A hiatal hernia occurs when part of the stomach pushes up through the hiatus. This is the small gap in your cat's diaphragm that allows the oesophagus to connect to the stomach. Your cat's sphincter muscles usually prevent the stomach pushing though the hiatus, but when these muscles are weak or damaged, the functioning of the oesophagus becomes impaired and your cat can become seriously ill if the condition is not treated.
Because cats are often allowed to go outside on their own, there's a chance they will have an accident when you're not around. One of the more common accidents cats face is being hit by a vehicle, which, understandably, can be a very serious event.
If you have an outdoor cat, particularly if you live near to a road, it's important to be on the lookout for signs they might have been in an accident involving a vehicle, as it's not always immediately obvious.