What is Fosphenytoin?

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  • Written By: Maggie J. Hall
  • Edited By: Susan Barwick
  • Last Modified Date: 13 May 2020
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Administered by intramuscular or intravenous injection, physicians typically use fosphenytoin in a hospital setting for the short-term treatment of generalized convulsions and seizures. Surgeons might also administer the medication to prevent or stop seizures during neurosurgery. Considered a prodrug, or a chemical precursor, fosphenytoin sodium requires metabolic processes, that naturally take place in the body, to transform it into phenytoin. The epileptic seizure treatment is usually not recommended in persons having kidney or liver malfunction as patients may experience toxicity or organ damage.

Following the metabolic process that converts fosphenytoin into phenytoin, up to 99 percent of the medication binds to the plasma protein, albumin. Complete conversion following intravenous administration occurs in about two hours, while conversion following intramuscular injection may take up to four hours. The actions and effects of fosphenytoin generally mimic the pharmacology of oral phenytoin.

Once administered, fosphenytoin acts as a sodium channel blocker on the surface of neural cells. When these channels remain open too long, too many sodium ions pass through, triggering a rapid flow of stimulating neurotransmitters. These, in turn, allow excessive amounts of calcium to enter the cells, causing a toxic effect that initiates convulsive and seizure activity. By limiting sodium ion access, fosphenytoin prevents the chemical chain reactions contributing to cellular hyperstimulation and damage due to possible toxic conditions. The medication’s action concerning ion regulation may pose a hazard for patients having certain cardiac conditions.

Fosphenytoin usually weakens the conduction system in the cardiac atria and ventricles, exacerbating symptoms in patients diagnosed with heart disease, and contributing to a possible life-threatening condition. When infused intravenously in high enough doses, the medication also produces hypotension. As metabolism of the drug occurs in the liver, individuals having hepatic impairment may exhibit jaundice, organ enlargement, and swollen lymph glands. Patients might also experience anemias as the medication decreases the number of red and white blood cells along with platelets.

Health care providers might require periodic blood tests to monitor cells counts, but patients should report any signs of unusual bruising or bleeding. Phosphorus is a by-product produced during fosphenytoin metabolism, which presents a danger for patients with kidney malfunction. Common adverse effects associated with the medication include injection site reactions due to the drug’s high alkalinity. Headache, dizziness, nausea, and possible vomiting are also typical reactions. Following administration, individuals should exercise caution when standing or walking as the medication might cause a lack of physical coordination.

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