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).

Inclusion Criteria:

- 18-60 years old

- Postural Tachycardia Syndrome: Heart rate increase >30 bpm from supine within 10 min
of standing, in the absence of orthostatic hypotension (>20/10 mmHg fall in blood
pressure), with chronic symptoms (> 6 months), and in the absence of other acute cause
of orthostatic tachycardia.

- Able and willing to provide informed consent

- Female premenopausal subjects must utilize adequate birth control and willingness to
undergo serum beta-hCG (human chorionic gonadotropin) testing

Exclusion Criteria:

- Use of acetaminophen or acetaminophen-related drugs (over-the-counter) in the 24 hours
prior to the study.

- Hypertension (>150 mmHg systolic and >100 mmHg diastolic) based on history or findings
on screening.

- Orthostatic hypotension (consistent decrease in BP >20/10 mmHg with 10 min stand)

- Pregnancy

- History of type 1 or type 2 diabetes mellitus

- Cardiovascular disease, such as myocardial infarction within 6 months, angina
pectoris, significant arrhythmia (sinus tachycardia is not excluded), deep vein
thrombosis, pulmonary embolism

- History of serious neurologic disease

- Impaired hepatic function (aspartate amino transaminase and/or alanine amino
transaminase >1.5 x upper limit of normal range)

- Impaired renal function (serum creatinine >1.5 mg/dL)

- Hematocrit <28%

- Any underlying or acute disease requiring regular medication that could possibly pose
a threat to the subject or make implementation of the protocol or interpretation of
the study results difficult

- Inability to comply with the protocol

Healthy control subjects

Defined as subjects without any significant past medical history, non-smokers, and on no
chronic medications at the time of the study. Healthy control subjects will be age- and
BMI-matched to the POTS patients.

Positive control

Patients with complete autonomic neuropathy (pure autonomic failure) will be enrolled as
positive control. This condition is defined as complete autonomic failure based on AFT
(autonomic function test) and norepinephrine plasma levels less than 100 pg/ml.