Muscle biopsies in dermatomyositis and polymyositis: practical relevance of analyzing different levels of histological sections of the same muscular compartment

Introduction: It is frequent in medical practice to have findings with normal aspects in histological muscle biopsies from patients with dermatomyositis (DM) or polymyositis (PM). This happens because, for example, the inflammatory infiltrate occurs in foci. Objectives: To evaluate the morphological and histological inflammatory infiltrate in various histological section levels. In addition, to correlate these findings with patients’ clinical, laboratory and therapeutic data. Methods: Cross-sectional study in which muscle biopsies from 34 patients were evaluated (DM and PM). From each muscle/patient biopsy block, three levels of histological sections were made (I, II, III) with 400-μm interval between adjacent levels (I × II, and II × III). Semi-quantitative analyses were performed in the following parameters between the adjacent levels: muscle fiber features, conjunctive tissue, vessels, presence of inflammatory cell infiltration. Results: Time spans between muscle biopsy and symptom onset of DM and PM patients were 5.5 and 3.5 months, respectively. All histological parameters analyzed varied between levels and did not correlate with the demographic, clinical, laboratory and therapeutic data before muscle biopsy (p > 0.05). Conclusion: Our results stress the importance of evaluating different levels of histological sections from the same muscle biopsy block, in order to minimize possible false-negative results. In addition, the data reinforce that besides the inflammatory infiltrate, the other histological parameters analyzed also occur in foci, justifying the dissociation between these parameters and clinical patients.