Diseases of the skeletal muscle.

After the advances created by the use of cryostat sections and histochemistry 60 years ago, muscle histopathology is now living a real renaissance. In the field of genetic neuromuscular disorders, muscle biopsy analysis is fundamental to address questions about pathogenicity and protein expression when new genes are discovered through next-generation sequencing approaches. Moreover, the identification of the same gene mutated in previously considered distinct histopathologic entities imposes a constant reassessment of morphologic boundaries in several groups of disorders. In other fields like the acquired inflammatory myopathies, histologic analysis nowadays helps to affirm a diagnosis, set up therapeutic strategies, and verify the success of immunosuppressive treatment. In this exciting scenario morphologists are definitely key figures in the neuromuscular field. The objective of this chapter is to give an overview on morphology of the most frequent and recently identified muscle conditions, stressing the importance that only a combined analysis of clinical findings, muscle histology, and specific ancillary investigations is effective in reaching a precise diagnosis and orienting therapy.