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Fosphenytoin is a medication used to manage seizures (treatment of generalized tonic-clonic status epilepticus, focal [partial] onset seizures, or generalized onset seizures, and for the prevention and treatment of seizures occurring during neurosurgery). It is a water-soluble phenytoin prodrug that can be employed in situations where phenytoin use is not advisable. It is in the hydantoin anticonvulsant class of medications. This activity outlines the indications, action, and contraindications for fosphenytoin as a valuable agent for neurosurgery prophylaxis, short-term oral phenytoin substitute, and treatment for status epilepticus. This activity will highlight the mechanism of action, adverse event profile, off-label use, and monitoring pertinent for healthcare team members in managing patients with seizures and related conditions. Objectives: Identify the mechanism of action of fosphenytoin. Describe the adverse effects of fosphenytoin. Review the appropriate monitoring for a patient taking fosphenytoin. Summarize interprofessional team strategies for improving care coordination and communication to advance fosphenytoin and improve outcomes. Access free multiple choice questions on this topic.

Fosphenytoin has a black box warning due to its activated form of phenytoin. In adults, exceeding 150 mg phenytoin equivalents per minute may lead to severe cardiac outcomes, particularly hypotension and cardiac arrhythmias. Therefore, clinicians should perform cardiac monitoring up to 30 minutes after IV infusion. In the case of cardiac abnormalities, the discontinuation or reduction of the drug dosage may be necessary.[31] Fosphenytoin has the same warning signs as phenytoin. The first to manifest during an acute overdose are nystagmus, ataxia, and dysarthria. The calculated doses appear to differ from patient to patient; however, nystagmus is evident at as little as 20 µg/mL doses, with lethargy and dysarthria presenting at upwards of 40 µg/mL. The Lethal dose of fosphenytoin is 2 to 5 grams in adults. In acute overdosage, the possibility of other CNS depressants, including alcohol, should be considered.[19] Treatment is nonspecific since there is no known antidote to fosphenytoin or phenytoin overdosage. The clinicians should examine the adequacy of the respiratory and circulatory systems and ensure relevant supportive measures. According to product labeling, in severe intoxication in the pediatric population, total exchange transfusion is used. Nephrologists should consider hemodialysis in severe cases.[32] Formate and phosphate, the metabolites of fosphenytoin, may lead to signs of toxicity following overdosage. Formate toxicity is associated with severe anion-gap metabolic acidosis. Phosphate in toxic amounts can cause hypocalcemia with paresthesia, muscle spasms, and seizures. Therefore, ionized calcium levels should be monitored to guide the treatment of hypocalcemia.[33]

Fosphenytoin is the prodrug formulation of the commonly used drug phenytoin. Although it is considered a safer formulation of phenytoin, it still maintains a similar adverse drug reaction profile. Fosphenytoin carries a black box warning for severe cardiac outcomes. Severe hypotension and cardiac arrhythmias are significant risk factors of fosphenytoin overdose exceeding 150 mg phenytoin equivalents per minute. In such cases, clinicians should provide support as per ACLS guidelines.[34] [Level 2] All the healthcare providers involved in taking care of the patient receiving fosphenytoin should be accurate with its indications, pharmacology, adverse effects, monitoring, and toxicity. Internists, neurologists, or intensivists should prescribe fosphenytoin for appropriate indications. Nurses should check IV compatibility and administer fosphenytoin according to product labeling. Pharmacists should ensure proper dosing and report to clinicians in case of drug overdose or interactions. Management of severe hypotension and cardiac arrhythmias requires ICU care under critical care physician supervision. The nephrologist consultation is necessary for severe acute overdose requiring hemodialysis or exchange transfusion. Clinicians should suggest that pregnant patients register in the North American Antiepileptic Drug (NAAED). This registry gathers information about the adverse drug reactions of antiepileptic drugs during pregnancy, enhancing patient safety and outcomes.[35] As illustrated above, multiple healthcare providers, including clinicians(MDs, DOs, NPs, PAs), specialists, and nurses, are involved in caring for patients receiving fosphenytoin. Therefore an interprofessional team approach would achieve maximum efficacy and minimize adverse drug reactions translating to better patient outcomes. [Level 5]