Fanconi syndrome

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Background

  • Fanconi syndrome is a generalized dysfunction of the proximal renal tubule resulting in impaired reabsorption of glucose, amino acids, phosphate, bicarbonate, uric acid, potassium, sodium, and low-molecular-weight proteins.
  • It produces a type 2 (proximal) renal tubular acidosis and may cause life-threatening hypokalemia, metabolic acidosis, dehydration, and bone disease.[1] It is not the same as Fanconi anemia, which is a separate inherited bone marrow failure syndrome.
  • The proximal convoluted tubule (PCT) normally reabsorbs ~65% of filtered sodium, water, bicarbonate, glucose, amino acids, phosphate, and uric acid
  • In Fanconi syndrome, global PCT dysfunction causes urinary wasting of all these solutes simultaneously[1]
  • Pathophysiology centers on mitochondrial dysfunction → ATP depletion → failure of Na⁺/K⁺-ATPase and energy-dependent transport systems[1]
  • Can be inherited or acquired; acquired forms are more relevant to the emergency physician

Inherited causes

  • Cystinosis — most common inherited cause (presents in infancy, 6-18 months)
  • Wilson disease
  • Hereditary fructose intolerance
  • Galactosemia
  • Glycogen storage diseases
  • Lowe syndrome (oculocerebrorenal syndrome)
  • Dent disease
  • Fanconi-Bickel syndrome
  • Tyrosinemia type 1
  • Mitochondrial cytopathies

Acquired causes (most EM-relevant)

  • Medications (most common acquired cause):[1][2]
    • Tenofovir (TDF; especially in HIV patients with pre-existing renal impairment) — most commonly encountered drug cause today
    • Ifosfamide — particularly in children after cumulative doses
    • Cisplatin, carboplatin
    • Valproic acid — especially with polytherapy and prolonged use
    • Expired tetracyclines (degradation products are directly nephrotoxic)
    • Adefovir, cidofovir, didanosine
    • Aminoglycosides (gentamicin)
    • Lenalidomide, streptozocin
  • Heavy metals:
    • Lead poisoning (most common heavy metal cause in children)
    • Cadmium, mercury, platinum, uranium
  • Paraproteinemia:
  • Other:
    • Renal transplant
    • Toluene exposure (glue sniffing)
    • Aristolochic acid (herbal medicines)
    • Paraquat poisoning

Clinical features

  • Presentation varies widely depending on the underlying cause, severity, and chronicity
  • May range from an incidental laboratory finding to life-threatening electrolyte emergency

Acute/ED presentation

  • Hypokalemia — may be severe and life-threatening; case reports of cardiac arrest from Fanconi-related hypokalemia[3]
  • Metabolic acidosis — non-anion gap (hyperchloremic) from proximal (type 2) RTA (bicarbonate wasting)
  • Polyuria, polydipsia (from impaired water/sodium reabsorption)
  • Dehydration, volume depletion
  • Muscle weakness (from hypokalemia and hypophosphatemia)
  • Nausea, vomiting
  • Fatigue, malaise

Chronic/subacute presentation

  • Children:
    • Failure to thrive, growth retardation
    • Rickets (hypophosphatemic; from phosphate wasting + impaired 1,25-dihydroxyvitamin D synthesis in PCT)
    • Bone pain, pathologic fractures
    • Polyuria, polydipsia
  • Adults:
    • Osteomalacia (bone pain, pathologic fractures, proximal muscle weakness)
    • Chronic fatigue, muscle weakness
    • Kidney stones (hypercalciuria in some forms)
    • Progressive chronic kidney disease
  • Cystinosis-specific: corneal cystine crystals, hepatomegaly, hypothyroidism, retinal depigmentation, growth failure

Key laboratory pattern ("the Fanconi fingerprint")

  • Glycosuria with NORMAL serum glucose (not diabetes — inappropriately low renal glucose threshold)
  • Generalized aminoaciduria
  • Phosphaturia with hypophosphatemia
  • Bicarbonaturia with non-anion gap metabolic acidosis (proximal RTA)
  • Hypokalemia (renal potassium wasting)
  • Hypouricemia (uric acid wasting)
  • Low-molecular-weight proteinuria (β₂-microglobulin, retinol-binding protein)

Differential diagnosis

Proximal Renal tubular acidosis

  • Isolated proximal RTA (type 2) without full Fanconi features
  • Renal tubular acidosis (type 1)
  • Type 4 RTA (hypoaldosteronism)

Other causes of non-anion gap metabolic acidosis

  • Diarrhea
  • Ureteral diversion
  • Carbonic anhydrase inhibitors (acetazolamide)

Other causes of unexplained hypokalemia

Other causes of glycosuria with normal glucose

  • Renal glycosuria (isolated SGLT2 mutation)
  • Pregnancy


Renal tubular disorders

  • Salt-wasting tubulopathies
    • Gitelman syndrome — distal convoluted tubule (NCC defect); hypokalemia, hypomagnesemia, hypocalciuria, metabolic alkalosis
    • Bartter syndrome — thick ascending limb (NKCC2/ROMK/ClC-Kb defect); hypokalemia, hypercalciuria, metabolic alkalosis
    • Liddle syndrome — collecting duct (ENaC gain-of-function); hypokalemia, hypertension, metabolic alkalosis
  • Renal tubular acidosis
  • Inherited disorders of tubular transport
    • Cystinuria — proximal tubule amino acid transport defect; recurrent cystine stones
    • Fanconi syndrome — proximal tubule generalized dysfunction; glucosuria, aminoaciduria, phosphaturia
    • Nephrogenic diabetes insipidus — collecting duct (aquaporin/V2R defect); polyuria, hypernatremia
    • Dent disease — proximal tubule (ClC-5 defect); low molecular weight proteinuria, nephrocalcinosis
  • Acquired tubulopathies

