Fanconi syndrome

Overview and Pathophysiology

Fanconi syndrome (FS), also known as De Toni-Debré-Fanconi syndrome, is a generalized dysfunction of the renal proximal tubules leading to excessive urinary wasting of amino acids, glucose, phosphate, uric acid, bicarbonate, and low-molecular-weight (LMW) proteins.
Most solute transport in the proximal tubule is mediated by receptor-mediated endocytosis or sodium gradient-dependent transport systems.
Proximal tubular cells have extensive endocytic receptors, notably megalin and cubam (composed of amnionless and cubilin), which are critical for the reabsorption, degradation, and recycling of filtered proteins.
Disruption of the endocytic pathway, such as altered endosomal acidification mediated by the vacuolar-type H+-ATPase and the ClC-5 chloride channel, compromises receptor recycling and decreases solute reabsorption.
Proximal tubule transport processes consume immense energy; thus, genetic defects, drugs, or toxic substances that inhibit oxidative phosphorylation in mitochondria cause nonselective impairment of most transport mechanisms, resulting in FS.

Patients classically present with profound growth faltering, which is driven by a complex interplay of malnutrition, hypokalemia, hypophosphatemia, and metabolic acidosis.
Polyuria, polydipsia, and recurrent episodes of dehydration are common and result primarily from osmotic diuresis caused by excessive urinary solute losses.
Hypophosphatemia combined with impaired 1-alpha hydroxylation of 25-hydroxyvitamin D leads to the development of rickets in children, manifesting with bowing deformities, wrist widening, and a rachitic rosary.
Defective bicarbonate reabsorption in the proximal tubules results in hyperchloremic (normal anion gap) metabolic acidosis, known as proximal renal tubular acidosis.
Sodium reabsorption is diminished, which may cause hypotension and volume contraction, secondarily activating the renin-angiotensin-aldosterone system and exacerbating potassium wasting, leading to severe hypokalemia.
Glucosuria is universally observed despite normal blood glucose levels.
Generalized aminoaciduria is present, defined by a fractional excretion of amino acids >5%.
Patients frequently display hypercalciuria and hyperuricosuria, along with prominent LMW proteinuria.

The causes of FS are broadly divided into hereditary, acquired, and exogenous or toxic categories.

Category	Specific Etiologies and Characteristics
Hereditary (Inborn Errors of Metabolism)	Cystinosis: The most common identifiable cause of FS in early childhood; presents at 6-12 months of age with severe tubulopathy, polyuria, and corneal cystine crystals.
	Dent Disease: An X-linked recessive disorder characterized by LMW proteinuria, hypercalciuria, nephrocalcinosis, and rickets.
	Lowe Syndrome: An X-linked disorder presenting with congenital cataracts, developmental delay, and severe FS.
	Other Metabolic Disorders: Galactosemia, hereditary fructose intolerance, tyrosinemia type I, Wilson disease, glycogen storage disease type I, and mitochondrial cytopathies.
Acquired (Endogenous)	Often secondary to immunological disorders (e.g., Sjögren syndrome, TINU syndrome), nephrotic syndrome, multiple myeloma, or paroxysmal nocturnal hemoglobinuria.
	Can be observed transiently in untreated children with distal renal tubular acidosis.
Exogenous (Drugs and Toxins)	Medications: Alkylating agents (ifosfamide), platinum compounds (cisplatin), aminoglycosides, tenofovir, adefovir, valproic acid, and suramin.
	Toxins: Chinese herbs containing aristolochic acid, heavy metals (lead, cadmium, mercury), and paraquat.

Urine analysis reveals the characteristic biochemical "fingerprint" of generalized proximal tubular failure: glucosuria with normal blood glucose, generalized aminoaciduria, phosphaturia, and marked LMW proteinuria.
Blood tests confirm hypokalemia, hypophosphatemia, and hyperchloremic metabolic acidosis with a normal anion gap.
Specific diagnostic testing is dictated by suspected etiology, such as measuring leukocyte cystine levels for cystinosis, checking serum copper and ceruloplasmin for Wilson disease, or performing targeted genetic panels (e.g., CTNS, CLCN5, OCRL1).
Ophthalmologic evaluation is critical to identify corneal cystine crystals (cystinosis), congenital cataracts (Lowe syndrome), or Kayser-Fleischer rings (Wilson disease).

The primary goal is the identification and specific treatment of the underlying cause, such as cysteamine for cystinosis, avoidance of offending sugars in galactosemia and hereditary fructose intolerance, copper-chelating agents for Wilson disease, or withdrawal of nephrotoxic drugs.
Symptomatic management requires replacing the massive renal losses to promote normal growth and prevent skeletal deformities.
Free access to water and salt supplementation is mandatory to prevent severe dehydration and hypovolemia.
Correction of proximal renal tubular acidosis requires large quantities of alkali supplementation, typically 2 to 15 mEq/kg/day, administered in multiple divided doses as sodium or potassium citrate/bicarbonate.
Severe hypokalemia necessitates continuous supplementation with potassium salts (chloride, gluconate, or citrate).
Hypophosphatemic rickets is managed with oral phosphate supplements combined with active vitamin D analogs (e.g., calcitriol or alfacalcidol) to maximize gastrointestinal absorption.
Treatment with recombinant human growth hormone may be considered to improve linear growth in selected patients with severe growth faltering who do not respond adequately to electrolyte and metabolic correction.