Neurodegenerative Disorders

CHAPTER 63

Neurodegenerative Disorders

Duchenne Muscular Dystrophy

INTRODUCTION/ETIOLOGY/EPIDEMIOLOGY

• Duchenne muscular dystrophy (DMD) is an X-linked recessive condition (boys affected) characterized by progressive muscle atrophy and weakness, caused by absence of dystrophin.

• Without dystrophin, the sarcolemma of muscles (crucial for the stability of the cell membrane) is not protected from injury during forceful contractions; as a result, muscle fibers become necrotic and inflamed and are replaced by fat and fibrous tissue.

• Prevalence is 1 per 3,500 boys.

• Rarely, DMD occurs in girls with X inactivation or Turner syndrome.

• Family history is positive in approximately 65% of cases.

SIGNS AND SYMPTOMS

• Becomes clinically evident between 3 and 6 years of age

• Muscle weakness develops symmetrically and is first noticed in the proximal muscles, often the hip extensors.

• Early symptoms include toe walking, delayed ambulation, frequent tripping and falling, and difficulty with running and climbing stairs.

• Later in the disease

— Contractures develop, especially of the hip abductors, then hip and knee flexors and ankle dorsiflexors.

— Progressive inability to walk and ultimate dependence on mobile devices or wheelchair by 7 to 16 years of age

— Scoliosis develops in about 95% of patients with DMD (Figure 63-1).

■Scoliosis progresses rapidly, especially after a child loses walking ability.

■Curve patterns are long, sweeping, and associated with pelvic obliquity (wheelchair sitting is an issue).

■Spine radiographs are indicated in nonambulatory patients and those with spinal asymmetry.

Figure 63-1. Progression of scoliosis in Duchenne muscular dystrophy over a 5-year period after cessation of walking ability. A, 12 degrees. B, 38 degrees. C, 105 degrees.

— Muscle pain accompanies the progressive physical disability, leading to diminished participation and enjoyment of activities of daily living. Pain may also result from overly zealous passive range of motion therapy and prolonged sitting with inability to shift weight for comfort.

• Physical examination findings

— Ankle equinus is an early overt sign of DMD, leading to toe walking.

— Waddling, wide-based gait because of weakness and an attempt to attain stability

— Pseudohypertrophy of the calves, which results from replacement of muscle tissue by fat and fibrous tissue (Figure 63-2)

— Gowers sign is present in DMD—difficulty rising from a seated position on the floor without using arms to push hips and knees into extension because of weakness of the pelvic girdle and proximal thigh muscles (see Chapter 4, Physical Examination, Figure 4-3). Some describe this as the child “walking” the hands up the legs to raise the trunk to an upright position from the floor.

— Positive Trendelenburg sign (see Figure 4-24) is also common—when the child stands on one leg, there is a drop in the non–weight-bearing hemipelvis, indicating a weakness of the gluteal muscles and hip abductors of the standing leg.

— Meryon sign also is a common physical finding, wherein the child slips through when the examiner attempts to lift from under the axilla.

Figure 63-2. Pseudohypertrophy of the calf muscles in a child with Duchenne muscular dystrophy.

• Associated non-musculoskeletal findings

— More than 90% of children with DMD have abnormal electrocardiography findings such as sinus tachycardia, cardiac hypertrophy, and diminished QRS complex.

— Mitral valve prolapse is also a characteristic finding because of papillary muscle involvement.

— Reduced pulmonary function, including diminished expiratory muscle strength

— Mild intellectual disability is common, with the average IQ approximately 80.

DIFFERENTIAL DIAGNOSIS

• Becker muscular dystrophy (BMD)

— Presents at a later age with less severe symptoms

— Dystrophin levels are diminished, not absent as in DMD.

• Facioscapulohumeral muscular dystrophy

— Occurs in either sex

— Mild involvement

— Usually presents in the second decade

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