Serum creatine phosphokinase determination is the most specific test for muscular dystrophies because elevated levels are indicative of muscle disease. Creatine phosphokinase (CPK) is a tissue-specific enzyme found in significant levels in skeletal muscle, the heart, and the brain. The isoenzyme CPK-MM, or CPK-3, is most specific for skeletal muscle and is tested when muscular dystrophy is a clinical concern. Three elevated creatine phosphokinase levels obtained at 1- month intervals are diagnostic for muscular dystrophy. Early in the disease process, these levels can be 50-300 times greater than normal levels, but the values tend to decrease as the overall muscle mass decreases. Creatine phosphokinase levels are highest in Duchenne muscular dystrophy, with less elevation noted in Becker muscular dystrophy.
The incidence of muscular dystrophy varies, depending on the specific type under consideration. Diagnosis requires a comprehensive medical history, noting particularly the distribution of weakness, age of onset, family history, and disease-specific features. Duchenne muscular dystrophy is the most common muscular dystrophy and is inherited in an X-linked recessive pattern; one-third of these cases are due to a spontaneous new mutation. The prevalence of Duchenne muscular dystrophy is one case per 3500 live male births. Becker muscular dystrophy, also inherited in an X-linked recessive pattern, is the second most common form, with a prevalence of one case per 30,000 live male births. The prevalence of Duchenne muscular dystrophy is about three times higher than that of Becker muscular dystrophy. Other forms of muscular dystrophy include myotonic dystrophy and facioscapulohumeral muscular dystrophy, which have a similar worldwide prevalence among populations of 100,000 persons.
In Duchenne muscular dystrophy, typically no abnormality is noted in the patient at birth. Positive family history warrants close continuity of pediatric medical care. Concerning clinical signs occur at age 18 months to 2 years, and primarily address motor milestones. Patients are late walkers. Muscle weakness may be noted in the hips, pelvis, and legs. A history of difficulty standing and an observed unsteady, waddling gait with trouble climbing stairs may be noted. Caregivers may notice enlarged calf muscles, known as pseudohypertrophy, because these muscle tissues are abnormal and may contain scar tissue. The classic Gowers sign, which is an indicator of proximal muscle weakness, persists after age 3 years. Learning disabilities are also seen in boys with Duchenne muscular dystrophy and occur in attention focusing, verbal skills, and emotional interaction.
Scoliosis is a common problem in patients with muscular dystrophy. It affects all patients with Duchenne muscular dystrophy. Loss of ambulation at age 6 to 12 years heralds the beginning of its development. The scoliosis progresses quickly and can have a significant impact on the respiratory and cardiac systems. Obligate wheelchair use is in place by age 12 years. The spine curvature is usually thoracolumbar or lumbar with associated pelvic obliquity, thoracic kyphosis, and lumbar hyperlordosis. The abnormal sagittal alignment may cause difficulty with seating systems, even modified systems, and the rapid progression of the scoliosis requires frequent wheelchair adjustments to minimize discomfort. Braces are not effective in progressive paralytic or neuromuscular curves, and surgery is often indicated.
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Cite this: Germaine L. Defendi. Fast Five Quiz: Inherited Conditions - Medscape - Oct 29, 2018.