The path to a Billion Dollar + company, page-271

When I posted earlier I hadn't read the other study you found, which is the English IIH data from 2002 until 2016. I note that one of the authors is Alex Sinclair

It states that the English incidence of IIH by 2016 was 4.7 per 100,000 which is somewhat lower than the Welsh stats.

If Invex are choosing to estimate the incidence of IIH overall in the UK & Europe to be around 3.7 per 100,000 then that is how they are arriving at their 24,000 target patient number.

You would have to think the up to date incidence number is likely to be higher than 3.7 but I suspect the problem for them is that the data is not available (publicly at least) in most European countries.