Disorders of Muscle

Abstract

The chapter begins with a discussion of the major types of muscle: skeletal muscle, smooth muscle, and cardiac muscle. Then, it discusses ways to distinguish muscle weakness from other causes of weakness. The chapter focuses on four broad categories of muscle diseases: muscular dystrophy due to genetic abnormalities, channelopathies with abnormal sodium, calcium or potassium membrane ion channels, inflammatory myopathies, and secondary endocrine myopathies. The chapter ends with a complete discussion of four important types of muscle diseases: Duchenne muscular dystrophy, dermatomyositis, statin myotoxicity, and primary hyperkalemic periodic paralysis. For each disease, attention is paid to their pathophysiology, major clinical features, major laboratory findings, and principles of management and prognosis.

A mother brings her 2-year-old toddler in for evaluation to the pediatrician with the complaint that her son is leaning forward while walking. She reports that he was the product of a normal birth and largely normal development. He reached early milestones (holding head up and rolling over) at the normal times, but his walking was delayed until 16 months of age. She is now concerned that he is walking “funny”. On exam, the little boy is alert and his sitting posture is normal. When he begins to walk, he has a waddling gait with excessive lordosis in his trunk. The pediatrician is concerned and orders blood tests including creatine kinase (CK).

Video Legend

This video shows a 9 year-old boy with Duchenne Muscular Dystrophy.

Segment 1: Motor Exam

• Gower’s maneuver getting up from floor. Due to the proximal muscle weakness, he begins on hands and knees with feet wide apart. Then straightens his legs and braces his hands on thighs in order to stand.

• Difficulty leaving the ground while jumping due to proximal leg weakness.

• Pseudohypertrophy of calf muscles is seen.

Segment 2: Gait Exam

• Waddling gait due to weakness of hip girdle muscles.