Phenobarbital

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Sections available in full article Name, Class of drug, Description, Uses, Administration, Routes of administration, Dosage, Pharmocokinetics, Normal, Precautions, Contra-indications, Use with care, Interactions, Adverse reactions, Sources, Publications,
Contributors Dr Kyle Braund BVSc MVSc PhD FRCVS DipACVIM
Dr Laurent Garosi DVM DipECVN MRCVS
Synonyms Epiphen Phenobarbitone

Name

  • Phenobarbital sodium.

Class of drug

  • Anticonvulsant.
  • Long-acting barbiturate.
  • Schedule IV controlled drug (non-narcotic basic class) and available by prescription only.

Uses

Action

  • Increases the threshold of electrical excitability and by decreasing the duration of after discharges in the motor cortex.
  • Inhibits kindling and increases threshold.
  • Increases responsiveness to inhibitory post-synaptic GABA-ergic interneurones by increasing chloride conductance in the cell and by reducing glutanate-induced depolarization of excitatory neurons.
  • Blocks pre-synaptic entry of calcium ions into nerve cells.

Indications

  • Drug of first choice in management of seizures Seizures.
  • Occasionally as a tranquilizer.
  • The initial drug of choice for treating idiopathic seizure disorders Epilepsy: idiopathic in both dogs and cats and is also used adjunctively for the emergency treatment of acute seizure disorders secondary to other causes, eg strychnine, tetanus Tetanus , meningitis Meningitis.

Adverse reactions

Effects of overdosage

  • Hepatotoxicity is rare (mostly seen in dogs on chronic treatment with phenobarbitone serum level above 35 mg/L) and is characterized by sedation, ataxia, anorexia, icterus, acites or coagulopahy with increased pre-and post-prandial serum bile acids, low albumin, increased ALT, ALP, bilirubin.
  • Hepatotoxicity may be reversible if detected early and phenobarbitone withdrawn.

Other reported reactions

  • Drowsiness/sedation.
  • Ataxia.
  • PU/PD/PP (usually dose related).
  • Nystagmus.
  • Restlessness or hyperexcitability.
  • Drug rash.
  • Hyperprothrombinemia.
  • Osteomalacia.
  • PU is thought to be due to central inhibition of ADH release.
  • PP is believed to result from suppression of the satiety center in the ventromedial hypothalamus.
  • Phenobarbital has been shown to induce coagulation defects in cats by reducing vitamin K-dependent clotting factors (II, VII, IX, X).
Dogs
  • Neutropenia.
  • Thrombocytopenia.
  • Anemia.
  • Splenomegaly.

Investigation of adverse reaction

Serum phenobarbitone

  • Monitor levels if toxicity or lack of seizure control is encountered 2 hours before and 2 hours after dosing.
  • Therapeutic serum levels vary with different laboratories, but are in the region of 0.015-0.04 mg/ml.
Thyroxine
  • Decreases total T4 and free T4 (due to increased biliary excretion and possibly an increase in deiodination of T4). TSH only shows a compensatory increase while total T3 has minimal fluctuation and cholesterol increases toward the upper limits of the normal range.
Liver enzymes
  • Liver enzyme elevations may take 5 weeks up to 7 months to return to normal after discontinuing this drug.
  • Monitor yearly for animals on long-term therapy.
  • Phenobarbitone treatment does not affect adrenal function tests (ACTH stimulation test and low dose dexamethasone test) despite acceleration in dexamethasone metabolism.

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