Congenital and acquired myopathies
A group of congenital pathologies is often associated with defects in enzyme systems or cellular organelles.
- Lipid myopathies. They arise due to a genetic deficiency of the carnitine system, when the enzyme carnitine palmitoyltransferase fails to regulate $\beta$-oxidation of fatty acids. Morphologically, this manifests as fatty dystrophy of the tissue and weakness. A specific threat is the development of rhabdomyolysis (necrosis) during physical exertion. Myocyte destruction triggers a cascade: myoglobin release $\rightarrow$ entry into plasma $\rightarrow$ renal filtration $\rightarrow$ myoglobinuria. The result is myoglobinuric nephrosis and severe renal failure.
- Mitochondrial myopathies. Based on mtDNA mutations (transmitted from the mother). They manifest in youth. Characteristic features are limb weakness and paralysis of external eye muscles (ptosis, ophthalmoplegia). The main morphological marker is the accumulation of abnormal mitochondria (fragmented, irregularly shaped, with crystalline inclusions) under the sarcolemma.
Acquired myotonic dystrophy has an endocrine origin. It develops against the background of parathyroid insufficiency and hypocalcemia. ATP deficiency and damage to the endoplasmic reticulum lead to an excess of intracellular calcium, which causes painful local muscle contractures.
Toxic and endocrine lesions
Toxic myopathies play a more significant clinical role than secondary inflammatory processes (myositis, which regresses after curing the primary infection or trauma).
- Thyrotoxic myopathy. Accompanies thyroid dysfunctions. In autoimmune thyroiditis, focal necroses and regeneration of myofibrils are observed, along with an abundance of lymphocytes in the stroma. In chronic toxic goiter, fibers vary in size, fatty and vacuolar dystrophy occur, and stromal sclerosis develops. Periodic paralyses in men are distinguished separately: due to sodium channel mutations, potassium levels drop, and the sarcoplasmic reticulum in cells expands sharply with the formation of vacuoles.
- Alcoholic myopathy. Ethanol directly destroys the sarcolemma and organelle membranes, and also impairs microcirculation, causing hypoxia. In skeletal muscles, this can provoke acute rhabdomyolysis with sharp pain. Weakness persists between attacks. The greatest danger is alcoholic cardiomyopathy (death of some fibers, compensatory hypertrophy of the remaining ones, and diffuse sclerosis), leading to cardiac death.
- Drug-induced myopathy. Most commonly induced by steroid therapy. It manifests as selective atrophy of type II muscle fibers and progressive weakness.
Pseudoparalytic myasthenias
This group unites pathologies characterized by impaired synaptic transmission of impulses to the muscle fiber.
- Myasthenia gravis. Has an autoimmune genesis. The body produces antibodies that bind to acetylcholine receptors of the postsynaptic membrane and fix the complement system on them. This leads to membrane damage. Clinically, the process starts with the extraocular muscles (diplopia, ptosis), followed by leg weakness. It is often associated with thymus pathology (hyperplasia or thymoma).
- Lambert-Eaton myasthenic syndrome. Unlike the previous form, there are no antibodies to receptors here. The problem lies in the insufficient release of acetylcholine vesicles in response to an action potential. It is accompanied by high levels of IgG and progressive muscle weakness. It often acts as a paraneoplastic syndrome in small cell lung cancer.
Soft tissue tumors (of muscular origin)
Neoplasms form from both smooth and striated muscle.
Smooth muscle tumors:
- Leiomyoma. A benign encapsulated node with expansive growth. It consists of mature cells forming haphazard bundles. The stroma is prone to hyalinosis. Typical locations are the myometrium and the gastrointestinal tract.
- Leiomyosarcoma. A malignant variant. The cells are sharply atypical, polymorphic, actively dividing, and form muscle symplasts. It grows infiltratively (although it looks like a node), metastasizes hematogenously early, and frequently recurs.
Striated muscle tumors:
- Rhabdomyoma. A rare benign tumor. It occurs mainly in children. Preferred locations are the thickness of the limb muscles, myocardium, and the root of the tongue.
Briefly: answers to topical questions
Which malignant tumors develop from striated muscle tissue?
A malignant tumor, rhabdomyosarcoma, develops from striated muscle tissue.
- Rhabdomyosarcoma is a relatively rare malignant tumor with pronounced cellular polymorphism and sharp nuclear atypia.
Tumor cells resemble embryonic muscle cells, and pathological mitoses are found in them. Typically, the neoplasm grows as a node in the thickness of the limb and trunk muscles. Atypical locations include the retroperitoneal tissue, mediastinum, and urogenital tract. Extensive metastasis and frequent recurrences are characteristic of the tumor, and the prognosis is often poor.
What types of primary progressive muscular dystrophies exist?
According to the cited sources, primary progressive muscular dystrophies include:
- Duchenne muscular dystrophy — associated with the absence of dystrophin, damage and death of muscle fibers, and their replacement by connective tissue; in late stages, most fibers are replaced by adipose and connective tissues.
- Becker muscular dystrophy — caused by mutations in the dystrophin gene, has a milder course, and affects older boys and adolescents.
Other diseases listed in the original answer are not presented in the sources as types of primary progressive muscular dystrophies.
What types of primary and secondary myositis are distinguished in pathomorphology?
Pathomorphology distinguishes primary (idiopathic) and secondary myositis.
Secondary myositis develops as a complication of infectious diseases, muscle or bone trauma, acts as a symptom, and regresses as the underlying pathology cures.
Primary lesions are grouped into idiopathic inflammatory myopathies, among which are:
- Primary polymyositis — inflammation with a predominance of cytotoxic CD8+ T lymphocytes in the endomysium.
- Primary dermatomyositis — a lesion with an infiltrate in the perimysium and perivascular space.
- Juvenile dermatomyositis — a form of the disease in childhood.
- Inclusion body myositis — a pathology with a leading role of cellular immunity.
- Myositis ossificans — a specific form of inflammatory myopathy.
- Giant cell myositis — an idiopathic inflammatory lesion.
The classification also includes localized myositis and myositis associated with eosinophilia.
The rhabdomyolysis cascade can be remembered by the rule of the "Four M's": Muscle breakdown — Myoglobin in blood — Myoglobinuria (renal filtration) — Myoglobinuric nephrosis.
Class and exam questions
What is the difference between myasthenia gravis and Lambert-Eaton syndrome?
In myasthenia gravis, antibodies block muscle acetylcholine receptors. In Lambert-Eaton syndrome, the receptors are free, but the release of the neurotransmitter itself (acetylcholine) from the nerve terminal is impaired.
Why are physical exercises dangerous in carnitine palmitoyltransferase deficiency?
Physical exertion with impaired fat oxidation provokes acute muscle necrosis (rhabdomyolysis). This leads to the release of a huge amount of myoglobin, which is fraught with acute renal failure.
How to distinguish leiomyoma from leiomyosarcoma under a microscope?
Leiomyoma consists of mature cells without marked cellular atypia, whereas leiomyosarcoma is characterized by high mitotic activity, the presence of giant multinucleated cells, and pronounced polymorphism.
What else to study on this topic
- Pathogenesis of electrophysiological disturbances in alcoholic cardiomyopathy.
- Secondary inflammatory myositis in infectious diseases.
- Morphological differences between fibroleiomyoma and classical leiomyoma.
- The association of Lambert-Eaton myasthenic syndrome with small cell lung cancer.
- Muscle fiber types and their selective involvement in drug-induced myopathies.
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