Image copyright AFP Image caption In a letter, a group of mental health professionals said Mr Trump was incapable of serving as president

It seems an incredible question to ask of a man who ran a multi-billion-dollar business and vanquished seasoned political opponents on his way to highest office in the US. But experts are debating the mental health of the US president.

The discussion of Donald Trump's mental health has come to the fore following an open letter from dozens of professionals who say his "grave emotional instability" makes him unfit for the presidency.

The call seems to break a long-standing rule among experts of not diagnosing public people, and has been condemned by a leading psychiatrist, who described the "psychiatric name-calling" as an insult to the mentally ill.

What is it about?

Debate over Mr Trump's mental fitness is nothing new, and existed even before his election, last November.

But the majority of mental health professionals have refrained from making public statements, following a self-imposed principle known as the "Goldwater rule", adopted by the American Psychiatric Association (APA) in 1973.

It prohibits psychiatrists from giving diagnosis about someone they have not personally evaluated. It was instated after a magazine asked thousands of experts in 1964 whether Republican nominee Barry Goldwater was psychologically fit to be president.

The APA warned last year that breaking the rule in trying to analyse the candidates in the presidential election was "irresponsible, potentially stigmatising, and definitely unethical".

Image copyright Reuters Image caption Psychiatrists were warned against making assumptions about the mental health of the candidates

But now, some professionals have spoken out, including those who have signed a petition asking for Mr Trump's removal. It has now gathered more than 23,000 signatures.

Some have suggested that Mr Trump has Narcissistic Personality Disorder (NPD).

People with this condition often show some of the following characteristics, according to Psychology Today:

Grandiosity, a lack of empathy for other people and a need for admiration

They believe they are superior or may deserve special treatment

They seek excessive admiration and attention, and struggle with criticism or defeat

What is new?

In a letter to the New York Times, 35 mental health professionals warned that the "grave emotional instability" indicated in Mr Trump's speech and actions made him "incapable of serving safely as president".

They said experts had remained silent because of the Goldwater rule, but that it was time to speak out. "This silence has resulted in a failure to lend our expertise to worried journalists and members of Congress at this critical time. We fear that too much is at stake to be silent any longer."

Image copyright AP Image caption The letter says Mr Trump has demonstrated an inability to tolerate different views

The letter added: "Mr Trump's speech and actions demonstrate an inability to tolerate views different from his own, leading to rage reactions. His words and behaviour suggest a profound inability to empathise.

"Individuals with these traits distort reality to suit their psychological state, attacking facts and those who convey them (journalists, scientists)."

Earlier this week, Democratic Sen Al Franken said that "a few" of his Republican colleagues had expressed concern to him about Mr Trump's mental health. The concerns, he said, stemmed from questions about the president's truthfulness and the suspicion that Mr Trump "lies a lot".

Why the controversy?

Apart from the apparent break of the Goldwater rule, other professionals say the psychiatric diagnosis of Mr Trump is an insult to the mentally ill.

Also in a letter to the NYT, Dr Allen Frances, who helped write the Diagnostic and Statistical Manual of Mental Disorders IV, one of the main key manuals used to classify mental disorders, said that "most amateur diagnosticians have mislabelled" Mr Trump with the diagnosis of "narcissistic personality disorder".

"He may be a world-class narcissist, but this doesn't make him mentally ill, because he does not suffer from the distress and impairment required to diagnose mental disorder."

He added: "Mr Trump causes severe distress rather than experiencing it and has been richly rewarded, rather than punished, for his grandiosity, self-absorption and lack of empathy.

"It is a stigmatising insult to the mentally ill (who are mostly well behaved and well meaning) to be lumped with Mr Trump (who is neither).

"Bad behaviour is rarely a sign of mental illness, and the mentally ill behave badly only rarely. Psychiatric name-calling is a misguided way of countering Mr Trump's attack on democracy."