CNN’s Chief Medical Correspondent Dr. Sanjay Gupta called Friday for Joe Biden to undergo full neurological and cognitive testing after his disastrous debate performance–and revealing serious medical concerns about the president’s state.

Gupta, a neurosurgeon, wrote in an extended essay for CNN that “as a brain specialist, it was concerning to watch President Joe Biden” during the debate and said that a dozen colleagues had been in touch with him since to share their worries. In the essay he referred to “dementia” and “parkinsonism,” although he made clear he was offering observations, not a diagnosis.

The CNN correspondent is one of the nation’s highest-profile doctors, making his intervention one which will cause concern for the White House, which has brushed aside calls for details of Biden’s health, putting his performance down to a “cold,” while Biden himself told donors “I nearly fell asleep on stage.”

Gupta listed a series of behaviors which he had noticed during the debate and which other doctors he has spoken to have also raised as concerns. They were, he wrote, “confused rambling; sudden loss of concentration in the middle of a sentence; halting speech and absence of facial animation, resulting at times in a flat, open-mouthed expression.”

Elizabeth Frantz/Reuters

Gupta wrote that doctors had reached out to him to say that Biden should have detailed neurological and cognitive testing and that the results should be released to the public. No detailed results were released by the White House in February when it said “extremely detailed” neurological tests had shown no cause for concern. There was no suggestion there had been cognitive tests of any kind.

But said Gupta, testing is now needed to find out whether there is a “simpler” reason for Biden’s rambling and frozen-faced performance, “or if there is something more concerning.”

“As a doctor, I would want to understand the possibility of underlying dementia because, over the past several years, we have learned that there are medical treatments and lifestyle changes that can delay and, in some cases, even reverse the symptoms of the disease,” the neurosurgeon wrote.

He said that the two brain aneurysms Biden suffered in earlier life may increase the risk of cognitive decline but said that ageing's trajectory “varies from person to person.”

Gupta brings unusual authority to the call for Biden to release results of detailed tests. He had such close ties to the Obama White House that he was reported to have been offered the role of Surgeon General in early 2009, but withdrew his name before a nomination could proceed. In 2008 he was one of the doctors to review the medical records of Sen. John McCain, the Republican candidate who at 71 was seen as unusually elderly. He and others gave McCain a clean bill of cognitive health and he continued to serve in the Senate until his death in 2018 from glioblastoma.

Emily Elconin/Reuters

The CNN correspondent also pointed out that Trump's performance in the debate included some similar patterns, "including nonsensical rants as well as confusing names and current events."

And he pointed out that while both men have now lived past the average American agespan of 78.4 years, both have other healthy traits of not smoking and drinking. Trump's father died of Alzheimer's when he was 93.

Mounting questions over Biden’s cognitive fitness will be the central focus of his Friday interview with ABC News’ George Stephanopoulus. Biden’s aides are promoting it as an opportunity for him to make a direct case that he is fit and cognitively alert. But a series of mangled sentences in recent days will have done little to calm nerves among Democrats.

They included stumbling in an interview aired Thursday with WURD, a Black radio station in Philadelphia, when he mangled a sentence to say “I’m proud to be, as I said, the first vice president, first Black woman, to serve with a Black president,” and referring–during a July 4th White House barbecue–to Donald Trump as “my colleagues.”

Biden aides, however, have pushed back hard on the idea that his verbal stumbles should be of concern. Campaign director of rapid response, Ammar Mufasa, posted on X Thursday: “This is absurd. It was abundantly clear what the president meant. This would be considered a perfectly normal speech pattern for any other person in America, and has certainly been normal for Joe Biden for his entire career.”