Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE left early from the 9/11 commemoration in New York City Sunday as she felt “overheated.” The video of her leaving the event shows Clinton leaning against a concrete post and then her legs appearing to buckle under her as she is helped into a vehicle.

What happened?

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The medical term for fainting is syncope. Syncope is the temporary loss of consciousness and posture, due to a temporary insufficient flow of blood (and hence oxygen) to the brain.

If the feeling of faintness and light-headedness and weakness comes but there is no loss of consciousness, the event is called near-syncope.

The video posted on Twitter by Zdenek Gazda shows what appears to be near-syncope or an “almost faint.” Unless you were with Clinton in the van, it’s impossible to know if she actually did faint.

Hillary Clinton 9/11 NYC pic.twitter.com/q9YnsjTxss — Zdenek Gazda (@zgazda66) September 11, 2016

As a doctor, I see near-syncope and syncope routinely — this happens during and after procedures fairly routinely. With near-syncope it is pretty easy to intervene, as Mrs. Clinton’s team did, and prevent the faint.

Why do people faint or almost faint?

The lack of blood flow to the brain that triggers fainting can be caused by a wide variety of things, from anxiety or eating a massive meal all the way to very serious heart problems.

The Journal of Geriatric Cardiology published a pretty definitive list of potential syncope causes in this useful review article about older adults.

The Cliffs Notes version is that fainting is either due to the nervous system, blood volume, or the heart. Neurally or nervous system-mediated syncope happens when the autonomic nervous system gets twitchy and messes up vascular tone or heart rate causing blood to pool in the legs. It’s a reflex gone awry.

The nervous system has all kinds of reflexes to control heart rate and blood pressure that are constantly fine tuning the vascular system behind the scenes and sometimes the signals get messed up. This is the most common cause of fainting for someone of Clinton’s age — 68.

Orthostatic hypotension is another common cause of syncope and it is basically the opposite of neutrally mediated syncope because it is a sign of sluggish or impaired autonomic reflexes. This can happen when there isn’t enough volume in the system (dehydration) or it can be due to medications (diuretics, anti-hypertensive therapy), especially when superimposed upon the natural aging-related changes in autonomic reflexes.

Cardiac problems can also lead to fainting.

I'm a surgeon. I once fainted in the OR. I was still fit to operate once I felt better. — Jennifer Gunter (@DrJenGunter) September 12, 2016

A few hours after the event, Clinton’s physician, Dr. Lisa Bardack, released a statement saying that Clinton was diagnosed with pneumonia two days earlier and was “put on antibiotics, and advised to rest and modify her schedule.”

Dr. Bardack’s statement indicated that Mrs. Clinton’s near-syncope was the result of being overheated and dehydration.

What does this mean for Hillary Clinton’s health?

The recent pneumonia may have been another factor in triggering Mrs. Clinton’s near-fainting episode, especially considering she was standing for about 90 minutes on a warm and humid day in a suit. If she was wearing Kevlar, that would have obviously contributed to the heat.

Mrs. Clinton is under the care of a physician and had a thorough work-up when she fainted in 2012 and a documented annual exam in 2015. As a result, we can exclude every heart condition as a cause.

Using the San Francisco Syncope Rule, which looks at risk factors like previous congestive heart failure, Clinton is likely at low risk from this event as she doesn’t have any of the medical conditions that typically predict a worse outcome, such as congestive heart failure or a low blood count, based on her doctor’s letter in 2015.

Should Clinton have gone to an Emergency Department

Whether a person who almost-faints or faints should go to the emergency department depends on many factors such as the specific cause, whether they are recovering appropriately or have they injured themselves, if they have any underlying illness, and their age.

This quote from a review article in American Family Physician sums it up nicely:

Patients at low risk of adverse events (i.e., those with symptoms consistent with vasovagal or orthostatic syncope, no history of heart disease, no family history of sudden cardiac death, normal electrocardiographic findings, unremarkable examination, and younger patients) may be safely followed without further intervention or treatment.

