In a small, airless office high up in the warren-like National Hospital for Neurology and Neurosurgery in Queen Square, central London, Suzanne O’Sullivan opens a laptop and nudges it in my direction confidingly. “Somewhere on here, I have some videos of patients having seizures,” she says, sounding (you can’t fail to notice it) somewhat excited. And then, more quickly: “Don’t worry, they all gave permission.” After this, she sets to fiddling, a little frustratedly, with the machine’s mouse. O’Sullivan, a neurologist whose particular area of expertise is epilepsy, may know an awful lot about what Emily Dickinson called the depths of the brain (“The Brain is deeper than the sea…”), but she isn’t, it has to be said, much of a hand with the wifi.

Finally, the videos start to play. In the first, a young woman is lying on her side in her bed in the telemetry ward (patients who are suffering from severe seizures can spend days in this unit, during which, wires attached to their heads, they are watched by highly trained neurophysiology technicians 24 hours a day). She is quite calm and still, but one of her hands is raised, the fingers moving, I now realise, in such a manner as to suggest she is working the valves of an invisible trumpet. “This is a fidgeting seizure,” says O’Sullivan, all of her attention on the screen. “But you wouldn’t really know it was a seizure at all if I hadn’t told you. It’s fairly subtle.” This is true. The ghostly fluttering lasts 20 seconds at most, and then it is over: so fleeting, I might have imagined it.

The fidget seizure comes from the temporal lobe, which is an emotional part of the brain

The second video is more dramatic. This time, it is a young man who lies in bed. He is asleep, deeply so – and then, suddenly, he is not. Up he jumps, skittering across the room, his arms and legs jerking wildly as if he is performing some kind of crazy dance. This goes on for 10 seconds, after which, just as quickly, the movements cease. Returning to consciousness, he becomes aware that he is tangled in the wires that are recording his brain waves; a nurse helps him to free himself. “That was a hypermotor seizure,” says O’Sullivan. “The fidget seizure comes from the temporal lobe, which is an emotional part of the brain. That’s why that kind of seizure is often accompanied by déjà vu, fear, panic. But this kind comes from the frontal lobe, which produces motor symptoms, so you can’t miss it. This happens to him three times a night.”

In the final video, a man is lying on his back in bed, a vision of peacefulness until the left side of his face begins to jerk rhythmically, as if someone were pulling on an invisible string threaded through his upper lip: a sight that might be comical if you didn’t know that this was something over which he has no control. When it stops – again, a nurse is already by his side – he looks dazed, as if he is not quite sure where he is, or why. “That’s a very specific seizure,” says O’Sullivan. “The kind that might make it hard for people to understand why it is so disabling. But imagine if that happened to you at work every day.” For a few moments, she continues to look at him, and then she turns to me, almost embarrassed. “Oh, I could show you seizures for an hour!” she says, with a grin. “They don’t look like seizures on the telly, do they? People know about the big, dramatic kind, but they don’t know about the tiny ones. What they fail to remember is that our brains do everything: when your hair stands on end, when you wink, everything. So if you’ve got a disease of the brain, like epilepsy, that means almost anything can be a symptom of a seizure.”

‘If you’ve got a disease of the brain, like epilepsy, that means almost anything can be a symptom of a seizure’: a model used by Suzanne O’Sullivan at the National Hospital for Neurology and Neurosurgery. Photograph: Phil Fisk/The Observer

O’Sullivan, a consultant who divides her time between the National Hospital for Neurology and the Sir William Gowers Centre in Chalfont St Peter in Buckinghamshire (an NHS unit that is part of the Epilepsy Society), became something of a public figure in 2016, when her surprising and powerful book about psychosomatic illness, It’s All In Your Head, won the £30,000 Wellcome prize (“Oh, it was the best thing ever. I was like the Queen of Sheba”). Now she has written a second, more challenging volume, Brainstorm: Detective Stories from the World of Neurology, in which she attempts to describe the complex, elusive experiences of some of her patients, and her various struggles to diagnose and treat their sometimes highly unusual symptoms. Along the way, she instructs us in the bottomless mysteries of the brain, telling us some of what scientists know about it (relatively little), but also what they don’t (there is so much yet to be discovered; we are not even close to 50% of the way there). “I hope the book will teach people about the brain,” she says. “How it’s organised, how it might work. But much more important to me are the people in it. These patients have disabilities that are unique to them, and which are very hard to explain to other people. I want to communicate their stories, to reveal what it’s like to live with an often invisible disability, with a condition that it is still very hard for doctors to treat.”

If I have a clinic with 10 people in it, there might be one or two where I might not feel entirely confident

How often, these days, does she find herself totally baffled by a patient’s symptoms? (O’Sullivan, the warmest, most approachable kind of doctor you could meet, has been a consultant since 2004.) “That happens about two times out of every 100,” she says. “But if I have a clinic with 10 people in it, there might be one or two where I might not feel entirely confident. You see, you’re basing so much on their ability, or that of their mother or partner, to describe something that may be very odd.” Luckily, she loves nothing more than to listen. “The story is everything, and that helped with the writing of the book. Each one has an inbuilt drama, a feeling of ‘What is going to happen next?’” By the time patients get to see O’Sullivan – they are referred from all over the country – they’re usually desperate. Earlier, she took me to meet a young woman who is staying in the video telemetry unit (let us call her Emily): “Emily was breezy, wasn’t she? And she looked so healthy, so bonny. But she is here to see if surgery is likely to offer a cure for her seizures. To get to that point – it means the drugs have failed – you have to be quite desperate.”

