Laboratory Studies
After a thorough history has been taken and a physical examination has been performed, a diagnosis of BMD may be confirmed with the following lab study sequence:
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Serum creatine kinase shows moderate to severe elevation (that is, 5-100 times the normal level)
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Dystrophin gene deletion analysis shows specific exon deletions in about 98% of cases; test methods include the multiplex polymerase chain reaction assay, southern blot analysis, and fluorescent in situ hybridization
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Muscle biopsy with dystrophin antibody staining demonstrates the presence of dystrophin in variable percentages; this may be helpful in the young child with no maternal history
A study by Zhang et al indicated that serum creatinine levels are significantly higher in patients with BMD than in those with DMD. The study was conducted using biochemical and genetic data, as well as Vignos scale scores (used to assess motor function), from 212 boys with dystrophinopathy. [20]
Laboratory evaluation is generally confirmatory of BMD if the patient possesses a phenotype that is consistent with muscular dystrophy and has a family history of the Becker form of the disease. Laboratory and phenotypic expression confirm sporadic cases. A clinical picture of muscular dystrophy, coupled with a preserved ambulatory status beyond age 16 years, is consistent with a diagnosis of BMD.
Genetic testing and next-generation sequencing technology may aid in diagnosis. [21, 22] Other laboratory studies that may be indicated include the following:
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Liver function screen for aspartate transaminase and alanine transaminase
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Muscle biopsy
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Standard histology
Imaging Studies
Spinal radiographs may be performed to follow the progression of scoliosis, particularly during adolescence.
A report by Marty et al indicated that quantitative cardiac magnetic resonance imaging (MRI) is an effective means of assessing structural and functional myocardial impairments in patients with BMD, with 26% of patients in the study demonstrating a significant reduction in the left ventricular ejection fraction, as well as a greater dyssynchrony index than controls. Moreover, native T1, T2, and extracellular volume fraction were significantly higher in patients with BMD than in controls, serving as quantitative biomarkers associated with inflammation and fibrosis. [23]
Other Tests
See the list below:
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Electromyography may be indicated. [24]
Expect normal nerve conduction with possible borderline-to-low motor evoked responses.
Expect increased insertional activity with myopathic motor unit action potentials (ie, short duration, low-to-normal amplitude, rapid recruitment, decreased units).
An electrodiagnostic study will facilitate a distinction between a muscular and a primary nerve process (eg, anterior horn cell disease, hereditary polyneuropathies).
Electromyography also may assist in identifying which muscle groups would be optimal for biopsy.
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An electrocardiogram/echocardiogram may show cardiomyopathy and/or arrhythmia. Dilated cardiomyopathy manifests after age 20 years; the risk progressively increases with age.
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Pulmonary function testing may reveal bellows failure caused by progressive weakness.
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Associated restrictive disease may be seen with scoliosis or a poorly compliant chest.
Histologic Findings
Standard muscle biopsy alone does not support a diagnosis of BMD. Histologic changes — specifically, findings of degenerating muscle fibers, a variation in fiber size, focal necrosis, regeneration, and a proliferation of connective tissue, as well as fatty replacement of degenerated muscles — point to a muscular dystrophy.
