Thanks for your question regarding your 14 year old Beagle cross girl who has started having some neurlogical issues with a pretty acute onset
It is unlikely this is anything to do with the vaccination on Tuesday - although it is interesting that these are almost coinciding with the vaccination
Given your girl's age - we do have to consider a brain tumor in this instance - even though she isn't showing any more obvious signs like head pressing, a head tilt or ataxia
If you had your girl insured, then an MRI would tell us a lot in this instance, however this in itself would require an anesthetic and your girl would be more at risk under a GA given her age now
I would also be concerned about possible meningitis here as well.
To help rule this out however, a spinal tap would be needed to collect some cerebrospinal fluid for cutlure and cytology
It is good that that Cushings test (I assume ACTH) was normal as this rules out Cushings and Addisons disease - however these aren't typical symptoms for either
Hopefully your Vet has run a full standard profile to check the electrolyte levels, glucose levels and internal organ function as well as a CBC to help rule out infection and possible anemia
This would definitely be the next best step if this hasn't been done yet
You do have the option of MRI and/or CSF tap if you can afford it - however we have to consider the risks of putting a 14 year old girl with neurological issues under a general anesthesia - it is quite high risk
In the meantime, your Vet may want to try increasing the dosage of gabapentin to see if this can help settle the tremors - or alternatively try an antiseizure medication like Keppra or phenobarbitone
If all the bloodwork is normal, it is less like to be an organ, electrolyte or infectious issue - however particularly for meningitis, the CBC can typically be normal for this and only a CSF tap can diagnose this
Given her age however, we really do need to consider a possible tumor here. This could be a primary brain tumor or even a nerve sheath tumor - both of which are hard to diagnose without a tool like MRI
Best of luck with your girl and I do hope that things improve for her
Please just reply if you need any further info won't you.