Diffuse, fracturing systemic skeletal histiocytosis of unknown type: a novel metabolic bone disease.

We describe a novel disease of diffuse skeletal histiocytosis associated with multiple fragility fractures and high osteoclast activity. Clinical, radiographic, biochemical, genetic, and histopathological investigations were performed to characterize the diagnosis of an Asian man who presented with hip fracture and diffuse skeletal lytic lesions. After excluding malignancy and other common metabolic bone diseases, open bone biopsy yielded several pathological samples all showing extensive skeletal histiocytosis likely to explain the diffuse axial and appendicular lytic lesions. Rare disorders such as Langerhans histiocytosis, Erdheim-Chester disease, and diffuse cystic skeletal angiomatosis were excluded through careful pathological examination and lack of CD1a and S-100 staining. Whole exome sequencing did not yield diagnostic findings to explain this likely acquired disease. High markers of osteoclast activity suggested excessive focal bone resorption but normalized after zoledronic acid treatment. A novel disease of skeletal histiocytosis with high bone turnover is differentiated from other histiocytic and lytic skeletal diseases.