The patients in Brainstorm are of many different ages and backgrounds; all that they have in common is that their symptoms are frequently bizarre. Donal is a school caretaker whose seizures involve a hallucination in which he sees what looks like the seven dwarves. August is a clever, rebellious young woman whose seizures cause her suddenly to run at speed, putting her in great danger (she is oblivious to fences and walls). Sharon, having been treated for epilepsy, turns out not to have the condition at all: her seizures are dissociative, her brain switching off for a moment, perhaps as a way of avoiding remembering something she doesn’t want to. (I was amazed by this, but it is not uncommon: at least one-fifth of people attending an epilepsy clinic with seizures do not have epilepsy.) “That’s a very delicate conversation,” says O’Sullivan. “You have to make sure the patient understands that you’re not accusing them of doing it on purpose, that you know they’re just as disabling – just as real and uncontrollable – as an epileptic seizure. Those suffering dissociative seizures are more likely to be admitted to intensive care – and they’re much harder to treat because psychiatric therapies are harder than taking a tablet.”

O’Sullivan, who grew up in Dublin, the city where she trained as a doctor, always knew neurology was the speciality for her. “The nervous system is beautiful,” she writes in her book. Epilepsy was noted as a brain disease by Hippocrates in 400BC, but it was a long time – millennia – before doctors recognised the condition, which affects about 1% of the population, as a vital investigative tool for the brain. Only in the 19th century did a neurologist called John Hughlings Jackson determine that the condition’s cause was a sudden electrical discharge spreading through the brain, at which point epilepsy became a symptom rather than a disease, the features of each seizure being representative of the part of the brain that was engulfed in the offending electrical discharge. This led neurologists to follow seizures to their source and to extrapolate the function of the brain from it; thus, they put together rudimentary maps of the brain.

‘The nervous system is beautiful’: Suzanne O’Sullivan’s model brain from the National Hospital for Neurology and Neurosurgery. Photograph: Phil Fisk/The Observer

The development of anaesthetic, allowing surgeons, if they were lucky, to find the source of the problem – a tumour, a lesion – moved things along. So, too, much later, did Cat scans and MRI scans. But we still don’t know what causes most brain diseases, nor do we know how to reverse them. O’Sullivan still does what the pioneers did a century ago. She interprets stories, extrapolating from a patient’s account of their symptoms, a diagnosis. At which point, she may be able to help – or not.

In the book, Maya, a woman in her 60s who has suffered from debilitating epilepsy since she was 10 – her seizures involve screaming, chewing and the loss of speech and movement in one hand – has surgery to remove an abnormal area of her left temporal lobe, and a miracle happens (except, of course, it is not a miracle, but the result of huge and hard-won expertise): she is cured. For the first time, she can move freely, alone. But another, much younger patient, Mike, having been offered smaller odds of success for a similar operation, opts not to have surgery, fearing the risk; his epilepsy is milder, and he can live with it. “Surgeries have really increased in the last 20 years,” says O’Sullivan. “But sometimes funny things happen. Patients have set up their whole lives around their epilepsy, and they slightly miss it; they must re-adapt. Also, when you remove the tissue, you’re inevitably removing healthy as well as unhealthy tissue, and that brings a risk. I am hoping the technology will arrive soon that will enable Mike to have the operation without opening up his brain, burning just a tiny bit out.” What about the drugs? Haven’t they improved? “They’re nicer to take. They have fewer side effects. But still, they only work for about 70% of patients.”

The minute you apply biology to your attraction to another person, it becomes unattractive

How quickly will things change? “We’re in the infancy of figuring out how to heal damage once it happens,” she says. “If you have a scar on your brain, we can’t do anything about it. But there are glimmers on the horizon. One is our ability to heal genetic diseases. Another is our increasing understanding of neuroplasticity. We used to think the brain was fixed, that it didn’t change after you were an infant. However, we know now that it is possible to create new connections, for functions to move around. If we could figure out how to promote that… But we’re a million miles away at the moment, so protect your brain!” She would never cycle without a helmet. “I am more careful than other people, yes. I don’t do high-risk things, though I am terribly bad about crossing the road without looking properly. But the flipside is that I worry less about brain function. You do get less good at learning things as you get older, but I know I’m not alone in that. It’s not a sign of anything degenerative.”

And perhaps there are some things that it is better not to know. “Of course, I want to understand how to cure diseases. But there is something worrying about the idea of reducing every part of your personality to a biological mechanism. If you do, what are you? If you could manipulate happiness and sadness, what would that mean? I am quite happy to be romantic in that way.” Is love a kind of brain disease? She smiles. “Well, the minute you apply biology to your attraction to another person, it becomes unattractive. I don’t need to know these things. But you needn’t worry. We’re a long way from understanding such feelings.”

Before I leave, we talk about the NHS: every technician and specialised nurse I have met today has come to Britain from another country. “My world is quite rarified,” she tells me. “We have no A&E here, for instance. But I don’t think there is any doubting that the NHS is in crisis. There are already a phenomenal number of empty doctor and technician posts, and our potential inability to recruit from abroad in the future is not going to help that.” All this said, though, you would never take O’Sullivan for a pessimist. Doom is not her mode. It’s impossible to imagine her whingeing without turning it into a joke. Something pulls her along, and you can almost feel it: her fascination with her work, her deep and abiding interest in those who come to her for help. “The life so short, the craft so long to learn,” said Hippocrates. Patience is everything in her realm. But equally, there is no time to waste.

Brainstorm: Detective Stories from the World of Neurology by Suzanne O’Sullivan is published Chatto & Windus at £16.99. To order a copy for £14.44, go to guardianbookshop.com