Excerpt from Endocrine Myopathies

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Background

A myopathy, simply, is any abnormal state of striated muscle. Clinically, the patient generally experiences muscle weakness, pain, cramps, muscle tenderness, and spasms in various degrees.

Disease of the endocrine system, including the thyroid, parathyroid, suprarenal, and pituitary glands, the ovaries, the testes, and the islands of Langerhans of the pancreas, usually results in multisystem signs and symptoms. A myopathy very often is present, and it rarely may be the presenting symptom.

Major categories of endocrine myopathy include those associated with (1) adrenal dysfunction (as in Cushing disease or steroid myopathy); (2) thyroid dysfunction (as in myxedema coma or thyrotoxic myopathy); (3) parathyroid dysfunction (as in multiple endocrine neoplasia); (4) pituitary dysfunction; and (5) islands of Langerhans dysfunction (as in diabetic myopathy from ischemic infarction of the femoral muscles). Steroid myopathy is the most common endocrine myopathy.

Articles on the specific endocrine diseases that may result in myopathy may be found in detail in this and the Medicine journals of eMedicine.

Pathophysiology

Although abnormal endocrine states usually present with muscle weakness—most often proximal weakness—the exact pathophysiology remains incompletely understood. Even histologic analysis and electromyographic testing may not show consistent, reproducible abnormalities in all cases, although some patterns are recognized and are discussed in the sections below.

Adrenal dysfunction

• The etiologies of hypoadrenalism are many, including infection, inflammatory disease, and tumor. Notably, adrenal failure may follow pituitary failure.
• In hypoadrenalism, neurological manifestations such as disturbances of behavior and mentation are prominent; myopathy is not likely to be a presenting finding.
• Factors contributing to muscle weakness in adrenal insufficiency include circulatory insufficiency, fluid and electrolyte imbalance, impaired carbohydrate metabolism, and starvation.
• The etiologies of hyperadrenalism include pituitary or ectopic overproduction of adrenocorticotropic hormone (ACTH), adrenal tumors, or exogenous corticosteroid administration. Pituitary ACTH hypersecretion (ie, Cushing disease) is caused by a corticotroph microadenoma in 90% of patients and by a macroadenoma in most of the rest.

Thyroid dysfunction

• Thyroid hormone deficiency states result in neurological syndromes that vary depending on the age of onset of the deficiency. Muscle weakness occurs most prominently in the adult forms of myxedema.
• Thyroid hormone excess also results in myopathy. Thyrotoxic myopathy is believed to be secondary to a disturbance in the function of the muscle fibers from increased mitochondrial respiration, accelerated protein degradation and lipid oxidation, and enhanced beta-adrenergic sensitivity due to excessive amounts of thyroid hormone.
• The heterogeneity of the endocrine myopathies is illustrated nicely by Rodolico and colleagues, who described 10 patients with primary autoimmune hypothyroidism presenting solely with myopathy.

Parathyroid dysfunction

• Hypoparathyroidism causes tetany, with or without carpopedal spasm. The pathophysiology may involve either deficiency of parathyroid hormone or inability of the hormone to have an effect at end-receptors because of dysfunction of the hormone receptors.
• Hyperparathyroidism does not cause tetany but results in muscle wasting and myopathy (ie, proximal muscle weakness). The pathophysiology is oversecretion of hormone, frequently from a parathyroid adenoma.
• Myopathy related to parathyroid dysfunction appears to result from altered parathyroid hormone (PTH) level and impaired action of vitamin D.

Pituitary dysfunction

• The myopathy from pituitary disease may be a result of secondary adrenal dysfunction and/or other endocrine disturbance such as thyroid dysfunction.
• Hypopituitarism as well as hyperpituitarism may result from multiple causes, from simple trauma, or from infection or tumor.

Polymyalgia rheumatica (PMR) and temporal arteritis (TA): Although research is just beginning, Imrich and colleagues note that age-related changes in the neuroendocrine system could represent a pathogenic factor for PMR and/or TA in genetically disposed.

Frequency

United States

In general, endocrine myopathies are recognized increasingly. However, the exact incidence and prevalence are unknown. Patients with endocrine dysfunction frequently complain of fatigue and weakness. These symptoms are referred to as a "myopathy" despite lack of defined histologic or electrophysiologic criteria fulfilling such a diagnosis. In fact, many of these patients show only muscle atrophy without muscle degeneration. Corticosteroid myopathy is the most common endocrine-related myopathy. Patients who have myopathy as the sole manifestation of endocrine dysfunction may sometimes have a delayed diagnosis.

International

As in the United States, the exact frequency is not known as the myopathies are heterogeneous.

Mortality/Morbidity

• Myopathy may result in weakness and/or pain. Either may significantly influence the quality of life and impair daily function. Myopathy also may result in muscle atrophy.
• Mortality is related to the underlying cause of myopathy. For example, myxedema coma may have a mortality rate between 50% (if treated aggressively) and 100%.

Sex

• Hyperparathyroid myopathy - Female-to-male ratio 2:1
• Hyperthyroid myopathy - Female-to-male ratio 1:1
• Iatrogenic steroid myopathy - Female-to-male ratio 2:1
• Hypothyroid myopathy - Female-to-male ratio 5:1
• Cushing myopathy - Depends on the etiology of Cushing syndrome

Age

• Hyperparathyroid myopathy - Peak incidence 40-60 years
• Hyperthyroid myopathy - Peak incidence 20-60 years
• Hypothyroid myopathy - Incidence increases after 40 years
• Cushing myopathy - Peak incidence 20-40 years

Please click here to view the full topic text: Endocrine Myopathies