eMedicine Specialties > Orthopedic Surgery > Pediatrics

Muscular Dystrophy

Author: Twee Do, MD, Assistant Professor, Department of Pediatric Orthopedic Surgery, University of Cincinnati College of Medicine; Director, Neuromuscular Orthopedic Services, Cincinnati Children's Hospital Medical Center
Contributor Information and Disclosures

Updated: Mar 31, 2009

Introduction

History of the Procedure

Muscular dystrophy (MD) is a collective group of inherited noninflammatory but progressive muscle disorders without a central or peripheral nerve abnormality. The disease affects the muscles with definite fiber degeneration but without evidence of morphologic aberrations.

The first historical account of MD was reported by Conte and Gioja in 1836.¹ They described 2 brothers with progressive weakness starting at age 10 years. These boys later developed generalized weakness and hypertrophy of multiple muscle groups, which are now known to be characteristic of the milder Becker MD. At the time, however, many thought that Conte and Gioja described tuberculosis; thus, they did not achieve recognition for their discovery.

In 1852, Meryon² reported in vivid details a family with 4 boys, all of whom were affected by significant muscle changes but had no central nervous system abnormality when examined at necropsy. Meryon subsequently wrote a comprehensive monograph on MD and even went on to suggest a sarcolemmal defect to be at the root of the disorder. He further suspected that the disorder is genetically transmitted through females and affects only males.

Guillaume Duchenne was a French neurologist who was already famous for his application of faradism (the use of electric currents to stimulate muscles and nerves) in the treatment of neurologic disorders when he wrote about his first case of MD.³ In 1868, he gave a comprehensive account of 13 patients with the disease, which he called "paralysie musculaire pseudo-hypertrophique." Because Duchenne was already held in high esteem for his work in faradism and for his contributions to the understanding of muscle diseases, one of the most severe and classic forms of MD, Duchenne MD, now bears his name.

The advancement of molecular biology techniques illuminates the genetic basis underlying all MD: defects in the genetic code for dystrophin, a 427-kd skeletal muscle protein (Dp427). These defects result in the various manifestations commonly associated with MD, such as weakness and pseudohypertrophy. Dystrophin can also be found in cardiac smooth muscles and in the brain (accounting for the slight mental retardation associated with this disease).⁴

Problem

Despite minor variations, all types of muscular dystrophy have in common progressive muscle weakness that tends to occur in a proximal-to-distal direction, although there are some rare distal myopathies that cause predominantly distal weakness. The decreasing muscle strength in those who are affected may compromise the patient's ambulation potential and, eventually, cardiopulmonary function.

In addition, structural soft-tissue contractures and spinal deformities may develop from poor posturing caused by the progressive muscle weakness and imbalance, all of which can further compromise function and longevity. Equinovarus contractures start as flexible dynamic deformities and advance to rigid contractures. This altered anatomy prevents normal ambulation, proper shoe wear, and transfers (how patients can be picked up to transfer out of their chair).

Once wheelchair bound, patients with MDs tend to develop worsening contractures and rapidly progressive scoliosis. On average, for each 10° of thoracic scoliosis curvature, the forced vital capacity (FVC) decreases by 4%.⁵ In a patient with an already-weakened cardiopulmonary system, this decrease in FVC could rapidly become fatal.

The goal of orthopedic management is, therefore, to preserve or prolong patients' ambulatory status for as long as possible. This goal can be achieved with soft-tissue releases for contractures and early stabilization of the spine.

Frequency

United States

The incidence rates of muscular dystrophies vary depending on the specific type. Duchenne MD is the most common MD and is sex-linked, with an inheritance pattern of 1 case per 3500 live male births.^6,7 One third of cases occurs as a result of spontaneous new mutations.⁸ Becker MD is the second most common form, with an incidence of 1 case per 30,000 live male births.⁹ Other types of MD are rare. For example, limb-girdle dystrophy occurs in only 1.3% of patients with MDs.

