Definition and Overview
A muscle biopsy is a procedure wherein a small piece or amount of muscle tissue is taken and examined in a laboratory. Combined with a good clinical history and physical examination, it helps determine or diagnose certain conditions affecting the muscles, such as an infection or a neuromuscular disease.
Who Should Undergo and Expected Results
Weakness is a feature of many disease conditions but is a major symptom of neuromuscular conditions. Disorders of the muscle may also present with pain, changes in muscle power, or decreased ability to move the muscles. Patients who present with these symptoms may need a muscle biopsy in order to obtain a precise diagnosis of their condition.
In cases of neuromuscular diseases, a muscle biopsy is conducted to differentiate a neuropathy from a myopathy. A neuropathy is a condition wherein the disease affects the nerves that supply the muscles while a myopathy is when the problem is in the muscles themselves. There are classic differences in the clinical presentation of these conditions; for example, weakness is usually noted in the distal muscles in neuropathies, while it is more proximal in myopathies. However, in many instances, the clinical presentation may not be as clear as symptoms may overlap and some individuals may develop combined disorders. In these cases, muscle biopsy plays an important role in the diagnosis.
A muscle biopsy may also diagnose certain infections of the muscles, such as toxoplasmosis as well as certain systemic diseases or conditions affecting connective tissues, such as polyarteritis nodosa. The results of the biopsy can then be used to formulate a treatment plan for the patient.
Aside from these, a muscle biopsy may also be performed to evaluate an individual with a known myopathy. For example, patients undergoing steroid therapy for a known inflammatory myopathy may have to be subjected to a muscle biopsy if they develop progressive weakness. Repeat muscle biopsies may also be necessary to monitor the disease and determine a patient’s response to a specific therapy.
In many cases, a muscle biopsy alone may not be adequate to diagnose the disease. An array of other laboratory examinations, such as electromyography and genetic testing, may have to be performed to obtain a definitive diagnosis.
How is the Procedure Performed?
A muscle biopsy is a fairly simple and straightforward procedure and is usually performed under local anesthesia. Children, however, may have to be subjected to general anesthesia for this procedure.
A specific muscle must be selected prior to the performance of the biopsy. Selection will depend on disease distribution, which helps in identifying the muscles that are likely to be affected by the disease.
After the administration of anesthesia, a needle is used to gather the necessary muscle tissue. It can be performed by using either a thin needle attached to a syringe (fine needle biopsy) or a slightly bigger biopsy needle (core needle biopsy), depending on the location of the targeted muscle and the required amount of biopsy sample. The physician may perform several passes of the needle to ensure that an adequate amount of muscle tissue is obtained. In cases where the muscle to be examined lies deep in the body or a bigger amount of tissue is required, an open muscle biopsy may be necessary. This makes use of a small incision to reach the muscle and obtain an adequate sample.
The extracted tissue is then sent to the laboratory where it is evaluated under a microscope. A variety of stains can be applied and immunohistochemical and biochemical studies performed to determine the predominant type of muscle fibers affected, which can then lead to the identification of the exact disease that the patient is afflicted with. If more information is needed to ensure the accuracy of the diagnosis, special tests may also be conducted to identify pathologic microorganisms, such as parasites. If considering a metabolic myopathy, the tissue can be sent to specialised centers to determine enzymatic activity. Depending on the number of tests that need to be performed, the results of the biopsy can be made available to the patient within days or a couple of weeks.
Possible Risks and Complications
After the procedure, the patient may experience some discomfort or pain in the area where the muscle sample is taken, although this is usually tolerable. Slight swelling may also occur, which subsides after several days. Anti-inflammatory and pain medications are usually given after the procedure.
Bleeding is also another possible risk. In most cases, this resolves on its own or after adequate pressure has been applied on the biopsy site. If the patient has a bleeding disorder or is taking any medications, it is imperative to inform the physician before the procedure. There are minimal complications with a properly performed muscle biopsy and the risks are usually not serious.
Shepich JR. Muscle biopsy. In: Pfenninger JL, Fowler GC, eds. Pfenninger & Fowler's Procedures for Primary Care. 3rd ed. Philadelphia, PA: Elsevier Mosby; 2010:chap 228.
Narayanaswami P, Weiss M, Selcen D, David W, Raynor E, Carter G, et al. Evidence-based guideline summary: diagnosis and treatment of limb-girdle and distal dystrophies: report of the guideline development subcommittee of the American Academy of Neurology and the practice issues review panel of the American Association of Neuromuscular & Electrodiagnostic Medicine. Neurology. 2014 Oct 14. 83(16):1453-63.