“What’s up?” I asked casually, ushering Thunderbolt and owner into the exam room.
“This may sound weird,” his mom replied, ” but last night I suddenly noticed one side of his head is all caved in.” By this time Thunder, a sweet ten-year-old Labrador-Beagle mix, was on the table where I could gaze into those big brown eyes. Yup. No doubt about it. Thunder had marked facial asymmetry. The muscles on the right side of his forehead were more pronounced than on the left, where the bony outline of his skull was clearly visible beneath the skin.
“Is he acting sick?” I continued my exam.
“Nope,” she assured me. “He’s feeling fine.” Everything else checked out normally. Thunder’s problem was unilateral, meaning only one side was affected, and symptoms were confined to the head. The first question in asymmetry cases is . . . which side is normal? Sometimes it’s hard to tell. Was the right side swollen? Or the left atrophied? In Thunder’s case the left side was clearly abnormal.
“Probably something wrong with the facial or trigeminal nerve,” I pronounced, rubbing my own forehead. Those pesky cranial nerves. Even 30 years ago in school I would mix them up. “I need to refresh my memory,” I admitted candidly and went off to review cranial nerves as well as inflammatory diseases of the masticatory muscles.
Unilateral facial nerve paralysis is a relatively common condition that often appears to occur spontaneously, especially in cocker spaniels, corgis, boxers, and domestic longhair cats. It can also occur as the result of trauma, middle or inner ear infections, or cancer. Signs on the affected side include drooping lip and ear, inability to fully close the eyelid, and reduced blink reflex. The neurological deficits lead to excessive drooling and difficulty eating normally, with food falling out of the side of the mouth. Chronic cases may develop sideways deviation of the whole face and are prone to eye ulcers and infections.
I stared at Thunder. His eyelid closed normally. I touched the corner. He blinked. His lips were symmetrical. I pinched them. He reacted. I tossed him a liver treat. He scarfed it down neatly. I pulled out my otoscope. Most dogs with deep ear infections will have other symptoms — a head tilt, loss of balance, or pain. Thunder showed none of these but I thought I should check. “Eardrums look fine,” I reported. “This doesn’t look like facial nerve paralysis.” Even if it were, unilateral facial nerve paralysis is usually idiopathic, meaning there is no known cause, nor is there specific treatment. Some dogs recover function spontaneously over time. Others never do, but manage just fine, even with a permanent deficit.
My next consideration was masticatory muscle myositis. MMM is an autoimmune inflammatory condition involving the chewing muscles on the head. Affected muscles are either swollen or atrophied, depending on the stage. Could Thunder have MMM? I reviewed my texts. Ah, that’s right. MMM is almost always bilateral and painful, presenting as an inability to open or close the mouth. Diagnosis involves muscle biopsy or a pricey test looking for those autoantibodies. With early diagnosis, prognosis is good, treating with corticosteroids to suppress the abnormal immune response. But Thunder’s mouth worked perfectly well. I pried it wide open just to be sure. No pain. No problem. No masticatory muscle myositis.
“Okay,” I concluded. “Must be trigeminal neuritis.” Cranial nerve number V, the trigeminal, is also involved with enervating the head muscles. That must be it. This time I searched my online veterinary database. Shoot. Trigeminal neuritis is also usually bilateral and manifested by a sudden inability to close the mouth. Affected animals have a constantly dropped jaw, leading to excessive drooling and messy table manners. No one knows the cause, although an autoimmune etiology is suspected and there is no specific diagnostic test other than ruling out other problems, and waiting. Most dogs recover fully within a month as long as they are given the necessary supportive care to eat and drink throughout that period. As I sifted through discussions of canine facial asymmetry, I stumbled across case after case of middle-aged dogs with the exact same signs as Thunder. Unilateral muscle atrophy of the forehead with no other symptoms. I read a number of cases, then went back to the exam room somberly.
“I think Thunder may have a trigeminal nerve sheath tumor.” I let the news sink in. “That’s cancer affecting the nerve covering. I’ll talk with the specialists and get you details.” The next day I reviewed options with Thunder’s mom. “The oncologist agrees,” I said. Considering Thunder’s age and symptoms, trigeminal nerve sheath tumor was far and away the most likely diagnosis although there was still a slim chance it could be unilateral immune-mediated trigeminal neuritis. “At this point she advises seeing a neurologist. An MRI can confirm the diagnosis and determine the location and extent of the tumor.”
“If it is cancer, what can we do?” his owner asked
Standard treatment is radiation therapy, five days a week for three weeks. A newer option is a “gamma knife” procedure — a much larger dose of radiation targeted at a much tinier area. This process consisting of only one to three treatments but is twice the cost and only performed at a few institutions. “There’s not much data about prognosis with either treatment,” I sighed. “The oncologist said a lot of her patients have done well . . . living one or two years.”
That didn’t sound like a very long time to either Thunder’s mom or me. She needed time to consider the choices. If she wasn’t going to go for radiation treatment, then we could just wait and see. If Thunderbolt improved, it was neuritis. With cancer, we would expect his symptoms to progress eventually. He might have seizures or other neurological abnormalities. It might become painful or prevent normal eating and drinking. But for now, those big brown eyes and wagging tail revealed nothing but a happy, albeit lopsided, dog wondering if he could have another liver treat.