[Rare osteological diseases in the rheumatological consultation: hypophosphatasia and phosphate loss syndromes].

Metabolic bone diseases cause bone and joint pain and are manifested as rheumatism. Typical for the rare genetic disease hypophosphatasia is a reduced activity of alkaline phosphatase (AP), where the variable residual activity causes the heterogeneous symptoms (e.g., arthralgia, myalgia and fractures). It is indicated by repeatedly low AP measurements. The diagnosis requires a meticulous medical history and laboratory-based clarification in order to rule out other differential diagnoses. Although supportive measures form the basis of treatment, costly enzyme replacement therapy is a possible treatment option for severe forms. Multidisciplinary care under the direction of a rheumatologist experienced in osteology or an osteologist is crucial in order to provide adequate care to affected patients. Phosphate loss syndromes due to overactivity of fibroblast growth factor 23 (FGF-23) lead to deformities of the lower extremities and short stature (in congenital disorders), bone and muscle pain, muscular weakness and pathological fractures, depending on the time of occurrence during life. In genetic forms of the disease (especially X‑linked hypophosphatemia), supplementation with calcitriol and phosphates and, if necessary, complex corrective surgery in adolescence are traditional treatment methods, which are increasingly being replaced by treatment with antibodies against FGF-23. The acquired variant is a paraneoplastic phenomenon from small mostly benign mesenchymal tumors, which clinically shows a relatively acute course with severe bone pain, pathological fractures and muscle weakness in previously healthy patients and can ideally be cured by resection of the tumor. The disease can be suspected by significantly reduced serum phosphate levels and narrowed down with further laboratory diagnostics. In our opinion, the measurement of calcium, phosphate and alkaline phosphatase should be part of the primary laboratory diagnostics performed by rheumatologists and the follow-up of pathological findings is indicated.