Diagnosis and Monitoring
Several types of tests can be used to diagnose and monitor patients with muscular dystrophy (CDC 2012; NINDS 2011):
- Creatine kinase (CK) testing, also known as creatine phosphokinase (CPK), is the most specific test for muscular dystrophy. CK is an enzyme that helps regulate cellular energy and is particularly concentrated in muscle cells, where a lot of power needs to be produced. In patients with muscular dystrophy, the muscle cells become damaged and release creatine kinase into the blood stream. In DMD and BMD patients, blood creatine kinase levels are at least 10–20 times higher than in normal individuals, and often much higher (Bushby 2005). Early in the disease process, CK levels are 50–300 times greater than normal levels, but the levels tend to decrease as muscle mass decreases. All patients suffering from muscular dystrophy incur CK elevation during active disease, and the finding of 3 elevated levels obtained one month apart is diagnostic for the disease.
- Patients with muscular dystrophy also have higher blood levels of aldolase, another enzyme concentrated in muscle cells, which is involved in the cellular metabolism of fructose and glucose (GHR 2013).
Genetic testing can confirm a diagnosis for muscular dystrophies in which an affected gene has been identified and a laboratory test exists. Specific methods used include the following.
- The polymerase chain reaction (PCR) is a method that examines a patient’s DNA to identify mutations in the affected genes. PCR can be used to detect more than 98% of existing genetic deletions, and it can be performed within 24 hours.
- If a PCR test detects a genetic defect, another type of test, called Southern blot, may be used for confirmation (Bakker 2013).
A small piece of muscle is removed and analyzed to help distinguish muscular dystrophy from other muscle diseases. Until the advent of molecular biology techniques, muscle biopsy was the definitive test for diagnosing and confirming muscular disease.
- Immunofluorescence is a technique used to stain proteins of interest (ie, dystrophin) in thin sections of tissue, usually collected from a muscle biopsy.
- Electron microscopy can visualize changes in the integrity of cellular structures inside muscle cells (Kyriacou 1997; NINDS 2011).
- In electromyography (EMG), a tiny needle containing an electrode is used to record the electrical activity between a nerve and its muscle.
- Nerve conduction velocity studies measure how quickly electrical impulses are communicated in nerves.
- Repetitive stimulation studies measure how well a muscle responds to electrical stimulation, a measurement of muscle function (NINDS 2011).
- Muscle ultrasound can examine damaged muscles, but not all the muscles can be evaluated using ultrasound techniques. Alternatively, magnetic resonance imaging (MRI), a noninvasive, painless procedure, allows the visualization of all muscles. In recent studies, magnetic resonance spectroscopy (MRS), another noninvasive approach, has shown promise for the evaluation of skeletal muscle involvement in DMD (Finanger 2012).
- Exercise tests help to track functional decline over time.
- Cardiac MRI can evaluate heart complications that accompany most muscular dystrophies. It has become the gold standard for characterizing heart anatomy and function (Otto 2012).
- Audiograms are necessary in patients diagnosed with the infantile form of fascioscapulohumeral muscular dystrophy for the early detection of hearing loss (Tawil 2008).
- Surveillance by ophthalmoscopy is needed in patients with fascioscapulohumeral muscular dystrophy to detect the presence of fluid in the eye caused by retinal telangiectasias (Tawil 2008).
- Periodic electrocardiography is recommended, usually every 2 years, because patients with certain forms of muscular dystrophies develop heart defects like cardiomyopathy (Amato 2011; Rocha 2010).