Muscle Contraction Disease (Myoclonus) in Dogs

Myoclonus, or a myoclonic seizure, is an uncommon form of seizure. The more common form of seizure is known as a tonic-clonic seizure, previously known as a grand mal seizure. This type of seizure has a two-step process; the first stage is loss of consciousness, then the body jerks rhythmically for several minutes. With a myoclonic seizure, the first step is skipped and the jerking motions will be exhibited without loss of consciousness. This may affect the whole body or it may target only specific muscle groups.

Myoclonus is an uncommon seizure disorder characterized by sudden jerking motions in which the animal retains consciousness during the seizure.

The myoclonic seizure will present differently than a typical tonic-clonic seizure. You may see any or all of the following signs if your pet is experiencing myoclonus. The myoclonic seizures are often triggered by flashing lights and sudden sights or sounds that may startle the canine.

• Constant movement of a specific group of muscles
• High-pitched vocalization
• Involuntary twitching of limbs
• Jerking of the head backward
• Rapid shuttering of head
• Sudden jerking movement
• Uncontrolled rhythmic “bouncing” 

Types

There are several disorders and diseases that can cause myoclonic seizures, or that have myoclonus as a symptom. Two of the most common disorders that cause myoclonus in dogs are canine distemper and Lafora’s disease.

Canine Distemper

• A highly contagious viral disease that can be found worldwide
• Distemper is often fatal, and those dogs that survive often develop lifelong neurologic disorders, including the frequent development of myoclonic seizures

Lafora’s disease

• A late-onset form of epilepsy that is characterized by myoclonus
• Some dogs with Lafora’s disease will develop tonic-clonic seizures later on
• Recent research indicates that problems with blood sugar regulation may play a role in the development of Lafora’s

Dogs acquire the distemper virus from contact with an infected animal. Distemper can affect not only canines, but also the bear, weasel, elephant and primate families. Domestic dogs are considered to be the reservoir species for this highly contagious virus, and can continue shedding the virus for several months after initial infection. Although distemper induced myoclonus may start during or shortly after the disease, it is also common for neurologic disorders to be delayed for weeks or even months. 

Lafora’s disease is caused by a genetic mutation that can occur in any breed and either gender. Signs of this disorder usually don’t develop until the dog reaches somewhere over seven years old, and miniature wire-haired dachshunds, basset hounds, and beagles are predisposed to developing this unusual form of epilepsy. Myoclonic seizures may be induced by toxins, infections, or trauma to the brain or spinal cord as well, albeit more rarely. 

Diagnosing the seizures themselves as myoclonic can be done by simple observation, however diagnosing the underlying cause of the disorder can be more complicated. 

Your veterinarian will get a full history of your pet from you, including when the symptoms began and under what circumstances. Your dog will also undergo a thorough physical exam, and tests will be run to analyze blood chemistry and check for imbalances or toxins in the system. A neurologic examination may be taken as part of the physical. X-rays may be examined to screen for tumors, and a sample of the patient’s cerebrospinal fluid may also be analyzed. 

Depending on the situation, your veterinarian may recommend additional imaging tests such as a CT scan, MRI or a nerve conduction study. If Lafora’s disease is suspected, tests will be run to determine if the mutation is present and a biopsy of the liver, muscle or nerve will reveal if any Lafora bodies can be identified. The liver is the most reliable of the biopsy sites for Lafora’s disease.

Any underlying conditions, such as toxins or active infections, will need to be addressed either before or concurrently to addressing the myoclonus itself. Once this has been completed, your veterinarian will assess the severity of the condition to determine what steps will need to occur next. 

If the seizures are mild and infrequent further treatment may not be required. If the disorder becomes more difficult to live with antiepileptic medications such as phenobarbital or potassium bromide can be prescribed to manage the symptoms. Although these medications are often quite effective, they can have a degenerative effect on the liver over time. Some dogs may respond positively to immunosuppressive therapy with glucocorticoids as well. The strain of the disorder in the beagle breed is particularly resistant to drug therapy. 

Research shows a possible connection between the severity of Lafora’s disease and the amount of simple carbohydrates in the diet. Diets lower in simple carbohydrates may slow the progression of the disorder and starchy or sugary treats may exacerbate the symptoms.

Seizures are often made more frequent and severe if the patient is under stress, so removing some of the stressors in your pet’s life may reduce the number of attacks. Pheromone sprays and diffusers may be recommended to further reduce their stress levels. Having your dog don sunglasses designed for canines may also reduce the number and severity of the episodes when walking in sunlight. Although myoclonus is usually not curable, it is often manageable with medication and patience. In some cases, the shaking is not medically controllable, and if the patient’s quality of life is severely adversely impacted, euthanasia may be recommended.