Usually, diagnosis begins with evaluation of the patient’s personal and family history, and proceeds with physical and neurological examinations that test reflexes and strength. The exams can detect problems with muscle tone and contraction, and the histories can bring to light patterns of inheritance and conditions that might have aggravated the muscle problems in the past.
Given this information, a doctor can sometimes distinguish an inherited myopathy from other diseases that affect muscle function, such as muscular dystrophies and neurological disorders. To specifically identify the myopathy and plan an appropriate course of treatment, the doctor can use several specialized tests.
Genetic (DNA-based) tests, usually performed on a blood sample, are available for many of the genetic mutations that underlie various myopathies. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century.
A muscle biopsy, the removal of a small piece of muscle tissue, is sometimes performed. The sample is examined for physical signs of muscle disease. Under the microscope, muscles affected by various myopathies have fairly distinct appearances.
Also, a muscle biopsy can be used to see how isolated muscles respond to different potentially harmful conditions. For example, to determine a patient’s susceptibility to malignant hyperthermia, a biopsied muscle can be tested for its reaction to potentially dangerous anesthetic drugs.
A muscle’s activity can be measured in the body by electromyography (EMG), which involves observing the electrical signals that a muscle produces during contraction. A needlelike electrode inserted into the muscle “reads” the electrical signals and sends them to a monitor called an oscilloscope. Although somewhat uncomfortable, the technique is useful for diagnosing channelopathies, which can show telltale abnormal signals on the oscilloscope.
Severe bouts of weakness related to a disturbance of thyroid function sometimes are confused with inherited periodic paralysis. Your doctor may order blood tests to check your thyroid function, especially if a family history of this condition is lacking.
Doctors frequently measure the level of thyroid-stimulating hormone (TSH) in a blood sample to determine whether the myopathy is endocrine-related or to rule out the endocrine myopathies.