If you acquire a medical condition you’d never heard of before, then naturally you wonder how big the club is. Am I alone or any there many like me? And so I have often wondered how many people in the world have homonymous hemianopia.
I was very grateful to an Australian correspondent who sent a link to an interview with Alex Leff. Dr Leff is at the Institute of Neurology at University College London. He and his colleagues have developed online therapies for people such as myself who are hemianopic – “half-blind” – in other words they cannot see anything, or much, to the right of their world.
Hemianopics have problems such as getting about in the home or the street, with reading (they may become alexic – unable to read or understand text), and with locating objects. The UCL Neurology Institute’s therapies, Read-Right and Eye-Search, try to help with reading and with locating objects.
I tried Read Right and enjoyed the experience a lot, but it didn’t help me. I was convinced, and by the experimental evidence, that these are very beneficial courses, but I am probably not typical and found my own solutions, which work for me, and which I plan to explore in a future article. It would be wrong to say ordinarily sighted people are all alike; but true perhaps that every visually impaired person is impaired in their own way.
In the interview Dr Leff also confirmed something I’d found and been quite surprised by at the time.
Getting statistics about prevalence of a condition – how many, per 10 000 say, suffer from it – is not simple. At first I innocently thought there must be an “official” figure. But actually counting people that fit into some medical category is very hard. You rely on clinical reports, which will be incomplete, and difficult to collate, and many patients with the given condition will not present.
After some hunting around I’d found this study from a population of 3500+ persons 49 or over in two urban districts west of Sydney Australia. This took in more than 80% of those in the target populations and seems to be about the most useful evidence we have about how common hemianopia is. According to Dr Leff this is indeed the best source available.
What did the study find? About 8 people per 1000 had hemianopsia, or a form of it.
The frequency in the general population must be lower, because most hemianopia originates in stroke, and the risk of stoke increases markedly with age. But some younger people have stroke, some have sports or other accidents which damage the visual cortex at the back of the head, some may suffer tumours that affect vision, and a small number of people are congenitally hemianopic.
So as a very rough guess – but who can do better? – I would say perhaps 3 per 1000 in the general population have hemianopia. So uncommon but not rare – there must be about 200 000 people affected in the United Kingdom, and worldwide maybe 20 million. A pretty big club!