Abstract
Myopathy refers to diseases derived from the muscle. Myopathies have different causes and may have acute, subacute, or chronic clinical presentations. They are progressive but have different time courses and affect proximal or distal muscle groups; some of them also affect heart muscle, leading to cardiomyopathy. In an increasing number of myopathies, the genetic background and the abnormal or missing muscle proteins have been identified and are cornerstones for the diagnosis. Other important diagnostic features are high CK values and abnormalities on electrophysiologic examination and muscle biopsy. In a few myopathies—for example, inflammatory myopathies and lipid storage myopathies—specific pharmacologic treatment is available. Rehabilitation of patients with myopathy includes physical and occupational therapy as well as assistive and orthotic devices. Different exercise training can be recommended in all patients with myopathy.
Keywords
Exercise, muscle atrophy, muscle biopsy, muscle fatigue, muscle weakness
| Synonym | |
| |
| ICD-10 Codes | |
| G71.0 | Muscular dystrophy, congenital |
| G71.19 | Myotonic disorders |
| G72.2 | Myopathy due to other toxic agents |
| E34.9 | [G73.7] Endocrine myopathy |
| G72.49 | Inflammatory myopathy, not elsewhere classified |
| G72.9 | Myopathy, unspecified |
| M33.90 | Dermatopolymyositis, unspecified, organ involvement unspecified |
| M33.20 | Polymyositis, organ involvement unspecified |
Definition
Myopathy is the common name for diseases derived from the muscle. Myopathies have different causes and different courses, that is, they may have acute, subacute, or chronic presentations ( Table 136.1 ). Myopathies affect proximal or distal muscle groups; some of them also affect heart muscle, leading to cardiomyopathy. Many myopathies are inherited disorders ( Table 136.2 ); in an increasing number of disorders, the genetic background and the abnormal or missing muscle proteins have been identified during the last decades. An updated table of genes contributing to myopathies is published online. Muscle diseases are rare, with a prevalence of approximately 50 per 100,000. Myopathy is more common in females than in males, although Duchenne muscular dystrophy is found only in males because of an X-linked inheritance.
| Muscular Dystrophies |
|
| Congenital Myopathies |
|
| Metabolic Myopathies |
|
| Inflammatory Myopathies |
|
| Endocrine Myopathies |
|
| Drug-Induced/Toxic Myopathies |
| Myotonic Syndromes |
|
| Disorder | Inheritance |
|---|---|
| Duchenne muscular dystrophy | X-linked recessive |
| Becker muscular dystrophy | X-linked recessive |
| Facioscapulohumeral dystrophy | Autosomal dominant |
| Scapuloperoneal dystrophy | X-linked dominant |
| Limb-girdle dystrophy | Autosomal recessive/dominant |
| Oculopharyngeal dystrophy | Autosomal dominant |
| Distal myopathy/muscular dystrophy | Autosomal dominant/recessive |
| Congenital muscular dystrophy | Autosomal recessive/dominant/sporadic |
| Congenital myopathies | Autosomal recessive |
| Myotonic syndromes | Autosomal dominant |
| Metabolic myopathies | Autosomal dominant/recessive, X-linked recessive |
Muscular Dystrophies
Muscular dystrophies are inherited disorders of muscle due to abnormal structural muscle proteins—for example, dystrophin in Duchenne muscular dystrophy and sarcoglycans, calpain, and dysferlin in different limb-girdle dystrophies. They are characterized by an early onset and a progressive course.
Congenital Myopathies
Congenital myopathies form a clinically heterogeneous group characterized by slowly progressive or non-progressive muscle weakness and manifested in the neonatal period. Based on diagnostic findings on muscle biopsy, these are divided into three main groups: nemaline myopathy, core myopathy, and centronuclear myopathy. Recent molecular studies have revealed a genetic heterogeneity.
Metabolic Myopathies Including Mitochondrial Myopathies
Metabolic myopathies form a clinically heterogeneous group of muscle disorders resulting from inherited defects in intracellular energy production. They may be manifested as cramps and myoglobinuria. Patients with cramps and myoglobinuria often have disorders in the glycogen or lipid metabolism pathways. They may be asymptomatic at rest with symptoms developing after exercise. Mitochondrial myopathies may be a part of a neurologic syndrome often involving the central nervous system and may also play important roles in neurodegenerative disorders such as Huntington disease and sporadic Parkinson disease.
Inflammatory Myopathies
Inflammatory myopathies are characterized by inflammatory changes in the muscle and are associated with infections or an immunologic process. They are divided into polymyositis, dermatomyositis, and inclusion body myositis. The course is acute or subacute and is almost always associated with an elevated serum creatine kinase (CK) level.
Drug-Induced and Endocrine Myopathies
Drug-induced myopathies are caused by different drugs—for example, colchicine, azidothymidine (AZT), chloroquine, hydroxychloroquine, and corticosteroids. Myalgia, myopathy, and even rhabdomyolysis due to intake of statins, the cholesterol-lowering agents, have been reported. Endocrine myopathies include both hyperthyroid and hypothyroid myopathies as well as myopathy due to hyperparathyroidism.
Myotonic Syndromes
A number of disorders are associated with clinical or electrical myotonia. Myotonia is inability to relax after vigorous effort. The myotonic disorders are divided into myotonic dystrophies and “pure” myotonia. The myotonias are inherited disorders due to alterations in ion channels in the muscle and seldom give rise to persistent muscle weakness. Myotonic dystrophy (MD 1) or Steinert disease exists in a congenital and an adult form. Individuals with myotonic dystrophy may not notice any problems until adolescence or early adult life. The first symptom may be difficulty in releasing an object due to myotonia. Progressive weakness with onset in distal muscles follows. The genetic explanation to MD 1 is an expansion of a CTG repeat in chromosome 19 ( DMPK gene). The gene encodes a protein kinase that occurs in different tissues, leading, for example, to cardiomyopathy. The clinical affectation is correlated to the number of repeats, and the number of repeats increases between generations, leading to a clinical anticipation. There is also a more rare form with later onset, a slower course, and proximal muscle weakness (MD 2) with a CCTG repeat in chromosome 3 ( ZNF9 gene) as genetic background.
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