Myopathy is a common term for a muscle disease that is unrelated to any disorder of innervation or neuromuscular junction, with a wide range of possible etiologies. Practically all types of myopathy result in weakening and atrophy of skeletal muscles – particularly those closest to the center of the body (also known as the proximal muscles), such as the thigh and shoulder muscles.

The myopathies can be divided into hereditary and acquired disorders. Hereditary group encompasses muscular dystrophies, congenital myopathies, metabolic myopathies, mitochondrial myopathies, as well as myotonias and channelopathies. Conversely, inflammatory, endocrine and toxic myopathies belong to the acquired group.

Hereditary myopathies

Muscular dystrophies represent a heterogeneous group of hereditary illnesses affecting both children and adults, with at least 30 different genes responsible for the disease development. These diseases are characterized by muscle wasting and weakness, with elevated levels of creatine kinase (CK). The diseases show a dystrophic pattern (i.e.degenerative pattern with necrosis and extensive fibrosis) and an involvement of the central nervous system.

Congenital myopathies are also a genetically and clinically heterogeneous group of conditions, originally classified according to unique morphological changes observed in the muscle tissue. Still, no necrotic or degenerative changes are present in congenital myopathies (in contrast to muscular dystrophy) and CK levels are often normal. This group of myopathies includes some well-established conditions such as nemaline myopathy, central core disease, X-linked myotubular myopathy and centronuclear myopathy.

Metabolic myopathies comprise a diverse group of disorders which arise as a result of defects in cellular energy metabolism, including the vital breakdown of fatty acids and carbohydrates to generate adenosine triphosphate (mainly via mitochondrial oxidative phosphorylation). Consequently, the three main categories of metabolic myopathies are fatty acid oxidation defects, glycogen storage diseases, and mitochondrial disorders due to respiratory chain impairment.

Mitochondrial myopathies are also a large group of variegated disorders resulting from primary dysfunction of the mitochondrial respiratory chain and subsequently causing muscle disease. This group of illnesses has a myriad of different phenotypes and genetic etiologies, and can frequently present with multi-system dysfunction. Typical examples of mitochondrial myopathies include severe Pearson syndrome, Kearns–Sayre syndrome and progressive external ophthalmoplegia which can manifest in late adulthood.

Genetic defects in the genes that code for calcium, sodium, potassium and chloride channels in skeletal muscles result in the periodic paralyses, the nondystrophic myotonias, and the ryanodinopathies. Researchers have expanded and clarified the role of gating pore current in channelopathy pathogenesis; hence this group of diseases now includes myotonia congenita, paramyotonia congenita, hyper and hypokalemic periodic paralysis, potassium-aggravated myotonia, as well as Andersen-Tawil syndrome.

Acquired myopathies

The idiopathic inflammatory myopathies constitute a subset of autoimmune connective tissue diseases primarily affecting the muscle, along with a myriad of extra-muscular manifestations. These conditions be sub-classified into dermatomyositis, polymyositis and inclusion body myositis, according to differences in clinical and histopathological features. The muscle pathology shows characteristic inflammatory exudates of variable distribution within the muscle fascicle, and there is a variable degree of CK elevation and irritative myopathy.

Muscle weakness and myopathy can also be found in endocrinologic conditions. For example, it is commonly found in acromegaly as a result from a combination of the direct effect of growth hormone excess on muscle, but also from other metabolic derangements as well (such as hypoadrenalism, hypothyroidism or diabetes mellitus). Cushing's disease, characterized by overproduction of hormones by the pituitary and adrenal glands, can also cause myopathy.

Myopathy is also included among the potential side-effects and toxicities associated with the certain lipid lowering agents (such as 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors), but also corticosteroids or alcohol. However, the precise mechanism of statin-induced muscle toxicity remains unclear, with potential contributing mechanisms such as decreases in mevalonate pathway products, alterations in gene expression related to apoptosis (programmed cell death) and genetic predisposition.