History and Physical Examination
In Duchenne muscular dystrophy (MD), unless a sibling has been previously affected to warrant a high index of suspicion, no abnormality is noted in the patient at birth, and manifestations of the muscle weakness do not begin until the child begins to walk. Three major time points for patients with Duchenne MD are as follows [15] :
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When they begin to walk
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When they lose their ability to ambulate
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When they die
Phases of Duchenne muscular dystrophy
The child's motor milestones may be at the upper limits of normal, or they may be slightly delayed. Some of the delays may be caused by inherent muscle weakness, but a component may stem from brain involvement.
Although the association of intellectual impairment in MD has long been recognized, it was initially thought to be a result of limited educational opportunities. [16] Psychometric studies have since revealed a definitively lower intelligence quotient (IQ) in patients with Duchenne MD despite equalization of educational opportunities. [17] The average IQ in patients with Duchenne MD is 85 points on the Wechsler Adult Intelligence Scale (WAIS), compared with 105 points in healthy populations. [10, 16, 17, 18]
In addition to mental deficits, another milestone delay is the patient's age at ambulation. Children with Duchenne MD usually do not begin to walk until about age 18 months or later. In the Dubowitz study, [10] 74% of children with Duchenne MD manifested the disease by age 4 years. By age 5 years, awareness increases as the disease is manifested in all affected children when they experience difficulty with school-related activities (eg, getting to the bus, climbing stairs, reciprocal motions during activities).
Other early features include a gait abnormality, which classically is a waddling, wide-based gait with hyperlordosis of the lumbar spine and toe walking. The waddle is due to weakness in the gluteus maximus and gluteus medius muscles and the patient's inability to support a single-leg stance. The child leans the body toward the other side to balance the center of gravity, and the motion is repeated with each step.
Hip extensor weakness also results in a forward tilt of the pelvis, which translates to a hyperlordosis of the spine to maintain posture. The child then walks on tiptoes because it is easier to stay vertical with an equinus foot position than on a flat foot, though no real tendo Achillis contracture exists at this early point.
Gradually, the child is observed to have increasingly noticeable difficulty with step taking. Frequent falls without tripping or stumbling often occur and are described as the feet being swept away from under the child. The child then begins having problems getting up from the sitting or supine position, and he or she can rise to an upright stance only by manifesting the Gowers sign.
The Gowers sign is a classic physical examination finding in MD and results from weakness in the child's proximal hip muscles. To get up from a sitting or supine position, the child must first become prone on the elbows and knees. Next, the knees and elbows are extended to raise the body. Then, the hands and feet are gradually brought together to move the body's center of gravity over the legs. At this point, the child may release one hand at a time and support it on the knee as he or she crawls up their legs to achieve an upright position.
Although the Gowers sign is a classic finding in Duchenne MD, it is by no means pathognomonic; other types of MD and disorders with proximal weakness may also cause this sign.
While still ambulatory, the child may have minimal deformities, including iliopsoas or tendo Achillis tightness. Mild scoliosis may be present if the child has an asymmetrical stance. Upper-extremity involvement rarely occurs in the beginning, although proximal arm muscle weakness may be evident on manual strength testing. When upper-extremity involvement manifests in later stages of Duchenne MD, it is symmetrical and, along with distal weakness, usually follows a rapid worsening of the child's condition toward being wheelchair bound.
The second important phase in Duchenne MD is the loss of ambulation. This usually occurs between the ages of 7 and 13 years, with some patients becoming wheelchair-bound by age 6 years. With early initiation of steroid treatment, prolongation of ambulation potential has been well documented. However, If children with MD are still ambulating well after the age of 13 years, the diagnosis of Duchenne MD should be questioned, because these patients may have Becker MD, the milder form of MD.
In Emery's work, [11] the 50th percentile for loss of ambulation in patients with Duchenne MD was age 8.5 years, with the 95th percentile at 11.9 years and the 99th percentile at 13.2 years. With the child's loss of ambulation, there is usually a rapidly progressive course of muscle or tendon contractures and scoliosis.
