A 25-year-old man comes to your office complaining that “I feel weak when I do my exercise routine in the morning. I like to work out hard like a soldiere.” He also tells you that “some times my urine is red and it scares me.” After some discussion, he informs you that he takes a rest period when the weakness occurs and the symptoms resolve. But, the symptoms reap pear when he resumes exercise. The man is obviously frustrated and says, “Doc, I can’t stay in shape if this keeps happening.” What is the most likely diagnosis?
Differentials • Anemia, muscular dystrophy, myasthenia gravis, polymyositis
Relevant Physical Examination Findings • Burning pain, cramping, and exhaustion in legs and arms during strenuous exercise • Normal muscle tone and strength bilaterally • No enlargement of the liver or spleen
Relevant Lab Findings • Blood chemistry: creatine phosphokinase (CPK) 100 U/L (high); hematocrit (Hct) 45% (normal); hemoglobin (Hgb) 15.2 g/dL (normal); blood urea nitrogen (BUN) 15 mg/dL (normal) • Urinalysis: red blood cells (RBCs) 20–30 (high); glucose high • Diagnostic anaerobic exercise test showed muscle pain, cramping, and exhaustion; increase in CPK; and no increase in blood lactate. • Muscle biopsy showed elevated glycogen content and low phosphorylase activity.
Diagnosis: McArdle Disease
• McArdle Disease. McArdle disease (type V glycogenosis) is an autosomal recessive dis ease and results from a deficiency in muscle glycogen phosphorylase, causing exercise induced muscle pain and cramping. Myoglobulinuria results from a breakdown of muscle protein in an attempt to liberate amino acids for conversion to glucose. Myoglobin con tains heme, binds oxygen, and provides oxygen to muscle for oxidation. BUN and meas ures the ability of the kidney to excrete the nitrogenous waste products produced by the body.
• Duchenne muscular dystrophy (DMD) is a genetic disease that shows X-linked recessive inheritance. The DMD gene is located on the short (p) arm of chromosome X in band 21 (i.e., Xp21) and encodes for the dystrophin protein. Dystrophin anchors within skeletal muscle fibers to the extracellular matrix, thereby stabilizing the cell membrane. A muta tion of the DMD gene alters the normal function of dystrophin, leading to progressive mus cle weakness and wasting. The muscle weakness begins as early as 1 year of age.
• Myasthenia gravis is an autoimmune disease characterized by circulating antibodies against the nACh receptor (nicotinic acetylcholine receptor; anti-nAChR) and decreased number of nACh receptors. Clinical findings of myasthenia gravis include muscle weakness that fluctuates daily or even within hours, and extraocular muscle involvement with ptosis and diplopia being the first disability.
• Polymyositis is a connective tissue disease. Clinical findings of polymyositis include pro gressive, bilateral weakness of the proximal muscles.