Myasthenia Gravis in Dogs – Definition
Weak muscles or sudden fatigue in dogs, more technically referred to as Myasthenia gravis, is a syndrome that involves skeletal muscle weakness in the absence of obvious nervous system abnormalities. It usually comes and goes in separate waves, and typically is aggravated by activity and relieved by rest. Affected dogs do not have the normal number of functional receptors for a certain neurotransmitter, acetylcholine, which makes their muscles unable to receive and act on nerve impulses that normally are associated with movement. These dogs generally don’t have obvious muscle deterioration or atrophy nor do they typically show any signs of having lost the sensations of feeling or touch. However, the defect in the communication channels between their nerves and skeletal muscles causes dogs with myasthenia gravis to suffer generalized muscle weakness, which is made worse with activity or exercise. They also are predisposed to extreme fatigue.
Causes of Myasthenia Gravis in Dogs – Biological Progression
Myasthenia gravis is caused by a reduction or deficiency in the number of cellular receptors for a specific neurotransmitter, acetylcholine, at the junctions between nerve endings and skeletal muscle cells. Skeletal muscles are those that are attached to bones and typically cross over at least one joint. They are made up of bundles of fibers that have the power to contract and produce movement and locomotion. Each muscle fiber receives its own nerve impulses, which trigger contraction and lead to motion. Impulses travel down nerves and release chemicals called “neurotransmitters” or “neuromuscular transmitters” into the tiny spaces between the nerve endings and the muscle fibers, which are known as “neuromuscular junctions”. There are many different chemical substances that act as neuromuscular transmitters. Each neurotransmitter binds with a specific mirror-image receptor on a muscle fiber cell, fitting together much like pieces of a complex jigsaw puzzle. This causes the muscle fiber to change chemical energy into mechanical energy, resulting in contraction of the fiber. Stimulation of multiple muscle fiber bundles stimulate the muscles to either relax or contract, depending on the neurotransmitters that are involved. Coordinated movement requires effective communication between nerves and skeletal muscles through the transfer and binding of many specific neuromuscular transmitters. Dogs with myasthenia gravis either don’t have the normal number of skeletal muscle cell receptors for the neurotransmitter acetylcholine when they are born (congenital myasthenia gravis), or the receptors that they do have are defective or have been damaged at some point after their birth (acquired myasthenia gravis).
Causes of Myasthenia Gravis in Dogs – In Real Time
Canine myasthenia gravis can be congenital, which means that it is present at the time a puppy is born, although this is fairly uncommon in companion dogs. Congenital myasthenia gravis is considered to be a genetic disorder, in large part because it occurs more commonly in certain breeds. It is caused by mutations in the genes that code for the acetylcholine receptors on skeletal muscle fiber cells. Myasthenia gravis can also be acquired during a dog’s lifetime. This is much more common than the congenital type of the disease – especially in mature animals between 3 and 10 years of age, depending on their breed (large and giant breed dogs tend to become physically “older” at a younger age than their smaller counterparts). Acquired myasthenia gravis is an autoimmune disorder. This means that the dog’s immune system attacks and destroys its acetylcholine receptors. Why this happens remains a medical mystery and is the subject of ongoing scientific research.
Prevention of Myasthenia Gravis in Dogs
Unfortunately, there currently is no known way to prevent dogs from being born with congenital myasthenia gravis or from developing the disease as they mature adult.
Myasthenia Gravis in Dogs – Symptoms and Signs
Dogs affected by congenital myasthenia gravis usually develop symptoms somewhere between 6 and 8 weeks of age, while dogs with acquired congenital myasthenia gravis typically are affected either between 1 and 4 years or after 9 years of age, depending on their breed. Dogs with this disease generally progressively become quite weak. They have difficultly standing up after they’ve been lying down and have trouble moving around once they are able to rise. They also become easily fatigued. The symptoms tend to get worse after they exercise or engage in even mild to moderate activities. Dogs suffering from myasthenia gravis usually are not able to engage in vigorous activities, at least not for any prolonged period of time.
