If your cat is exhibiting tremors of her head or limbs, these may appear to be symptoms similar to those experienced by humans with Parkinson's disease; however, while there is no actual Parkinson's condition identified in cats, there are various conditions which can occur in older felines that can cause tremors.
It would be advisable to have your cat evaluated in person by her vet, so you can find out more details about her condition, if she is indeed, showing symptoms of tremors. Severe arthritis, disc degeneration, tumors of the brain or spine, toxins present in the system, and chemical imbalances are a few of the conditions which may be responsible for tremors. Sometimes, they may also be identified as 'idiopathic tremors', meaning they have no known cause, so further testing and research would be necessary.
After your regular vet sees your kitty for an in person examination and some routine senior blood work, it may be necessary to consult with a veterinary neurology specialist, depending on the results of the tests and your vet's recommendations. Your vet should be able to make a reliable referral to someone in your area, or you can search for a veterinary neurologist on your own, here: http://www.acvim.org/websites/acvim/index.php?p=3
I hope all will be well with your furry girl!
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