After nearly a decade of keeping it under wraps, Olympic medalist Katie Ledecky has shared her POTS diagnosis with the world.

The athlete, who has won 14 Olympic medals for swimming, the most of any female Olympian, said she has POTS (postural orthostatic tachycardia syndrome).

In "Just Add Water: My Swimming Life," Ledecky's new memoir, which was published by Simon & Schuster in June, she wrote that the disease can cause "dizziness, fainting and exhaustion."

US OLYMPIAN SAMMY SULLIVAN SAYS ARMY SERVICE HELPED HER WIN BRONZE MEDAL FOR RUGBY

Here's more. 

POTS is a disturbance in the autonomic nervous system, which controls some of the normal regulatory functions of the body, according to Dr. Blair Grubb, a cardiologist and expert on POTS at The University of Toledo College of Medicine and Life Sciences.

"When the person stands, gravity will try to displace downward roughly 20% to 30% of the body's blood volume," he told Fox News Digital. 

In response to this displacement, the brain tells the heart to beat faster and more forcibly, and tells the blood vessels in the lower half of the body to tighten, or constrict, to three times the level they were previously, the doctor said.

"This allows for accumulation of much more blood than normal in the lower half of the body," Grubb said.

SLEEP DOCTORS SAY OLYMPIC ATHLETES' CARDBOARD BEDS COULD HAVE ‘DISASTROUS’ IMPACT

As more blood is displaced downward, the brain gets less and less oxygen. 

Ledecky discussed this effect in her book. She wrote, "I pool blood in the vessels below my heart when I stand. My body then releases extra norepinephrine or epinephrine, which adds additional stressors on my heart, making it beat faster."

Individuals with a genetic trait called Ehlers-Danlos syndrome (also known as joint hypermobility syndrome) appear to be more susceptible to developing this condition, according to Grubb. 

"However, POTS is frequently triggered by a viral infection, such as Epstein-Barr virus or COVID-19," he told Fox News Digital.

One of these infections can trigger an autoimmune response, in which the body's immune system attacks itself and produces antibodies that interfere with the ability of blood vessels to tighten, the doctor said.

POTS can also occur on its own, without any obvious triggers.

A POTS patient’s symptoms will depend on how much blood is displaced downward, experts say.

"It can vary from mild cases where your heart races and you get a little dizzy upon standing, all the way to presenting as a disabling condition, disallowing patients from being upright," Valerie Iovine, PT, a physical therapist at Strive Physical Therapy in Philadelphia, told Fox News Digital.

"It can also change over the course of life, but can also change day to day or week to week."

The disorder’s name — postural orthostatic tachycardia syndrome — literally translates to "when you become more upright, your heart races," noted Iovine, who treats many patients with POTS and also has the disorder herself.

"The heart will flutter in an attempt to properly oxygenate the brain," she said. 

"But in addition to the dizziness, headaches, passing out or near passing out, many with this disorder don't realize that it can account for other issues, like temperature dysregulation, blood pressure dysregulation and GI dysfunction."

In more extreme cases, patients may have difficulty thinking, concentrating or remembering — sometimes called brain fog, according to Grubb.

WITH SOME OLYMPIC EVENTS POSTPONED, SEINE RIVER WATER QUALITY POSES CONCERNS

People may also experience blurred vision, black spots in their visual field, tunnel vision and headache. 

"Even greater displacement of blood can cause the individual to lose consciousness," Grubb said.

The primary treatment for POTS is increasing water and salt intake, Grubb said. 

"It is also important to recondition the patient through exercise, building the strength in their lower extremities," he said.

Some medications, such as midodrine and droxidopa — known as vasoconstrictors — can work to tighten blood vessels and increase blood return to the heart, according to Grubb. 

WHAT IS ANGELMAN SYNDROME? COLIN FARRELL’S SON IS LIVING WITH THIS RARE DISEASE

Still other drugs, such as fludrocortisone or desmopressin, work by increasing the volume of fluids available for the heart to pump. 

"Medications such as pyridostigmine facilitate nerve transmission and help the nervous system work more effectively to maintain normal vascular function," Grubb added.

Many often use the term "exercise intolerance" when describing symptoms of POTS, but Iovine said that "poorly prescribed and dosed exercise" is the problem.

"Exercise is the best management for POTS," she said.

"I would argue that for these patients, movement is medicine."

In her book, Ledecky noted that swimming can be an effective treatment for POTS, writing that "reclined aerobic exercise, such as swimming, and strengthening your core, can provide relief."

Seeking care from a cardiologist and a physical therapist is essential, according to Iovine.

The cardiologist can help to get vital signs like heart rate and blood pressure under control, she said, while a knowledgeable physical therapist can help to manage symptoms and increase upright tolerance. 

"Things like proper hydration, extra electrolytes, and being able to keep cool can also help manage symptoms," Iovine advised.

"The heat will make the blood vessels expand, making it harder to get the blood up the head against gravity," she said.

"POTS can be a challenge for anyone — from people with complicated pre-existing conditions, all the way to the top athletes, like Katie Ledecky," Iovine told Fox News Digital. 

The disease can often cause a flu-like feeling after exercise, something called post-exertional malaise/post-exertional symptom exacerbation (PEM/PESE). 

"This is when the autonomic nervous system can have a reflexive reaction to stress or exercise, either making existing symptoms worse or creating a new host of issues in people with POTS," Iovine said. 

"This can pose an issue in the sense of rigorous exercise for an Olympic athlete — or in other cases, exertion may be as simple as getting out of bed and walking to the kitchen."

POTS causes a "dynamic disability," Iovine noted.

"One day, it may allow you to swim like an Olympic athlete, and other days, [it will] have you stuck in bed or even in a wheelchair."

POTS is particularly challenging in athletic activities that involve frequent "start and stop" activities, such as basketball, volleyball, soccer and field hockey, added Grubb.

Both experts emphasized that POTS is a "real illness."

"People who were previously quite healthy develop severe limitations and disabilities," Grubb said.

CLICK HERE TO SIGN UP FOR OUR HEALTH NEWSLETTER

"They can experience disabling fatigue, exercise intolerance, heat intolerance, palpitations, lightheadedness, dizziness, fainting and brain fog."

Iovine said that due to the "invisibility" of the disease combined with the high heart rate and variability of presentations, it is often dismissed as a function of mental health.

"Patients are often turned away from proper care, made to believe it is all in their heads," she said.

"POTS is a very real condition, and the good news is that there are very real management strategies as well," Iovine went on.

For more Health articles, visit www.foxnews/health

While there is no cure for POTS, many patients are able to manage their symptoms and return to their daily activities.

Added Iovine, "Build up your care team and your confidence to advocate for your health and keep to your regimented routines for management."