This page addresses the symptoms of inherited and endocrine myopathies , as well as the following signs:
Some of the congenital inheritable myopathies cause severe, general muscle weakness that creates problems with basic activities like swallowing and breathing.
Other inheritable myopathies cause episodes of muscle weakness or stiffness that are milder and more localized, and temporary in nature.
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 myopathies 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.
Symptoms of endocrine myopathies  include weakness and atrophy (shrinking) of the muscles around the shoulders and hips, muscle stiffness, cramps, slowed reflexes, and in severe cases, muscle breakdown.
For more on the specific signs and symptoms of each type of myopathy, please see the individual disease pages.
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With the exception of Andersen-Tawil syndrome , the myopathies almost never affect heart muscles directly. However, 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 one form of periodic paralysis  (the hypokalemic form), attacks of weakness are associated with a decrease in blood potassium level, whereas another type of periodic paralysis (the hyperkalemic form) causes an increase. Either change can indirectly cause an irregularity in the rhythmic contraction of the heart.
Andersen-Tawil syndrome, in contrast, can directly damage the heart, since it results from an inherited defect in a potassium channel (see Causes/Inheritance ) that is found in both heart and skeletal muscle. People with Andersen-Tawil may have heart rhythm disturbances, even if their blood potassium level is normal.
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People with myopathies can experience a range of adverse reactions to certain anesthetic drugs used during surgery. Although these drugs sometimes just aggravate the myopathy, they also can produce potentially fatal reactions, such as malignant hyperthermia, which refers to a dangerously high increase in body temperature.
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. See Medical Management .
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