Delayed diagnosis of plasma cell disorder-related Fanconi syndrome in young adults presenting as osteomalacia: report of two cases with normokalemia and normal haematological parameters at the time of presentation

Adult-onset hypophosphatemic osteomalacia is rare and diagnosis is frequently delayed. Fanconi syndrome (FS) due to monoclonal gammopathy is a well-recognized, but rare cause of hypophosphatemia. The relatively young age of patients and normal routine hematological parameters often results in late recognition of this treatable disease entity. Low phosphorus, elevated alkaline phosphatase, mildly impaired renal function and hypokalemia are often the only abnormalities on routine evaluation. We summarize the clinico-pathological features of two cases who initially presented with fractures and proximal myopathy and were subsequently found to have FS secondary to light chain proximal tubulopathy. Atypical features like absence of hypokalemia at presentation and elevated Fibroblast Growth Factor 23(FGF 23), a marker of oncogenic osteomalacia were noted. Marked clinical improvement and recovery of renal parameters were evident with phosphate supplements and chemotherapy for the plasma cell disorder. FS due to monoclonal gammopathy may present with atypical features and diagnosis may be challenging