The scariest kind of death, I’ve always thought, is the drop-dead kind: the sudden stop of a perfectly healthy person’s heart as they go about their daily business. This is what happened to Mary Ann Kenny’s 60-year-old husband, John, who was out jogging in the “dazzling spring sunshine” of Killiney, a picturesque coastal suburb of Dublin, when he unexpectedly collapsed.

Kenny discovered something was wrong only when she turned on her “old-fashioned button Nokia” after a lengthy work meeting and noticed she had six missed calls from unknown numbers. The worst part, in her own account, was the realisation that she would have to break the news to their two young sons.

That tragedy took place ten years ago, in the spring of 2015. Kenny’s psychiatric crisis came soon after, and is the focus of her memoir, The Episode. To write it, the university lecturer, now 60, painstakingly mined material from her extensive electronic patient record, which “runs to eight volumes and 875 pages” and contains the intimate details of the medical rabbit hole she fell down after John’s death. Many of the notes contained in her file, she emphasises with a dignified kind of frustration, read like “a zookeeper’s or experimental psychologist’s observations about a caged and frightened animal”.

Book cover for "The Episode: A True Story of Loss, Madness and Healing" by Mary Ann Kenny.

To begin with, Kenny’s grief appeared run-of-the-mill. After the initial shock of losing her husband she suffered many of the usual symptoms — reduced appetite, sleeplessness, irregular periods — but otherwise she diligently worked her way through various coping mechanisms: holding a thoughtful funeral, reading grandly titled self-help books, chatting to a therapist who had “an infectious laugh and grandfatherly air”.

Visits to the gates of Ayesha Castle (now called Manderley Castle) became a vital ritual too. “I would place minuscule bunches of bluebells and sprigs of white, pink and red valerian picked at the roadside inside the narrow mock arrow slit above the spot where he had collapsed and died,” she writes. For brief moments, “the agony would recede”.

Something about Kenny’s grief beast was different, however. More dangerous. After initial resistance to using drugs to feel better, fearful of addiction, she tried antidepressants, but a single dose caused her to wake in the middle of the night “drenched in sweat, weak with nausea”. Other symptoms appeared thick and fast, most notably an “excruciating and persistent burning” underneath her skin and, later on, a weeks-long bout of constipation, which eventually resulted in a number of “involuntary bladder explosions”.

Gripped by paranoia, Kenny became convinced that she had poisoned her children. “I was tormented by an image of my evil self standing over the cooker,” she recalls, “pouring pills into the simmering food with the depraved intention of causing harm.”

Four months after John’s death, and by then on the radar of medical professionals, Kenny agreed to spend a couple of weeks in a psychiatric hospital. Rather than being encouraged to talk about positive memories of her life before John or to forget for a while the pressures of her everyday life, such as organising childcare or communicating with her employers about why she was away, treatment was almost entirely focused on pill-popping. “I lay there in frozen fear,” she remembers, “facing away from the world, my arms wrapped tightly around my torso.”

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After a brief spell of freedom, having told “white lies” about feeling better, Kenny was rehospitalised on the grounds of “suicidal ideation”. (In truth, she never really considered the act, simply wondered “how anyone could master the seemingly impossible logistical challenge of throwing themselves under a speeding vehicle at exactly the right moment”.)

More tests, more drugs and more probing questions followed, and it didn’t take long for things to go from bad to worse. By October 2015 Kenny was “close to physical and mental collapse”, ingesting a daily “cocktail of antidepressants, anxiolytics, sleeping tablets and antipsychotics” but finding no relief from “the endless churning” in her brain.

Exhausted and confused, she became “prepared to confess to anything”, happily admitting to appalling things she knew weren’t true, such as planning to push her sons off a cliff. “Even while I was gobbing them out,” Kenny admits, “I almost marvelled at the ridiculousness of my disclosures.”

At one point, after being transferred to an intimidatingly high-security ward, Kenny cringed with guilt at the idea of inconveniencing nursing staff and “terrified of causing further fuss” mopped up her vomit by herself. She feared she would never be allowed home, and that her children would remain for ever on the Child Protection Notification System, considered “at ongoing risk of significant harm”.

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Thankfully, Kenny did recover, though it was slow and difficult, like stepping cautiously across a recently mopped floor. How much of her depression was really grief and how much of it grew out of being cooped up in what she describes as “a system that medicalised my distress” is unclear, even to her, but she insists that despite the blow of John’s death and the intensely distressing nature of her psychosis, the psychiatric treatment she received “was the most traumatising of all”.

This is a brave, elucidatory book, which proves that breakdowns aren’t always shouty and obvious, and that anyone can fall suddenly and silently off the cliff of sanity. We are lucky that Kenny is a skilled enough writer to recount her all-encompassing mental murk with such clarity and impressive detachment, because there’s much to learn from this candid account of a life so nearly ruined.

The Episode: A True Story of Loss, Madness and Healing by Mary Ann Kenny (Sandycove £18.99 pp288). To order a copy go to timesbookshop.co.uk. Free UK standard P&P on orders over £25. Special discount available for Times+ members