When and why should a muscle biopsy be considered in the evaluation of mitochondrial disease?
While the ability to noninvasively reach a mitochondrial disease diagnosis utilizing clinical genetic testing in blood has vastly improved in recent years, many patients require further evaluations in other tissues. Clinically affected tissue such as skeletal muscle is the most common one biopsied, since myopathy is the most common symptom in mitochondrial disease and the tissue in which many of the diagnostic tests have been validated. Muscle is used to directly measure oxygen consumption as a way to objectively evaluate mitochondrial oxidative phosphorylation (OXPHOS) capacity in freshly isolated samples. More commonly, frozen samples are studied by spectrophotometry methods to quantify the enzymatic activities of individual electron transport chain (ETC) complexes, which has become the “gold standard” way to confirm the presence of suspected mitochondrial respiratory chain (RC) disease.
Studies on a muscle biopsy sample may provide valuable diagnostic information, even without clinically overt myopathic disease. Histologic assessment of skeletal muscle may provide specific histochemical and immunohistochemical evidence of a RC defect. Ragged red fibers (representing mitochondrial proliferation) and COX-negative fibers (reflecting deficient OXPHOS capacity) are canonical mitochondrial disease features, and paracrystalline inclusions seen on electron microscopy (representing mitochondrial creatine kinase crystal formation) are a pathognomonic finding. These tests predate genetic testing for mitochondrial disease.
Muscle biopsy analysis also allows for detection of conditions that may mimic mitochondrial myopathy or induce secondary mitochondrial dysfunction, such as fatty acid oxidation defects, glycogen storage disorders, and congenital and inflammatory myopathies. Measuring muscle mtDNA content permits identification of mtDNA depletion disorders. Measuring muscle coenzyme Q10 levels is performed to identify primary CoQ10 biosynthesis disorders. Of particular utility, muscle mtDNA genome sequencing is often necessary to identify pathogenic mtDNA mutations, which may not always be detectable in blood.
It is standard practice to perform a skin biopsy either instead of a more invasive procedure as an initial step or often at the time of obtaining a muscle biopsy. Skin cells are primarily used to establish primary fibroblast cell line cultures that are an ongoing source of cells for genetic, enzymatic, and a host of biochemical tests. While skin biopsies are less invasive and readily performed in the clinic using topical and subcutaneous anesthetic, not all mitochondrial disorders have detectable abnormalities in fibroblasts. When fibroblast testing is unrevealing, skeletal muscle biopsy may be required for diagnosis.
Muscle biopsies do have limitations. The muscle biopsy pathological and/or biochemical evaluation may be normal in genetically confirmed mitochondrial disease patients, particularly early in the disease course or in young children. Further, muscle morphology and histochemical abnormalities can be observed in secondary myopathic processes and over time in all people to some degree with aging. Biochemical RC deficiencies may be tissue-specific and influenced by the precise genetic etiology. For example, some mtDNA depletion disorders require liver biopsy (patients with a hepatocerebral mtDNA depletion disorder). There may also be apparent inconsistencies between testing modalities. Despite being invasive and limited, the muscle biopsy remains the gold standard for mitochondrial disease diagnosis, particularly for mtDNA diseases, where variable mtDNA mutation levels (also known as heteroplasmy) are often tissue-specific.
How is a muscle biopsy obtained?
Skeletal muscle can be obtained either via an open biopsy or a percutaneous needle biopsy procedure. Historically, vastus lateralis muscle open biopsies performed under general anesthesia in an operating room have been the gold standard. Limitations include invasiveness, cost, need for general anesthesia, higher risk of infection, need for suture removal, and a longer surgical scar.
What is a percutaneous needle muscle biopsy?
The development of the needle muscle biopsy technique by Bergstrom in the 1960s provided a viable alternative to open biopsy. Percutaneous needle biopsy has advantages including ease of performance in an outpatient clinic with local anesthetic (adults) or with only conscious sedation (young children) to avoid the risk of general anesthesia. Benefits include less time for the procedure, substantially lower cost, and a much smaller scar. Several passes are typically needed to obtain adequate tissue for comprehensive mitochondrial disease diagnostic evaluation.
Percutaneous needle muscle biopsy evaluation at CHOP
In CHOP’s Mitochondrial Medicine Frontier Program (MMFP), percutaneous needle muscle biopsy evaluation may be offered for older children and adult patients who have completed a mitochondrial disease clinical evaluation. Performed in a sterile outpatient clinical setting following insurance pre-authorization approval, a 5-millimeter Bergstrom muscle biopsy needle with suction is used to obtain muscle tissue samples in the vastus lateralis. Our MMFP approach prioritizes patient safety and ensures that muscle and skin biopsies are performed and processed in a manner that optimally preserves mitochondrial morphology, enzymatic activity, protein and DNA/RNA content to allow for the broadest range of tissue investigations. A multidisciplinary team reviews the biopsy results. When all results are completed, patients return to the MMFP clinic to discuss muscle biopsy results and determine appropriate next steps in patient diagnosis and management.
References and suggested readings
Zolkipli-Cunningham, Z, et al. Mitochondrial disease patient motivations and barriers to participate in clinical trials. PLoS One. 2018;13(5);e0197513.
Tarnopolsky, MA, et al. Suction-modified Bergstrom muscle biopsy technique: experience with 13500 procedures. Muscle Nerve. 2011;43(5); 717-725.