A Glossary of Common MG Terms

This list was compiled by The Myasthenia Gravis Foundation of California

Below is a list of definitions of common myasthenia gravis terms:

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

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

Automimmune 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

Bulbar MG - Myasthenic weakness involving the muscles for speech, chewing and swallowing.  The name derives from the fact that the nerve supply to these structures comes from the medulla or "bulb" in the lower brain

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 - A rare inherited group of various disorders of neuromuscular transmission clinically similar to autoimmune MG but without immunological origin

Corticosteroids (Steroids) - Hormones used to dampen the faulty immune response that occurs with myasthenia gravis

Crisis - A somewhat old-fashioned term for "acute respiratory insufficiency" developing over hours or days and 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.

Cyclosporine (brand-name Sandimmune) - An immunosuppressive medication available in liquid solution and in 25, 50, and 100 mg capsules, most often used as a steroid-sparing agent, allowing MG patients on prednisone to take less prednisone over the long term.  Expensive and a little more difficult to monitor, this drug is usually used if other medications fail.

Diplopia - The perception of two images in the field of vision.  Ocular myasthenia may affect the eye movements in vertical, horizontal or diagonal directions, with variable degrees of double vision.

Dysarthria - Difficulty speaking characterized by imprecise articulation of consonants.  It results from weakness of the muscles used for speech.

Dysphagia - Difficulty or an inability to swallow

Ephedrine Sulfate - A medication taken as a 25 mg capsule sometimes added to cholinesterase inhibitors to serve as an auxiliary medication in the treatment of MG

Genralized MG - MG affecting more than just the eye muscles (contrasted with purely ocular myasthenia)

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 junction.

Intravenous Immune Globulin (IV-IG) - A very expensive medical procedure involving intravenous infusion of human gamma globulin antibodies pooled from multiple donors, which results in rapid but temporary relief of MG symptoms.

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 (MG) - A disease characterized by fluctuating weakness of eye muscles, face muscles, chewing, swallowing, talking, 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.

Neonatal Myasthenia - Myasthenic weakness lasting 2-12 weeks in a newborn infant as the result of a passive transfer of acetylcholine receptor antibodies from the mother.

Ocular Myasthenia - An autoimmune condition characterized by variable weakness of the muscles of the eyelids and eye movements.  This leads to droopy eyelids (ptosis) and/or double vision (diplopia)

Plasmapheresis - Also called "Plasma Exchange"; An expensive medical procedure that separates the blood into its two major components; plasma (the liquid part of the blood) and cells.  The blood cells are then returned to the patient, with the plasma being replaced by artificial fluids.  Performed by a machine similar to that used for kidney dialysis.

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.

Ptosis - Droopiness of eyelids caused by muscle weakness.  In ocular myasthenia, ptosis is usually variable and worsens as the day progresses.

Remission - Health status whereby a patient is relatively symptom free and no longer needs any MG medication.

Repetitive Nerve Stimulation - This is an electrodiagnostic test that is used to diagnose myasthenia gravis and other disorders of neuromuscular transmission.  To perform this test, small metal disks (electrodes) are taped to the skin over a muscle.  Other metal disks are taped over the nerve that goes to this muscle.  Brief electrical impulses are given to the nerve to make the muscle contract.  At first the impulses are so small that they are not felt, but gradually they build up until they are strong enough to make the muscle fully contract.  Then, a series of 4 to 10 impulses are given at 2 or 3 times a second.  In myasthenia gravis, the muscle contractions become smaller during the 4 to 10 impulses.

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

Single Fiber EMG - This is a sensitive electrodiagnostic test which is used to diagnose myasthenia gravis and other disorders of neuromuscular transmission.  To perform this test, a special EMG needle is placed in a muscle.  The person is coached on how to gently contract the muscle.  The doctor finds and records from two muscle fibers that are activated by the same nerve fiber.  This is repeated for up to 20 recordings.  In myasthenia gravis, the muscle fibers are not activated as closely together as normal, due to the fatigue of the junction where the nerve meets the muscle.

Timespan (Mestinon Timespan) - A longer-acting 180 mg oblong capsule, which provides a slow release of pyridostigmine bromide.  It is usually prescribed for nighttime usage so that a patient does not have to awaken during the night to take Mestinon 60 mg doses.

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, opening of a drooping eyelid.

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.  Surgical approaches include:

Thymoma - Tumor of the thymus gland found in 10-15% of patients with myasthenia gravis - Types of thymoma include:

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

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Please note that any medical views expressed in this article are those of the individual author and do not reflect any official opinion of the Myasthenia Gravis Association of Western PA.  Each person's situation is unique.  If you have any medical questions, please discuss them with your doctor, who best knows your situation.

This article was reprinted with the permission of the Myasthenia Gravis Foundation of California and may not be reprinted without written permission from their organization.  They can be reached at (800) 574-7884 or you can visit their website at www.myasthenia-ca.org.

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