Both these questions are very difficult to answer as there are so few cases in the world to be able to make an accurate estimate. At present, it is thought that there are only a few thousand cases diagnosed in the world but many undiagnosed or misdiagnosed. Based on the experience from other rare diseases it is possible that the prevalence of PTHS in the population is 1 in 225,000 – 300,000 or roughly 4 in a million. The population of the UK is about 65 million so the incidence of PTHS in the UK should be about 260 cases. It was estimated that about 200 cases of PTHS were diagnosed in the UK in July 2014 and the estimate has risen to about 300. Pitt Hopkins UK was in contact with about 60 families in Feb 2016 and approaching 100 in 2020 and so possibly with about a third of the families.
You will see other numbers on the internet so some explanation is needed for these figures. Sometimes numbers are given by an approximation of names known. This may be needed for grants etc. We can then say for sure that there are hundreds of people diagnosed as we can get this information from numbers on the international support groups.
In the UK we have a different situation as testing is done through the National Health Service. Our genetic centres are able to confer more easily than in some other parts of the world. In 2014 most regional genetic centres had diagnosed about 7 to 8 cases and from this, it was estimated there were at least 200 cases in the UK at that time. As said above we are in contact with 62 families. It should be understood that not every family makes contact with a support group or organisation or even know they exist, especially if they may have been told by their doctor they don’t know of any support group in the UK.
The numbers known in The Netherlands were around 50 in 2014 and as the population is a quarter of that in the UK the number of 200 in 2014 in the UK matched quite well. Using the recognition of a syndrome in one, smaller country, to calculate the frequency in other countries is a legitimate approach.
We cannot name a few thousand around the world but in the UK we could name 90-100 which is about a third of the number thought to be diagnosed. (Obviously for privacy reasons we won’t )
Of course, there are many who have PTHS in the UK, probably adults, that are not diagnosed at all or possibly misdiagnosed!