Jonathan Swift spent many of his early years in the Irish and English countryside. He was presumably struck at this time of the year, when bare trees are silhouetted against evening skies, that in an ageing hedgerow oak it is obviously the top of the tree that decays first.

The writer was only 50 when he made his comment. At the time he was back living in Ireland, but although he had suffered from Meniere’s disease (attacks of deafness, tinnitus and vertigo) for some years, he had probably only recently noticed the start of some intellectual deterioration. Contrary to popular belief, patients with early Alzheimer’s are usually all too aware of their declining memory and the problems of forgetfulness. Only when the disease becomes established, and categorised as moderate, do they lose this insight and become unaware of other personality changes.

President Reagan spoke very bravely of his Alzheimer’s, and was aware from an early stage, of the problems it produced (both his brother and mother had the same trouble). The onset of his Alzheimer’s could have been precipitated by the consequences of the blood loss he suffered after the attempt on his life.

Swift is known to more people as the author of Gulliver’s Travels and the originator of Lilliput than for any other reason. Gulliver’s Travels wasn’t published until 1726, by which time he had already been a prominent figure in the literary, political, philanthropic and theological worlds of Ireland and England for more than 30 years.

Swift had had a notoriously riotous youth, but was in time ordained, became the Dean of St Patrick’s Cathedral in Dublin, was a famous essayist, and a defender of the rights of the Irish. But when younger, the gossip surrounding his sex life would rival that of a 21st-century politician. He was buried beside Stella, his long-standing female confidante. Last week I was wandering through Trinity College Dublin while searching for my lost mobile telephone and diary. This reminded me of Swift’s observation about old trees dying from the top downwards.

One hundred and fifty years after Swift died Dr Alois Alzheimer, an Austrian physician, described the dementia that bears his name.

Alzheimer’s patient, a woman named Auguste, showed signs of dementia early, but she was severely disabled by the age of 51. Swift, on the other hand, went on to do much of his best work after he made his original observation, but didn’t die from Alzheimer’s until his mid-sixties.

The definition of the stages of the disease has changed over the past ten years. Until recently dementia was divided into seven stages; now it is usually graded as mild, moderate or severe, partly according to the patient’s score when taking the Mini Mental State Exam. The MMSE, a matter of answering simple questions and obeying straightforward commands, is as important for older people as A levels are for their grandchildren but, rather than grades, they are awarded a score out of 30.

When he made his observation Swift would have fitted into the first or second of the seven categories. At these stages it is always questionable if there is nothing more of a problem than that of the almost inevitable benign memory loss and forgetfulness of ageing. The sufferers appear perfectly normal to others, are astute enough to cover up any lapses but, although forgetting familiar names, would continue to function socially. Swift was certainly working effectively and pursuing his political causes with enthusiasm, vigour and even aggression when he made his comment. Aggression and other personality changes are later symptoms of Alzheimer’s.

Patients whose Alzheimer’s is still classified as mild may find it so difficult to find the right words, or to remember people’s names, that work becomes impossible. They may have such an appalling memory, and have so little ability to concentrate, that they are no longer able to enjoy reading or a social life, but even so would not be considered eligible for NHS medication.

It used to be the intention, and in my opinion still should be, to treat patients early so that they may live as full a life for as long as possible. This policy seems to be good medical practice and sound common sense, but the National Institute for Health and Clinical Excellence (Nice) has recommended that anti-Alzheimer’s drugs should be started only once the dementia could be described as moderate, even though Aricept is effective in mild as well as moderate Alzheimer’s.

There are two other drugs that work in the same way as Aricept (donepezil) — Reminyl (galantamine) and Exelon rivastigmine. But only when moderate Alzheimer’s is diagnosed — and patients are, for example, unable to manage their personal affairs, are always agitated, obviously intellectually disabled, with poor knowledge of current events and find travelling by themselves an ordeal — is NHS treatment to be sanctioned.