“I think,” Patricia Utermohlen is saying, at her kitchen table in a flat overlooking the Regent’s Canal in Maida Vale, “that painting is all about memory.” She has more evidence for this assertion than most. At a hardly believable 85, she remains a teacher of art history, but more than that, her husband, William Utermohlen, was a painter who, in 1995, at the age of 61, developed Alzheimer’s disease. Over the subsequent dozen years of his life, Patricia watched him struggle to hold on to his art, and to his memory, and ultimately be defeated by it. “Even the time he was beginning to be ill, he was always always drawing, every minute of the day,” she recalls. “I say he died in 2000, because he died when he couldn’t draw any more. He actually died in 2007, but it wasn’t him by then.”

One wall of a small and startling exhibition devoted to William Utermohlen’s work in London documents that decline in painful detail. The sequence begins with a picture that he made soon after he had been diagnosed. In the painting, he sits beneath the skylight in the studio where he worked, along the corridor from the kitchen table at which I now sit with his wife. And, all at sea, he grips the edges of the table in bright Van Gogh yellow, head slightly bowed, “in anger and fear”, as Patricia says. After that, the self-portraits become an exercise in depicting the fragmenting and dissolving self, of death in life. A nurse who loved Utermohlen’s work encouraged him to keep working, to try to draw Alzheimer’s from the inside. In the portrait of 1996, a year after his diagnosis, he has become a haunted figure, eyes still alive, but slightly disembodied, wearing a red shirt of pain. Thereafter, each successive year offers a self-portrait still recognisably the painter but retreating into more dislocated elements, until by 1999 only the shadow of the eyes and a whited-out dome of skull remain.

In the context of the rest of the show, this is one of the saddest pictures you are likely to see. Though a respected painter and teacher all his life, Utermohlen was never given the recognition that his talents deserved. He first came to Britain as a GI from his native Philadelphia in the late 1950s. He married in 1962, and stayed. In the 1960s he was a bold figurative painter, in a time of abstraction and pop art. His series of large-scale canvases of the cantos of Dante’s Inferno or mummers in Philadelphia carry a powerful charge, but seem almost out of time. By the late 1980s he had already begun a retreat from these large-scale public paintings into the confines of his own head, long before the diagnosis of his illness.

Looking back now, his wife can see premonitions of this retreat in paintings that Utermoh­len made five years before he was diagnosed. These “conversation pieces” show the artist in this flat, half asleep and reflected in a mirror while his wife reads in bed, or typically absent from the chatter around this table, beginning to be lost in his own thoughts. I wonder if she thought he knew something was wrong even then. “I think he did,” she says, “or it was as if he was trying to communicate that idea to himself somehow.”

In some ways she was the last to know, or at least “when you are living with someone you don’t really want to admit”. She was forced to concede that something was wrong when her husband visited his dealer in Paris and couldn’t find his way to the Louvre, which he was in and out of all the time. It was only then that she discovered that had been travelling out to his studio in Old Street every morning for perhaps a year and staring at a canvas that he no longer knew how to begin to fill.

Soon after Utermohlen was diagnosed, he and his wife swapped their London flat for one in Rome – “our last abroad holiday”, as she says now – and toured the galleries and museums as they had always done. “Bill didn’t seem that interested until we got to Velázquez,” she recalls. “The portrait of Pope Innocent. He sat there in front of it for 45 minutes, as if trying to get it straight in his head. As it turned out, I don’t think he was seeing Velázquez but he was seeing that famous version of it made by Francis Bacon”, in which the pope melts into anguished abstraction. When he got back to the studio, Utermohlen painted a self-portrait that makes reference to Bacon’s half-human figures, the mouth forming the desperate O of a silent scream. “That was really the last time he was still cognisant enough to be able to make the connections to another artist,” she says.

As his illness progressed, the artist was cared for and kept under the observation of a team at the National Hospital for Neurology and Neurosurgery in Queen Square, central London, led by Professor Martin Rossor. The neurologist subsequently published a couple of papers about Utermohlen’s case with a colleague, Dr Sebastian Crutch. The papers noted the ways in which his creative instincts, his profound vocabulary of depicted emotional states and his efforts to explain the world to himself through drawing persisted long after the doctors would have anticipated. “The example of continued artistic endeavour at a stage when Alzheimer’s has blunted the craftsman’s most precious tools offers a testament to the resilience of human creativity,” they concluded.

Those words don’t begin to describe the heroic efforts to preserve identity that Utermohlen’s later work conveys, but they have pointed the way to art-based therapies for Alzheimer’s sufferers that have achieved some positive results. In the course of their work the neuroscientists looked for comparable patterns in the late work of other artists: they make tentative suggestions that you can see the same kind of degeneration in Willem de Kooning’s painting, but the art historian in Patricia Utermohlen is not convinced. “I don’t think it was Alzheimer’s with de Kooning,” she says; “it must have been a different kind of dementia, the outlines remained so precise. To my knowledge, Bill is the only one who has carried on so far into the illness.”

One of the stranger aspects of this process, she says, with a bleak smile, is that he was a devotee of Dürer-style precision in his drawing, with an almost scientific rigour, and yet the later work becomes so much looser. “It’s odd,” she says, “because he hardly ever thought of his German ancestry, but toward the end he becomes a kind of German abstract expressionist. He might have been quite amused by that, I think.

"William Utermohlen" runs until 26 May at GV Art London, London W1

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