A combination of symptoms such as tachycardia, dizziness, fatigue, pain, shortness of breath, and anxiety can be indicative of either Postural Orthostatic Tachycardia Syndrome (POTS) or Inappropriate Sinus Tachycardia (IST).
So, what’s the difference?
POTS vs. IST
A normal resting heart rate usually falls between 60 and 100 beats per minute (bpm). Tachycardia refers to a heart rate that exceeds the normal range. Palpitations refer to the feeling or sensation that your heart is skipping beats or beating unusually fast.
IST is a condition where your heart is beating fast for no obvious reason (while you’re at rest and not even exerting yourself). IST is a diagnosis of exclusion. In other words, diagnosing IST involves ruling out other potential causes first. If no other cause is found, IST is identified by a resting heart rate consistently above 100 bpm with an average 24-hour heart rate over 90 bpm. People with IST often experience palpitations along with tachycardia.
POTS is a condition that causes your heart rate to greatly increase when you go from supine (lying face up) to sitting to standing. In addition to the fast heart rate, the position changes can also induce other symptoms such as dizziness, fatigue, exhaustion, or even fainting. The criteria to receive a POTS diagnosis as an adult is to have a 30 bpm increase from supine to standing. For the adolescent population, the criteria is a 40 bpm increase.
What is Dysautonomia?
POTS and IST are caused by a dysfunction in the autonomic nervous system, known as dysautonomia. The autonomic nervous system (ANS) is an overarching system that’s responsible for maintaining homeostasis, or equilibrium, in the body. The ANS controls and regulates involuntary bodily functions like heart rate, blood pressure, and digestion.
The ANS is divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is the “fight or flight” response that prepares the body for stressful situations. It is responsible for increasing heart rate and blood pressure, sending blood to muscles, and increases alertness. The parasympathetic nervous system is the “rest and digest” system that works to conserve and restore energy by lowering heart rate and blood pressure, inducing relaxation, and increasing digestive activity. Dysautonomia usually reflects an imbalance or lack of equilibrium between the sympathetic and parasympathetic systems.
What We See Clinically
At CFNC, we see patients with many different forms of dysautonomia, with POTS and IST being among the most common. While these conditions present very similarly from a symptom perspective, the diagnostic criteria is different and we usually find that the dysfunction of the nervous system is also different. To put it simply: similar symptoms, different diagnostic criteria, different root causes.
Many people with dysautonomia experience symptoms related to the cardiovascular system. However, the underlying root cause for these symptoms involves a disruption in neuro-cardiac communication (communication between the brain and the cardiovascular system). The brain may not be sending or receiving the correct signals needed in order to properly regulate vital functions like heart rate and blood pressure.
In some cases of dysautonomia, there are structural issues within the cardiovascular system itself. These structural problems can also impact how the cardiovascular system functions and responds to signals from the brain. These patients can also benefit from improved neuro-cardiac communication.
When evaluating patients at CFNC, our doctors have found a significant trend in functional neurological abnormalities for both IST and POTS.
In the cases of IST, it is often seen that the dysfunction in the brain is concentrated in the mesencephalon or midbrain. This area of your brainstem is responsible for many functions, including but not limited to pain modulation, consciousness, sleep rhythms, pupillary responses, eye movements, DOPAminergic and GABAergic centers. (DOPAminergic and GABAergic centers refer to specific areas in the brain that utilize different neurotransmitters - dopamine and gamma-aminobutyric acid - to regulate various nervous system functions.) Your midbrain is in constant communication with your limbic centers, cortex, sympathetic ganglion and more. Dysfunction of this part of the brain could lead to increased heart rate.
With POTS, we find neurological dysfunction often resides in the pontomedullary areas of the brainstem and can produce very similar symptoms. The pontomedullary area of your brainstem is responsible for eye movements, motor and sensory control to the face, sound conduction, swallowing, taste and coordination to the tongue and throat, and motor control of some neck muscles.
Both your midbrain and your pontomedullary regions communicate with your hypothalamus, which is responsible for relayed communication from the nervous system to the endocrine system. This is where we find an overlap in these conditions through hypothalamic-pituitary-adrenal dysfunction, specifically with disruption of your sleep wake cycles and hormones further leading to complication of both symptoms and neurological function. (Check out our blog post highlighting the cross-talk between the nervous, immune and endocrine systems.)
Our Approach
At Carolina Functional Neurology Center, we work to provide the most comprehensive treatment for all of our patients. Our goal is to look further at your neurological function and pinpoint the root cause of your symptoms. With the right combination of therapies, nutrition, supplementation and lifestyle changes, we believe that everyone can improve their symptoms and live a more functional life.
If you are interested in speaking with one of our doctors about your symptoms, please contact us.