Myoclonus is defined as a sudden onset jerking motion. How is this different from a tremor? Tremors tend to have a creshendo, decreshendo appearance where as myoclonus has been described as "square form" impulse with sudden, rapid onset, sustained contraction, and sudden stop. Okay, fine, it's not a tremor. How do I diagnose myoclonus?
There are four common types:
1) Physiologic - the classic example is hiccups.
2) Epileptic- occur with or around epileptic seizures
3) Idiopathic - unknown origin or cause (rare in veterinary medicine...or we're just missing it.)
4) Symptomatic - due to an underlying pathology. For example, distemper virus.
Neuroanatomic lesion localization:
1) Cortical - usually associated with myoclonic seizures.
2) Subcortical (brainstem) - "falling asleep" myotonia. In this case, people or animals will suddenly "jerk" just as they fall asleep. Classically, this has been thought to occur because the cortical functions "turn off" before the brainstem functions but this may not be the complete story.
3) Peripheral - an example is hemifacial spasms, or ocular spasms (have you ever had your eyelid twitch annoyingly and you cannot stop it?)
Veterinary neurologists are often consulted for symptomatic and epileptic myoclonus. Although there is a population with idiopathic myoclonus, it is rare. Symptomatic myoclonus occurs at rest, asleep, or during motion. Symptomatic myoclonus does not stop with sleep! The animal may appear aware or unaware of the twitch. If the contraction is happening in a body part used for walking or eating it may be life limiting. If not, animals may live with a myoclonus with minimal disruption. Epileptic myoclonus occurs around a time of seizure activity and is therefore often managed with anticonvulsant management.
Distemper virus is a common cause of canine myoclonus and CNS infections (various causes) have been cited as causes of feline myoclonus. There is no treatment available for distemper virus therefore, patients are monitored for progressive neurologic signs (mentation changes, gait changes, seizures, other) and euthanized if signs are progressive. If the signs are not progressive, usually myoclonus is not a reason for euthanasia.
That's all I have for you today! I hope you enjoyed this TidBit about myoclonus and feel better prepared when you see it in your exam room. :)
Thank you for reading! May you have a wonderful, twitch-free holiday week!
Canine Distemper Causing Lameness?
An interesting case report came across my desk awhile ago so I thought I might share some key points with you this week. (Green L, Cook L, et al. Distemper encephalomyelitis Presenting with Lower Motor Neuron Signs in a Young Dog. JAAH 2020)
Signalment: 4 month old spayed female dog
History: The puppy was presented to the neurology service with a several day history of right thoracic limb monoparesis. Key findings in the neurologic examination included axial pain on palpation, thoracic limb lameness, weak withdrawal of the affected limb and a paw replacement deficit in that limb. Absent cutaneous trunci reflex was also noted on the right side. Other than the neurologic findings, the only other interesting finding on physical examination was a mildly febrile state of 102.7F) and crusting of the foot pads.
She was initially managed conservatively however signs progressed to inappropriate mentation which was suspected to be due to the fentanyl patch. This was removed and signs improved however the clients elected to proceed with additional testing at that time.
Neuroanatomic lesion localization? You tell me!! (see below when ready)
Differential diagnoses listed at this time included non-infectious inflammatory conditions (MUE), infectious meningomyelitis, and hemorrhage or trauma.
Diagnostic testing
Spinal MRI: Abnormalities in the central spinal cord
Brain MRI: Unremarkable
CSF: Mild pleocytosis cell count 10/ul: reference < 5/ul) with normal protein and RBC.
Infectious disease testing: Neospora IFA was negative however the Toxoplasma IgG was markedly elevated with a borderline IgM. Parvovirus testing and cryptococcus antigen testing was negative. PCR on CSF for distemper (CDV) was positive. A whole lot of other infectious diseases were tested for, and negative.
Treatment with clindamycin, sucralfate, gabapentin, tramadol, famotidine, metronidazole and a single dose of dexamethasone was started.
Progression: She was discharged 4 days later, but returned due to progressive mentation changes and worsening ambulation. During evaluation she had a focal seizure. (Now we have multifocal neuroanatomic lesion localization!). She was humanely euthanized and submitted for necropsy.
Necropsy: Findings consistent with CDV were found and in addition, immunohistochemistry of the C5 and lumbar spinal cord and strong nuclear and cytoplasmic brown staining for CDV and no staining for Toxoplasma.
What is so noteworthy about this case?
She was partially vaccinated. Did you know that 40% of dogs with confirmed distemper infection, in one study, were vaccinated? (Tipold A: 1995).
