Fosphenytoin is a prodrug and is metabolized quickly to phenytoin after administration. The conversion of fosphenytoin to phenytoin involves the release of phosphate. In fact, each mmol of fosphenytoin releases 1 mmol of phosphate.
Clinical Question
Are patients at risk for hyperphosphatemia after fosphenytoin loading?
Data
There are only two cases of reported hyperphosphatemia.
A 17-year old African-American male with end-stage renal disease developed acute hyperphosphatemia to 3.9 mmol/L (12.1 mg/dL) following the IV administration of 1000 mg of fosphenytoin for an idiopathic complex partial seizure
An infant in status epilepticus had marked hyperphosphatemia 8.4 mmol/L (25.9 mg/dL) after a 5-10 fold dosing error.
Bottom Line
Despite the phosphate load from fosphenytoin administration, hyperphosphatemia is very rare and probably associated with renal insufficiency and dosing errors.
References
McBryde KD, et al. Hyperphosphatemia due to fosphenytoin in a pediatric ESRD patient. Pediatr Nephrol. 2005;20(8):1182-5. [PMID 15965770]
Rose R, et al. Fosphenytoin-induced bradyasystole arrest in an infant treated with charcoal hemofiltration [abstract]. J Toxicol Clin Toxicol. 1998;36:473.
