This 50-year-old patient presented with a two year history of bilateral upper extremity muscle weakness. They also complained of muscle aches and cramps, tingling and numbness. EMG showed features of a chronic myopathic process. NCS showed features of severe symmetric axonal sensory peripheral neuropathy. Their home medications included rosuvastatin.
What do the optically clear spaces represent in Figures #1 and #2?
A. Freezing artifact
B. Autophagic vacuoles
C. Glycogen
D. Lipid
The irregular optically clear spaces within the muscle fibers are due to freezing artifact.
A higher magnification image of an NADH-TR stained frozen section shows that the clear spaces are present in both muscle fiber types (dark type 1, and paler type 2).
Answer: Freezing artifact
The optically clear spaces represent freezing artifact: the formation of ice crystals within the muscle fibers which displace the sarcomeres. Freezing artifact hampers the morphologic evaluation of muscle biopsies.
Typically, muscle biopsies are “snap frozen” by plunging the tissue into isobutene (2 methyl butane) cooled to ~-155 °C by liquid nitrogen. The cooled isobutene transfers the freezing temperature to the tissue quickly as it remains in direct contact with the tissue.
In contrast, freezing the tissue directly in liquid nitrogen does not transfer the freezing temperature as well because it loses contact with the tissue when it “boils” away from the warmer tissue surface.
A standard cryobath bath used for intraoperative frozen sections does not cool the muscle biopsy down quickly enough, and produces significant freezing artifact.
Freezing artifact can be partially corrected by briefly (but completely) thawing the biopsy tissue followed by re-freezing in liquid nitrogen cooled isobutane.
Reference(s)/Additional Reading
Nix JS, Moore SA. What Every Neuropathologist Needs to Know: The Muscle Biopsy. J Neuropathol Exp Neurol. 2020 Jul 1;79(7):719-733. doi: 10.1093/jnen/nlaa046. Erratum in: J Neuropathol Exp Neurol. 2021 Mar 22;80(4):387. PMID: 32529201; PMCID: PMC7304986
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