Symptoms of Myasthenia Gravis – What the Owner Sees
Most cases of myasthenia gravis involve generalized muscle weakness and accompanying fatigue that affects many skeletal muscles throughout a dog’s body. There is a focal form of this disease that mainly affects the muscles of mastication, which are those muscles involved with the act of chewing and swallowing. Owners of dogs with myasthenia gravis may notice one or more of the following clinical signs, either in young puppies or mature animals:
- - Generalized progressive weakness (often episodic; waxes and wanes; comes and goes in waves; worsens with activity or exercise and resolves with rest; usually most pronounced in the hindquarters)
- - Difficulty standing up from a lying position
- - Staggering; abnormal swaying gait
- - Exercise intolerance
- - Breathing difficulties (dyspnea; due to aspiration pneumonia)
- - Varying degrees of noisy breathing sounds (can be due to laryngeal paralysis)
- - Difficulty swallowing (dysphagia); repeated attempts to swallow
- - Excessive drooling (ptyalism)
- - Megaesophagus (enlarged, dilated esophagus)
- - Coughing (due to aspiration pneumonia)
- - Regurgitation (distinguish from vomiting)
- - Voice change
- - Sleeping with the eyes open
- - Sudden collapse
Dogs at Increased Risk of Developing Myasthenia Gravis
Congenital myasthenia gravis is considered to be at least partly a hereditary condition in Smooth-Haired Miniature Dachshunds, Jack Russell Terriers, Springer Spaniels and Smooth Fox Terriers, although it certainly can occur in other breeds. Acquired myasthenia gravis also can affect dogs of any breed. However, it most frequently is seen in Terriers, Akitas, German Short-Haired Pointers, Chihuahuas, Dachshunds, Scottish Terriers, German Shepherd Dogs, Labrador Retrievers, Golden Retrievers, Newfoundlands and Great Danes. Dogs that have hypothyroidism, thyroid gland tumors (thymomas), low circulating levels of platelets or Addison’s disease (hypoadrenocorticism) also are more likely to develop acquired myasthenia gravis as they mature.
Signs of congenital myasthenia gravis usually show up when a puppy is between 6 and 8 weeks of age, which means that in most cases the breeder should be able to recognize the disorder by the time that young puppies are scheduled to go to their new home. Acquired myasthenia gravis can show up anytime; small and medium breeds most commonly show signs between ages 1 and 4 or after 9 years, while large and giant breed dogs typically become symptomatic a bit earlier due to their shorter average overall life-spans.
Diagnosing Myasthenia Gravis - Initial Evaluation
A veterinarian who is presented with a dog showing signs of generalized skeletal muscle weakness, or showing weakness that is localized to its facial muscles, will take a thorough history from the animal’s owner. It will be especially important for the owner to tell the doctor when these signs first showed up, whether they are constant or come and go and whether they change in severity over time. The veterinarian will perform a thorough physical examination that includes a neurological evaluation. She also may recommend routine blood and urine tests, including a complete blood count, serum chemistry profile and urinalysis, as part of the initial work-up. The results of these tests can help to rule out metabolic and other potential causes of the dog’s condition, although they won’t be able to help confirm a definitive diagnosis of myasthenia gravis. The dog’s thyroid and adrenal gland function can also be tested on blood samples. Chest radiographs (X-rays) may be taken to look for evidence of aspiration pneumonia, megaesophagus or a mass near or adjacent to a thyroid gland. Ultrasonography can also be useful.
