Additional unintended useful information can be obtained from many histochemical stains used in the routine evaluation of skeletal muscle biopsies.  In addition to identifying amyloid deposition, Congo red stain provides some morphologic detail.  In this case, granular brown lipofuscin pigment can be seen in the atrophic muscle fibers under routine light microscopy.  The lipofuscin pigment is also autofluorescent, showing a white-yellow color under fluorescence microscopy.

Congo Red stained frozen section of skeletal muscle under routine light microscopy (top) and fluorescence microscopy with FITC filter (bottom); original magnification: 400x each.