It's not often that I make a diagnosis at the garage. But just a few weeks ago, while queuing for the car wash at a local service station, the penny dropped about a patient whose case has been puzzling me for several years. Let me explain.

The world of business is something that has always been a real mystery to me. I can do the doctoring just fine, but when it comes to entrepreneurial business - all that buying and selling and spreadsheets and creative accounting - I am something of a dunce. And so patients like Tom who has clearly made something of a fortune out of the magical world of business, are all the more intriguing. I suppose I have looked on with a touch of envy at his never ending supply of top-of-the-range cars, but console myself with the thought that my life as a doctor is actually more interesting than his life in business, however successful it might be.

And Tom isn't in the best of health. Getting his blood pressure under control has been an absolute nightmare. Whatever I prescribe, nothing seems to improve it. I know that his medication regimen is now quite complex, but I've written out clear charts for him about what should be taken when - and yet nothing seems to work. He insists that he is following my instructions and advice. It's most odd.

And then, at the car wash, I saw him and his beautiful car two places in front of me in the queue. The garage had just fitted a new super de luxe machine, and Tom was looking at the instructions with an air of total bewilderment.

And I saw, and overheard, him go to one of the garage staff who was nearby and say 'I'm sorry - I don't have my reading glasses - could you explain the new instructions to me?'

And the penny dropped. I was as certain as I could be that he didn't use reading glasses, and so there could only be one explanation. Perhaps he couldn't read at all. After all, this is a problem that is far more common than most of us would believe, and would explain fully the fact that all my written instructions on his antihypertensive medication were not getting anywhere.

And, just a couple of weeks' later, when he came in to see me, I was pondering as to how I might broach this subject. And so I did what I usually do in this situation - without asking a question, and towards the end of the consultation which had revealed his blood pressure as worryingly high as usual, I simply made a statement. 'Tom,' I said, 'I've noticed over the years that many incredibly successful people struggle with reading.' And I just left it at that.

He could have looked puzzled. He could have looked cross. Instead he just looked through me into the middle distance as his eyes very slowly filled with tears. And very quietly he said: 'so you know'.

So I told the story about the garage, and he nodded and said: 'I've never dared tell anyone. I'm just so ashamed.' He went on to explain how his wife did all the paperwork for the business, and not a soul realised that he really could not read more than the simplest word. I began to understand the power that drove the character played by Kate Winslet in the film The Reader who would rather go to prison as a war criminal than admit to illiteracy.

I suspect that this is another problem to which the old medical cliché of 'there's a lot of it about' is much more common than we realise. But only time will tell if I now have a better chance of getting his blood pressure down.