Hydrocephalus means water on the brain. I'm not going to get into the medical reasons why it happens in fetal development because it's passed on genetically and can be avoided completely.
This genetic disorder is something to take very seriously because A. there is a test for it B. it's lethal to the foal and C. birthing a Hydrocephalus affected baby can be traumatic or terminal for your mare.
Cure: Test your mare before breeding and If you have a carrier mare avoid breeding with a carrier stallion. Problem is solved.
As you probably guessed from above Hydrocephalus is a genetically transferred disorder. It's recessive so the only way to get an affected foal is to breed 2 carriers together. You'll have a 25% chance of an affected/dead foal and 50% chance of a carrier foal. It's not worth the risk that you'll get the 25% chance of a non affected non carrier foal. It's irresponsible to take that risk.
The reason the birthing is so traumatic to the mare, possibly causing dystocia, c section or death to the dam, is because the cranium of a hydrocephalus foal can be enlarged significantly
As you can see in the next photo the brain cavity was opened up to show it's not formed completely and the fluid in that cavity was drained.
The KFPS/FHANA has started testing the all approved stallions and posting the results. I have compiled the published and known carriers on the stallion pages. If you have a positive mare and have picked a stallion that you want to breed to that is missing information please contact the KFPS directly to see if your positive mare is a risky match with the desired stallion. I believe they will not give out any stallion information if your mare is not tested, so please get your mare tested.
Here is the form for the FHANA horses to be tested FORM (email me if the link doesn't work or firstname.lastname@example.org)to contact the KFPS directly for all other books (bb1,2, C and D books) please go to www.kfps.nl and contact them directly
IF you do not want to go through the KFPS or you do not have a pure Friesian this is a company that will test for Hydrocephalus cariers regardless of breed.
Scientific article that you can look at for more detailed information.