Cryptic crystals: rare finding of crystal-storing histiocytosis in a gastric biopsy

Crystal-storing histiocytosis (CSH) is a disorder characterised by non-neoplastic histiocytes filled with intracytoplasmic crystals, which are usually composed of immunoglobulins. It often points to an underlying lymphoproliferative or plasma cell disorder. We present a case of a middle-aged woman who had an endoscopy as part of investigation for haematemesis and rectal bleeding. Light microscopy of the gastric biopsy showed an expanded lamina propria containing large polygonal-shaped cells with small uniform nuclei and dense granular eosinophilic cytoplasm with scattered rhomboid appearing crystalline material. Special stains, immunohistochemistry, and electron microscopy were performed, with electron microscopy demonstrating abundant rhomboid intracytoplasmic inclusions, confirming a diagnosis of CSH. We discuss the diagnostic approach used in this case and review the current literature on the utility of electron microscopy for CSH.