a high prevalence of impaired sudomotor function in the POTS patients who reported GI symptoms suggesting that abnormal post-ganglionic sympathetic function could play a role in the pathophysiology of these GI abnormalities.

We previously defined a subgroup of POTS patients in whom we detected a partial peripheral autonomic neuropathy primarily affecting lower extremities (neuropathic POTS). These subjects had decreased norepinephrine spillover in response to sympathetic activation and abnormal sweat volumes and prolonged latency detected by quantitative sudomotor axon reflex (QSART). Recently, Gibbons and Freeman (2013) strengthen the definition by providing histological evidence of neuronal damage with the inclusion of skin biopsies with specific staining for autonomic dense fiber and sensitivity assessment.

In Neuropathic POTS there is evidence of impaired vasomotor tone in different specific vascular bed, particularly the splanchnic circulation. Tani et al. reported reduced splanchnic vascular resistance and increase in resting mesenteric blood flow providing evidence of splanchnic denervation.

In summary, there is evidence of post-ganglionic sympathetic denervation is a subset of patients with POTS. The most current definition are based on the presence of abnormal sudomotor and sensitivity assessment.

The sympathetic nervous system (SNS) provide innervation to the enteric ganglia, the circular muscles of sphincters, and the mucosa of the stomach and intestines. The SNS also negatively regulate the motor and secretory functions of the gastrointestinal (GI) tract. Browning and Travagli (2014) reported that the absence of sympathetic inhibitory innervation causes excessive and uncoordinated activity in the GI tract. Indicating that a preserved ANS (autonomic nervous system) regulation of the GI tract is crucial for the maintenance of normal GI motility.

In addition to regulating the motor function, the SNS and parasympathetic nervous system (PNS) regulate the postprandial GI peptides secretion by enteroendocrine cells (EEC). EECs are the first line components of the Brain-Gut axis. Multiple peptides, such as incretins (GLP-1, GLP-2, GIP), and PYY (peptide YY) are important for the maintenance of glucose homeostasis. They are secreted by a different type of EEC in the GI tract. Prior to their absorption, nutrients in the GI lumen are important stimuli for peptide secretion in the ileum in rats, pigs , and humans. These peptides are secreted before the bulk of ingested meal reaches to the ileum, suggesting the presence of a neuronal/endocrine pathway in GI tract.

In summary, the SNS through innervation the gut smooth muscle; ENS (enteric nervous system) and EECs negatively regulate the GI motor function and incretins secretion which impact glucose homeostasis.

Evidence from animal models showed that when rats underwent removal of the superior autonomic mesenteric ganglia that contains mostly SNS neurons and were challenged with an oral glucose gavage; plasma insulin and C-peptide secretion were increased compared with controls (non-ganglionectomised rats). Furthermore, glucose levels were much lower in the ganglionectomised rats suggesting that the SNS splanchnic innervation plays a critical role in the maintenance of glucose homeostasis. The increased secretion of insulin and C-peptide levels in this model could be explained by an increase in incretin hormonal release. Additional studies using isolated guineas pig ileum (in vitro model) showed that GLP-1 secretion is inhibited by SNS nerve stimulation which is mediated by α-adrenergic receptors.

In summary, in the absence of sympathetic tone on ENS and EECs the incretins secretion increases which may cause low levels of plasma glucose.

The focus of the present proposal is to determine glucose homeostasis, GI motility, and their association with GI and cardiovascular symptoms in POTS patients versus healthy controls. Furthermore, we will determine differences in these outcomes in POTS patients with and without evidence of postganglionic sympathetic fiber neuropathy.

The glucose homeostasis will be evaluated by a modified oral glucose tolerance test (OGTT). In addition, we will assess GI symptoms and hemodynamics before and after oral glucose (at minute 0, 30, 60, 90, and 120). The plasma levels of GI peptides (GLP-1, GLP-2, PYY, glucagon, C-peptide, insulin) will be measured in different time points after oral glucose. Gastric emptying will be evaluated by acetaminophen absorption test (AAT). The LPS (lipopolysaccharide), LBP (lipopolysaccharid-binding protein), sCD14, and I-FABP (faty acid-binding protein) as GUT cells damage markers will be measured at baseline. The following technics will be used in this study:

1. Oral glucose tolerance test (OGTT): In the case of OGTT, subjects will be given a ready-to-use test solution (TRUTOL® 75, Thermo Scientific, USA) containing 75 g glucose dissolved in 300 mL water, immediately after fasting blood sampling. They will be instructed to drink the test solution within 5 mins. Blood samples will be drawn at 5, 10, 15, 30, 60, 90, and 120 minutes after drinking the ready-to-use test solution. Gastric emptying will be measured by acetaminophen absorption test.
2. Acetaminophen absorption test (AAT): Acetaminophen (20 mg/kg) will be given to patients. Serum acetaminophen will be determined by fluorescence polarization immunoassay. This assay uses a six-point calibration curve, and the detection limit is 4 µmol/L. The coefficient of variation is less than 5%. Estimation of the rate of gastric emptying was based on serum concentrations of acetaminophen in the blood samples collected. An algorithm that transforms serum concentrations of paracetamol into estimates of gastric emptying was applied. This algorithm adjusts for first-pass metabolism, unequal distribution and individual rate of elimination, and provides estimates for the percentage of meal emptied from the stomach as a function of time.
3. Gastrointestinal symptoms scoring: The 2-page questionnaire is based on elements from a questionnaire that have been validated with some modifications. The questionnaire contains 17 questions on the frequency of GI symptoms that have been troublesome in the preceding 6 months. The frequency of each symptom is rated on seven-point Likert scale from no discomfort to very severe discomfort.
4. Hemodynamic symptoms scoring: Hemodynamic symptoms will be measured by using the Vanderbilt POTS Symptom Score. The patients will be asked to rate the severity of 9 symptoms on a 0-10 scale (with 0 reflecting an absence of symptoms). The sum of the scores at each time point will be used as a measure of symptom burden. The 9 symptoms are: mental clouding, blurred vision, shortness of breath, rapid heartbeat, tremulousness, chest discomfort, headache, lightheadedness, and nausea. This symptom score has been previously used by our center, and the symptoms were chosen as they reflect common complaints of patients with POTS.
5. Glucose and insulin levels: Glucose levels will be measured with a glucose analyzer (YSI Life Sciences, Yellow Springs, OH).
6. GI peptides measurements: The plasma designated for GLP-1 measurement will be supplemented with aprotinin (1,000 kallikrein inactivation unit (KIU)/ml) and dipeptidyl peptidase-4 inhibitor (20 μl/ml plasma; Millipore, St. Charles, MO). Plasma insulin, c-peptide, glucagon, GIP, active GLP-1 (7-37 and 7-36 amide), peptide YY, pancreatic polypeptide, and leptin were measured by multiplex immunoassays (Luminex, Millipore).