Musculoskeletal System

Skeletal Muscle - Inflammation

Narrative
Comment:
Inflammation of skeletal muscle can occur as a result of numerous types of injury, including physical trauma (e.g., injection sites, bite wounds, and blunt trauma), exposure to myotoxins or infectious agents, and ischemia, thrombosis, or myofiber necrosis. Inflammation can exhibit various morphologic patterns. It can be primarily interstitial, with little or no myofiber necrosis, or can be the predominant feature, with little inflammation.

In NTP studies, there are five standard categories of inflammation: acute, suppurative, chronic, chronic-active, and granulomatous; abscesses are diagnosed as suppurative inflammation. In acute inflammation (Figure 1 and Figure 2), the predominant infiltrating cell is the neutrophil, though fewer macrophages and lymphocytes may also be present. There may also be evidence of edema or hyperemia. The neutrophil is also the predominant infiltrating cell type in suppurative inflammation (Figure 3 and Figure 4), but the neutrophils are aggregated, and many of them are degenerate (suppurative exudate). Cell debris (both from the resident cell populations and from infiltrating leukocytes); proteinaceous fluid containing fibrin, fewer macrophages, occasional lymphocytes, and/or plasma cells; and possibly an infectious agent may also be present within the exudate. Grossly, these lesions would be characterized by the presence of pus. In the tissue surrounding the exudate, there may be fibroblasts, fibrous connective tissue, and mixed inflammatory cells, depending on the chronicity of the lesion. Lymphocytes predominate in chronic inflammation. Lymphocytes also predominate in chronic-active inflammation, but a significant number of neutrophils are also present (Figure 5 and Figure 6). Both lesions may contain macrophages. Granulomatous inflammation is another form of chronic inflammation, but this diagnosis requires the presence of a significant number of aggregated, large, activated macrophages, epithelioid macrophages, or multinucleated giant cells (Figure 7 and Figure 8).

Since inflammation can occur in response to, or result in, myofiber necrosis, myopathic changes in addition to edema and/or hemorrhage often occur concurrently. An inflammatory response is necessary to effectively repair damaged tissues; however, the nature, duration, and intensity of this response will crucially influence the overall outcome of repair.
Recommendations:
Inflammation should be diagnosed and graded whenever it is considered a primary lesion. It may be diagnosed as a secondary lesion (e.g., secondary to necrosis) if it is particularly severe or more severe than expected relative to the severity of the primary lesion. The diagnosis should include the type of inflammation (e.g., acute, chronic, chronic-active) as a modifier. Generally it is not necessary to include a site modifier unless it is needed to separate two distinct lesions. Associated lesions, such as vascular lesions, foreign bodies, or infectious agents, should be diagnosed separately. Lesions secondary to the inflammation (e.g., necrosis) and lesions that are part of the inflammatory process (e.g., edema or hemorrhage) should not be diagnosed separately unless warranted by severity but should be described in the pathology narrative.
References:

Berridge BR, Van Vleet JF, Herman E. 2013. Cardiac, vascular, and skeletal muscle systems. In: Haschek and Rousseaux’s Handbook of Toxicologic Pathology, 3rd ed (Haschek WM, Rousseaux CG, Wallig MA, Bolon B, Ochoa R, Mahler MW, eds). Elsevier, Amsterdam, 1635-1665.

Greaves P. 2007. Musculoskeletal system. In: Histopathology of Preclinical Toxicity Studies, 3rd ed. Elsevier, Oxford, 160-214.

Greaves P, Seely JC. 1996. Non-proliferative lesions of soft tissues and skeletal muscle in rats, MST-1. In: Guides for Toxicologic Pathology. STP/ARP/AFIP, Washington, DC.

Greaves P, Chouinard L, Ernst H, Mecklenburg L, Pruimboom-Brees IM, Rinke M, Rittinghausen S, Thibault S, von Erichsen J, Yoshida T. 2013. Proliferative and non-proliferative lesions of the rat and mouse soft tissue, skeletal muscle, and mesothelium. J Toxicol Pathol 26(3 suppl):1S-26S.

Abstract: http://www.ncbi.nlm.nih.gov/pubmed/25035576

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Abstract: http://www.ncbi.nlm.nih.gov/pubmed/21798099

McDonald MM, Hamilton BF. 1990. Bones, joints, and synovia. In: Pathology of the Fischer Rat: Reference and Atlas (Boorman G, Eustis SL, Elwell MR, Montgomery CA, MacKenzie WF, eds). Academic Press, San Diego, 193-207.

Percy DH, Barthold SW. 2007. Mouse. In: Pathology of Laboratory Rodents and Rabbits, 3rd ed. Blackwell, Ames, IA, 88-89.

Vahle JL, Leininger JR, Long PH, Hall DG, Ernst H. 2013. Bone, muscle, and tooth. In: Toxicologic Pathology: Nonclinical Safety Assessment (Sahota PS, Popp JA, Hardisty JF, Gopinath C, eds). CRC Press, Boca Raton, FL, 561-587.

Van Vleet JF, Valentine BA. 2007. Muscle and tendon. In: Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals, 5th ed, Vol 1 (Grant MG, ed). Elsevier, Edinburgh, 185-280.

  • Image of inflammation in the skeletal muscle from a female Swiss CD-1 mouse in a chronic study

    Skeletal muscle - Inflammation, Acute in a female Swiss CD-1 mouse from a chronic study. A neutrophilic infiltrate has led to necrosis and loss of muscle fibers.

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