International

The incidence internationally is similar to that of the US for most of the dystrophies, except for the oculopharyngeal type, which is more common in French Canadians than in other groups.¹⁰ Distal MD tends to occur in Sweden.

Etiology

Classification of types of muscular dystrophy 

The etiology of MD is an abnormality in the genetic code for specific muscle proteins.¹¹ They all are classified according to the clinical phenotype, the pathology, and the mode of inheritance. The inheritance pattern includes the sex-linked, autosomal recessive, and autosomal dominant MDs. Within each group of heritable MDs (see below), several disorders exist. These are characterized by the clinical presentation and pathology.

Heritable MDs include the following:

• Sex-linked MDs
  • Duchenne
  • Becker
  • Emery-Dreifuss
• Autosomal dominant MDs
  • Facioscapulohumeral
  • Distal
  • Ocular
  • Oculopharyngeal
• Autosomal recessive MD – limb-girdle form

Genetic defects and dystrophin

In the X-linked forms of MD, such as the Duchenne and Becker dystrophies, the defect is located on the short arm of the X chromosome.¹² Hoffman and coworkers identified the locus of the defect in the Xp21 region, which includes approximately 2 million base pairs.⁵ The gene codes for Dp427, which is a component of the cytoskeleton of the cell membrane.

Dystrophin is distributed not only in skeletal muscle but also in smooth and cardiac muscles and in the brain. The large size of the dystrophin gene explains the ease at which spontaneous new mutations can occur, as in Duchenne MD. The large size also allows mistakes in protein synthesis to occur at multiple sites.

Defects that interfere with the translation reading frame or with the promoter sequence that initiates synthesis of dystrophin lead to an unstable, ineffective protein, as in Duchenne MD. Disruption of the translation process further down the sequence leads to production of proteins of lower molecular weight that, although present, are less active and result in the milder variety of Becker MD.

As with Duchenne MD, Emery-Dreifuss MD is a sex-linked recessive disorder, but its defect is localized to the long arm of the X chromosome at the q28 locus.¹³ Some authors, however, have cited case reports of similar findings in Emery-Dreifuss that were transmitted in an autosomal dominant pattern.¹⁴ However, this finding is more of an aberration than a normal observation in Emery-Dreifuss MD.

In autosomal recessive conditions such as limb-girdle MD, the genetic defect is localized to the 13q12 locus.

In the autosomal dominant facioscapulohumeral MD, the defect is at the 4q35 locus. In distal MD, it is at the 2q12-14 loci.¹⁵

Pathophysiology

Multiple proteins are involved in the complex interactions of the muscle membrane and extracellular environment. For sarcolemmal stability, dystrophin and the dystrophin-associated glycoproteins (DAGs) are important elements.^16,17

The dystrophin gene is located on the short arm of chromosome X near the p21 locus and codes for the large protein Dp427, which contains 3685 amino acids. Dystrophin accounts for only approximately 0.002% of the proteins in striated muscle, but it has obvious importance in the maintenance of the muscle's membrane integrity.⁵ Dystrophin aggregates as a homotetramer at the costomeres in skeletal muscles, as well as associates with actin at its N-terminus and the DAG complex at the C-terminus, forming a stable complex that interacts with laminin in the extracellular matrix. Lack of dystrophin leads to cellular instability at these links, with progressive leakage of intracellular components; this results in the high levels of creatine phosphokinase (CPK) noted on routine blood workup of patients with Duchenne MD.

Less-active forms of dystrophin may still function as a sarcolemmal anchor, but they may not be as effective a gateway regulator because they allow some leakage of intracellular substance. This is the classic Becker dystrophy. In both Duchenne and Becker MD, the muscle-cell unit gradually dies, and macrophages invade. Although the damage in MD is not reported to be immunologically mediated, class I human leukocyte antigens (HLAs) are found on the membrane of dystrophic muscles; this feature makes these muscles more susceptible to T-cell mediated attacks.

Selective monoclonal antibody hybridization was used to identify cytotoxic T cells as the invading macrophages; complement-activated membrane attack complexes have been identified in dystrophic muscles as well. Over time, the dead muscle shell is replaced by a fibrofatty infiltrate, which clinically appears as pseudohypertrophy of the muscle. The lack of functioning muscle units causes weakness and, eventually, contractures.