Hypokalemia


Acid-base disorders

Evaluation

Workup

  • BMP: hypokalemia, hypophosphatemia, low bicarbonate (non-anion gap metabolic acidosis), low uric acid; BUN/creatinine for renal function
  • ABG/VBG: non-anion gap metabolic acidosis (hyperchloremic); urine pH may be <5.5 (unlike distal RTA) if serum bicarbonate is below the reabsorptive threshold
  • Urinalysis:
    • Glucosuria (with normal serum glucose — the hallmark clue)
    • Proteinuria (low-molecular-weight)
  • Urine electrolytes: elevated urine potassium (transtubular potassium gradient >7 suggests renal K⁺ wasting)
  • Urine amino acids: generalized aminoaciduria (elevated excretion of virtually all amino acids)
  • Fractional excretion of phosphate (FePO₄): elevated (>5%)
  • Fractional excretion of uric acid: elevated (>10%)
  • Serum phosphate, uric acid, calcium, magnesium, vitamin D (25-OH and 1,25-OH), PTH
  • Urine β₂-microglobulin — marker of proximal tubular injury[1]
  • ECG: assess for hypokalemia-related changes (U waves, flattened T waves, ST depression, prolonged QT, arrhythmias)
  • Additional workup directed at underlying cause:
    • Medication review (tenofovir, valproic acid, ifosfamide, expired tetracyclines)
    • Serum and urine protein electrophoresis, free light chains (multiple myeloma)
    • Serum copper, ceruloplasmin (Wilson disease)
    • Lead level
    • White blood cell cystine level (cystinosis)
    • Slit-lamp examination (cystine corneal crystals)

Diagnosis

  • Clinical diagnosis based on the constellation of: glycosuria with normoglycemia + generalized aminoaciduria + phosphaturia + bicarbonaturia + hypokalemia[4]
  • No single test is diagnostic; the pattern of proximal tubular losses is key
  • Once Fanconi syndrome is identified, the underlying cause must be sought

Management

Emergency management (acute presentations)

  • Hypokalemia:
    • Aggressive IV and PO potassium repletion; may be profoundly refractory due to ongoing renal losses[3]
    • Continuous cardiac monitoring if K⁺ <3.0 mEq/L or symptomatic
    • Target serum K⁺ >4.0 mEq/L
    • Consider amiloride or spironolactone to reduce renal potassium wasting
  • Metabolic acidosis:
    • Oral or IV sodium bicarbonate or sodium citrate/potassium citrate (Bicitra, Polycitra)
    • Large doses may be required (10-15 mEq/kg/day in children) because bicarbonate is lost in the urine
    • Caution: bicarbonate repletion may worsen hypokalemia (drives K⁺ intracellularly and increases distal delivery); replete potassium FIRST or concurrently
  • Dehydration: IV fluid resuscitation; may need large volumes due to polyuria
  • Hypophosphatemia:
    • Oral phosphate supplementation (Neutra-Phos, K-Phos)
    • IV phosphate if severe (<1 mg/dL) or symptomatic
  • Hypoglycemia: IV dextrose if present (hereditary fructose intolerance may present with hypoglycemia)

Identify and treat the underlying cause

  • Drug-induced: discontinue the offending agent — this is the most important intervention for acquired Fanconi syndrome[1]
    • Tenofovir: switch to tenofovir alafenamide (TAF) or alternative antiretroviral
    • Valproic acid: consider alternative antiepileptic
    • Expired tetracyclines: discard
    • Recovery after drug withdrawal may take weeks to months[2]
  • Heavy metal poisoning: chelation therapy as appropriate (see Lead poisoning)
  • Multiple myeloma/paraproteinemia: hematology/oncology referral for treatment of underlying disease
  • Cystinosis: cysteamine (Cystagon) — reduces intracellular cystine accumulation; early initiation improves renal outcomes

Long-term supplementation

  • Bicarbonate/citrate: oral sodium bicarbonate or potassium citrate for chronic metabolic acidosis
  • Phosphate: oral phosphate supplements
  • Vitamin D: calcitriol (1,25-dihydroxyvitamin D) — PCT dysfunction impairs conversion of 25-OH to active form
  • Potassium: oral supplementation; potassium-sparing diuretics if refractory
  • Nephrology follow-up for ongoing management and monitoring of renal function

Disposition

  • Severe hypokalemia (K⁺ <2.5 mEq/L), symptomatic hypokalemia, or cardiac arrhythmias: admit to monitored setting; continuous telemetry; serial electrolytes[3]
  • Severe metabolic acidosis (pH <7.2): admit for IV bicarbonate and electrolyte management
  • Significant dehydration or hemodynamic instability: admit for IV resuscitation
  • New diagnosis of Fanconi syndrome: may require admission or urgent outpatient workup depending on severity; nephrology consultation
  • Stable, known Fanconi syndrome with mild electrolyte abnormalities: outpatient management with close nephrology follow-up, medication adjustment, and return precautions for weakness, palpitations, or syncope
  • Medication-induced Fanconi: ensure offending drug is held and arrange nephrology and primary care follow-up for drug substitution and monitoring of recovery

See Also

External Links

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Fanconi Syndrome. StatPearls. 2025. PMID: 30521243
  2. ↑ 2.0 2.1 Fanconi Syndrome. Medscape. 2024.
  3. ↑ 3.0 3.1 3.2 A 57-Year-Old Female Presenting With Cardiopulmonary Arrest Secondary to Severe Hypokalemia From a Fanconi-Like Syndrome. Cureus. 2024;16(3):e55705. doi:10.7759/cureus.55705
  4. ↑ Fanconi Syndrome. Merck Manual Professional Edition. 2024.
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