Introduction
Background
Phenytoin is a commonly prescribed anticonvulsant used to treat most types of seizure disorders and status epilepticus, with the exception of absence seizures.
Historically, phenytoin has been used as an antidysrhythmic agent, especially in the treatment of digoxin toxicity. It has recently fallen out of favor because of the advent of digoxin antibody fragments. Similarly, phenytoin is no longer considered appropriate for the management of toxin-induced seizures or for the treatment of alcohol withdrawal syndrome.
Pathophysiology
Mechanism of action
Phenytoin blocks voltage-sensitive sodium channels in neurons. This action leads to a delay in neuronal electrical recovery. Phenytoin's inhibitory effect is dependent on the voltage and frequency of neural cell firing by selectively blocking the neurons that are firing at high frequency. Phenytoin prevents the electrical spread of a focus of irritable tissue from entering normal tissue.
Phenytoin administration has been associated with toxic effects. Phenytoin toxicity depends on the route of administration, duration, exposure, and dosage. The route of administration is the most important determinant of toxicity. Phenytoin may be administered orally or intravenously. In addition, fosphenytoin may be administered intramuscularly.
Pharmacokinetics
Phenytoin is a weak acid and has erratic GI absorption. Following ingestion, phenytoin precipitates in the stomach's acid environment; this characteristic is particularly important in the setting of an intentional overdose. Peak blood levels occur 3-12 hours following single dose ingestion.
The parenteral form of phenytoin is dissolved in 40% propylene glycol and 10% ethanol and adjusted to a pH of 12; sodium hydroxide is added to maintain solubility. Extravasation of the solution may cause skin irritation or phlebitis. Phenytoin administered intravenously at a rate higher than 50 mg/min may cause hypotension and arrhythmias. These complications are believed to be secondary to the diluent, propylene glycol.
Phenytoin has a volume of distribution of 0.6 L/kg and is extensively bound to plasma proteins (90%). Blood levels of phenytoin reflect only total serum concentration of the drug. Only the free unbound phenytoin has biological activity.
Population groups that are predisposed to elevated free phenytoin levels include neonates, elderly persons, and individuals with uremia, hypoalbuminemia, or hyperbilirubinemia. These patients may exhibit signs of toxicity when drug levels are within the therapeutic range (see Lab Studies). Certain medications can interfere with phenytoin levels.
Hepatic microsomal enzymes primarily metabolize phenytoin. Much of the drug is excreted in the bile as an inactive metabolite, which is then reabsorbed from the intestinal tract and ultimately excreted in the urine. Less than 5% of phenytoin is excreted unchanged in the urine. Individuals with impaired metabolic or excretory pathways may exhibit early signs of toxicity.
Phenytoin metabolism is dose dependent. Elimination follows first-order kinetics (fixed percentage of drug metabolized during a per unit time) at the low drug concentrations and zero-order kinetics (fixed amount of drug metabolized per unit time) at higher drug concentrations. This change in kinetics reflects the saturation of metabolic pathways. Thus, very small increments in dosage may result in adverse effects.
Frequency
United States
Of the 4145 toxic exposures reported in the 2003 Annual Report of the American Association of Poison Control Centers' Toxic Exposure Surveillance System (AAPCC TESS), 2173 were unintentional toxicities, 1398 were intentional, and 460 were reported as adverse reactions.1
Mortality/Morbidity
Death or severe morbidity rarely occurs with an intentional overdose as long as the patient receives good supportive care.
Of the 4145 reported toxic exposures in 2003, 2756 were treated in a health care facility. Of this subset of patients, 850 had no significant outcome, 1035 had minor effects, 743 had moderate morbidity, 98 had major morbidity, and 10 resulted in fatality.1
Race
No scientific evidence has demonstrated that outcomes of acute toxicity are based on race.
Sex
Phenytoin is a category D drug. Various congenital anomalies have been reported from usage during pregnancy (see Fetal hydantoin syndrome). No scientific data have demonstrated that effect or outcome of acute toxicity is based on sex.
