Just when you thought he had finally maxed out his Bank of Crazy credit card, Donald Trump finds an inventive way to hike the limit.

The latest front opened by the Donald in his ceaseless war against sanity is medical. But before we come to his physician’s letter, a recap for anyone with the sense and taste to have bypassed this development in the presidential campaign.

Recently, Trump and his helpers in the straitjacketed media have been focusing on Hillary Clinton’s health. Hillary, so far as anyone with respect for facts understands, is in good shape. Her own doctor, Lisa Bardack, released a letter last year advising that she has recovered from the cerebral blood clot she suffered in 2009; listing a few trivial conditions and the medications she takes; and concluding that the Democratic contender is fit to lead the free world should America in its wisdom so decide.

Despite the detail and precision, and though no one has cause to doubt Bardack’s competence or honesty, the letter failed to assuage Trumpian concerns that Hillary is gravely unwell, if not dying.

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The Donald, not to mention such faithful bondsmen as Rudy Guiliani and the moron-manipulating far-right online “news” resources, heavily hint at a neurological disorder, possibly Parkinson’s. In their defence, the diagnosis is predicated on as rare and definitive a symptom as Hillary coughing a bit now and again in public.

Once upon a time, such a categorical bill of health from as grand and senior a doctor as Bardack would have killed the rumours. In our post-objective-truth age, however, the confirmation that Hillary is healthy is taken by the nebbishes who festoon online message boards with sexist, racist and everythingist gibberish for incontrovertible proof of the opposite.

These same saddoes are pleased, meanwhile, to trust the rival medical letter of the hour. This one is from a certain Harold Bornstein, who has served as Donald Trump’s physician for more than 30 years in succession to his late father, Jacob.

One curiosity here is that Bornstein is a gastroenterologist, though this may be less curious than it looks. Trump has been dangerously full of it, after all, for a long time. Anyway, even more eccentric than Dr Bornstein not being a general practitioner is Dr Bornstein himself. Check out the images online. In some, he looks a bit like The Dude, Jeff Bridges’ eternal stoner in The Big Lebowski, after someone spiked his White Russian with dodgy acid. In others, his riotous hairstyle – a bonding agent there, presumably, with his patient – reminds you of Christian Bale’s wildly elaborate tonsorial arrangement in American Hustle. In others, he might be one of the guys in a 1970s cop movie serial killer identity parade who is instantly overlooked for being too obviously sent from central casting. In all, he looks demented.

More deranged than his appearance, miraculously, is the letter he wrote – or transcribed – last December, which he now justifies on the grounds that Trump told him he had five minutes to compose before a limo arrived to collect it. Even that, given the text, seems an eon.

“To whom my concern,” begins the good doctor, as if in the midst of one of those mini-strokes (TIAs, as we doctors call them, or transient ischaemic attacks) which Trump fans enjoy ascribing to Hillary. Bornstein then describes Trump’s blood pressure and laboratory test results were – forgive the baffling medical jargon; in the context it’s unavoidable – “astonishingly excellent.” Remind you of anyone, that brand of hyperbole?

“His physical strength and stamina are extraordinary,” continued the doctor, before concluding: “If elected, Mr Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency.”

It is this closing remark which may make the conspiracy theorist wonder if Trump began the chat with a brusque, “Now look, Bornstein, there’s a car coming for the letter in five minutes, so take this down …”. What kind of medic would declare, even equivocally, that Trump is healthier than 44 men he has never examined, either because they were not his patients, or because they had ceased to have a pulse long before he emerged from medical school?

And is the 70-year-old Trump really healthier than Teddy Roosevelt, as others have asked, when in 1901 that hyper-fit woodsman entered the White House at 42?

Bornstein’s claim to know the unknowable suggests that any letter of the kind would carry more weight were it signed by any other bearer of a medical title on earth. Dr Pepper for one. Dr Dre for another.

And so it spirals on, this massively entertaining, hugely terrifying collision between worlds (the real one and the parallel universe of Trump’s creation) masquerading as a presidential election. Hillary’s health is good, and there is no reason to doubt the same of Trump, but who will say the same about the condition of the US body politic?