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Endocrine Myopathies Workup

  • Author: Wayne E Anderson, DO, FAHS, FAAN; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
 
Updated: Aug 25, 2014
 

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 levels may be normal or increased.

  • Hypothyroidism: Creatine kinase usually is elevated markedly.
  • Hyperthyroidism: Creatine kinase usually is normal.
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Imaging Studies

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

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Other Tests

Electromyography (EMG) may reveal the presence of a myopathy, although 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 disease.

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 find 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.
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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.[8, 9] Histologic changes associated with endocrine myopathies are variable and rarely are specific. There is a striated muscle protein that may prove to be a disease progression marker.[10]

  • Hyperthyroidism - Normal histology versus nonspecific findings
  • Hypothyroidism - Nonspecific type II muscle fiber atrophy, occasionally with glycogen storage
  • Steroid myopathy - Nonspecific type II muscle fiber loss, sometimes with lipid storage
  • Thyrotoxic periodic paralysis - Vacuolar dilation of the sarcoplasmic reticulum
  • Corticosteroid therapy - Rapid evolving myopathy with myosin-deficient muscle fibers. (This was reported by al-Lozi et al in 5 patients who received corticosteroid therapy.)[11]
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Contributor Information and Disclosures
Author

Wayne E Anderson, DO, FAHS, FAAN Assistant Professor of Internal Medicine/Neurology, College of Osteopathic Medicine of the Pacific Western University of Health Sciences; Clinical Faculty in Family Medicine, Touro University College of Osteopathic Medicine; Clinical Instructor, Departments of Neurology and Pain Management, California Pacific Medical Center

Wayne E Anderson, DO, FAHS, FAAN is a member of the following medical societies: California Medical Association, American Headache Society, San Francisco Medical Society, San Francisco Medical Society, International Headache Society, California Neurology Society, San Francisco Neurological Society, American Academy of Neurology, California Medical Association

Disclosure: Received honoraria from Teva for speaking and teaching; Received grant/research funds from Allergan for other; Received honoraria from Insys for speaking and teaching; Received honoraria from DepoMed for speaking and teaching.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Neil A Busis, MD Chief of Neurology and Director of Neurodagnostic Laboratory, UPMC Shadyside; Clinical Professor of Neurology and Director of Community Neurology, Department of Neurology, University of Pittsburgh Physicians

Neil A Busis, MD is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine

Disclosure: Nothing to disclose.

Chief Editor

Nicholas Lorenzo, MD, MHA, CPE Founding Editor-in-Chief, eMedicine Neurology; Founder and CEO/CMO, PHLT Consultants; Chief Medical Officer, MeMD Inc

Nicholas Lorenzo, MD, MHA, CPE is a member of the following medical societies: Alpha Omega Alpha, American Association for Physician Leadership, American Academy of Neurology

Disclosure: Nothing to disclose.

Additional Contributors

Dianna Quan, MD Professor of Neurology, Director of Electromyography Laboratory, University of Colorado School of Medicine

Dianna Quan, MD is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine, American Neurological Association

Disclosure: Nothing to disclose.

Acknowledgements

The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous author Ling Xu, MD to the development and writing of this article.

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