She had hyperesthesia associated with her neurologic signs. Pain is a very unusual finding for a distemper case and the authors suggest this is the first case report of confirmed CDV in a dog with limb pain.
The progression from limb to brain is unusual but not unreported.
Take Home Message
A young dog, with multifocal neurologic findings, or findings of spinal pain in conjunction with neurologic findings, may have CDV. I suggest adding this disease to your ever-growing-list of infectious diseases that can cause spinal pain in dogs.
Neuroanatomic lesion localization answer: C6-T2 radiculopathy or neuropathy. There is no evidence of spinal cord involvement initially so a myelopathy is less likely.
As always, thanks for reading! I hope you have a NON-PAINFUL week, that isn't LAME! (Sorry...I just couldn't help myself.)
Canine Distemper
Etiology
Viral replication initially begins in lymphoid tissue and induces marked immunosuppression. Virus then reaches the CNS through the choroid plexus, ependymal cells and perivascular spaces 1-3 weeks after infection by virus infected lymphocytes and monocytes. What is the significance here? Neurologic signs often follow GI and respiratory signs.
Signalment
Distemper should be strongly suspected in an unvaccinated young dog with neurologic signs and with a history of recent GI and/or respiratory disease. Myoclonus, a repetitive twitching of muscles, is a common indication of current or previous distemper infection. Distemper can also be present – and a diagnostic challenge – in older, vaccinated dogs with no history of systemic disease prior to neurologic signs. Older dogs typically have a demyelination, with chronic, slowly progressive, signs of a myelopathy.
Clinical Signs
Respiratory and GI signs occur 1-3 weeks prior to CNS signs. The presence and pattern of illness depend primarily on the viral strain and the age and the immunocompetence of the patient. (Different strains but only one serotype means that exposure to one strain protects dogs against any subsequent strain.) There are 3 different scenarios:
Dogs that develop an early, effective immunological response recover from mild or no neurological signs. Approximately 50% of dogs have a subclinical neurologic course
Dogs that are unable to mount an immunological response suffer severe systemic illness, including acute encephalitis, leading to death within about 3 weeks of exposure. These dogs will have seizures, blindness, and other signs of grey matter disease.
Dogs with delayed immunologic response don’t develop acute illness but may develop a chronic, persistent infection, characterized by chronic encephalitis or myelitis.
In acute disease, infection of neurons and microglia and astrocytes leads to mostly grey matter damage. In chronic infection, the immune response to persistent viral infection leads to inflammation and demyelination. Vaccine induced distemper is associated with a mild encephalitis in dogs vaccinated with modified live vaccines.
Diagnostic Tests
Active or inactive chorioretinitis may be evident on ophthalmoscopic examination.
CSF varies from normal to having increased protein and lymphocytic pleocytosis during active infection
Distemper antibody titers or PCR in CSF, blood and urine can be helpful. False positive urine and blood PCR results may occur within 3 weeks of vaccination.
PCR on CSF is the most diagnostic test for active infection, however myoclonus, demyelination and seizures can be residual neurologic signs after the acute infection has been cleared. Therefore, a negative PCR on CSF does not suggest that the pet NEVER had distemper, it merely suggests that there isn't detectable virus at that point in time. CSF titers (IgG/IgM) can be quite useful in this situation when compared to the serum titers. That said...what do we do with this information?
Immuno-histochemistry on hyperkeratotic foot pad, conjunctiva, respiratory epithelium or CSF WBC can be diagnostic.
Treatment
Supportive care is important. Anticonvulsant drugs should be employed if seizures are present. Quality of life may be limited and poor if seizures are present and progressive. Myoclonus does not necessitate euthanasia (and should be differentiated from seizures) but it may negatively affect the quality of life depending on the muscles involved. I have personally seen distemper cause myoclonus of the jaw with repeated opening and closing (it looks like a pet gasping for air), flexion of the lips (with resultant wear of the teeth due to grinding) and abdominal contraction (like a hyperactive cutaneous trunci reflex) in dogs with confirmed distemper. No treatment resolves myoclonus at this time.
Although this is a rare disease due to vaccination, we have had a resurgence in our area due, I suspect, to an increase in pet adoptions from endemic areas such as the southeastern USA. Vaccination can prevent infection for most pets!
Have you treated distemper? How did it go? Reach out and let me know if you have any questions or comments on this disease.
For those of you in my referral zone - please note that I will be closed and not responsive to email or telephone Monday-Thursday September 6-9th as we celebrate Rosh Hashanah (Jewish New Year). I apologize for any delayed responses during this time and, as always, appreciate your patience!