Most authors have recommended posterior spinal fusion at 20° when the vital capacity is at its best. [19, 10, 13, 20, 21] However, some reports showed that respiratory function after spinal fusion did not significantly differ. [22, 23, 24, 25, 26] The investigators concluded that respiratory failure resulted from muscle weakness and not the mechanical bellows of the chest cage, as was previously assumed.
Duchenne MD is a terminal disease in which death usually occurs by the third decade of life (mostly from cardiopulmonary compromise), despite steroid treatment. [10] The most common inciting event is a respiratory infection that progresses extremely rapidly despite its initial benign course. The resultant respiratory failure can easily occur from the underlying progressive nocturnal hypoventilation and hypoxia or from an acute cardiac insufficiency.
Additional clinical findings in Duchenne muscular dystrophy
Other clinical findings in Duchenne MD include absent deep tendon reflexes in the upper extremities and patella (though the tendo Achillis reflex remains intact even in the later stages of this disease), pain in the calves with activity (< 30% of patients), pseudohypertrophy of the calf (60%), and macroglossia (30%). Cardiopulmonary involvement is present from the beginning of the disease stages, but the findings are not so clinically obvious. Electrocardiographic (ECG) tracings show right ventricular strain, tall R waves, deep Q waves, and inverted T waves. [27]
Other types of muscular dystrophy
Becker MD is similar to Duchenne MD, but because patients have some measure of functioning dystrophin, the manifestations of Becker MD occur later and are more mild. Patients tend to live past the fourth or fifth decades.
Emery-Dreifuss MD is an uncommon sex-linked dystrophy that presents with early contractures and cardiomyopathy in affected patients; the typical presentation involves tendo Achillis contractures, elbow flexion contractures, neck extension contractures, tightness of the lumbar paravertebral muscles, and cardiac abnormalities. Death may occur in the fourth or fifth decade as a result of first-degree atrioventricular (AV) block, a condition that is usually not present at the initial presentation of this disease.
Autosomal dominant distal MD is a rare form of MD and tends to become apparent in those aged 30-40 years; it is more commonly found in Sweden than in any other country and can cause a mild weakness that affects the arms before the legs.
Autosomal dominant facioscapulohumeral dystrophy causes facial and upper-extremity weakness, and scapulothoracic motion is decreased, with winging of the scapula. This type of dystrophy can occur in both sexes and appear at any age, though it is more common in late adolescence.
Autosomal dominant oculopharyngeal dystrophy appears in those aged 20-30 years. The pharyngeal muscle involvement leads to dysarthria and dysphagia, which may necessitate palliative cricopharyngeal myotomy. The ocular component comprises ptosis, which may not become obvious until the patient's mid life.
None of the autosomal dominant conditions significantly affect longevity.
Complications
Complications of MD usually include the following:
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Contractures and early wheelchair dependence, even in patients who develop minor musculoskeletal injuries (eg, ankle sprain) and are immobilized
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Weakness - Prolonged immobilization worsens the clinical weakness caused by MD and ultimately renders the patient nonambulatory; this exacerbates any developing contractures and potentiates the development of osteopenia or osteoporosis that can lead to limb and vertebral fractures
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Osteoporosis and fractures (see Management of Osteoporosis and Fractures)
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Scoliosis
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Cardiopulmonary failure
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Schematic of the sarcomere with labeled molecular components that are known to cause limb-girdle muscular dystrophy or myofibrillar myopathy. Mutations in actin and nebulin cause the congenital myopathy nemaline rod myopathy, and the mutations in myosin cause familial hypertrophic cardiomyopathy. Image courtesy of Dr F. Schoeni-Affoher, University of Friberg, Switzerland.
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Gomori trichrome–stained section in patient with myofibrillar myopathy. Note the abnormal accumulations of blue-red material in several muscle fibers.
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Left: The photomicrograph is a muscle biopsy with normal emerin immunostaining. Right: The micrograph is from a patient with X-linked Emery-Dreifuss muscular dystrophy. Note the absence of nuclear staining as well as the hypertrophied and atrophied muscle fibers.
