Soon after Suzanne O’Sullivan had left medical school in Dublin, she met a patient named Yvonne, whose mysterious illness appeared to bear little relation to any of her previous studies.

Yvonne, she was told, had been stacking the fridges in a supermarket when a colleague had accidentally sprayed a fine mist of window cleaner in her face. She tried to wash her eyes, left work and went to bed early, hoping they would feel less sore the next day. But when she woke up, her vision was worse – everything was so blurry she struggled to read the time on the clock. Twenty-four hours later, she could not tell night from day.

O’Sullivan’s colleagues assumed Yvonne was faking it, perhaps for some kind of lawsuit. “There’ll be no Oscar for that performance,” one muttered

Except after six months of examinations, doctors could find nothing wrong with Yvonne’s eyes. She was eventually admitted to the neurology unit where O’Sullivan was working. During the observations, Yvonne’s eyes would flicker between her husband and the doctors; as the consultant moved an ophthalmoscope close to her eyes, she blinked. It certainly seemed like her eyes were responding to her surroundings, yet she continued to claim that she was enveloped in an impenetrable darkness.

O’Sullivan’s colleagues assumed she was faking it, perhaps for some kind of lawsuit. “There’ll be no Oscar for that performance,” one muttered after they had left the ward. O’Sullivan herself was unconvinced. “I liked Yvonne. I felt sorry for her. But I did not believe she was blind,” she writes in her new book It’s All in Your Head, recently shortlisted for the Wellcome Book Prize.

Now she knows better. At the Royal London Hospital she has become an expert in “psychosomatic” illnesses. She has treated people who are paralysed from the waist down, or who have such severe cramp in their fingers that their hand has become little more than a claw; one woman could not even empty her bladder without a medical catheter. Yet when doctors look, they can find no physiological cause – suggesting the problem originates in the mind, not the body.

In this light, it’s perfectly possible that Yvonne really wasn’t conscious of what she was seeing – somehow, her unconscious mind was discarding the information before she became aware of it.

Keen to know more, I spoke to O’Sullivan about her career and her recent book detailing these remarkable case studies.

Although O’Sullivan had seen patients like Yvonne from the very beginning of her career, the interest intensified once she started specialising in epilepsy. Patients would often come to her suffering from debilitating seizures that left them writhing around on the floor, limbs flailing, helpless, yet when they came in to the hospital for neurological tests, they showed none of the signature brain activity of epilepsy; they were “psychogenic” seizures. “It became obvious no one is talking about these problems that are consistently common, but which are getting no attention in medical journals or medical conferences,” she says. “This is a very neglected area.”

That silence brings stigma, meaning that most patients end up feeling insulted by the diagnosis. “Their first reaction is ‘you think I’m doing it on purpose’ or ‘you think it’s not real’ or ‘I could stop it if wanted’,” O’Sullivan says. “There’s this perception that there is a degree of wilfulness”.

If you find their symptoms hard to believe, consider the many profound ways that your emotions already move your body. O’Sullivan points out that every time we cry, or laugh, or shake with anger, if we feel so sad that it is hard to lift ourselves from our bed, or queasy with nausea after we hear of a friend’s food poisoning, then we are experiencing psychosomatic phenomena first-hand. In fact, up to 30% of people visiting their family doctor or GP (and 50% of women seeing their gynaecologist) are enduring symptoms that cannot be explained with a physical cause – suggesting they may be psychosomatic in origin. The difference is that for most of us those feelings pass and we can return to our normal lives, but for O’Sullivan’s patients they are exaggerated and chronic, lasting months, years, even a lifetime.

They really are disabled. They are more disabled than most people with a physical disease

She emphasises that the psychological origin does not make the blindness, fatigue, seizures or paralysis any less debilitating. “They really are disabled. They are more disabled than most people with a physical disease.”

Consider Camilla, a lawyer in London, who had been diagnosed with epilepsy before O’Sullivan managed to show that it was psychogenic. She described how humiliating she found the seizures; how people try to help by sitting on her to stop her limbs from flailing about, and stick their fingers down her throat to stop her choking; one man knelt beside her, asking if she was OK, before stealing her mobile phone. “But do you know what happens all the time? People video me on their mobile phones and walk away laughing,” she told O’Sullivan. The more you know about these patients, the harder it is to believe that anyone would deliberately expose themselves to this humiliation.

O’Sullivan has nevertheless met a few fakers, such as Judith, who claimed to be suffering seizures as the side-effect of chemotherapy for leukaemia years before. Hoping to get to the bottom of the story, O’Sullivan called her into a hospital ward, where a video camera could film a seizure when or if it occurred. Sure enough, at 9:15 in the evening, a nurse found Judith lying on the floor unconscious, falling so hard that she had apparently fractured her hand. When O’Sullivan played back the video, however, she saw there had been no seizure at all. Judith had simply raised her hand and hit it hard against the wall four times, before lying down gently on the floor, pulling down a plate with her to attract the nurse. Indeed, looking at the medical records, it became clear that Judith had never suffered from leukaemia, either.

What kind of mental torment would make someone invent an illness?

Although such patients with “factitious” illnesses may add to the stigma of people like Camilla or Yvonne, O’Sullivan still feels compassion for these people. After all, what kind of mental torment would make someone behave in this way? Even if Judith hadn’t suffered from leukaemia, perhaps she had witnessed it in someone else – an experience she found difficult to process in any other way? “Factitious illness is one of most serious disorders I know,” O’Sullivan says.

There is currently precious little research on the best way to treat psychosomatic illness, but she tends to refer her patients to psychiatrists or to a cognitive behavioural therapist (CBT) who might be able to unknot the distress or trauma that is leading to the illness.

And there are some real success stories. Not all cases can be pinned to a specific event, but Camilla realised that her seizures may have been linked to the death of her young son, setting her on the path to recovery. Yvonne, meanwhile, seemed to have been struggling to juggle the stresses of work, children, and an overbearing husband, and as she learned to cope with those upsets, her sight slowly returned.

Patients with paralysis or muscle spasms can also respond well to physiotherapy. “They need to learn how to use their legs again,” O’Sullivan says. It’s often an uphill struggle; particularly with seizures, relapse is common. “We have to give ongoing support and reassurance.”

One of O’Sullivan’s biggest concerns is the rate of misdiagnosis – doctors overlooking the psychological origin of these illnesses, instead prescribing the patients drugs, or even admitting them for harmful surgery. She says this may come from the doctors’ fear; somehow, it seems more serious missing a physical cause of an illness than a psychological cause – yet the damage can be just as great.

“The people who have been told they have epilepsy – they will be put on a toxic medication for at least a couple of years, before they accept it’s not going to work.” Over that time, the condition has become a part of the patients’ life – they’ve told their friends, families, and employer, making it much harder to accept the new diagnosis. “You become wedded to it through your own belief and the doctor’s belief,” O’Sullivan says. “You are getting treatment you don’t need for a disease you don’t have, and you are deprived of the treatment you do need – you’re not referred for CBT, a physiotherapist or a psychiatrist.”

Perhaps inspired by meeting Yvonne at the start of her career, she would like to raise awareness of psychosomatic illnesses at the earliest stages of a doctor’s training. “My feeling is that it needs to start at medical school-level,” she says. “I must have encountered these patients thousands of times but I have no recollection of being told how I should help them.”

For the time being, she hopes that her book will at least spark that conversation; so far, she has already found that a small number of patients are becoming more accepting of the diagnosis, and are less afraid of the stigma attached to it. “I have my fingers crossed that it will be a talking point, that people will feel less ashamed.”