Myopathy
What is it?
Myopathies are primitive muscle diseases characterized by a structural and functional alteration of muscle fibers. They may be related to a genetically determined degenerative process such as muscular dystrophies; result from an altered cellular metabolism (such as glycogenosis, mitochondrial myopathies, as well as hormonal disorders such as during dystothyroidism); caused by alteration in the excitation-contraction coupling of muscle fibers (such as channelopathies); result from inflammatory process directed against muscle tissue (such as polymyositis, dermatomyositis and myositis by included bodies); or be toxic (due to the use of drugs such as statins).
Clinical consequences include progressive slimming of the muscle masses of the four limbs, most frequently in a symmetrical fashion and affecting the proximal musculature. Patients also complain of impaired muscle strength, especially in raising their arms above their heads, walking or climbing stairs, or getting up from a chair without the aid of their arms. In some cases, the distal muscles of the limbs may be predominantly affected (distal and included body myopathies), or the facial musculature with difficulty in whistling, eyelid closing, cheek dimpling (facio-scapulo-humeral dystrophy), or vice versa eyelids that tend to remain half-closed (mitochondrial myopathy).
Other patients may complain of a delay in the ability to decontract muscles after contraction that is voluntary (e.g., holding the handlebars of a bicycle) or caused by mechanical stimulation (dystrophy or myotonic syndromes). It should borne in mind that some myopathies may also include systemic symptoms, with skin (skin rash), cardiac (hypertrophic or dilated cardiomyopathy, arrhythmias, inflammation), ocular (cataract, retinopathy), endocrine (dysthyroidism, infertility), cerebral (cognitive impairment, epilepsy) disorders.
Which are the symptoms?
The main symptom generally is weakness (hyposthenia) of the large proximal muscles, in the upper and/or lower limbs. Sometimes, especially in inflammatory and dysmetabolic forms, muscular pain (called myalgia) and/or cramp-like syndrome may be associated. Hyposthenia is associated with a progressive muscular hypotrophy of the same muscles. Muscular hypertrophy (or pseudohypertrophy in some dystrophies) can occur in rare cases. Another frequently present symptom is fatigability, when the patient is able to perform an exercise but gets tired quickly. Finally, abnormalities of muscle contraction may be observed, such as myotonia (difficulty releasing a muscle after contraction) and involuntary muscle movements. It should not be forgotten that sometimes myopathies are associated with systemic symptoms that can affect various organs, especially the heart and lungs, and, in rarer cases, the dermis, brain, and eyes.
- muscle weakness
- muscle atrophy (rarely hypertrophy)
- fatigability
- myalgia and/or cramps
- abnormalities of muscle contraction
- possible systemic symptoms
How is it diagnosed?
A correct diagnosis requires a thorough neurological examination, determining certain dosage of muscle enzymes in blood and inflammatory, metabolic and hormonal indices, as well as electromyographic examination. If indicated, muscle MRI may be helpful. Muscle biopsy and a series of genetic tests allow in most cases to complete the diagnostic process by identifying the cause of the disease.
Suggested exams
How is it treated?
The treatment of myopathies depends on their cause. If the cause is acquired, as in inflammatory forms, anti-inflammatory drugs (such as cortisone), immunomodulatory drugs (such as intravenous immunoglobulins) or immune-suppressive and biological drugs (monoclonal antibodies) can be used. If the cause is an endocrine disorder, such as dystothyroidism, this dysfunction needs simply to be corrected. For metabolic forms, there are some drugs that improve mitochondrial function, while for glycogenosis, enzyme replacement therapy is available for type 2 form. In channelopathies, there are certain drugs that stabilize the muscle membrane while improving symptoms. In myotonic forms, there are symptomatic drugs that reduce the disorder. For muscular dystrophies, drugs that can be used in the forms known as Duchenne dystrophy have become available only recently. Many of these drugs are still in experimental trials, and may be used within clinical trials (oligonucleotides, monoclonal antibodies, gene therapy), and in the coming years their use may be expanded to various forms of muscular dystrophy.
Where do we treat it?
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