Cervical Disc Herniation Associated Myoclonus in Dogs

Intervertebral disc herniation (IVDH) is the most common cause of cervical pain in small breed dogs. The most common clinical presentation is cervical pain with a normal neurologic examination, however in a few dogs gait deficits, paw replacement deficits, or reflex deficits can seen. Myoclonus, or a sudden onset, repetitive muscle contraction is seen in about 4% of dogs in a recent study from France. This muscle contraction is frequently confused for seizure behavior by clients so be on the look out for it! The classic presentation is a small breed dog that stops an activity, demonstrates myoclonus, and then resumes it's activity. Other signs of cervical pain (yelping, low head carriage, reduced range of motion) are often present when clients are questioned, so be sure to ask! 

What is the Significance of Cervical Myoclonus with IVDH?

The presence of myoclonus did not change the prognosis or outcome for the 20 patients in the recent study (JAVMA 2023: 261:4: 511-516.). Surgical correction resulted in less recurrence of signs, and immediate resolution in the post operative period compared to medical management. Approximately 25% of of medically treated dogs experienced another episode of myoclonus considered to be distinct from the original presentation. Medical management consisted of NSAIDs, gabapentin and, for some, tramadol. 

What is the Take Away?

• Myoclonus can occur with mechanical or chemical irritation of cervical nerve roots 

• Myoclonus does not affect prognosis

• Surgical management remains the recommended treatment for rapid resolution of signs of pain and reduction in relapse/recurrance

• French Bulldogs were over represented in this study!! (Again - See TidBit Tuesday in March for the list of Frenchie spinal cord diseases

As always, thank you for reading! I am thrilled to see the lovely weather on the horizon this week and hope you have a chance to enjoy some of it, too. 

Myoclonus - what does it mean?

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! 

How to handle a jerk

Have you evaluated an elderly patient with a history of sudden, unprovoked jerk movements lasting several seconds and not been sure what to make of it? It's easy to disregard this information because it doesn't fit into one category but perhaps, we shouldn't.

Myoclonic "jerks" can occur as a part of an epileptic syndrome (myoclonic epilepsy) or as a separate movement disorder. As a movement disorder, they may arise from neuromuscular, spinal, cortical or subcortical origin and therefore may have many different etiology. A report in Cavalier King Charles Spaniels came out recently (https://doi.org/10.1111/jvim.16404) which detailed myoclonic jerks in older CKCS. This study was not conclusive about the origin of the jerk, but suggested a cortical or subcortical origin. Several of the dogs in this group had idiopathic epilepsy, diagnosed with MRI/CSF/blood work prior to the onset of the myoclonic jerk so it is possible this is simply a manifestation of their epileptic syndrome. However, several of the dogs were on treatment with primary anticonvulsant drugs (imepitoin and phenobarbital) and the seizures improved but the myoclonic jerk worsened! This suggests a more likely non-epileptic origin. This study was SMALL so it is much too early to draw conclusions about the cause but it does provoke thought.

Here is what I took away:

1. Older adult or geriatric onset myoclonic jerks may be seizure, or non-seizure in origin
2. Phenobarbital (and imepitoin) didn't help
3. Levetiracetam helped in 3 or 6 cases. This could mean it is epileptic or non-epileptic in origin, remember!
4. If non-epileptic in origin, myoclonic jerks do not warrant treatment as they are unlikely to result in progressive neurologic disease (but knowing if they are non-epileptic is difficult)
5. Myoclonic jerks are seen as rapid movements of the face, head or thoracic limbs that are several seconds in duration and do not have a pre or post ictal period associated with the signs. Not sure what I'm talking about? Follow the link to the article and scroll to the supplementary material. There are two videos attached to help you visualize what is being noted.

So what do I do?
1. If the neurologic examination is ABnormal - suggest diagnostic imaging of the brain or spinal cord to determine if pathology is present to account for the jerk motion.
2. If the neurologic examination is normal, consider non-epileptic jerks and either start levetiracetam or monitor if infrequent.

Thanks for reading and have a great week! Stay cool out there and watch out for jerks!

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!