Chihuahua Health Problems
There are several diseases/disorders that many small dogs (especially the Chihuahua) are born with or that develop with age.
Luxated Patellas
The most common health problem, especially among Chihuahuas, are Luxated Patellas, where the kneecap slips in and out of the tendons and muscles surrounding the bone. Your chihuahua may have it in one or both of its hind legs, but it is most common in one or the other. There are several levels of severity. If your chihuahua whimpers or constantly limps due to his/her luxated patellas, it is probably time for surgery. Some chihuahuas, however, have Luxated Patellas their entire life and never need surgery. You want to ask the pet store if the chihuahua you are looking to buy has luxated patellas because surgery costs anywhere from $1200-$3500. My advice? Consult with your veterinarian, especially if chi exhibits pain.
Here is a video of a chihuahua with luxated patellas.
Hypoglycemia
This is when your chihuahua’s blood sugar level becomes low. This can cause a wide variety of symptoms such as shaking, vomiting, seizures, passing out, and others. Your vet will probably recommend that you keep honey on hand in order to feed your chi small amounts of honey throughout the day in order to prevent low blood sugars. It is always important to talk to your vet in order to check for individual needs and medical treatment options. Medical treatments may not be the same for all chihuahuas.
Hydrocephalus
Hydrocephalus is when fluid in the brain fails to drain into the circulatory system, which therefore causes build up of fluid in the brain. This then cause the skull to enlarge. If your chihuahua has a dome-shaped head or has a fontanel that does not close, your chihuahua may have this condition. Check with your vet in order to make sure his/her brain is draining correctly.
Here is a picture of a chi with hydrocephalus, from Veterinary Answers:

