Printer Friendly

Facts About Myopathies.

QUESTIONS AND ANSWERS Part I

WHAT ARE MYOPATHIES?

The word myopathy means "disease of muscle." More specifically, myopathies are diseases that cause problems with the tone and contraction of skeletal muscles (muscles that control voluntary movements.) These problems range from stiffness (called myotonia) to weakness, with different degrees of severity.

Some myopathies, especially when they're present from birth, have life-threatening complications. But, with time and physical therapy, some people born with myopathies can gain muscle strength. Others often can manage their symptoms through medication, lifestyle modifications, or use of orthopedic and respiratory equipment.

This booklet focuses on six inheritable myopathies (myopathies that can be passed from parent to child) in MDA's program.

* myotonia congenita (Thomsen's disease and Becker type)

* paramyotonia Congenita (Eulenberg's disease)

* periodic paralysis (hyperkalemic, hypokalemic, normokalemic)

* nemaline myopathy (rod body disease)

* myotubular myopathy (centronuclear myopathy)

This booklet also contains information on two noninheritable myopathies caused by abnormal activity of the thyroid gland -- hypothyroid myopathy and hyperthyroid myopathy.

WHAT CAUSES THE INHERITABLE MYOPATHIES?

These inheritable myopathies are caused by mutations, or changes, in genes -- the blueprints for making proteins that are necessary for our bodies to function correctly. Genes are responsible for building our bodies; we inherit them -- along with any mutations or defects they have -- from our parents, and pass them on to our children. (For more information on how myopathies are inherited, see "Does It Run in the Family?")

In the inheritable myopathies, genetic mutations cause defects in various proteins necessary for muscle tone and contraction.

WHAT ARE MUSCLE TONE AND CONTRACTION, AND WHAT CONTROLS THEM?

Contraction is the forceful shortening or tightening of muscle, which pulls on the joints to cause movement. In other words, when your brain "tells" a muscle to move, you cause it to contract, and it's then able to do what you're asking.

Muscle tone refers to a readiness for contraction that makes resting muscle resistant to stretching. Or, a toned muscle holds its shape and its elasticity and is able to respond by contracting when you want it to move. Bodies with poor muscle tone appear "floppy." Good muscle tone is important for posture and coordination.

A skeletal muscle's tone and contraction depend on its ability to respond to stimulation from nerve cells, which are responding to signals sent from the brain, such as your decision to move your hand or leg. A muscle is actually a bundle of individual muscle cells, each one stimulated by a single nerve cell.

The process of muscle contraction begins when the nerve cells release chemical signals onto the muscle cells. These signals cause the opening of ion channels, pores in each muscle cell's outer surface that open and close to regulate ion movement. Ion channels allow certain ions -- charged molecules of sodium, calcium, potassium or chloride -- to pass into and out of the muscle cell, creating electrical currents. Opening of sodium and calcium channels causes an electrical excitation that will lead to contraction, while opening of potassium and chloride channels keeps the excitation from occurring.

The purpose of the electrical excitation is to stimulate the opening of still more channels that release calcium from internal compartments in the muscle cell. Finally, the freed calcium ions trigger muscle contraction by stimulating the sliding action of filament proteins.

These rodlike proteins run lengthwise within the muscle cell and are anchored at opposite ends by scaffolds called z-discs. When the filament proteins slide past each other -- by an energy-dependent, ratchetlike mechanism -- they cause shortening of the muscle cell and shortening (contraction) of the whole muscle.

If this process is disrupted at any stage between the nerve's signaling the muscle and the filament proteins' action, the muscle loses its normal capacity for tone and contraction. At one extreme, the muscle might be limp and weak, and at the other extreme, the muscle may be unable to relax.

Many of the inheritable myopathies are caused by mutations that interfere with ion channels, preventing necessary ions from flowing through the muscle cells. These disorders (myotonia congenita, paramyotonia congenita, periodic paralysis and central core disease) are sometimes called channelopathies.

Central core disease seems to damage, and thus weaken, muscles by causing an excess release of internal calcium. The other channelopathies interfere with normal muscle tone and contraction by causing over- or underexcitation.

