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Dog: Myasthenia Gravis

General information

Other common/scientific names: MG

Myasthenia gravis is a neuromuscular disease that interrupts the way nerves communicate with muscles. In the normal dog, nerve cells transmit a signal from the spinal cord to the muscles that a dog uses to move his body called skeletal muscles. This signal causes the muscles to contract or relax. In order for the signal or impulse to be transmitted between nerve cells, a chemical or neurotransmitter called acetylcholine (ACh) is needed. Special receptors must recognize and pick up the acetylcholine so it can be utilized. Without this chemical, nerve impulses cannot be transmitted and muscles cannot cause movement. This eventually leads to weakness and paralysis.

Causes

Myasthenia gravis can be either congenital (present at birth) or acquired. With the congenital form, the pup is born without a normal number of acetylcholine receptors. This form of MG is seen in Jack Russell terriers, Springer spaniels, Samoyeds and Smooth Haired Fox terriers. Acquired MG, on the other hand, is an immune-mediated disorder where the dog’s own body produces antibodies which destroy the acetylcholine receptors. Dogs with acquired MG may also have cancer of the thymus gland. The congenital form is less common and affects pups as early as two months of age. The acquired form targets older dogs (9-10 years of age). Both forms may be inherited.

Cardinal symptom

Muscle weakness

Symptoms

Muscle weakness that worsens with exercise and improves with rest is the main clinical sign of myasthenia gravis. Because the esophagus is lined with skeletal muscle, dogs with MG may develop a weakened esophagus. This can lead to difficulty swallowing, regurgitation and megaesophagus. Chronic regurgitation can lead to aspiration pneumonia which is caused when regurgitated food is inhaled into the lungs. Aspiration pneumonia is a common finding in dogs with MG. Other signs include muscle weakness involving the face and the inability to blink. Cycling females seem to have worse clinical signs due to hormone influence.

Diagnosis

An antibody test is used to diagnose myasthenia gravis. Using this test, a blood sample is obtained and tested for the antibodies responsible for destroying the ACh receptors. The presence of these antibodies is diagnostic for myasthenia gravis. Other tests used less commonly include a drug response test and muscle biopsy. Chest radiographs should be taken to check for an enlarged thymus and megaesophagus.

Treatment

Myasthenia gravis is treated with a medication which inhibits the enzyme used to break down ACh thereby increasing the level of ACh in the body. Along with this medication, elevating the food and water can reduce the incidence of regurgitation. Immunosuppressive medications are also used. Dogs with aspiration pneumonia will need to be treated with antibiotics.

Prognosis

Myasthenia gravis is a serious disease and prognosis is guarded. Early diagnosis and treatment is associated with a better outcome. Aspiration pneumonia is the primary cause of death in dogs with myasthenia gravis.

Prevention

Dogs with myasthenia gravis should not be used for breeding. Additionally, spaying your female dog (if stable) can lessen clinical signs.

Tips

Regular recheck examinations and monitoring of your dog is critical to managing myasthenia gravis. Vaccinations may worsen this disease, so vaccinations should be postponed unless absolutely necessary.

Update version: 4/24/2014, © Copyright by www.enpevet.de
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