Case Presentation: 8 year old MC Lab was presented to you for acute, progressive unilateral temporalis muscle atrophy. The owners noted this atrophy over about 2 weeks but described is a “sudden”.
Physical examination: The dog has obvious atrophy of the right temporalis muscle, and an ulcer on the right cornea. The remaining physical exam was unremarkable
Neurologic examination:
Mentation: Normal
Cranial nerves: Right temporalis muscle atrophy, reduced corneal and palpebral reflex OD, the remainder was normal.
Spinal reflexes: normal
Postural reactions: Normal paw replacement in all four limbs
Palpation: No signs of pain
What is the neuroanatomic lesion localization?
We have a right sensory eye problem (trouble blinking and reduced sensory reflex to the cornea) along with muscle atrophy. This appears to be a sensory and motor CN 5 neuropathy (Trigeminal).
Differential diagnosis: Neoplasia (trigeminal nerve sheath tumor), neuritis (Neospora, toxoplasma).
We did an MRI and found a mass that was involving CN 5. Based on the MRI appearance a trigeminal nerve sheath tumor was suspected. What does this mean for the dog? Luckily there as an excellent paper published in 2017 by Katie Swift and colleagues. This paper reported the findings of 27 dogs with nerve sheath tumor suspected based on clinical and MRI findings. In this study, 15 dogs received stereotactic radiation therapy (SRT) and 10 dogs were conservatively managed. See the summary below for details.
Signalment
Mean age: 9 years
Mixed breeds and Labradors were overrepresented, but no strong breed predisposition emerged.
Masticatory Muscle Atrophy: 26/27 dogs (96%) had atrophy and most were unilateral.
Intracranial Signs: 13/27 dogs (48%), including: seizures, mentation changes, circling, ataxia, postural reaction deficits.
Ocular Disease was present in 12/27 dogs (44%) including corneal ulceration, keratoconjunctivitis sicca (KCS) and some had previous ocular surgery secondary to neurotrophic disease
Outcome
The 10 dogs that had conservative management received a mixture of steroids, anticonvulsants or observation only. These patients had stable or slowly progressive clinical atrophy and little to no improvement in neurologic deficits. Dogs with conservative management had a median survival time of 12 days. However, 4 dogs were euthanized immediately after the diagnosis and selection bias likely selected for more severe cases into this group. Three dogs survived 250-577 days!
The 15 dogs that underwent SRT showed some improvement following treatment (pre-existing ulcers improved, but some dogs developed new ulcers or KCS) and atrophy did not improve in 14/15 dogs. That said, both dogs with seizures became seizure free! The median survival was 441 days (14.5 months) with a 1 year survival rate of 55-60%.
This study supports MRI-diagnosed trigeminal nerve sheath tumors as predominantly centrally extending, locally aggressive cranial nerve tumors characterized by unilateral masticatory muscle atrophy, frequent ocular complications, and eventual brainstem progression. Stereotactic radiation therapy provided the greatest apparent benefit in dogs with intracranial neurologic signs and yielded a median survival of approximately 14.5 months, but it rarely restored trigeminal nerve function or reversed muscle atrophy. There was not a statistically significant difference in survival (surprising, right?) but this doesn’t mean radiation isn’t effective. The findings support SRT as a reasonable palliative-to-definitive local therapy, while highlighting the need for earlier diagnosis and prospective studies comparing stereotactic versus conventionally fractionated radiation protocols.
Thanks for reading! I’m back from the ACVIM Forum in Seattle Washington last week and have lots of really exciting cutting edge research to share with you over the next few months. Now that we’re into summer you know that my schedule changes with my kid’s camp schedule. If you cannot find an appointment soon enough PLEASE EMAIL me. I will do my best to work your patient as often as possible!