Visiting Vet: Was it GME?

Sometimes neurological diagnoses can’t pin it down.

A labrador can still enjoy a favorite spot at home while being monitored for a neurological disease. — Mitchell Griest

The first episode happened when Tidbit was 2 years old. In the middle of the night, the young Labrador retriever was jumping off an armchair where she had been sleeping, when she abruptly fell down. Her hind end looked “wobbly,” and she was acting almost as if she were drunk. Within a few minutes, she was back to normal but then, three hours later, it happened again. The following morning was Martin Luther King Day. My office was closed, so Tidbit’s family sought an emergency visit with the veterinary practice covering holiday urgent care. A thorough exam, including basic blood tests, showed all within normal limits. Tidbit’s owners were advised to continue observing her and to consult a neurologist should the problem recur.

Three weeks later, it happened again. Same chair. Same symptoms. Same rapid resolution. “Maybe her legs are just falling asleep?” I asked dubiously, when they brought her to me the next morning. “Or could be some kind of back or hip pain?” But I could not find any abnormalities, orthopedic or otherwise. Every bit of Tidbit looked like a happy, healthy young dog. Could she periodically be ingesting something toxic, causing the wobbliness? Mushrooms? Garbage? Medications? Marijuana? But if any of these were the culprits, the incoordination should last much longer, and there should be other symptoms, such as vomiting, twitching, and/or altered mental status. I concurred with the veterinarian who had seen her previously. This could be some kind of petit mal seizure, or other primary neurological disease.

Two years then passed uneventfully. Well, except for that time Tidbit was hospitalized after eating grapes. Yes, that’s right. Grapes can be toxic to dogs, causing irreversible kidney failure, but that’s a column for another day. Tidbit survived her grape ingestion with no adverse effects. Then, almost four years to the day from the first occurence, Tidbit’s owner called, reporting a much more profound neurological event. Tidbit had a persistent head tilt to the right side, and was off-balance. These signs are typical of canine idiopathic vestibular syndrome, an annoying but otherwise benign balance disorder, but Tidbit was young for that diagnosis, as it usually occurs in much older dogs. And she had other symptoms. “Her right eye doesn’t seem to blink,” her owner reported. “She keeps scratching at her right cheek, and occasionally bites at her right front leg.” Tidbit was also shaking her head, drooling, and lunging at her food in an odd, jerky manner. 

My physical exam confirmed all of Tidbit’s owner’s observations. Basic blood tests, including testing for Lyme and several other tick-borne diseases were all normal. Tidbit looked perky and happy, and did not seem to be in distress, but something was clearly wrong with her central nervous system. “I’m afraid this might be some type of meningoencephalitis,” I said. “You’re gonna need a neurologist if you want a definitive diagnosis.” Within 48 hours Tidbit was at VCA South Shore Animal Hospital for a neurology consult. She had right-sided facial nerve paralysis and vestibular disease of unknown cause. They advised an MRI for further evaluation of her brain and skull, cerebrospinal fluid tap and analysis, and more extensive testing for infectious diseases.

At the top of the differential diagnosis was a scary disease called granulomatous meningoencephalitis (GME), an inflammatory disease of the central nervous system. No one knows exactly what causes GME, but it is thought to be an autoimmune disease where the body starts attacking its own cells. No underlying infectious organisms have been implicated. It is conjectured that GME occurs due to a combination of genetic and environmental factors. It is most common in young to middle-aged small-breed dogs, but larger dogs account for about a quarter of the cases. There are three types of GME — ocular, focal, and multifocal or disseminated. Symptoms depend on the form, and can include seizures, neck pain, incoordination, altered mental status, walking in circles, blindness, listlessness, tilted head, facial abnormalities, and weakness. Prognosis also varies, depending on type. Disseminated GME tends to come on fast, and, sadly, dogs often die or are euthanized within months. The focal and ocular forms have a better prognosis, but there is no cure for GME, and affected dogs often need lifelong medication. There are also other types of noninfectious meningoencephalitis, with varying prognoses, all lumped together under the name meningoencephalitis of unknown origin (MUO).

Tidbit’s test results trickled in. Cerebrospinal fluid analysis, normal. Tests for viral, parasitic, and tick-borne diseases, negative. MRI of brain, ears, and head structures, unremarkable. So no evidence of cancer. No malformations of skull or brain. No inner or middle-ear issues. What else could it be? Well, dogs sometimes develop idiopathic facial nerve paralysis. Dogs also sometimes get idiopathic vestibular disease. In other words, either of those two conditions can affect a dog for no apparent reason, and do not necessarily indicate anything progressive or life-threatening. Still, Ockham’s razor tells us that given two competing theories, the simpler explanation is preferred. How likely is it that Tidbit suddenly got two independent idiopathic conditions? 

An abnormal MRI is helpful in making a presumptive diagnosis, but the only definitive test for GME is a brain biopsy — obviously something too invasive to consider. Although Tidbit’s MRI was normal, the neurologist felt GME and MUO could not be completely ruled out. Treatment usually starts with immune-suppressive doses of corticosteroids, such as prednisone. If there is a good response, the dose is gradually lowered to the minimum required to control symptoms. This can take many months, and relapses are common. For those that do not respond well, additional medications, including other immunosuppressant and/or chemotherapeutic agents, can be added. 

Tidbit’s family has given her the very best chance by having all the recommended tests and starting her medication. Sometimes even specialists can’t pinpoint a definitive diagnosis, or foresee the future. All we can do is wait and see and hope. For now, Tidbit is every bit a happy Labrador, still sleeping in that favorite armchair.