Phosphopenic osteomalacia of tumor genesis under mask of ankylosing spondylitis

Aim. To present a clinical case of osteomalacia associated with fibroblast growth factor-23-secreting tumor under the mask of ankylosing spondylitis (AS).Materials and methods. Clinical observation of a 31-year-old patient with long-time diagnosis of AS is presented. Underestimation of back pain cause at the initial stage of diseaseled to an erroneous diagnosis of AS. A thorough assessment of the anamnesis, additional examination using modern imaging methods in combination with laboratory analysis (low blood phosphorus level, hyperphosphaturia, normal value of C-reactive protein, erythrocyte sedimentation rate, negative HLA-B27), made it possible to establish the correct diagnosis of “mesenchymal phosphaturic tumor of the left foot (surgical intervention dated 11.26.2020), secondary hypophosphatemic tumor-induced osteomalacia complicated by multiple bone fractures”, to carry out timely treatment with full recovery.Results. The literature data on epidemiology, pathogenetic mechanisms, clinical manifestations and management approaches of tumor induced phosphopenic osteomalacia are presented. An algorithm for examining patients with suspected of this disease is described, taking into account the expression of somatostatin transmembrane receptors on the surface of a mesenchymal phosphaturic tumor.Conclusion. One of the rarest causes of specific back pain is osteomalacia, which can be caused by various diseases, for example, a tumor secreting FGF23 The complexity of the diagnosis lies in the non-specificity of clinical manifestations – generalized myalgia and myopathy, ossalgia, pathological fractures, etc. Timely diagnosis and radical treatment makes it possible to achieve stable remission with complete leveling of symptoms, therefore surgical excision of the tumor is the “gold” standard of therapy.