Age
Neonates and elderly patients are at greater risk for toxicity because of impaired metabolism and decreased protein binding.
- Decreased protein binding contributes to higher levels of biologically active medication at therapeutic measured plasma levels (see Lab Studies).
- Of the 4145 reported exposures in 2003, a total of 600 patients were younger than 6 years, 283 were aged 6-19 years, and 3228 were older than 19 years.1
Clinical
History
Establish if the toxicity is acute or chronic.
- Intentional overdose
- Important historical elements
- Time of ingestion
- Co-ingestants
- Motivation for ingestion
- Medications available in the household
- Paramedics or family members may be able to provide additional information (eg, medications, past medical history).
- Important historical elements
- Chronic toxicity
- Important historical elements
- Duration of administration
- Dosing
- Frequency
- Compliance (last dose and missed dose)
- Recent changes to their pharmacotherapy
- Important elements for patient query
- When symptoms began
- Severity of symptoms
- Exacerbating factors
- Associated problems
- Relieving factors
- Important historical elements
Physical
- Phenytoin may cause a febrile reaction, hypotension (during intravenous infusion), or bradycardia.
- Mouth -Gingival hyperplasia (chronic use), the most common adverse effect (20%)
- Neurologic
- Hyperreflexia or hyporeflexia
- Abnormal gait (bradykinesia, truncal ataxia)
- Respiratory distress
- Encephalopathy
- Meningeal irritation with pleocytosis
- Tremor (intention)
- Irritability or agitation
- Confusion
- Hallucinations
- Mental status varies from completely normal to the extremes of stupor and coma, particularly if co-ingestants are present
- Peripheral neuropathy (chronic use)
- Priapism
- Urinary incontinence
- Choreoathetoid movements
- Dysarthria
- Dysphagia
- Seizures (rare)
- Death (rare)
- Eyes
- Nystagmus (horizontal, vertical)
- Ophthalmoplegia
- Diplopia
- Miosis or mydriasis
- Hypersensitivity reactions
- Fever, rash, and lymphadenopathy, commonly observed together
- Systemic lupus erythematosus (SLE)
- Polyarteritis
- Polymyositis
- Eosinophilia
- Megaloblastic anemia
- Pseudolymphoma
- Lymphadenopathy
- Vascular - Phlebitis
- Skin
- Hirsutism
- Acne
- Rashes, can be mild, morbilliform, scarlatinoid or as severe as Stevens-Johnson syndrome
- Jaundice
- Facial or periorbital edema
- Erythema multiforme (EM)
- Toxic epidermal necrolysis (TEN)
- GI/abdomen
- Right upper quadrant tenderness
- Hepatomegaly
- Splenomegaly
- Nausea
- Vomiting
- Hepatitis
- Other
- Fetal hydantoin syndrome is manifested by broad nasal bridge, wide fontanelle, low hairline, cleft lip/palate, epicanthal folds, short neck, microcephaly, low-set ears, small or absent nails, dislocated hip, hypoplasia of distal phalanges, impaired growth, and congenital heart defects.
- Metabolic symptoms may include osteomalacia and hypothyroidism (chronic toxicity).
- Hypotension, bradycardia2 , myocardial depression, ventricular fibrillation, asystole, and tissue necrosis3 all have been associated with the IV formulation.
More on Toxicity, Phenytoin |
 Overview: Toxicity, Phenytoin |
| Differential Diagnoses & Workup: Toxicity, Phenytoin |
| Treatment & Medication: Toxicity, Phenytoin |
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| References |
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References
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Further Reading
Keywords
phenytoin toxicity, phenytoin poisoning, phenytoin exposure, anticonvulsant drug, seizure disorder, phenytoin overdose, elevated phenytoin levels, status epilepticus, phenytoin ingestion, seizure treatment, treatment of seizure
