The way to make a horror film scary is to keep the monster hidden, and this is how I’ve started to feel about my brain. Since I was a teenager my migraines have begun in the same way, with a white mark on my vision, as if I’ve looked too long at the sun. The mark grows until I can barely see, and then the headache comes, and then, well. In June, I woke with the familiar blind spot, but that afternoon it hadn’t changed, nor had it disappeared a week later, or a month. Eventually I had an MRI. Before I sailed into the space tube, I chose the Beach Boys to play through the headphones; as I closed my eyes to avoid the claustrophobia, the opening notes of In My Room, the sound of an ancient broadband connection just behind it. The next day I got a call from the neurologist. Rather than a migraine, he said, with unfamiliar graveness, I’d had a series of mini-strokes.

It’s odd to be shown evidence that something’s wrong with you when everything feels alright. The feeling reminded me of a similar unlikeliness five years ago – when all evidence said I was pregnant, but until the child actually arrived in blood and drama, the diagnosis remained to me a kindly theory. This time the oddness is a different shape to that growing bump – ghostly, unclear. I’m writing this with one eye shut, as the blind spot remains, revealed now to be where the blood vessel to the eye has been permanently damaged. I am seeing the scar of a stroke, its stain. If I concentrate I can make out its shape, a weary dribbling moose. In that first phone call I asked the doctor, if I hadn’t realised I’d had these strokes, how would I know if I was having another? Well, he said, you might feel… and then he calmly reeled off a list of symptoms – lightheadedness, headaches, dizziness, confusion – which all described the details of my daily life and many of the traits that keep me adorable.

So my day to day has become a BBC drama, where every episode is weighted with the possibility of murder, but there is no body. And though the soaring shots of cliffs and sea appear ominous, it’s just landscape and weather – if you turn the colour up it could be a postcard from the past, and if you hadn’t read the preview in the paper, you wouldn’t know there was any death at all. My cliffhangers are the weekly hospital tests to discover why something like this is happening to someone like me, in order to prevent another stroke that might be less mini, and each one comes with its own kindly doctor and a new understanding of a part of my body I’d taken for granted.

My heart, for example, which I’ve mainly relied upon for beating and love, has gathered sinister new significance. In a dark room a cardiologist spoons me and, together in a kind of romance, we watch his screen to see if the organ contains holes. My brain, until now a benign and useful friend, a mate, feels like a slinking presence. It delivered the blind spot like a ransom note in the post, a threat and a reminder of the power it wields. What could it turn off next? My ability to smell urine from a hundred paces? My preference for a dark chocolate biscuit? A pain that will change me?

It’s difficult to work out how to process this news, both for me and my friends. It’s bad, isn’t it, but not BAD bad. It’s like I’ve been shot, but somewhere silly, like in the earlobe or little toe. It would be easier for them if there was an obvious wound, a cast they could sign. It would be easier if it was something that was happening today, present tense, rather than something that has happened, past, only choosing to reveal itself now. It would be easier if I… felt something. I have headaches, but I always have headaches. I am tired, but I’m always tired. Yet suddenly I am being treated the way I was born to be treated – my boyfriend delivers a dressing gown to me on the stroke of seven, putting a cool hand on my head. At a recent hospital appointment he lifted me on to the bed when I fainted, and later at my request did an impression of the way my face went just before. As I approach the end of my 30s, I find my final vice is extreme sympathy.

My questions about what happens next are less technical more existential, as I consider a future of never quite knowing. Like a parent that moves their family out of the city after bingeing on stories of stabbings, am I destined for a bubble-like life, where every headache sends me spiralling into the internet? As someone who has always been careful to recognise, for fear of falling into those ancient holes of diets and regret, that we should live peacefully inside our changing bodies, I am now struck daily by the disconnect. Between what my brain tells me and what it really means. Between what I see and what I can’t, and between the way I feel and the way I am.

Email Eva at e.wiseman@observer.co.uk or follow her on Twitter@EvaWiseman