A 54-year-old woman showed up in the emergency room at Georgetown University Hospital with her husband, unable to remember the past 24 hours. Her newer memories were hazy, too. One thing she did recall: Her amnesia had started right after having sex with her husband just an hour before.

While sex can be forgettable or mind-blowing, for some people, it can quite literally be both at the same time. The woman, whose case was reported in the September issue of The Journal of Emergency Medicine, was experiencing transient global amnesia, a rare condition in which memory suddenly, temporarily, disappears.

People with transient global amnesia suffer no side effects, and the memory problems usually reverse themselves in the span of a few hours. It's a rare condition, affecting only about 3 to 5 people per 100,000 each year. But what makes transient global amnesia so eerie is that researchers aren't sure what causes it, or why patients remain otherwise chatty and alert while missing large chunks of their memories. [Inside the Brain: A Journey Through Time]

"We don't know very much about the cause," said Sebastian Ameriso, a neurologist at the Institute for Neurological Research in Buenos Aires, who was not involved in the 54-year-old woman's case. "It causes a lot of alarm, but this is not a stroke or an event that causes damage to the brain. It's almost always very benign."

Mind-erasing activities

Sex can trigger transient global amnesia, as can other physically strenuous activities. People in their 50s and 60s are the most likely to experience an episode, but strangely, most people with transient global amnesia have it only once. In most cases, the amnesia is anterograde, meaning people have trouble forming new memories. Sometimes, people also experience transient retrograde amnesia, forgetting some portion of their previous memories. In the case of the 54-year-old woman at the Washington, D.C., hospital, the last day was a fog, and she had been forgetful and confused since having sex.

As with most patients, the woman's brain scans using magnetic resonance imaging (MRI) and computed tomography (CT) showed nothing unusual and no damage to the brain. By the time she left the emergency room, her symptoms were almost gone.

The closest thing to an explanation researchers have for this sex-triggered amnesia is that the problem may not begin in the brain, but in the neck. In a January 2010 study published in the journal Stroke, Ameriso and his colleagues conducted sonograms of the necks of 142 patients who'd experienced transient global amnesia within the last week. They found that 80 percent of the patients had what is called insufficiency of the valves in the jugular vein.

This vein, which runs down the side of the neck, carries spent blood from the brain back to the heart. Valves in the veins prevent blood from flowing backward toward the head, but if the valves don't close sufficiently, blood could seep back upward.

Memory mysteries

The best guess for what might be happening is that patients unwittingly trigger the transient global amnesia by raising the pressure inside their abdomens. This is called the "Valsalva maneuver," familiar as the "bearing down" people might do when lifting weights, defecating or even having sex. The increased pressure increases the resistance to blood flowing down the jugular veins, and insufficient valves may allow deoxygenated blood to push back up the neck. Oxygen-poor blood then "piles up" in the veins draining the brain, especially in central brain regions that are key to memory formation. The result could be transient amnesia.

What this explanation doesn't cover is why most people with transient global amnesia experience it only once, Ameriso told Livescience.

"This doesn't explain why this would happen only once while we do this Valsalva maneuver many times during the day," he said.

Whatever the cause, transient global amnesia can be upsetting. In one case reported in 1964, a man lost his memory the moment he orgasmed, causing him to exclaim, "Where am I? What's happened?" [10 Surprising Sex Statistics]

People with transient global amnesia usually rush to the hospital in great distress, Ameriso said — which is not a bad thing, given that sudden memory loss can also herald a stroke or other serious neurological problems.

For doctors and patients alike the most important thing is a quick diagnosis, Ameriso said. Otherwise, patients can languish in the hospital for days, waiting anxiously for test results.

"It's important to be able to diagnose this very quickly, looking for the insufficiency in the veins," Ameriso said. "If you can confirm that this is the case, you can save a lot of money."

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.