Causes
Congenital (common in toy breeds: Chihuahua, Yorkshire Terrier, English Bulldog) due to a developmental abnormality. It can also be acquired through tumours, infections or haemorrhages.
Diagnosis
Ultrasound through the fontanelle (in puppies whose skull is still open), MRI or CT scan.
Treatment
Mild cases: medication to reduce fluid production and inflammation. Severe cases: surgery to shunt the excess fluid away. The prognosis is variable.
Prevention
Do not breed from affected animals (the congenital form has a hereditary basis).

