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November 12, 2021

EMG Needle Site

EMG Needle Site

Figure 1: Frozen section hematoxylin and eosin; 200x original magnification

Note the increased cellularity within the perimysium due to the presence of foamy macrophages (large arrow).

The adjacent muscle fibers show some variation in least mean diameter and a few atrophic regenerating fibers are present.

 

This middle-aged patient presented with a two year history of bilateral lower extremity pain and weakness predominately involving anterior thighs. Electrodiagnostic studies one month prior to muscle biopsy showed minimal myopathic features without evidence for neurogenic change. Lower extremity (3/5) and upper extremity (4/5) weakness and waddling gait were noted on physical examination.  Muscle biopsy of the left thigh was performed.

 

Based on the clinical history and morphologic alterations in Figures 1-5, which of the following is the best diagnosis?

A. Dermatomyositis

B. Needle EMG site

C. Polymyositis

D. Metastatic carcinoma

 

EMG Needle Site

Figure 2: Frozen section modified Gomori Trichrome; 200x original magnification

Note the increased cellularity within the perimysium due to the presence of foamy macrophages (large arrow).

The macrophages have a somewhat clear cytoplasm with some fine granular magenta staining material.

EMG Needle Site

Figure 3: Frozen section MHC1 (HLA-ABC); 100x original magnification

The muscle fibers toward the outside of the muscle fascicle show very mild increased sarcoplasmic and/or membranous granular immunostaining for MHC1 (i.e. suggestive of a perifascicular distribution of muscle fiber injury).

EMG Needle Site

Figure 4: Formalin Fixed Paraffin Embedded tissue section hematoxylin and eosin; 400x original magnification

Areas of fat necrosis with associated mononuclear and multinucleate histiocytes and sparse chronic inflammation were seen in the perimysial and epimysial fibroadipose tissue.

Figure 5: Formalin Fixed Paraffin Embedded tissue section hematoxylin and eosin; 100x original magnification

Note the areas of fat necrosis and an oval  cystic space surrounded by fibrosis.  The latter is felt to most likely represent tissue injury related to prior EMG needle.

 

Answer: Needle EMG site

Focal inflammatory changes related to prior needle EMG site or injection site may mimic the morphologic alterations of an inflammatory myopathy. Communication between the Neurologist performing the EMG and the Surgeon performing the muscle biopsy is critical to avoid biopsy sampling of such areas. Awareness of this potential pitfall is necessary for Pathologists and Neurologists evaluating muscle biopsies.

The possibility of partially sampled dermatomyositis was considered in this case, but was felt to be unlikely given the presence of fat necrosis and a needle track site. The overall morphologic features are not those of polymyositis. The cells within the perimysium have morphologic features of macrophages rather than the epithelioid cytomorphology of metastatic carcinoma. That said, some metastatic carcinomas may be subtle (for example metastatic lobular carcinoma from breast).

 

References

El Khoury M, Cherel P, Becette V, De Maulmont C, Costes V, Talma V, Hagay C. Unusual soft-tissue metastasis of an invasive lobular carcinoma mimicking fasciitis. AJR Am J Roentgenol. 2004 Mar;182(3):745-7. doi: 10.2214/ajr.182.3.1820745. PMID: 14975980.