Other types of MDs are caused by alterations in the coding of one of the DAG complex proteins. The gene loci coding for each of the DAG complex proteins is located outside the X chromosomes. Gene defects in these protein products also lead to alterations in cellular permeability; however, because of the slightly different mechanism of action and because of the locations of these gene products within the body, there are other associated effects, such as those in ocular and limb-girdle type dystrophies.

Presentation

In Duchenne muscular dystrophy, 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 when they begin to walk, when they lose their ability to ambulate, and when they die.¹⁸

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.¹⁹ Psychometric studies have since revealed a definitively lower intelligence quotient (IQ) in patients with Duchenne MD despite equalization of educational opportunities.²⁰ The average IQ in patients with Duchenne MD is 85 points, compared with 105 points in healthy populations, as determined by using the Wechsler Adult Intelligence Scale (WAIS).^6,19,20

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,⁶ 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 his or her 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 his or her tiptoes because it is easier to stay vertical with an equinus foot position than on a flat foot, although no real tendo Achillis contracture exists at this early point.

Gradually, noticeable difficulty with step taking by the child is observed. 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 Gower sign (see below).

The Gower 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 Gower sign is a classic physical examination 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. If children with MD are still ambulating after age 13 years, the diagnosis of Duchenne MD should be questioned, because these patients usually have Becker MD, the milder form of MD.

In Emery's work,⁷ 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 recommend posterior spinal fusion at 20° when the vital capacity is at its best,^21,6,9,22,23 However, recent and other reports showed that respiratory function after spinal fusion did not significantly differ.^{24,25,26,27,28} 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).⁶ 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.

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. Electrocardiogram (ECG) tracings show right ventricular strain, tall R waves, deep Q waves, and inverted T waves.²⁹

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 to 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, although it is more common in late adolescence.

Autosomal dominant oculopharyngeal dystrophy appears in those aged 20 to 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 affects longevity.

Indications

The indications for any operative intervention in patients with muscular dystrophy include making a diagnosis by means of muscle biopsy or prolonging the patient's function and/or ability to ambulate by specific procedures.

Until the advent of molecular biology techniques, muscle biopsy was the definitive test for diagnosing and confirming MDs. The histologic changes found in MDs depend on the stage of the disease and the muscle selected, of which the optimal site is the vastus lateralis, wherein a small lateral thigh incision is made.

Other indicated procedures include tendo Achillis and iliopsoas tenotomies for ease of fit into braces, tibialis posterior tendon transfers or tenotomies for more rigid equinovarus deformities of the foot, and segmental spinal stabilization for rapidly developing scoliosis (see Treatment, Surgical therapy).

Relevant Anatomy

The overall status of any patient must be considered before operative intervention is undertaken, and it becomes especially important in patients with muscle weakness, as in MD. For example, posterior spinal fusion to the pelvis straightens the scoliosis and allows better upright sitting balance. However, in patients with low vital capacity (<30%), the risks of pulmonary complications are much higher, and these risks may tip the scale in favor of not operating on the scoliosis.

Other examples include equinus contractures in patients who are very weak; tendon lengthening itself is necessarily a weakening procedure on the involved muscle. If the patient has to maintain a rigid equinus foot position for stability of gait and the tendon is lengthened by surgery, the patient will not be able to ambulate.

After scoliosis surgery, patients may need additional pulmonary support and an extended stay in the intensive care unit (ICU). Preoperative tracheostomy is usually not any more effective in early mobilization of dystrophic patients; if necessary, this procedure is performed only after the patient's condition has been stabilized and after a mold has been obtained for a hard brace with chest and abdominal cutouts.

Contraindications

In patients with muscular dystrophy, some relative contraindications to surgery include obesity, rapidly progressive muscle weakness, poor cardiopulmonary status, and a patient's lack of motivation for participating in postoperative rehabilitation programs.

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