A fifth myopathy, nemaline myopathy, is caused by mutations that affect filament proteins. When the filament proteins fail to do their jobs, muscles can't contract properly, causing a loss of tone and strength.

At least one myopathy (a type of myotubular myopathy) is caused by mutations in a muscle protein required for normal muscle development. When this protein is absent or inactive, the muscles don't form properly.

Some of the inheritable myopathies are congenital, meaning they cause problems from the time of birth. Others have a later onset, with symptoms appearing in childhood or adulthood.

By the way, myopathies aren't contagious, and they aren't caused by overexertion. However, exercise can aggravate some of the myopathies, because of mutations that change the way muscles respond to activity.

WHAT HAPPENS TO SOMEONE WITH AN INHERITABLE MYOPATHY?

Some of the congenital inheritable myopathies cause severe, general muscle weakness that creates problems with basic activities like swallowing and breathing. These problems can be fatal if not dealt with, but they can be treated with assistive medical devices like feeding tubes and mechanical ventilators.

Other inheritable myopathies cause episodes of muscle weakness or stiffness that are milder and more localized and temporary in nature. These episodes often can be managed through medication, or by careful control of exercise and diet.

Unlike muscular dystrophies, myopathies usually don't cause muscles to die but just keep them from working properly. Also, myopathies are usually nonprogressive -- that is, a myopathy usually doesn't grow worse over a person's lifetime. In fact, some children with myopa-thies gain strength as they grow older.

Finally, some myopathies can give people a listless facial expression, caused by weakness of muscles in the face. Myopathies have no effect on intelligence.

Special Issues in Inheritable Myopathies

Anesthesia: People with myopathies can experience adverse reactions to certain anesthetic drugs used during surgery. Although these drugs sometimes just aggravate the myopathy, they also can produce a potentially fatal reaction called malignant hyperthermia, which refers to a dangerously high increase in body temperature. People with central core disease are especially at risk for malignant hyperthermia because the two conditions are sometimes caused by the same ion channel defects.

Malignant hyperthermia is triggered by certain inhaled anesthetics (like halothane) and certain muscle relaxants (like succinylcholine). These drugs can intensify ion channel defects and boost muscle metabolism -- the set of chemical reactions that provides energy to muscle. The increased metabolism raises body temperature, and causes excessive contraction and rhabdomyolysis -- a process of acute muscle breakdown. The resulting leakage of ions and muscle proteins into the circulatory system can cause life-threatening damage to the heart, lungs and kidneys.

People with central core disease aren't always susceptible to malignant hyperthermia. Those who are susceptible won't experience malignant hyperthermia unless they're exposed to triggering anesthetics.

Before having surgery, people who have a personal or family history of central core disease -- or any other myopathy -- should consult their doctors about the risks of anesthesia and about the availability of "nontriggering" anesthetics.

Respiratory care: Nemaline myopathy and congenital (X-linked) myotubular myopathy cause weakening of the respiratory muscles (those that control the lungs). Therefore, people with either of these diseases probably will need to use mechanical ventilation to support breathing, and should have their breathing monitored regularly by a specialist.

Also, weak lungs are susceptible to infection, so signs of respiratory illness should be taken seriously.

Cardiac care: The myopathies almost never affect heart muscle directly, but sometimes they can cause indirect damage to the heart. In nemaline myopathy and congenital myotubular myopathy, an inadequate oxygen supply to the body during severe bouts of respiratory weakness can lead to heart problems. In hyperkalemic periodic paralysis, certain conditions can trigger potassium elevation in the blood, which can interfere with contraction of the heart.

People with these diseases should be wary of potential heart problems and have their cardiac function checked by a specialist.
COPYRIGHT 2001 Muscular Dystrophy Association, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2001 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Publication:Pamphlet by: Muscular Dystrophy Association
Article Type:Pamphlet
Date:May 1, 2001
Words:1356
Previous Article:Facts About Limb-Girdle Muscular Dystrophy (LGMD).
Next Article:Facts About Myopathies.
Topics:

Terms of use | Privacy policy | Copyright © 2018 Farlex, Inc. | Feedback | For webmasters