hypophosphataemia

see also:

• phosphate

• hypophosphatemia is usually asymptomatic even if severe, however, levels below 0.32mmol/L may cause symptoms such as seizures, muscle weakness, and even coma

• acute respiratory alkalosis (eg. hyperventilation)
  • perhaps the most common cause in hospital settings

• primary or secondary hyperparathyroidism
• vitamin D deficiency or resistance
• primary renal wasting - rare hereditary forms - no hypocalcaemia though
• Fanconi syndrome
• osmotic diuresis
• some other diuretics - acetazolamide, some thiazides
• some chemotherapy agents
• some iron infusions
• renal dialysis
• hereditary hypophosphatasia (HHP):
  • a molecular defect in the gene encoding tissue non-specific alkaline phosphatase (TNSALP)
  • low alkaline phosphate levels, increased pyrophosphate and decreased phosphylation of vitamin B6 and thus reduced entry of B6 into cells
  • severe hypophosphatasia is estimated to be 1:100,000 in a population of largely Anglo-Saxon origin;
  • higher prevalence in Spanish¹⁾ and in Mennonites in Canada
  • perinatal form: most fatal form; profound hypomineralization; stillbirth or neonatal death rates are high;
  • infantile form: poor feeding and inadequate weight gain; rickets; rib fractures; hypercalcaemia;
  • childhood form: delayed walking; premature loss of deciduous teeth or fusing of cranial sutures; rickets; myopathy; growth retardation;
  • adult form: may have features of childhood form; early loss of adult dentation; pseudogout / pyrophosphate arthropathy; stress fractures;

• poor intake is rarely a cause
• medications that bind phosphate eg. aluminium, magnesium
• steatorrhoea or chronic diarrhoea

• acute glycolysis causes increased phosphorylation within cells causing reduction in extracellular phosphate
  • fall more likely to be severe if underlying phosphate depletion
  • eg. insulin, glucagon, adrenaline, DKA
• acute post-parathyroidectomy / hungry bone syndrome

• see phosphate for phosphate replacement Rx
• if high PTH:
  • if low calcium then:
    • exclude pancreatitis, hypomagnesaemia, over-hydration
    • otherwise likely to be vitamin D deficiency Rx with vitamin D
  • if high calcium then either primary or tertiary hyperparathyroidism:
    • phosphate replacement
    • consider cinacalcet as a temporary measure to lower calcium levels and potentially improve phosphate levels before definitive surgery
    • surgical excision of parathyroid gland causing the hyper PTH