Hi, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. I'm so sorry to hear about your cat. For how long has she had this condition of not being able to open her mouth? Does everything else seem OK? Any weight loss? Any vomiting or diarrhea? Any drooling? Any coughing? Any excessive water consumption or excessive urination?
Thanks for your reply. Has she been diagnosed with any chronic disease like periodontal or other oral disease? Do you think that she can't truly open her mouth (which would be a muscular or neurological issue), do you think that her mouth is sore from a foreign body lodged somewhere in there or severe periodontal disease or a growth, or do you think that she is weak from some more generalized disease and just not wanting to open her mouth? Do any of these seem correct to you?
Whoa! I've never seen that, but will certainly do some research and be back soon! In the meantime, I'd offer her baby food meat that she can lap up. . .even with a little water added if the food is still too thick. . .poor baby!
Additional potential causes of locked jaw syndrome are masticatory muscle myositis, neoplasia, trigeminal nerve paralysis and central neurological lesions, temporomandibular joint luxation and dysplasia, osteoarthritis, retrobulbar abscess, tetanus, and severe ear disease. Treatment of locked jaw is directed towards the primary cause. This is from a study done on both dogs and cats with locked jaw syndrome. The first potential cause mentioned is ankylosis of TMJ arthritis. Cats don't usually get Masticatory Muscle Myositis. . .which would be the muscle disease first mentioned.
If you want to pursue a diagnosis and treatment, then you may need to see a specialist if your vet hasn't offered any further diagnostics. I'd recommend an internal medicine specialist. Immediately, your cat should have an pharyngostomy feeding tube placed which would allow you to put food directly into her stomach through this tube.
Has your vet tried to open her mouth under anesthesia? Has he tried a pain reliever like buprenex to see if her lock jaw is due to pain in her mouth or her TM joint? Does she have any indication that she has an abscess behind one of her eyes? In this case, her eye may be bulging.
I would talk to him about that then. . .opening her mouth under anesthesia. In my research, cat specialists all said that they don't know of any cases of myositis of the masticatory muscles, so I would think her problem is either oral pain or a retrobulbar abscess. Regardless of what is causing her problem, your little old girl needs immediate attention because she can't eat. This would be anesthesia to look to see if her muscles are truly frozen (which I highly doubt), and to place a pharyngostomy feeding tube. I'm sure your vet has seen retrobulbar abscesses. Do you notice any bulging of an eye?
I know that it is probably 11 PM there, but if you had a syringe you could use that to put water into her mouth (right in front would probably be easiest) and even baby food (either full strength or thinned enough with water to go through the syringe nozzle). Ideally, she should be seen tonight for supportive care preparing her for further diagnostics. Generally, though, after hours care is more expensive, so if finances are an issue. . .when are they not????. . .then you could try to support her as I said tonight, and call your vet first thing in the morning.