Steroid myopathy is a condition characterized by weakness of the proximal muscles of the arms and legs and the flexor muscles of the neck. It can be caused by excessive endogenous or exogenous steroids and can be either acute in onset or chronic.
Steroid myopathy (SM) was first described in 1932 by Cushing [1]. It can be caused by excessive endogenous or exogenous corticosteroids. Adrenal tumors are associated with excessive endogenous corticosteroid production while excessive exogenous corticosteroid is due to steroid treatment for medical conditions like asthma, chronic obstructive pulmonary disease, and autoimmune diseases such as polymyositis, connective tissue disorders, and rheumatoid arthritis. [2] [3] [4]. On an average, 12% of patients on steroid treatment develop SM [5] with glucocorticoid-induced myopathy being the commonest drug-induced myopathy [6]. Risk factors for the development of SM include the advanced age, patients with cancer, patients with respiratory muscle weakness, debilitation and physical inactivity.
SM can develop either acutely or insidiously. Acute SM is the rapid onset of progressive generalized weakness as well as proximal and distal muscles weakness [7] including respiratory muscle weakness occurring within a week after the onset of treatment with high-dose corticosteroids [8]. It is commonly seen in patients admitted to intensive care who are on mechanical ventilation with neuromuscular blockade. In chronic SM, there is an insidious onset of progressive proximal muscle weakness in the arms and legs. Pelvic girdle muscles are affected more than the arms with sparing of the sphincteric muscles and the muscles innervated by the cranial nerves. Patients may be unable to rise from chairs or climb stairs or raise their arms above their heads to perform activities. With time, myalgias become constant and studies have reported pulmonary symptoms secondary to weakness of the respiratory muscles like the diaphragm [8]. The pattern of SM is identical in iatrogenic SM as well as endogenous SM [5].
The workup in SM is focused on differentiating between endogenous hypercortisolism and exogenous glucocorticoid use. Laboratory tests include creatine kinase, lactate dehydrogenase (LDH), and aldose. In acute SM, patients have elevated levels of serum creatine kinase and aldolase [5] [9] with myoglobinuria. On the other hand, in chronic SM the serum levels of creatine kinase are normal but the urine creatinine clearance increases dramatically and can even appear several days prior to the appearance of symptoms of myopathy [2]. Myoglobinuria and rhabdomyolysis are absent in chronic SM.
Electromyography (EMG) and nerve conduction studies (NCS) in acute SM may reveal abnormal spontaneous activity, early recruitment, and small, polyphasic motor units as well as findings suggestive of neuropathy subsequent to high-dose corticosteroid treatment [10] [11].
Muscle biopsy in acute SM shows focal and diffuse necrosis of all types of muscle fibers without the involvement of type II fibers. In chronic SM, the muscle biopsy demonstrates specific atrophy of type IIb muscle fibers, an absence of inflammatory infiltrate, variations in fiber size with centrally placed nuclei, and occasional muscle necrosis [9].