Endocrine Myopathies Workup
- Author: Wayne E Anderson, DO, FAHS, FAAN; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE more...
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.
Imaging studies neither confirm nor exclude the presence of muscle disease. They may be of benefit in the diagnosis of endocrine disorders.
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.
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.
- 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.)
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