A 21-year-old Caucasian male suffers from weakness and gait disturbance. He noticed that he has had difficulty releasing the doorknob lately. Physical examination shows cataracts, frontal baldness, and gonadal atrophy. Biopsy shows muscle atrophy involving mostly type 1 fibers. The patient's condition is best described as which of the following?

Question

A 21-year-old Caucasian male suffers from weakness and gait disturbance.  He noticed that he has had difficulty releasing the doorknob lately.  Physical examination shows cataracts, frontal baldness, and gonadal atrophy.  Biopsy shows muscle atrophy involving mostly type 1 fibers.  The patient's condition is best described as which of the following?

Quizplus · Accepted Answer

Abnormally slow relaxation of muscles is called myotonia. Classic symptoms of myotonic muscular dystrophy are difficulty loosening one's grip after a handshake or inability to release the doorknob. Myotonic muscular dystrophy is the second most common inherited muscle disorder (Duchenne muscle dystrophy is the most common). Myotonic dystrophy is inherited as an autosomal dominant trait. It occurs because of an abnormal trinucleotide repeat expansion (CTG) of the gene that codes for a myotonia-protein kinase. The number of repeats increases with subsequent generation, causing increasingly severe disease in each generation (anticipation). Age of presentation varies from early childhood to late adulthood. Skeletal muscle involvement includes myotonia (sustained muscle contraction). Cataracts are seen in almost all patients-frontal balding and gonadal atrophy are other common features. On light microscopy, atrophy of muscle fibers is prominent. Type 1 fibers are more affected. Unlike Duchenne muscular dystrophy, necrosis of muscle fibers and fibrofatty replacement are not common in myotonic dystrophy. (Choice A) Denervation muscle atrophy is caused by axonal destruction. It occurs due to trauma, ischemia, or generalized disease (such as amyotrophic lateral sclerosis). Clinically, denervation leads to muscle paralysis and atrophy. (Choice C) Dermatomyositis and polymyositis are examples of inflammatory myopathies. (Choice D) Ion channel myopathies manifest with myotonia and episodes of hypotonic paralysis (often associated with exercise). No muscle atrophy is seen on light microscopy. PAS-positive intracytoplasmic vacuoles are found in these conditions. (Choices E and F) Neither myasthenia gravis, nor mitochondrial myopathies lead to myotonia with preferential atrophy of type 1 fibers. Educational Objective: Myotonic dystrophy is an autosomal-dominant disorder. It is caused by an increased number of trinucleotide repeats on myotonia-protein kinase gene. Sustained muscle contraction (myotonia), along with weakness and atrophy, is common. Cataracts are seen in almost all patients. Frontal balding and gonadal atrophy are other common features.

A 21-Year-Old Caucasian Male Suffers from Weakness and Gait Disturbance