Beagle pain syndrome is a disease of Beagles manifesting as spinal pain usually localized to the cervical area (neck), but which can be diffuse. Affected beagles are usually young (6-18 months) but recurrences of signs can occur in older dogs. The etiology is unknown but is likely an immune mediated meningitis/polyarteritis. A similar syndrome, steroid-responsive meningitis-arteritis, has been described in young Bernese mountain dogs, German shorthaired pointers, boxers, toller retrievers, and occasionally in other breeds.
Because of the positive response to immunosuppression (ie. steroid use), an immune mediated pathogenesis is likely. There may be a genetic component in the Beagle.
Clinical signs often consist of an acute to gradual onset of intermittent fever, anorexia (not eating), depression, hunched posture, reluctance to move, and moderate to severe neck pain evidenced by cervical rigidity, low head carriage, and 'guarding' of the neck. Some dogs scream out in pain with any movement. Chronically affected dogs may develop ataxia or paresis (difficulty in walking).
Cerebrospinal fluid findings may show blood-tinged fluid with erythrophagocytosis and increased neutrophils and protein. Often there is a profound neutrophilic pleocytosis with cell counts often exceeding 1000 cells/ul. Spinal fluid culture is negative for bacterial and fungal organisms.
Diagnostic imaging is often unremarkable but may show meningeal enhancement. (which is why the MRI didn't show anything).
Treatment consists of steroids, which Phoebe hasn't had before (I'm not sure why she was prescribed benazapril--does she also have congestive heart failure or high blood pressure?)
So I would definitely ask your vet to research Beagle Pain Syndrome and start Phoebe on high doses of steroids and see if her pain can get under control again quickly.