Clinical meaning
Fanconi syndrome is a generalized dysfunction of the PROXIMAL RENAL TUBULE resulting in impaired reabsorption of multiple solutes that were filtered at the glomerulus. Normally, the proximal tubule reabsorbs approximately 65% of filtered sodium and water, nearly 100% of filtered glucose and amino acids, 80% of filtered phosphate, and 80% of filtered bicarbonate. In Fanconi syndrome, damage to proximal tubular transport mechanisms causes these solutes to spill into the urine. The hallmark findings include: (1) GLUCOSURIA with NORMAL blood glucose (glucose appears in the urine because the proximal tubule cannot reabsorb it, NOT because blood glucose is elevated -- this distinguishes it from diabetic glucosuria), (2) AMINOACIDURIA (generalized loss of amino acids into the urine), (3) PHOSPHATURIA causing HYPOPHOSPHATEMIA (phosphate wasting leads to impaired bone mineralization → rickets in children, osteomalacia in adults), (4) BICARBONATURIA causing TYPE 2 (PROXIMAL) RENAL TUBULAR ACIDOSIS (loss of bicarbonate into the urine results in non-anion gap metabolic acidosis with normal anion gap), (5) HYPOKALEMIA from potassium wasting, (6) URICOSURIA causing hypouricemia, and (7) LOW-MOLECULAR-WEIGHT PROTEINURIA (proteins normally reabsorbed by the proximal tubule appear in the urine). CAUSES of Fanconi syndrome are classified as INHERITED or ACQUIRED: INHERITED causes include cystinosis (the most common inherited cause in children -- lysosomal cystine accumulation damages proximal tubular cells), Wilson disease (copper deposition), galactosemia, hereditary fructose intolerance, and glycogen storage diseases. ACQUIRED causes include MEDICATIONS (tenofovir -- the most common drug cause, used in HIV treatment; expired tetracyclines -- degradation products are toxic to tubular cells; cisplatin; ifosfamide; valproic acid), HEAVY METAL toxicity (lead, mercury, cadmium), MULTIPLE MYELOMA (light chain deposition damages tubules), kidney transplant rejection, and amyloidosis. The nurse must understand that Fanconi syndrome is NOT a disease of the glomerulus -- the glomerular filtration rate may be normal, especially early in the disease. The problem is the tubular REABSORPTION step. The clinical consequences primarily result from chronic electrolyte losses: hypophosphatemia causes bone disease, metabolic acidosis causes growth failure in children, and hypokalemia causes muscle weakness and cardiac arrhythmia risk.