A muscle biopsy is a surgical procedure in which one or more small pieces of muscle tissue are removed for further microscopic or biochemical examination. The procedure, often used in the diagnosis of a neuromuscular disorder, is considered "minor" surgery and is usually performed under local anesthetic.
A doctor is likely to call for a muscle biopsy after looking at preliminary blood tests, performing an electromyogram (EMG) and physical examination, and determining that the patient's symptoms indicate an underlying neuromuscular disorder. The muscle biopsy can help distinguish between muscular and neurological problems and can help pinpoint the exact neuromuscular disorder present.
Not everyone suspected of having a neuromuscular disease requires a muscle biopsy. In some cases, diagnosis can be made by symptoms and a DNA test based on a blood sample.
Open or needle biopsy
There are two types of muscle biopsy. The open biopsy involves the removal of one or more small pieces of muscle tissue with sharp scissors.
The neuromuscular specialist selects a muscle, usually the biceps, triceps, deltoid or quadriceps muscle, that should yield the most information about the disease. Usually moderately affected muscles are chosen; the weakest muscles may already be too degraded for analysis. The procedure involves a 2-to-3-inch incision, which is then closed with stitches and may feel sore for a few days.
In a needle biopsy, used since the 1960s, a pea-sized muscle sample is collected with a large bore needle. Although this is less invasive than the open biopsy, the doctor loses the ability to examine the muscle visually first, and the specimen collected is smaller.
Analyzing the sample
When the muscle samples are sent to a laboratory for analysis, the technicians cut them into many thin sections for examination. Using different tests on different sections, they look at the tissue's overall appearance, chemical activities in the tissue, and the presence or absence of critical proteins. The information these tests provide helps determine exactly what disease and what form of it the person has.
Histology tests (histo means tissue) employ chemical stains to see the muscle's overall appearance and the structure of the muscle cells. This analysis can yield information about muscle degeneration and regeneration, fiber type abnormalities, mitochondrial abnormalities, scar tissue, inflammation and other clues to specific disorders.
Histochemistry uses stains to detect chemical activities in the cells, including the actions of specific enzymes and metabolic processes. A lab that performs only histology may miss important metabolic abnormalities.
Immunohistochemistry uses antibodies to detect the presence or absence of proteins. This analysis can show whether the cells are missing dystrophin (indicating Duchenne or Becker MD), sarcoglycans (limb-girdle MD), merosin (congenital MD) or other proteins whose absence causes specific muscular dystrophies. Specific antibodies can also be used to identify the nature of inflammatory cells found in the muscle.
The lab may also use electron microscopy to get very high magnification views of the cellular structure, which can confirm structural abnormalities, like the presence of nemaline rods.
Finally, a DNA analysis can be performed on a muscle sample to detect a genetic mutation. Although a blood sample is usually adequate for a DNA test, a muscle sample may be needed to test for mitochondrial DNA mutations.
Yadollah Harati, a neurologist and director of the Muscle and Nerve Pathology Laboratory at Baylor College of Medicine in Houston, usually takes as many as five separate muscle samples from different regions of the muscle incision. Several are analyzed and at least one is frozen for future use. Harati believes no biopsy should be done unless the amount of tissue removed is adequate for a complete study.
Having at least three muscle samples gives the lab an adequate amount of tissue to work with. In some disorders, particularly "patchy" disorders like the inflammatory myopathies, signs of the disease may not be present in all regions of the muscles, so more samples give a better chance of accuracy.
It's important that the tissue samples be frozen promptly and properly after the biopsy and be stored carefully. If they're not handled and stored correctly, the results may be inaccurate.
Your doctor may occasionally recommend a new biopsy even though you've had one in the past, especially if you've been given a tentative diagnosis or now suspect your diagnosis was incorrect. With many new muscle-protein antibodies now available for testing biopsy samples, as well as new understanding of mitochondrial disorders and new DNA tests, a new biopsy may be desirable.
According to Harati, tissue that was frozen promptly after removal and maintained carefully is useful for many years. In autoimmune diseases, tissue changes over time in your body may necessitate a new biopsy for the most accurate diagnosis.
The analysis of a muscle biopsy sample is a very tedious and labor-intensive process in which many sections of the muscle must be cut, many different types of procedures performed, and the results carefully analyzed. Harati's lab usually performs a few basic histology tests immediately after the biopsy and then, based on these results, determines what further tests should be made.
His lab typically makes an initial report on the day of the biopsy and a full report in two to three weeks.
"Simply Stated" is a Quest column designed to explain some terms and basic facts about neuromuscular disease.
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