Duchenne Muscular Dystrophy (DMD)


There are a number of tests that can be conducted to reach a diagnosis of muscular dystrophy. These include laboratory studies to assess the levels of creatine kinase, also called creatine phosphokinase (CPK), and multiplex polymerase chain reaction (PCR) to analyze the hot spot exons in the causative dystrophin gene for each type of muscular dystrophy. There are also imaging studies that can be conducted such as ultrasonography. Other tests that may be done to diagnose or follow the progression of muscular dystrophy include electrocardiography (ECG), pulmonary function tests (PFTs), electromyography (EMG), and muscle biopsy.

Laboratory studies

There are several laboratory studies that can be performed to diagnose muscular dystrophy. 

CPK Determination

In most types of muscular dystrophy, serum levels of CPK are elevated. This is because CPK leaks into the bloodstream when muscle tissue is damaged. Although it cannot differentiate between different types of muscular dystrophy, CPK determination allows for the differentiation of muscular dystrophy from other types of neuromuscular conditions such as spinal muscular atrophy (SMA), in which serum CPK levels are usually normal.1 

The test requires a blood sample obtained via venipuncture. The normal values for total CPK are between 10 and 120 µg/L.2

Multiplex Polymerase Chain Reaction

Multiplex PCR can be used to screen for hotspot exon deletions in genes known to cause muscular dystrophy. For example, it can be used to detect deletions in the DMD gene, which lead to Duchenne or Becker muscular dystrophies.3

Quantitative multiplex PCR analysis can also be used to detect carriers of Duchenne and Becker muscular dystrophies.4

Imaging Studies

Imaging studies such as ultrasonography can be used to evaluate muscular changes associated with muscular dystrophy. It is a non-invasive method that allows for easy monitoring of disease progression over time. In muscular dystrophy, affected muscles show increased echogenicity, while the echogenicity of bones is decreased. Some patients may also have an interruption of delineation of the fascia.5,6

In patients with Duchenne muscular dystrophy (DMD), ultrasonography can be performed on the quadriceps, gastrocnemius, and soleus muscles.6

A study by Korean researchers showed that the grade of echogenicity at the mid-thigh level of DMD patients who were unable to rise from a sitting position was higher than in those who could rise, showing that ultrasonography findings also reflect functional abilities.7

Other Tests

Other tests that can be conducted to diagnose or follow the progression of muscular dystrophy as well as assess how the disease is affecting different systems in the body include ECG, PFTs, EMG, and muscle biopsy.

Electrocardiography

Electrocardiography, which records the electrical signals in the heart, is an important tool to measure cardiac health in patients with muscular dystrophy. Cardiac dysfunction occurs in many forms of muscular dystrophy. Electrocardiography, as well as cardiovascular magnetic resonance (CMR), can reveal cardiac involvement and allow for the initiation of cardioprotective medical treatments.8

In DMD and BMD, the most common ECG findings are conduction defects, supraventricular or ventricular arrhythmias, hypertrophy, and myocardial necrosis.

Pulmonary Function Tests

Respiratory dysfunction is also common in many forms of muscular dystrophy, and PFTs can be used to assess the respiratory status of patients at the time of diagnosis. Pulmonary function tests can also allow for monitoring of the progress and course of the disease and give an idea about prognosis.9

Electromyography

Electromyography, which measures the electrical impulse along nerves, nerve roots, and muscle tissue, can help confirm the diagnosis of muscular dystrophy by excluding other neuromuscular conditions such as motor neuron disease, neuromuscular junction disorders, or motor neuropathies. Electromyography can also provide information about the location, type, and severity of the underlying process causing muscle weakness. Finally, the EMG findings can help identify which muscles should be biopsied.10

Muscle Biopsy

Muscle biopsy is a common diagnostic tool that can identify muscular dystrophy. It can also help distinguish between a necrotizing, metabolic, or inflammatory myopathy, thereby allowing the right therapy to be started. 

The immunohistochemical staining of the sample can help identify the protein defect that causes the anomaly and guide the genetic testing necessary to reach a definitive diagnosis of the type of muscular dystrophy.11

Although electromyography often guides the decision on which muscles to biopsy, a sample is usually taken from the deltoid, biceps, and quadriceps muscles. 

There are 2 types of muscle biopsy: an open biopsy and a needle biopsy. 

A multidisciplinary team of specialists should work together to evaluate the biopsy. These include neurologists, surgeons, and pathologists.

References

  1. Spinal muscular atrophy (SMA) – diagnosis. Muscular Dystrophy Association. Accessed June 23, 2021. 
  2. Creatine phosphokinase test. Mount Sinai. Accessed June 23, 2021. 
  3. Singh R, Vijjaya, Kabra M. Multiplex PCR for rapid detection of exonal deletions in patients of Duchenne muscular dystrophy. Indian J Clin Biochem. 2006;21(1):147-151. doi:10.1007/BF02913084 
  4. Ioannou P, Christopoulos G, Panayides K, Kleanthous M, Middleton L. Detection of Duchenne and Becker muscular dystrophy carriers by quantitative multiplex polymerase chain reaction analysis. Neurology. 1992;42(9):1783-1790. doi:10.1212/wnl.42.9.1783
  5. Helmy H, Aboumousa A, Abdelmagied A, Alsayyad A, Nasr SA. The role of muscle ultrasound in helping the clinical diagnosis of muscle diseases. Egypt J Neurol Psychiatr Neurosurg. 2018;54(1):29. doi:10.1186/s41983-018-0039-6
  6. Aizawa H, Kozima S, Takagi A. Ultrasound imaging of muscles in Duchenne muscular dystrophy [in Japanese]. Rinsho Shinkeigaku. 1989;29(1):49-53. 
  7. Na YM, Bae KJ, Kang SW, Kim MY, Kang BC. Ultrasound findings in Duchenne muscular dystrophy disease. J Korean Acad Rehabil Med. 1997;21(3):572-578.
  8. Verhaert D, Richards K, Rafael-Fortney JA, Raman SV. Cardiac involvement in patients with muscular dystrophies: magnetic resonance imaging phenotype and genotypic considerations. Circ Cardiovasc Imaging. 2011;4(1):67-76. doi:10.1161/CIRCIMAGING.110.960740 
  9. Sharma GD. Pulmonary function testing in neuromuscular disorders. Pediatrics. 2009;123(Suppl 4);S219-S221. doi:10.1542/peds.2008-2952D
  10. Paganoni S, Amato A. Electrodiagnostic evaluation of myopathies. Phys Med Rehabil Clin N Am. 2013;24(1):193-207. doi:10.1016/j.pmr.2012.08.017
  11. Joyce NC, Oskarsson B, Jin LW. Muscle biopsy evaluation in neuromuscular disorders. Phys Med Rehabil Clin N Am. 2012;23(3):609-631. doi:10.1016/j.pmr.2012.06.006

Article reviewed by Debjyoti Talukdar, MD on July 1, 2021.