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Pharmacologic class: Hydantoin
Therapeutic class: Anticonvulsant
Pregnancy risk category D
Thought to regulate neuronal membrane by promoting sodium excretion from neurons. This action prevents hyperexcitability and excessive stimulation, which inhibits spread of seizure activity. Lacks general CNS depressant effect.
Injection: 150 mg in 2-ml vials (100 mg phenytoin sodium), 750 mg in 10-ml vials (500 mg phenytoin sodium)
Indications and dosages
➣ Status epilepticus
Adults: 15 to 20 mg phenytoin sodium equivalent (PE)/kg I.V. at 100 to 150 mg PE/minute as a loading dose, then 4 to 6 mg (PE)/kg I.V. daily for maintenance
➣ To prevent seizures during neurosurgery
Adults: 10 to 20 mg PE/kg I.M. or I.V. as a loading dose, then 4 to 6 mg PE/kg I.M. or I.V. daily for maintenance
• Hepatic disease
• Renal impairment
• Elderly patients
• Hypersensitivity to drug
• Adams-Stokes syndrome
Use cautiously in:
• hepatic or renal impairment, severe cardiac or respiratory disease
• elderly patients
• pregnant or breastfeeding patients (safety not established).
• Know that drug is a phenytoin prodrug and is given in PE units to avoid the need to perform molecular weight-based adjustments when converting between fosphenytoin and phenytoin sodium doses.
• For I.V. use, dilute in dextrose 5% in water or normal saline solution.
• Don't give faster than 150 mg PE/minute. Too-rapid infusion causes hypotension.
☞ Check ECG, vital signs, and overall patient status continuously during infusion and for 10 to 20 minutes afterward.
• When giving I.M., rotate injection sites.
CNS: ataxia, agitation, dizziness, drowsiness, dysarthria, dyskinesia, speech disorder, extrapyramidal syndrome, headache, nervousness, weakness, confusion, hyperesthesia, paresthesia, cerebral edema, coma, intracranial hypertension
CV: hypotension, tachycardia
EENT: diplopia, nystagmus, tinnitus
GI: nausea, vomiting, constipation, dry mouth, anorexia
GU: pink, red, or reddish-brown urine
Hematologic: lymphadenopathy, aplastic anemia, agranulocytosis, leukopenia, megaloblastic anemia, thrombocytopenia
Metabolic: hypocalcemia, hypokalemia, hyperglycemia, increased glucose tolerance
Musculoskeletal: back or pelvic pain, osteomalacia
Skin: hypertrichosis, rash, pruritus, exfoliative dermatitis, Stevens-Johnson syndrome
Other: gingival hyperplasia, altered taste, fever, facial edema, weight loss, injection site pain, allergic reactions
Drug-drug. Amiodarone, benzodiazepines, chloramphenicol, cimetidine, disulfiram, estrogens, felbamate, fluconazole, fluoxetine, halothane, influenza vaccine, isoniazid, itraconazole, ketoconazole, methylphenidate, miconazole, omeprazole, phenothiazines, phenylbutazone, salicylates, sulfonamides, tolbutamide, trazodone: increased fosphenytoin blood level
Antidepressants, antihistamines, opioids, sedative-hypnotics: additive CNS depression
Barbiturates, carbamazepine, reserpine: decreased fosphenytoin blood level
Corticosteroids, cyclosporine, doxycycline, estrogens, felbamate, methadone, quinidine, rifampin: altered effects of these drugs
Dopamine: additive hypotension
Lidocaine, propranolol: additive cardiac depression
Streptozocin, theophylline: decreased efficacy of these drugs
Warfarin: initial increase in warfarin effects in patients stabilized on warfarin therapy, followed by decreased response to warfarin
Drug-diagnostic tests. Alkaline phosphatase, glucose, hepatic enzymes: increased levels
Dexamethasone, metyrapone: test interference
Glucose tolerance test: decreased tolerance
Potassium, thyroxine: decreased levels
Thyroid function tests: decreased values
Drug-behaviors. Acute alcohol ingestion: increased drug blood level, additive CNS depression
Chronic alcohol ingestion: decreased drug blood level
• Be prepared to slow administration or stop therapy if significant cardiovascular reactions occur.
• Monitor neurologic status carefully, especially for evidence of increasing intracranial pressure.
☞ Assess for rash. Withhold drug and notify prescriber if it occurs.
• Monitor phenytoin blood level after drug has metabolized to phenytoin (about 2 hours after I.V. dose or 4 hours after I.M. dose).
• Monitor electrolyte levels.
• Evaluate blood glucose level. Watch for hyperglycemia in patients with diabetes.
• Inform patient that he may experience sensory disturbances during I.V. administration.
☞ Advise patient to immediately report adverse effects, particularly rash.
• Tell patient that drug may turn his urine pink, red, or reddish brown.
• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, and behaviors mentioned above.