WASHINGTON (REUTERS) - Three Americans released from North Korean captivity on Wednesday (May 9) are at risk of a range of symptoms, from crippling anxiety to severe depression, as a result of the trauma experienced during their imprisonment.

Half a dozen experts told Reuters the men could suffer from post-traumatic stress disorder, or PTSD, with many of them reacting to their sudden freedom much like soldiers returning from a war zone.

"Losing your freedom is a huge thing," said Dr Anne Speckhard, a professor of psychiatry at Georgetown University who has studied PTSD for decades. "I think we really underestimate how horrible it is for human beings (in captivity)."

The detainees, missionary Kim Dong Chul, and Mr Kim Hak Song and Mr Kim Sang Duk, also known as Tony Kim, were expected to arrive outside Washington early on Thursday morning after a flight from North Korea.

US officials have revealed little about the conditions under which the three were held and whether they endured any form of physical abuse.

In a tweet announcing their release, President Donald Trump said they "seem to be in good health".

In addition to the conditions of captivity, the severity of a prisoner's reactions to imprisonment can vary by the length of confinement, but also by the prisoner's psychological resiliency in the first place.

The detainees could suffer symptoms such as anxiety, depression, irritability and anger.

Some people held in captivity find they have trouble spending time in enclosed spaces - elevators, for instance - or among large crowds.

Many develop a kind of hyper-alertness, reacting strongly to perceived threats, and they can also suffer from reduced cognitive ability for a period of time. Flashbacks and nightmares about their experiences are common.

Those problems can also manifest themselves in physical ways, experts said. People who have endured long stretches of imprisonment may find they have trouble sleeping or eating after release.

"Not unlike soldiers, when they come home, and even though they're, quote, safe, their bodies are conditioned to react under enormous stress," said Dr Robert Schwarz, a psychologist who has researched PTSD.

"There's a whole series of biochemical reactions that go on. It's not just an emotional response."

In interviews, some former North Korean detainees have described hours of interrogation and said they suffered mental humiliation.

A few said the isolation led them to consider suicide.

Last year, former detainee Aijalon Gomes, 38, set himself on fire in a San Diego lot in November 2017 and died.

Mr Gomes was held for about seven months in North Korea after he crossed into the country from China in 2010 on what he saw as a religious mission.

He suffered from PTSD and had recently left a mental health programme against doctors' advice before his death.

Some former prisoners, like Mr Jeffrey Fowle, a tourist held for about six months in North Korea for leaving a Bible in a sailors' club, and Mr Kenneth Bae, who did hard labour during his two years in North Korea, have said their religious faith helped them survive their ordeals.

Dr Speckhard said faith, or any belief system, can give prisoners a source of strength, a crucial psychological support.

Mr Fowle said the "mental strain" was the worst part of his confinement in a guest house. But he also said he has not suffered any flashbacks or lasting psychological issues.

"Things have turned out well, and that is all behind me," he said.