Figure 1Muscle biopsies: Hematoxylin and eosin-stained section of the gluteus medius of Patient 1 shows several fibers harboring classic rimmed vacuoles (arrowhead) and non-rimmed (arrow) vacuoles (A), the latter of which contained periodic acid-Schiff (PAS)-positive material (B, arrow) and some membranous material. Similarly, triceps biopsy of Patient 2 demonstrates classic rimmed (arrowhead) and non-rimmed (arrow) vacuoles on modified Gomori trichrome staining (C), with some non-rimmed vacuoles containing PAS-positive material (D, arrow) and some membranous material. All observed PAS-positive material within the vacuoles was digested by diastase, consistent with glycogen. Unlike the classic rimmed vacuoles, these glycogen-filled vacuoles were acid phosphatase negative.