(Lancet 1998;352:391)
Definition: Nml serum [Phos] 2.8–4.5 mg/dL w/ slightly ↑ concentration in children vs. adults.
Pathophysiology
• Homeostasis by balance btw GI absorption (60–80% dietary Phos absorbed) and renal excretion (80% Phos reabsorbed in proximal tubule, coupled to Na)
• Controlled by PTH (inc renal secretion) and vit D (inc GI absorp, dec urine excretion)
Hypophosphatemia
• Etiology
• ↑ urinary excretion: HyperPTH, vit D deficiency, renal tubule defects (Fanconi, hereditary hypophosphatemic rickets [formerly known as vit D dependent rickets] caused by mutations in PHEX [X-linked], FGF23 [autosomal dominant], DMP1 [autosomal recessive], SLC34A3 [autosomal recessive hypophosphatemic rickets with hypercalciuria] etc.), metab or resp acidosis, acetazolamide (Nat Rev Nephrol 2010;6:657; Curr Opin Pediatr 2007;19:488)
• ↓ intestinal absorption: Dietary restriction, chronic antacid use, vit D def, chronic diarrhea, steatorrhea
• Internal redistribution: Hungry bone syndrome, respiratory alkalosis, recovery from DKA or malnutrition, refeeding syndrome (Pediatr Clin N Am 2009;56:1201), sepsis
• Clinical manifestations
• Rarely symptomatic unless severe (<1 mg/dL); generally due to ATP deficiency
• Muscle dysfunction w/ prox myopathy, dysphagia, and ileus; rarely rhabdo
• Respiratory failure 2/2 weakness of respiratory muscles
• Cardiac involvement w/ impaired contractility and CHF
• Heme abnormalities w/ hemolysis, thrombocytopenia, and impaired phagocytosis
• Renal dysfunction w/ calcium and magnesium wasting
• Neurologic dysfxn w/ metab encephalopathy, seizures, lethargy, confusion, coma
• Evaluation: Chem 7, Ca (iCa), Phos, Mg, vit D, PTH, UCa, UPhos, UCr, and urine pH
• Management
• Acute onset w/ severe depletion (<1 mg/dL) and/or symptoms treat w/ IV
• KPhos contains 4.4 mEq K+ and 3 mMol phosphate/mL. NaPhos contains 4 mEq Na+ and 3 mMol phosphate/mL
• Administer 0.1–0.2 mMol/kg phosphate over 6 hr, then maintenance infusion 15–45 mg/kg/d; observe for HypoCa and follow [Ca] × [Phos] product
• Chronic onset: NeutraPhos contains 250 mg (= 8 mMol) phosphate per packet, half as the Na salt and half as the K salt
• Cow’s milk has 1 mg Phos/mL
Hyperphosphatemia
• Etiology
• Inc exogenous load: Cow’s milk intake in infants, vit D intoxication, phosphate containing enemas, supplements
• Inc endogenous load: Tumor lysis syndrome, rhabdomyolysis, malignant hyperthermia (w/ anesthesia), hemolysis, acidosis
• Dec renal excretion: Renal failure, hypoPTH, Mg def, acromegaly, thyrotoxicosis
• Clinical manifestations
• Rapid increase in [Phos] can result in hypoCa and assoc symptoms
• [Ca] × [Phos] >70 results in soft-tissue deposition of calcium
• Evaluation: Serum electrolytes, BUN/Cr, Ca (iCa), Phos, Mg, vit D, PTH, ABG, UCa, UPhos, UCr, and urine pH
• Management
• Most effective therapy is reduction of intestinal absorption 2/2 dec dietary intake and use of phosphate binders (calcium carbonate, calcium acetate [PhosLo], Renagel [binding resin])
• If 2/2 cell lysis, management as per management of tumor lysis syndrome or rhabdomyolysis (IV fluid, consider mannitol)
• If severe symptoms and 2/2 renal failure, consider dialysis