Primary Fanconi renotubular syndrome

Primary Fanconi renotubular syndrome (also known as primary renal Fanconi syndrome or idiopathic Fanconi syndrome) is a rare inherited disorder affecting the proximal tubules of the kidneys. The proximal tubule is the part of the kidney responsible for reabsorbing essential substances—such as glucose, amino acids, phosphate, bicarbonate, uric acid, and low-molecular-weight proteins—back into the bloodstream after filtration. In this condition, the proximal tubular transport mechanisms are defective, leading to excessive urinary loss of these substances despite normal blood levels. This results in a constellation of findings including generalized aminoaciduria, glycosuria (glucose in the urine without diabetes), phosphaturia, bicarbonaturia leading to metabolic acidosis, and low-molecular-weight proteinuria. The clinical consequences of these renal losses can be significant, particularly in growing children. Chronic phosphate wasting leads to hypophosphatemic rickets or osteomalacia, causing bone pain, skeletal deformities, and impaired growth. Metabolic acidosis from bicarbonate loss contributes to failure to thrive, polyuria, polydipsia, and dehydration. Patients may also develop hypokalemia, which can cause muscle weakness. Over time, progressive kidney damage may occur in some forms, potentially leading to chronic kidney disease. The condition primarily affects the renal and skeletal systems, though systemic effects of electrolyte imbalances can impact multiple organ systems. Treatment is primarily supportive and aimed at replacing the substances lost in the urine. This includes phosphate supplementation combined with active vitamin D analogs (such as calcitriol) to treat or prevent rickets and osteomalacia, oral bicarbonate or citrate to correct metabolic acidosis, and potassium supplementation as needed. Adequate fluid intake is important to prevent dehydration. With appropriate and early management, many complications can be mitigated, though lifelong monitoring and treatment are typically required. Mutations in several genes have been identified in primary forms, including SLC34A1 (encoding the sodium-phosphate cotransporter NaPi-IIa), EHHADH, and HNF4A, among others. The condition must be distinguished from secondary Fanconi syndrome, which can be caused by metabolic diseases such as cystinosis, Wilson disease, or galactosemia, as well as by nephrotoxic drugs or heavy metals.

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