Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your cat but excessive vocalization in an older cat is a fairly common phenomnenon. While pain/discomfort (see #3 below) could be one explanation for this behavior, there actually could be several other different reasons for it as it well.
1. Cats with senility or dementia can vocalize a lot; I think it reflects the confusion and/or anxiety that they are experiencing. Many of these cats will experience memory loss, possibly inappropriate elimination, staring off into space or acting like they don't quite know where they are at times. Signs can be intermittent but the condition does tend to progress.
Treatment options are limited, unfortunately.
a) Feliway diffusers or a spray which are natural pheromones which can sometimes instill a sense of calm may be helpful. There a similar oral product called Composure Chews. These products are available on the internet or at most pet stores
b) Fish oil supplements such as Welactin may help some cats because of their anti-oxidant properties.
c). Specific cognitive supplements such as Neutricks, SAMe (Novifit) and Senilife may be useful as well as NuCat Senior supplement. They should be available on the internet.
2. Cats with hyperthyroid disease will often excessively vocalize. I don't have a good explanation for this (probably secondary to hypertension....see below) but I see it all the time. Blood work would diagnose this problem and there are good treatments for this disease.
3. Cats with arthritis pain or other pain in their bodies will vocalize. We are also somewhat limited in treating cats with this problem but options to consider would include:
a. Cosequin for Cats which is a joint supplement and availalble at many pet/grain stores or online.
b. Occasional use of anti-inflammatory drugs such as Onisor which is licensed for use in cats but can only be given for three days in a row.
There is a drug called Metacam but it is somewhat controversial in veterinary medicine. It's use can cause significant damage to the kidneys and should be used with great caution in older cats.
c. Adequan which is basically a stronger supplement but is an injection and needs to be given by your vet. 4. Cats with hypertension can behave in some odd ways. This is rarely a primary problem in cats but usually secondary to Hyperthyroid Disease (#2 above) or kidney issues or Diabetes.
d. Fish oil supplements as I mentioned above
e. Pain medication such as Buprenex can be very useful and could be given every day or only on the days that she needs it.
If you think this might be the problem, you could discuss trying drugs for a week or so to determine if there is improvement in the vocalization. Some cats will benefit from acupuncture believe it or not.
Blood work would help diagnose either one of these conditions.
And blood pressure measurements are routinely done in cats these days and they can be treated for it, similar to a human.
5. She is going deaf. I think these cats meow louder to hear themselves.
6. I always worry about a brain mass when older cats start to behave in odd ways but I only include this to be complete, not to alarm you.
I hope this helps to explain the possible causes for this behavior. Deb