President Bush's problem?

This summer, James Fallows published an article in the Atlantic Monthly in which he analysed both Bush and Kerry's styles of debate. After viewing videos of Bush debating 10 years ago, he concluded that his mangled words, made up words, and stumbling sentences are a recent phenomenon and that a decade ago he was highly articulate. Fallows dismisses the explanation that Bush suffers from dyslexia or another learning disability and suggests that the change is the result of "the sheer change in scale from being governor of Texas to being President of the United States," which left him overwhelmed by the issues.
A letter* by a doctor to be published in the Atlantic agrees that the diagnosis cannot be a learning disability -- which does not suddenly appear -- but suggests another explanation, which, if true, would be devestating both to Bush and to the country:

Slowly developing cognitive deficits, as demonstrated so clearly by the President, can represent only one diagnosis, and that is "presenile dementia"! Presentile dementia is best described to nonmedical persons as a fairly typical Alzheimer's situation that develops significantly earlier in life, well before what is usally considered old age. It runs about the same course as typical senile dementias, such as classical Alzheimer's--to incapacitation and, eventually, death, as with President Ronald Reagan, but at a relatively earlier age, President Bush's "mangled" words are a demonstration of what physicians call "confabulation," and are almost specific to the diagnosis of a true dementia. Bush should immediately be given the advantage of a considered professional diagnosis, and started on drugs that offer the possibility of retarding the slow but inexorable course of the disease.

The letter is to be published in the October edition and is already online.
(*I've linked to a copy of the letter on someone's website, because you have to register to see the letter in the Atlantic; for those of you who subscribe, click here.)