What do you do with Unilateral Muscle Atrophy?

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!

Unilateral Masticatory Muscle Atrophy

It's a Wednesday morning and you see on your schedule a 7 year old dog with unilateral temporalis and/or masseter muscle atrophy. What parts of the neurologic system could be involved?

1. Muscle: A problem with muscle function, termed a myopathy, can result in muscle atrophy. The most common cause of temporalis and masseter muscle atrophy in dogs is masticatory muscle myositis (MMM), which is caused from an immune mediated attack against the muscle fiber. This is a UNIQUE form of muscle inflammation because the proteins on these muscles are embryologically unique (2M fiber type) from all other muscles in the body. We care about that because it means that we can identify an antibody (AB) test that we can run that is highly specific AND sensitive! What else should we consider? Infectious myositis, secondary to neospora or toxoplasma infection would be my second choice differential for many dogs with this clinical presentation. In this scenario, the protozoa get into the muscle, set up a secondary inflammation (myositis) and muscle atrophy results. Sometimes you can see a mild positive on the aforementioned MMM AB titer test, but it is low, if positive at all. However, if you concurrently test for neospora and toxoplasma (serum titers) you can catch this "false" positive, and treat the correct disease.

Treatment for MMM: Immunosuppressive steroids for 30+ days, followed by a taper protocol.
Treatment for infectious myositis: clindamycin or sulfa antibiotics until negative or stable titers. This is often MONTHS of treatment!

2. Cranial nerve 5: A dysfunction of CN 5 can result in denervation atrophy of the temporalis muscles. On the exam, look closely for concurrent signs of a sensory neuropathy to the face and if present, a CN 5 dysfunction should be suspected.  This might include reduced blink reflex, reduced or absent corneal reflex, and reduced response to nasal stimulation compared to the unaffected side. The differential diagnoses list is much bigger but typically centers around a few common causes (neoplasia, neuritis, hypothyroidism, and trauma). To diagnose a CN 5 neuropathy the best approach is a thyroid panel,  CBC, serum biochemistry and then a brain MRI followed by a spinal tap, along with titers for infectious diseases (as indicated). Not all of those tests are needed for each patient so pick and choose as appropriate for your case. 

Treatment for CN 5 deficits: this varies and is dependent on the underlying cause. It is a bit hard to summarize TidBit Tuesday-style. :) 

These cases can be puzzling to sort out so please reach out if you feel a neurologic examination is helpful for your patient. Have a great week!

Temporalis and Masseter Muscle Atrophy

It's Tuesday at 10 am and you're about to see a 7 year old dog with unilateral temporalis and/or masseter muscle atrophy. What parts of the neurologic system could be involved?

1. Muscle: A problem with muscle function, termed a myopathy, can result in muscle atrophy. The most common cause of temporalis and masseter muscle atrophy in dogs is masticatory muscle myositis (MMM), which is caused from an immune mediated attack against the muscle fiber. This is a UNIQUE form of muscle inflammation because the proteins on these muscles are embryologically unique (2M fiber type, if you must ask) from all other muscles in the body. We care about that because it means that we can have a an antibody (AB) test that we can run that is highly specific AND sensitive! The second myopathic disease of the head muscles, that I see commonly, is myositis due to neospora or toxoplasma infection. Simply put, the protozoa get into the muscle, set up a secondary inflammation (myositis) and sometimes you can see a mild positive on the aforementioned MMM AB titer test. However, if you concurrently test for neospora and toxoplasma (serum titers) you can catch this "false" positive, and treat the correct disease.

Treatment for MMM: Immunosuppressive steroids for 30+ days
Treatment for infectious myositis: clindamycin or sulfa antibiotics until negative or stable titers.

2. Cranial nerve 5: A dysfunction of CN 5 can result in denervation atrophy of one (or both) temporalis muscles. On the exam, look closely for concurrent signs of a sensory neuropathy to the face and if present, a CN 5 dysfunction should be suspected. The differential diagnoses list is much bigger but typically centers around a few common causes (neoplasia, neuritis, hypothyroidism, and trauma). To diagnose a CN 5 neuropathy the best approach is a brain MRI followed by a spinal tap, along with titers for infectious diseases, thyroid panel and routine CBC, serum biochemistry. Not all of those tests are needed for each patient so pick and choose as appropriate for your case.

Treatment for CN 5 deficits: this varies widely and is dependent on the underlying cause. It is a bit hard to summarize TidBit Tuesday-style. :) Stay tuned for another email detailing all of the possible causes and treatments for cranial neuropathies.

I hope this TidBit Tuesday helps you focus your exam, and thoughts, when faced with a case of unilateral atrophy of the muscles of the head. This report was stimulated from recent conversations about this presentation so please reach out if you have a case, or questions about a case, with unilateral muscle atrophy. The more we talk with each other, the more we learn from each other!

Have a great week!