Congenital and Acquired Myopathies
A group of congenital pathologies is frequently linked to defects in enzyme systems or cellular organelles.
- Lipid myopathies. These arise from genetic deficiencies in the carnitine system, where the enzyme carnitine palmitoyltransferase fails to regulate fatty acid $\beta$-oxidation. Morphologically, this manifests as fatty change in the tissue and weakness. A specific threat is the development of rhabdomyolysis (necrosis) during physical exertion. Myocyte destruction initiates a cascade: myoglobin release $\rightarrow$ entry into plasma $\rightarrow$ renal filtration $\rightarrow$ myoglobinuria. The outcome is myoglobinuric nephrosis and severe renal failure.
- Mitochondrial myopathies. These are based on mtDNA mutations (transmitted maternally). They manifest in young adulthood. Limb weakness and paralysis of extraocular muscles (ptosis, ophthalmoparesis) are characteristic. The main morphological marker is the subsarcolemmal accumulation of abnormal mitochondria (fragmented, irregular shape, with crystalline inclusions).
Acquired myotonic dystrophy has an endocrine origin. It develops against the background of hypoparathyroidism and hypocalcemia. ATP deficiency and endoplasmic reticulum damage lead to intracellular calcium overload, causing painful local muscle contractures.
Toxic and Endocrine Disorders
Toxic myopathies play a more significant clinical role than secondary inflammatory processes (myositis, which regresses after successful treatment of the underlying infection or trauma).
- Thyrotoxic myopathy. This accompanies thyroid dysfunction. In autoimmune thyroiditis, focal necrosis and myofibril regeneration are observed, along with abundant lymphocytes in the stroma. In chronic toxic goiter (Graves' disease), fibers vary in size, fatty and vacuolar change develops, and interstitial sclerosis occurs. Periodic paralysis in men is distinguished separately: due to sodium channel mutations, potassium levels drop, and the sarcoplasmic reticulum in cells markedly expands, forming vacuoles.
- Alcoholic myopathy. Ethanol directly damages the sarcolemma and organelle membranes, and impairs microcirculation, causing hypoxia. In skeletal muscles, this can provoke acute rhabdomyolysis with sharp pain. Between episodes, weakness persists. 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. This has an autoimmune genesis. The body produces antibodies that bind to acetylcholine receptors on the postsynaptic membrane and fix complement, leading to membrane damage. Clinically, the process starts with extraocular muscles (diplopia, ptosis), followed by lower limb weakness. It is frequently associated with thymic pathology (hyperplasia or thymoma).
- Lambert-Eaton myasthenic syndrome. Unlike the previous form, there are no antibodies against the 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 frequently acts as a paraneoplastic syndrome in small cell lung cancer.
Soft Tissue Tumors (Muscle Origin)
Neoplasms originate from both smooth and striated muscle tissue.
Smooth Muscle Tumors:
- Leiomyoma. A benign encapsulated nodule with expansile growth. It consists of mature cells forming haphazard bundles. The stroma is prone to hyalinosis. Typical localization is the myometrium and gastrointestinal tract.
- Leiomyosarcoma. The malignant counterpart. Cells are markedly atypical, pleomorphic, actively dividing, and form muscle symplasts. It grows infiltratively (although it appears nodular), metastasizes hematogenously early, and frequently recurs.
Striated Muscle Tumors:
- Rhabdomyoma. A rare benign tumor, encountered mainly in children. Preferred localizations include the skeletal muscles of the extremities, myocardium, and base of the tongue.