Nerve and Muscle Biopsies
Nerve and muscle biopsies are subject to a variety of artifacts if they are not properly handled. Furthermore, the interpretation of nerve and muscle pathology frequently requires special studies, in-cluding electron microscopy and histochemistry. For these reasons, nerve and muscle biopsies are best handled directly by specialized laboratories. Each laboratory usually has its own protocol for handling these biopsies. A detailed discussion of these specimens is, therefore, beyond the scope of this book. However, you should be aware of a few basics in processing these specimens, in case you do not have access to a specialized neu-romuscular laboratory when the biopsy is done.
Muscle biopsies are usually obtained as two strips of muscle approximately 3.0 × 0.5 cm, taken parallel to the direction of the muscle fibers. A portion of tissue should be stretched to its normal in situ length, pinned to a card, and placed imme-diately (in the operating room) in 4% buffered glutaraldehyde. Do not mince this piece for elec-tron microscopy. The remainder of the biopsy should be transported in saline-moistened gauze (do not soak) to the pathology laboratory. A por-tion of the biopsy should then be immediately fresh-frozen and the remainder fixed in formalin. To ensure that the fiber size in the biopsy reflects the in situ diameter and to reduce artifacts caused by the extreme contractility of the tissue, this piece of muscle should also be stretched to its normal in situ length in the direction of the fibers. Thiscan be accomplished with a special muscle clamp or simply by pinning the stretched biopsy to a stiff index card. Be careful not to overstretch the tissue, as this can cause artifacts as well. Muscle biopsies frozen in liquid nitrogen tend to develop ice crystal artifacts, so it is best to freeze the tissue in 2-methyl-butane (isopentane) cooled to dry ice temperature or below. When submitting muscle biopsies in formalin for histology, remember to submit both longitudinal and cross sections.
Diagnostic nerve biopsies to determine the cause of a neuropathy are usually obtained as 3-to 5-cm-long strips of the sural nerve. As was true for muscle biopsies, these should be processed immediately by a specialized neuromuscular lab-oratory. You should handle these biopsies only when they cannot be handled by a specialized laboratory. Do not stretch these biopsies! In-stead, the biopsy is usually cut into four pieces. One piece should be submitted in 4% glutaralde-hyde for electron microscopy, one submitted in formalin for routine microscopy, another fresh-frozen and stored at 2708C, and the fourth saved fresh in saline-moistened gauze for the neuromuscular laboratory. If you have to cut a nerve, be careful not to squash it while cutting. Pressure on the nerve can induce the artifactual appearance of demyelinization. As was true for muscle biopsies, the sections submitted for histol-ogy should include both longitudinal and cross sections.
If a nerve biopsy is performed simply to docu-ment the transection of a nerve, the specimen should be entirely submitted in formalin for rou-tine paraffin embedding and sectioning.