The "soft spot" or fontanel often found on the top of a Chihuahua's head is
also called the
molera.  It is a normal part of the Chihuahua breed and some
other toy breeds as well.  Many dogs outgrow the molera and the skull fills in
by the time they are about a year old just as it does with human babies.  
Others have a molera their whole life and are perfectly normal, happy, healthy
dogs.  The presence of a molera is not considered a fault and does not affect
the dog's ability to be bred or shown.  The presence of a molera does
not
mean that a dog has hydrocephalus (also known as water on the brain).

The presence of a molera and the occurrence of Hydrocephalus are
completely separate and unrelated
.