NEUROMUSCULAR CASE OF THE MONTH - JULY 2012

SPECIAL FEATURE: Glossary for myasthenia gravis
Contributed by G. Diane Shelton DVM, PhD
Department of Pathology
University of California, San Diego

 

A diagnosis of myasthenia gravis (MG) exposes animal owners to a new vocabulary specific to this illness. At first, terms used to describe MG, its diagnosis and treatment, may seem daunting and difficult to understand. This glossary is an attempt to define MG terms using "owner-friendly" words. Informed owners are active participants in securing medical care for their animals, and with greater understanding, will comply with their veterinarian's important treatment directives. Each case of myasthenia gravis is unique. The best source of information regarding the diagnosis and treatment of MG is your veterinary specialist.

Acetylcholine - A chemical released by a nerve ending that activates a muscle cell to contract and generate force.

Acetylcholine Receptor - A protein substance on the muscle cell membrane which accepts acetylcholine from the nerve ending.

Acetylcholinesterase
- An enzyme located in the gap between a nerve ending and the muscle cell membrane whose function is to inactivate acetylcholine.

Acute Fulminating Myasthenia Gravis (MG)
- A rapidly progressive, severe form of MG in which generalized weakness occurs over hours to a few days. The weakness may be severe enough to require assisted mechanical support for breathing. Usually this respiratory insufficiency is the result of weakness of the diaphragm and intercostal (rib) muscles, but it can also occur if weak throat muscles obstruct the airway. Esophageal weakness (megaesophagus) may result in aspiration pneumonia.

Aspiration Pneumonia
- Pneumonia due to the entrance of foreign matter, such as food particles, into the lung airways.

Assay for Binding Antibody to Acetylcholine Receptor
- A laboratory test, designed to assist in the diagnosis of autoimmune myasthenia gravis, which measures the level of circulating antibodies that bind to acetylcholine receptors.

Autoimmune Disease -
A disorder caused when the body fails to recognize itself and mounts an immune attack (usually reserved for invading bacteria or a viral infection) producing antibodies against its own tissue. In MG, the acetylcholine receptors are the victims of this misdirected immune attack.

Azathioprine (brand-name Imuran)
- An immunosuppressive medication available as a 50 mg tablet, most often used as a steroid-sparing agent, allowing MG patients on prednisone to take less prednisone over the long term.

Blink Reflex (palpebral reflex)
- A reflex elicited by touching the eyelid and observing for a blink. This response fatigues or is absent in animals with MG.

Cholinesterase Inhibitors (Anticholinesterases)
- Medications that provide temporary symptomatic relief of MG weakness by decreasing the activity of acetylcholinesterase, thus allowing more acetylcholine to accumulate at the neuromuscular junction.

Congenital Myasthenic Syndromes (CMS)
- A rare inherited group of various disorders of neuromuscular transmission clinically similar to autoimmune MG but without immunological origin.

Corticosteroids -
Hormones used to dampen the faulty immune response that occurs with myasthenia gravis.

Dysphagia -
Difficulty or an inability to swallow.

Focal (Bulbar) MG -
Myasthenic weakness involving the muscles necessary for vocalization, chewing, and swallowing and transportation of food to the stomach with sparing of the limb muscles necessary for ambulation.

Generalized MG
- MG affecting bulbar and limb muscles

Immunosuppressive Drugs -
Medications which modulate or suppress the body's immune system thus suppressing the autoimmune disease by reducing the level of circulating antibodies available to attack acetylcholine receptors at nerve-muscle junctions.

Megaesophagus (Esophageal Dilatation
)- A large, flaccid esophagus resulting from weakness of the muscles involved in transporting food from the mouth to the stomach.

Mestinon (Pyridostigmine Bromide)
- The most commonly used cholinesterase inhibitor, available in a 60 mg tablet for oral use and as a syrup for patients with swallowing difficulties.

Myasthenia Gravis -
A disease characterized by fluctuating weakness of eye muscles, face muscles, chewing, swallowing, vocalization, breathing, neck or limb muscles, made worse by use of those muscles and improved at least partially by rest of the same muscles, with little muscle atrophy and no sensory abnormalities.

Prednisone
- A corticosteroid similar to cortisol, one of the body's natural steroid hormones.

Prostigmin (Neostigmine Bromide)
- A less frequently used cholinesterase inhibitor available as a 15 mg tablet, as well as three different injectable concentrations.

Regurgitation -
The passive expulsion of food or fluid from the esophagus. This must be distinguished from vomiting which is an active process preceded by hypersalivation, retching, and abdominal contractions.

Remission
- Clinical remission refers to the health status whereby a patient is relatively symptom free and no longer needs any MG medication. Immune remission refers to the absence of detectable acetylcholine receptor antibodies in a patient that was previously positive for these antibodies.

Repetitive Nerve Stimulation -
This is an electrodiagnostic test that is used to diagnose myasthenia gravis and other disorders of neuromuscular transmission. Anesthesia is required in dogs and cats.

Seronegative MG
- Myasthenia gravis is clinically suspected in patients with negative results from the Assay for Binding antibody to Acetylcholine Receptor.

Timespan -
A longer-acting 180 mg capsule which provides a slow release of pyridostigmine bromide. It is usually prescribed for night-time usage.

Tensilon
- Also known as "edrophonium chloride." Medication used in a pharmacological test to confirm the clinical diagnosis of myasthenia gravis. Tensilon is injected intravenously to look for a brief, but measurable, improvement in weakness; for example, return of a blink reflex.

Titer
- The numeric count of serum antibodies as measured by the Assay for Binding Antibody to Acetylcholine Receptor.

Thymectomy -
An operation to remove the thymus gland. In veterinary medicine this is usually reserved for animals with thymoma.

Thymoma -
Tumor of the thymus gland found in approximately 3% of dogs and 30% of cats with MG.

Thymus
- A flat, H-shaped gland lying behind the breastbone (sternum) and in front of the heart.

This glossary was adapted with permission from the Myasthenia Gravis Foundation of California, 5675 Telegraph Road, Suite 230, Los Angeles, CA 90040


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