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Neuromuscular Disorders

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A 65-year-old man was diagnosed with lung cancer 6 months ago. Over the past 2 months, he has had worsening severe proximal muscle weakness. He is most likely to have which of the following?

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a. Dermatomyositis

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b. Trichinosis

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c. Multiple sclerosis (MS)

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d. Progressive multifocal leukoencephalopathy (PML)

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e. Myasthenia gravis

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The answer is a. Dermatomyositis occurs as a paraneoplastic syndrome in about 15% of cases overall. Among those older than age 40, the proportion of paraneoplastic cases increases to 40% for women and 66% for men. Tumors underlying dermatomyositis may develop in the lungs, ovaries, gastrointestinal tract, breasts, or other organs, but the CNS is generally not the site of a tumor associated with dermatomyositis. Because of the higher probability of malignancy in adults with dermatomyositis, patients diagnosed with this inflammatory disease should routinely undergo a variety of diagnostic studies, including rectal and breast examinations, periodic screens for occult blood in the stool, and hemograms. Sputum cytologies, full-body computed tomography (CT) scan, and urine cytologic studies are recommended by some physicians. Both PML and MS are strictly CNS diseases. Trichinosis is a parasitic disease that involves skeletal muscle and may produce substantial weakness, but it is not associated with any tumors.

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A 2-year-old male child has recently been diagnosed with the most frequent type of muscular dystrophy. The parents are highly educated people, but not in the medical field. They have many specific and detailed questions. Which abnormal gene is responsible for their child's condition?

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a. Glucose-6-phosphatase

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b. Hexosaminidase B

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c. Myosin

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d. Dystrophin

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e. Actin

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The answer is d. Duchenne dystrophy has been incontrovertibly linked to the gene, located on the X chromosome, that makes dystrophin. The more profound the disturbance of this gene, the earlier the disease becomes symptomatic. The gene for dystrophin has single or multiple deletions in affected children. Women who are probable carriers of the defective gene can be checked for heterozygosity and given genetic counseling. Chorionic villus biopsy at 8 to 9 weeks can determine whether a fetus that is at risk for the deletion actually carries it.

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A 67-year-old woman has noticed blurry vision and weakness over the past 4 months. Her symptoms are always worse toward the end of the day. She undergoes a neuromuscular evaluation, including nerve conduction study (NCS)/electromyography (EMG), which shows a decrementing response of compound muscle action potential to ...

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