Myasthenia gravis usually is diagnosed as a result of a thorough neurological examination. A drug called edrophonium chloride can be used as a diagnostic tool. When it is injected into a dog, this drug blocks the activity of the enzyme that normally breaks down the neurotransmitter, acetylcholine. This leads to higher concentrations of acetylcholine at the dog’s neuromuscular junctions. If the dog’s muscle strength improves noticeably within several minutes of receiving the injection, it probably has myasthenia gravis. A blood test for this disease is also available. It measures the level of antibodies to the dog’s acetylcholine receptors that are circulating in its blood. This blood test is considered to be the gold standard for diagnosing the acquired form of myasthenia gravis in dogs. Electrodiagnostic tests, such as repetitive nerve stimulation tests and single-fiber electromyography, are available at veterinary teaching hospitals and some specialized referral clinics. The best way to definitively diagnose congenital myasthenia gravis in dogs is to examine tissue biopsies taken from the dog’s muscles. Pathologists can quantify the number of normal and abnormal acetylcholine receptors from muscle tissue biopsies.
Fortunately, with current techniques, it is not especially difficult to diagnose myasthenia gravis in our companion dogs. In addition, unlike with many other canine diseases and isorders, this one is often treatable or at least manageable. This should provide at least some degree of comfort to those who own animals affected by either congenital or acquired myasthenia gravis.
Goals of Treating Myasthenia Gravis in Dogs
The goals of treating a dog that has myasthenia gravis are to improve the transmission of acetylcholine between its nerve endings and muscle fiber receptors, improve its comfort and muscle stability and eliminate or at least reduce any acquired immune-related causes of its condition.
Treatment Options for Dogs with Myasthenia Gravis
A dog with myasthenia gravis may or may not need to be admitted to the hospital, depending on the severity of its disease. Intensive inpatient care is often necessary if the dog has aspiration pneumonia or is unable to eat or drink without always regurgitation. Aspiration pneumonia generally is treated with intensive injectable and oral antibiotic therapy, administration of oxygen, placement of an intravenous catheter, administration of intravenous fluids and supportive nursing care. Dogs that can’t hold down their food may require placement of a gastrostomy tube so that their nutrition and hydration can be maintained. Hospitalization can also help the medical team figure out the right dosage and combination of drugs to manage the dog’s condition.
Several drugs are available to increase the concentration or prolong the action of acetylcholine at the neuromuscular junction sites. This helps resolve the generalized muscle weakness that almost always is associated with myasthenia gravis. Most of these medications, which are called anicholinesterase drugs, can be given orally or by injection. Corticosteroids can also be helpful in certain cases. If a thymus gland tumor is contributing to the dog’s condition, it may be able to be removed surgically. Surgery should only be done after the dog is stabilized. Dogs with associated hypothyroidism can be put on oral thyroid replacement therapy, and dogs with associated megaesopohagus can be treated for that disorder.
Dogs with myasthenia gravis should be fed with their head elevated, which is most easily accomplished by putting their front legs on a ramp or stepstool and placing their food and water bowls in raised feeders. Dogs with acquired megaesophagus often benefit from administration of immunosuppressive drugs such as steroids, although these medications can have some serious side effects that should be discussed with the dog’s veterinarian before treatment begins. Affected dogs will self-limit their activity due to muscle weakness and pneumonia, if it is present. Foods of different consistencies – such as hard dry food, soft wet or canned food, soaked kibble or gruel – can be tried to determine which one or ones the dog tolerates the best.
Prognosis and Outlook
If the underlying cause of myasthenia gravis is identified and successfully treated, the outlook for dogs with the acquired form of the disorder is fairly good with 6 to 9 months of treatment. Aspiration pneumonia is the most common and serious adverse side effect in dogs with this disease. The prognosis for dogs that have difficulty swallowing and chronic regurgitation is guarded, because those conditions increase the risk of aspiration pneumonia dramatically. Dogs with thymomas that cannot be completely surgically removed to eliminate or control the signs of myasthenia gravis also have a guarded prognosis.
Owners of dogs with myasthenia gravis should recognize that while the disease often is treatable, sometimes it can’t be cured. Treatment can require many months of special feeding and administration of special medications. Having a concerned, conscientious, dedicated owner is one of the most important factors in a dog’s successful recovery from this disorder.