Dr. James Grisolia, chair of neurology at Scripps Mercy Hospital in San Diego, said, "We don't know why, but the very parts of our brain that control memory ... are also the same parts that get the most stubborn, hard-to-control seizures, with huge implications for our memory."
In most cases, memory loss seems to be instigated by a series of smaller seizures over a long period of time. Propst had been diagnosed with epilepsy before the grand mal seizure that robbed her of her memory, and it's likely that the minor seizures she experienced were instrumental steps toward the major event.
Smaller seizures were what chipped away at the long-term memory of 42-year-old Mary Smith of Clinton, Mass. Smith was diagnosed with epilepsy when she was a senior in college, at the age of 21. But she says that as early as age 15, she started displaying the signs of psychomotor seizures.
"I would say all of my epileptic seizures have actually contributed to long-term memory loss," she said. Among the casualties are nearly every recollection of her college years.
"I don't remember being in college, but I have the pictures," she noted. There are other memories that she says she cannot recall -- things that she says she should remember but simply can't, like giving birth to her son.
"He's almost 20. I had him when I was 23," she said. "Twenty years old, but I remember hardly anything about his childhood."
"It is easy to feel really alone," said Smith, who, despite her memory loss, maintains good relationships with her friends and family and works as an adult education teacher.
Part of this isolation could be attributable to the fact that, for most people, memory is a significant part of identity.
"Memory remains one of the many unplumbed mysteries of the brain," Grisolia of Scripps Mercy Hospital said. "Our memories, accumulated over a lifetime, speak more to the heart of who we are than any external set of biologic, demographic or socioeconomic data about us."
Propst, too, said her memory loss has made it difficult in some cases to relate to others.
"It's hard to relate to other people if you don't have memories to share," Propst said.
Even with absent memories, however, the past has a way of catching up. Propst said that since the Denver Post covered her story last week, she has received a number of calls from old acquaintances who want to talk to her about her past life.
"As a custodian, I'm pretty invisible," Propst said. "I think people tend to think that I'm not very competent."
"I want to get together with someone who says, 'Yes, you were smart, you made a difference.'"
Once gone, are memories lost forever? Not all doctors think so. Ewing, her doctor, said he believes that somewhere, deep in Propst's brain, the memories she lost still exist.
"If you could find a way to reconstruct that connection, I think that they're there. I just don't think she has the ability to cross the bridge. ... Whatever switch it was that allowed her to access her memories, if we can throw it back, I have no doubt that these memories are still there. They should still be there; the brain area looks like it's intact."
But, if given the chance, Propst said, she would not want her old memories back.
"Actually, I haven't tried to reconnect with many of them," she said. "They really have nothing to do with my life now."