As Clinton recovered quickly, is likely very educated about her health, probably knew why she felt faint (the heat and her pneumonia), her decision to go to her daughter’s and call her physician seems reasonable.

After all, she has been worked up previously for fainting. It’s possible her doctor may have other tests planned and it is also possible that her doctor feels no more tests are indicated. It is also possible that Clinton and her physician had a plan for what to do if she felt faint again.

Should Clinton have disclosed her pneumonia when she was diagnosed?

Community-acquired pneumonia is a common infectious disease. If it’s not serious enough to require admission and not incapacitating; it is hard to see how it is anyone’s business but Hillary Clinton’s.

It certainly isn’t something that disqualifies anyone from being President. Conspiracy theories and wild conjectures about Clinton’s health are so common it’s entirely possible that she didn’t want to make an issue about it or was concerned that this illness that affects 5.6 million Americans each year would somehow be used to make her appear uniquely weak.

Q: How exactly does near syncope or pneumonia make you medically unfit for office?

A: It doesn't — Jennifer Gunter (@DrJenGunter) September 12, 2016

Many people feel they have to work when they should be resting. Others don’t have a choice because of lack of sick leave. Some feel that if they take a few days off because of illness that they will be perceived as weak. It’s possible Clinton thought she was feeling well enough to work.

Many women, myself included, can empathize with her.

Fainting is scary and dramatic, but it’s often “nothing.” Really.

Seeing someone collapse or almost collapse is scary, but very often it looks worse than it is medically speaking. In this witnessed situation (standing for 90 minutes in a suit on a warm day) with an previous work-up for fainting and a recent diagnosis of pneumonia with a prompt recovery there is no medical cause to sound an alarm for a secret sinister illness.

Think about the last time you knelt for too long then stood up quickly and felt woozy and had to grab something. That was near-syncope. It happened because the blood pooled in your legs and then when you stood up quickly for some reason it didn’t get back to your heart fast enough and your brain was temporarily short changed. It doesn’t mean you are on death’s door it means you knelt for too long and stood up too quickly. Sometimes a faint or an almost faint is just that, an event with no underlying serious health issues.

If Hillary Clinton is a “fainter” then so is George H.W. Bush.

George W.H. Bush vomited and fainted at a State dinner in 1992. He felt warm (experienced near-syncope as Clinton did). He thought he would be okay, but he fainted. U.S. Air Force General James Martin (age 52) fainted during a Pentagon press briefing in February of this year. Like Mrs. Clinton he was dealing with a respiratory illness.

The risk of fainting increases with age for both men and women, but lots of young people faint in the heat (for example models at the Yeezy 4 fashion show).

Nixon in hospital during 1960 campaign. Had staph infection. pic.twitter.com/YYa5OOkY5H — Greg Mitchell (@GregMitch) September 12, 2016

More than 20% of people older than 75 will experience syncope.

So if Presidents are going to be over the age of 75 then some of them might faint. If Presidents are going to get the flu then some will faint. If presidents are going to be people, then some will faint.

Could Mrs. Clinton wear short sleeves to events that are hot? That’s what I do. But I appreciate why Clinton might be hesitant to do this given how Michelle Obama Michelle LeVaughn Robinson ObamaTo honor Justice Ginsburg's legacy, Biden should consider Michelle Obama National Urban League, BET launch National Black Voter Day The Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill MORE has been criticized repeatedly for wearing sleeveless dresses.

The biggest medical concerns with fainting are the following:

1) Is there some concerning underlying medical cause? In this situation, the answer is not likely given her previous medical work up.

2) Was there a need for medical resuscitation? No, she recovered very quickly with supportive care.

3) Was there an injury? No.

Mrs. Clinton felt faint. It was dealt with appropriately. It looked dramatic, but it’s ok.

And so is she.

Dr. Jen Gunter is an OB/GYN doctor and writer, and is based in San Francisco. You can follower her on Twitter at @DrJenGunter or on her blog.

The views expressed by Contributors are their own and are not the views of The Hill.