Phenytoin is contraindicated in absence seizures?
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The textbooks confirm this clearly. Here is the full answer:
Phenytoin and Absence Seizures
Yes - phenytoin is contraindicated in absence seizures (petit mal epilepsy). Multiple authoritative sources confirm this:
- Goodman & Gilman states: "Phenytoin is effective against focal and generalized tonic-clonic seizures, but not generalized absence seizures."
- Katzung explicitly warns: "Phenytoin may worsen other seizure types in idiopathic generalized epilepsies, including absence epilepsy and juvenile myoclonic epilepsy, and in Dravet syndrome."
- Henry's Clinical Diagnosis confirms: "It is ineffective in treating myoclonus, absence (petit mal), and atonic seizures."
Why Does Phenytoin Fail in Absence Seizures?
Phenytoin's primary mechanism is blockade of voltage-gated sodium channels (Na⁺ channels) - it prolongs channel inactivation and prevents high-frequency neuronal firing. This works well for focal and tonic-clonic seizures.
Absence seizures, however, are generated by T-type calcium (Ca²⁺) channel activity in the thalamo-cortical circuits. Phenytoin has no effect on T-type Ca²⁺ channels, and can actually paradoxically increase absence seizure frequency.
Phenytoin is Also Ineffective/Contraindicated In:
| Seizure Type | Why |
|---|---|
| Absence (petit mal) | T-type Ca²⁺ channel mechanism; phenytoin worsens it |
| Myoclonic seizures | May worsen (seen in JME) |
| Atonic seizures | Ineffective |
| Dravet syndrome | Can worsen |
First-Line Treatment for Absence Seizures:
- Ethosuximide - selective T-type Ca²⁺ channel blocker; drug of choice for pure absence seizures
- Valproate (valproic acid) - broad-spectrum; preferred when absence co-exists with tonic-clonic seizures
- Lamotrigine - second-line option
Key exam point: Phenytoin is effective for focal seizures and tonic-clonic seizures but is contraindicated (and potentially worsening) in absence, myoclonic, and atonic seizures. Always choose ethosuximide or valproate for absence seizures.
(Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics; Katzung's Basic and Clinical Pharmacology, 16th Ed.; Henry's Clinical Diagnosis and Management)
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