The autonomic nervous system (ANS) manages involuntary functions like heart rate and blood pressure. When a person moves from lying down to standing up, the ANS must quickly adjust to gravity, which pulls blood toward the lower extremities. Orthostatic intolerance describes symptoms that occur upon standing and are relieved by lying down, indicating a failure in this ANS regulation. Both Orthostatic Hypotension (OH) and Postural Orthostatic Tachycardia Syndrome (POTS) fall under this category, representing distinct physiological responses to postural change.
Understanding Orthostatic Hypotension
Orthostatic Hypotension (OH) is defined by a significant, sustained drop in blood pressure when moving from a seated or lying position to a standing position. This sudden decrease in pressure means that not enough blood is reaching the brain, which causes the characteristic symptoms. The drop in blood pressure must be repeatable and measurable to confirm the diagnosis.
The specific diagnostic criteria require a drop in systolic blood pressure of at least 20 millimeters of mercury (mmHg) or a drop in diastolic blood pressure of at least 10 mmHg within three minutes of standing. This hemodynamic failure leads to symptoms such as dizziness, lightheadedness, and blurred vision, resulting from reduced blood flow to the brain. In severe cases, the lack of blood flow can cause syncope, commonly known as fainting.
In people with OH, the body’s compensatory mechanisms are inadequate or entirely failed. This condition is often associated with factors like dehydration, medication side effects, or underlying neurological disorders that directly impair the ANS response. The focus of OH is the failure to maintain adequate blood pressure, while the heart rate response may be absent or only mildly increased.
Understanding Postural Orthostatic Tachycardia Syndrome
Postural Orthostatic Tachycardia Syndrome (POTS) is a form of orthostatic intolerance characterized by an abnormal increase in heart rate upon standing. Unlike OH, the defining feature of POTS is this excessive heart rate response to postural change. This condition involves chronic symptoms lasting at least three months, which are relieved upon returning to a lying position.
The formal diagnostic criteria for adults require a sustained increase in heart rate of 30 beats per minute (bpm) or more within the first ten minutes of standing, or a heart rate exceeding 120 bpm, without meeting the blood pressure drop criteria for OH. For adolescents, the threshold is higher, at 40 bpm or more. This excessive tachycardia is the body overreacting to the pooling of blood in the lower body, attempting to increase cardiac output despite stable blood pressure.
POTS symptoms are often more widespread and chronic than those seen in simple OH. Patients frequently experience severe fatigue, “brain fog” or difficulty concentrating, headaches, nausea, and palpitations. These symptoms reflect systemic autonomic dysfunction and are not solely due to the transient lack of blood flow to the brain that characterizes OH.
The Crucial Distinction: Coexistence and Diagnostic Separation
The core distinction between Orthostatic Hypotension and Postural Orthostatic Tachycardia Syndrome lies in which physiological parameter fails most dramatically upon standing: blood pressure for OH, and heart rate for POTS. By definition, a person diagnosed with “Classical POTS” must not have a significant blood pressure drop that meets the criteria for OH. The presence of orthostatic hypotension necessitates an OH diagnosis, even if tachycardia is present.
However, the two conditions are not entirely mutually exclusive and can occur in the same individual. Some patients with POTS, particularly the hyperadrenergic subtype, may experience transient or intermittent blood pressure drops that do not meet the full OH criteria, or they may even have mild hypertension while standing. Furthermore, an individual’s condition might evolve, with a patient transitioning from exhibiting OH to meeting POTS criteria, or vice versa, necessitating ongoing clinical assessment.
Diagnostic separation is often achieved using the Tilt Table Test, which systematically monitors both heart rate and blood pressure while a patient is tilted upright. This test allows clinicians to precisely measure the magnitude and timing of the changes in heart rate and blood pressure. Accurate diagnosis is paramount because, while both involve autonomic dysfunction, the specific treatment strategies are tailored to address the primary failure, whether it is blood pressure maintenance or heart rate regulation.