The Anachronous Mom's
Giant Cell Arteritis page
Arteritis is very rare, and it usually occurs in people over about 66.  A good friend diagnosed it in herself at about 39/40 years of age.  Of course nobody would believe her (despite the fact that she's a maven, a programmer, very smart, and so forth) for ten years.  Ah, yes, the vagaries of diagnosis.

Just a couple of links on it.

One is from Neuroland, which has Neurology information  for physicians & health professionals.

And the other is from the National Organization for Rare Disorders.