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# Mylot%2525252525252525252525252525252525252525252525252525252525252525252525252525252525252525252525252525252525252525252525252525252520at (Mephenytoin)
## Overview
Mephenytoin is an antiepileptic drug (AED) belonging to the hydantoin class. It is a sodium channel blocker that stabilizes neuronal membranes, reducing the repetitive neuronal firing.
## Primary Indications
* Refractory partial seizures (focal seizures) with or without secondary generalization.
## Adult Dosing
* **Loading Dose:** Historically, a loading dose was used to achieve therapeutic serum concentrations rapidly. This is generally no longer recommended due to significant toxicity risks.
* **Maintenance Dose:** Typically initiated at 100 mg PO TID.
* Dose may be increased gradually, often weekly, by 100 mg/day based on clinical response and tolerance.
* Usual maintenance dose range: 200-600 mg/day PO divided into 2-3 doses.
* Maximum recommended dose: 800 mg/day PO. Doses exceeding this are associated with increased toxicity.
## Pediatric Dosing
* There is no established pediatric dosing for mephenytoin. Its use in children is generally discouraged due to a higher risk of adverse effects, particularly hematologic and dermatologic reactions.
## Dose Adjustments
* **Hepatic Impairment:** Use with caution and consider dose reduction. Mephenytoin is extensively metabolized by the liver.
* **Renal Impairment:** Use with caution.
## Contraindications
* Hypersensitivity to mephenytoin or other hydantoins.
* History of bone marrow suppression.
* History of significant hepatic disease.
## Adverse Effects
* **Common:** Drowsiness, dizziness, ataxia, nystagmus, confusion, headache, nausea, vomiting, constipation, rash.
* **Serious:**
* **Hematologic:** Aplastic anemia, agranulocytosis, leukopenia, thrombocytopenia, megaloblastic anemia (folate deficiency).
* **Dermatologic:** Severe skin reactions including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). Mephenytoin is associated with a high risk of these reactions.
* **Hepatic:** Hepatotoxicity.
* **Neurologic:** Paradoxical increase in seizure frequency, movement disorders (e.g., choreoathetosis).
* **Metabolic:** Gingival hyperplasia, hirsutism, osteomalacia.
## Key Drug Interactions
* **CYP450 Inducers/Inhibitors:** Mephenytoin is metabolized by CYP2C19 and can induce or inhibit other CYP enzymes. Interactions with numerous AEDs and other medications are possible.
* **CNS Depressants:** Additive sedative effects with alcohol, benzodiazepines, opioids.
* **Folic Acid Antagonists:** Mephenytoin can decrease serum folate levels.
## Monitoring
* **Therapeutic Drug Monitoring (TDM):** Serum concentrations should be monitored. Therapeutic range is generally considered 2-4 mg/L, but clinical response is paramount. Higher levels correlate with increased toxicity.
* **Complete Blood Count (CBC) with differential:** Baseline and regularly throughout therapy, especially with dose increases or symptoms of infection.
* **Liver Function Tests (LFTs):** Baseline and periodically.
* **Renal Function Tests:** Baseline and periodically.
* **Signs and symptoms of skin reactions:** Educate patients to report any new rash or blistering immediately.
* **Signs and symptoms of hematologic abnormalities:** Monitor for signs of infection, bleeding, or bruising.
## Clinical Pearls
* Mephenytoin is rarely used due to its narrow therapeutic index and high risk of serious adverse effects, particularly severe dermatologic and hematologic reactions.
* It is typically reserved for patients with epilepsy refractory to other AEDs.
* Initiate therapy at a low dose and titrate slowly.
* Educate patients thoroughly on potential side effects and the importance of reporting any new symptoms immediately.
* Consider folate supplementation due to the risk of folate deficiency.
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**Disclaimer:** This information is intended for educational purposes and does not substitute for professional medical advice. Always consult the most current prescribing information and guidelines for complete and up-to-date details before making any treatment decisions.
