AvIDP: typical histological PNS findings. The histological appearance of a healthy mixed fascicular nerve is displayed in figure A. Its endoneurium (E) contains large (LMF) and small (SMF) myelinated fibres that are confined by a peripheral rim of blue (azurophilic) compacted myelin. B: AvIDP leads to a breakdown of the myelin sheath while leaving the axons mainly untouched. Concomitantly with the myelin loss (arrow), an expansion of the eosinophilic extracellular matrix of the endoneurial sheath develops which results in a decrease of nerve fibre density. A, B: P = perineurium; scale bar = 100 μm.