About Neuromuscular Disorders > Diagnosing a Neuromuscular Disorder

Diagnosing a Neuromuscular Disorder

Diagnosing a neuromuscular disorder is a multi-step process and could involve many tests. In many instances more than one diagnostic test will have to be performed. Often, a patient's clinical symptoms will play an important role in diagnosing a neuromuscular disorder. Below is a list of the common types of tests done when diagnosing a neuromuscular disorder. Your own healthcare provider may perform others. Please consult your healthcare provider for more information.

Muscle Biopsy

A muscle biopsy is a minor surgical procedure done under local anesthesia (or general anesthesia for children) using either a needle or small incision to remove a small sample of muscle tissue. The procedure may be done to confirm a clinical diagnosis, distinguish between problems of nerve or muscle or identify a metabolic defect among other reasons. The sample of muscle tissue is sent to a laboratory where thin slices called "sections" can be examined under a microscope. The laboratory technique called immunostaining allows a laboratory scientist to determine if certain muscle proteins are decreased or missing.

Electromyography (EMG)

An EMG is a test used to record the electrical activity of muscles. When muscles are active they produce an electrical current that is usually proportional to the muscle activity. When someone has a disorder affecting his or her muscles, such as a muscular dystrophy, the EMG can detect abnormal muscle electrical activity. An EMG helps to distinguish between a problem that begins in the muscle or muscle weakness due to nerve disorders. There are two types of EMG tests: 1) intramuscular EMG: involves inserting a needle electrode through the skin into the muscle; 2) surface EMG: involves placing the electrode on the skin overlaying the muscle to detect the electrical activity of the muscle. In both methods the electrical activity is displayed on a screen (or audibly) for the clinician to interpret.

Nerve Conduction Velocity (NCV) Test

NCV, also known as a nerve conduction study or NCS, is an electrical test used to detect nerve disorders. The nerve is electrically stimulated by one electrode on the surface of the skin while other electrodes, which are placed a set distance along a limb from the first electrode, detect the electrical impulse from the first electrode. The distance between electrodes and the time it takes for electrical impulses to travel between electrodes are used to calculate the speed of impulse transmission. A healthy nerve signal travels at speeds of up to 120 miles per hour. A decrease in the speed of nerve conduction indicates a nerve disease.

Genetic Testing

Genetic testing is used to identify mutations (changes in the DNA sequence that affect function) in genes. The results of a genetic test can confirm or exclude a suspected genetic condition or help to determine a person's chance of developing symptoms of a genetic disorder or of passing on a genetic disorder to their offspring.

Types of genetic tests include gene tests, where individual genes or relatively short lengths of DNA or RNA are tested, and chromosomal tests, where whole chromosomes or very long lengths of DNA are tested.
  1. Gene tests
    Gene tests look for signs of disease in DNA or RNA from a person's blood or other body substances such as saliva or tissue. These tests look for large changes such as a gene that has a section missing or added, or small changes, such as a missing or added chemical base (subunit) within the DNA strand. Gene tests may also detect if there are duplicated regions of DNA or genes that are incorrectly turned off.

    Sometimes there is a correlation between what type of gene mutation and the effects on health caused by disease. This is called a genotype-phenotype correlation. For example, individuals who have Duchenne muscular dystrophy often have a frame-shift (or out of frame) mutation in the gene that codes for dystrophin. Individuals who have an in-frame mutation in the dystrophin gene may have a less severe disease, called Becker muscular dystrophy.

  2. Chromosomal tests
    Chromosomal tests can facilitate analysis of a large number of genes and large regions of genetic material. Types of chromosomal tests include:
    • Karyotype — This test gives a picture of all of a person's chromosomes from the largest to the smallest. This type of testing can identify changes in chromosome number and large changes in DNA structure.
    • FISH analysis (fluorescent in situ hybridization) — This test identifies certain regions on chromosomes using fluorescent DNA probes. FISH analysis can find small pieces of chromosomes that are missing or have extra copies. These small changes can be missed by the overall karyotype test.

Biochemical tests

Biochemical tests analyze certain protein levels or enzyme activities which are indicative of disease. These tests can be done on different types of samples from a person, including blood, urine, and muscle. A biochemical test looks for certain values which are abnormal (elevated or decreased from normal values). A clinician can look for patterns in these values to then narrow down the potential genetic cause.

An example of a biochemical test is the Creatine Kinase (CK) Test. CK, also known as a creatine phosphokinase (CPK), is an enzyme that is present in skeletal and cardiac muscle tissue. CK is used to generate energy in muscle fibers. When a CK blood (or serum) level is elevated it is often an indication of muscle damage caused by injury, muscular dystrophy, or cardiac problems. This test is performed by drawing a small amount of blood and sending it to a lab for analysis.

Muscle Strength Tests

Tests of muscle strength play an important role in the diagnosis of a neuromuscular disorder and are a routine part of a physical exam. Weakness in the muscle may indicate injury to the tendons that connect the muscle to the bone, injury to the nerves that send a signal to the muscles to move, or a generalized weakness of the muscle itself. A muscle test involves asking a person to move in a certain way while the clinician applies a resistive force. Examples of this include measuring the strength that someone can grip someone's hand.


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