Endocrine Myopathies Workup

Updated: Jul 02, 2021
  • Author: Shireen R Chacko, MBBS; Chief Editor: Nicholas Lorenzo, MD, CPE, MHCM, FAAPL  more...
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Workup

Laboratory Studies

Because the diagnosis is made by elucidating the underlying endocrine abnormality, laboratory studies are considered in relation to the most likely etiologies.

Laboratory studies measuring hormone levels may help distinguish one endocrine myopathy from another. These tests are best ordered in consultation with an endocrinologist.

Creatine kinase (CK) levels may be normal or increased in various endocrine disorders, as follows:

  • Hypothyroidism: CK is usually elevated. Interestingly, the degree of elevation of CK does not have a clinical correlation with the severity of myopathic symptoms, [56]  with mild CK elevations even seen in asymptomatic patients. [57]
  • Hyperthyroidism: CK is usually normal.
  • Glucocorticoid excess: CK is usually normal [7]  and, in fact, may be low in some cases. [58]
  • Adrenal insufficiency: CK is usually normal. [7]  
  • Hyperaldosteronism: There are case reports of elevated CK levels in hyperaldosteronism-associated myopathy. [13]
  • Hypoparathyroidism: CK may be normal or, rarely, elevated. [7]  
  • Hyperparathyroidism, primary and secondary: CK is usually normal. [7]
  • Oseomalacia: CK is usually normal. [7]
  • Acromegaly: Slight CK elevation may be present. [7]  
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Imaging Studies and Other Tests

Imaging studies

Imaging studies neither confirm nor exclude the presence of muscle disease. They may be of benefit in the diagnosis of endocrine disorders.

Other tests

Electromyography (EMG) may reveal the presence of a myopathy, but a normal examination does not rule out the diagnosis. Although commonly performed with nerve conduction study testing, needle EMG is direct invasive testing of muscle and therefore differs from nerve conduction study testing. Myopathy is a disorder of muscle, and the nerve conduction study portion of the electrophysiological examination should be normal; however, the endocrinopathies often also cause neuropathies or may be associated with other conditions (such as diabetes) in which neuropathies are common. This heterogeneity explains the great variability and lack of consensus regarding the electrophysiological findings in endocrine diseases.

Needle EMG examination preferentially studies the type I units, as these units fire selectively during weak muscle contraction. Thus, a disease process selectively involving type II units, such as steroid myopathy, may reveal no abnormalities on EMG.

EMG findings consistent with a myopathic process include the following:

  • Polyphasic motor unit potentials
  • Shortened duration of motor unit potentials
  • Decreased amplitude of motor unit potentials
  • Adrenal dysfunction: In cases of adrenomyeloneuropathy, a distinct and different disorder not otherwise considered in this article, nerve conduction velocity may be normal or decreased.
  • Hypothyroidism: EMG helps differentiate delayed muscle relaxation from myotonia.
  • Hyperthyroidism: EMG abnormalities may be found more proximally and are of the typical myopathic type. Motor conduction studies typically are normal, although some can show distal leg denervation.
  • Hyperparathyroidism: The usual finding is myopathic motor unit potentials and increased frequency of polyphasic potentials without spontaneous activity. However, patients with severe proximal weakness and bulbar involvement may have fasciculations and a reduced recruitment pattern with normal nerve conduction velocities. [7]
  • Acromegaly: 50% of patients have been reported to display myopathic changes on EMG. [7]  
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Histologic Findings

Muscle biopsy is considered mainly to exclude other treatable or congenital muscle diseases, including myotonic dystrophy or congenital myopathies. [59, 60] Histologic changes associated with endocrine myopathies are variable and rarely specific. There is a striated muscle protein that may prove to be a disease progression marker. [61]

Histologic findings are as follows:

  • Steroid myopathy: The characteristic finding is type II (fast-twitch) muscle fiber atrophy. Other changes that have been described include vacuolization and aggregation of mitochondria. [7, 62]
  • Thyrotoxic myopathy: Normal histology versus nonspecific findings.
  • Hypothyroidism-associated myopathy: Nonspecific type II muscle fiber atrophy, type 1 fiber hypertrophy, occasionally with glycogen accumulation.
  • Thyrotoxic periodic paralysis: Vacuolar dilation of the sarcoplasmic reticulum
  • Hyperparathyroid myopathy: Type II fiber atrophy without degeneration. [7]  
  • Osteomalacia: Non-specific, usually minimal findings. [7]  
  • Acromegaly: Isolated muscle fiber necrosis, increased glycogen and satellite cell hypertrophy may be seen. [7]   
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