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Anticonvulsants

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Anticonvulsants

Key Points

  • Anticonvulsants suppress abnormal electrical impulses in the cerebral cortex — they prevent seizures but do not cure epilepsy
  • Phenytoin (Dilantin): Narrow therapeutic index 10–20 mcg/mL; IV rate ≤50 mg/min (cardiac risk); NEVER mix with D5W; monitor for Stevens-Johnson syndrome
  • NEVER abrupt discontinuation — risk of status epilepticus (continuous seizures); always taper
  • IV phenytoin: Requires cardiac monitoring; infuse no faster than 50 mg/min in adults
  • Gabapentin: Structurally similar to GABA; used for neuropathic pain as well as seizures
  • Valproate (Depakote): Mood stabilizer and anticonvulsant; teratogenic (spina bifida risk) — avoid in pregnancy
  • In many first-time seizure cases, immediate long-term anticonvulsant initiation is not automatic; early neurology follow-up and individualized risk review are emphasized

Drug Class Overview

Anticonvulsants (anti-seizure drugs) reduce seizure frequency and severity by stabilizing neuronal cell membranes and suppressing abnormal electrical discharges. They work through three primary mechanisms:

  1. Increase seizure threshold in the motor cortex — harder for neurons to become excited
  2. Limit seizure spread — suppress transmission from one neuron to the next
  3. Decrease nerve impulse conduction velocity within neurons

Most anticonvulsants enhance the inhibitory neurotransmitter GABA or block excitatory sodium or calcium channels.

Seizure Classification (Guides Drug Selection)

  • Generalized seizures: Involve both brain hemispheres (tonic-clonic/grand mal, absence/petit mal)
  • Focal (partial) seizures: Begin in one brain area (simple focal, complex focal, secondary generalized)

Major Drug Classes

Hydantoins — Phenytoin (Dilantin)

Mechanism: Blocks voltage-sensitive sodium channels → reduces high-frequency neuronal firing.

Indications: Tonic-clonic (grand mal) seizures, focal seizures, prevention of post-neurosurgical seizures.

Narrow Therapeutic Index: Therapeutic serum level 10–20 mcg/mL — routine monitoring required.

IV Phenytoin — Rate Restriction

IV phenytoin must NOT exceed 50 mg/min in adults (1–3 mg/kg/min in pediatrics) — faster infusion causes severe hypotension and cardiac arrhythmias. Requires continuous cardiac monitoring during and after IV administration. NEVER mix with D5W (causes precipitation).

Adverse Effects:

  • CNS: Nystagmus (early toxicity sign), ataxia, slurred speech, drowsiness, confusion
  • Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN): Severe life-threatening skin reactions — discontinue immediately at first rash
  • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS): Fever, rash, lymphadenopathy, organ involvement
  • Long-term: Gingival hyperplasia (gum overgrowth), osteoporosis, phenytoin-induced vitamin D deficiency
  • Teratogenic: Fetal hydantoin syndrome — avoid in pregnancy

Barbiturates — Phenobarbital

Mechanism: Enhances GABA activity → increases chloride ion conductance → CNS depression.

Indications: Broad-spectrum anticonvulsant; also used for alcohol withdrawal.

Adverse Effects: Sedation, CNS depression, respiratory depression, dependence.

Carbamazepine (Tegretol)

Mechanism: Blocks sodium channels.

Indications: Focal seizures, tonic-clonic seizures; also used as mood stabilizer for bipolar disorder; trigeminal neuralgia.

Therapeutic Range: 4–12 mg/L.

Significant Drug Interactions: Induces CYP450 hepatic enzymes → reduces effectiveness of many other drugs (oral contraceptives, warfarin).

Adverse Effects: Stevens-Johnson syndrome, agranulocytosis, hepatotoxicity. Requires CBC and liver function monitoring.

Valproate / Valproic Acid (Depakote)

Mechanism: Enhances GABA; blocks sodium channels.

Indications: Broad-spectrum anticonvulsant (all seizure types); bipolar mania; migraine prophylaxis.

Therapeutic Range: 50–125 mcg/mL.

Teratogenicity: Neural tube defects (spina bifida) — contraindicated in pregnancy unless benefit clearly outweighs risk.

Adverse Effects: Hepatotoxicity (requires LFT monitoring), pancreatitis, thrombocytopenia, hair loss, weight gain.

Levetiracetam (Keppra)

Mechanism: Binds synaptic vesicle protein SV2A → reduces neurotransmitter release.

Advantages: Broad-spectrum; few drug interactions; no enzyme induction; approved in pregnancy (relatively safer).

Adverse Effects: Behavioral changes (irritability, aggression), somnolence, headache.

Gabapentin (Neurontin)

Mechanism: Structurally similar to GABA but does NOT bind to GABA receptors; modulates calcium channels.

Primary Uses: Neuropathic pain (diabetic peripheral neuropathy, postherpetic neuralgia), adjunct for partial seizures.

Adverse Effects: Dizziness, somnolence, ataxia — dose-dependent CNS depression.

Nursing Assessment

Before Administration:

  • Obtain seizure history: type, frequency, triggers, duration
  • Baseline blood levels for drugs with narrow therapeutic index (phenytoin, carbamazepine, valproate)
  • Liver and kidney function (most anticonvulsants are hepatically metabolized)
  • Pregnancy status — multiple anticonvulsants are teratogenic

During IV Administration (Phenytoin):

  • Monitor cardiac rhythm continuously
  • Infuse no faster than 50 mg/min in adults
  • Use NS (NOT D5W) for dilution
  • Have emergency resuscitation equipment available

Ongoing Monitoring:

  • Therapeutic drug levels (trough and peak where applicable)
  • Signs of toxicity: ataxia, nystagmus, slurred speech (phenytoin); behavioral changes (levetiracetam)
  • CBC and liver function (carbamazepine, valproate)
  • Skin: report any rash immediately — may indicate Stevens-Johnson syndrome

Patient Education:

  • Take medications exactly as prescribed — do NOT miss doses or stop abruptly
  • Abrupt discontinuation can cause status epilepticus
  • Avoid alcohol and other CNS depressants
  • May impair alertness — do not drive until drug effect is established
  • Carry medical alert identification
  • Phenytoin patients: maintain regular dental hygiene (gingival hyperplasia)
  • Report skin rash, fever, jaundice, unusual bruising immediately
  • Keep follow-up appointments after new-onset seizures (neurology follow-up is time-sensitive) and bring seizure/event records for therapy adjustment decisions

Self-Check

  1. A patient receiving IV phenytoin develops hypotension and bradycardia during infusion. What is the most likely cause, and what is the priority nursing action?
  2. A patient on long-term phenytoin develops a rash with blistering and mucosal involvement. What serious adverse reaction do you suspect, and what should be done immediately?
  3. Why must anticonvulsants never be discontinued abruptly, even if a patient feels seizure-free for a long time?

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