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Chlorphenamine for prolonged drug-induced extrapyramidal side effects in a dog
  1. Joao Miguel Frias1,2,
  2. Joanne Michou3 and
  3. Angela Fadda2,4
  1. 1Neurology and Neurosurgery, Royal Veterinary College - Hawkshead Campus, Hatfield, UK
  2. 2Neurology and Neurosurgery, Lumbry Park Veterinary Specialists, Alton, UK
  3. 3Anaesthesia, Lumbry Park Veterinary Specialists, Alton, UK
  4. 4Neurology and Neurosurgery, Langford Veterinary Services, Bristol, UK
  1. Correspondence to Dr Joao Miguel Frias; jfrias{at}


An eight-year-old male entire crossbreed dog, evaluated for resection of a mass at the base of the tail, underwent general anaesthesia for CT. Following preanaesthetic medication of 0.2 mg/kg butorphanol and 0.015 mg/kg acepromazine intravenously, anaesthesia was induced with 2 mg/kg alfaxalone and 0.3 mg/kg midazolam intravenously. Anaesthesia was uneventful, although during recovery the dog showed prolonged excitatory signs consisting of intermittent opisthotonic posture, limb dystonia, myoclonic jerks, paddling movements of all limbs, oculogyric movements and excessive vocalisation consistent with drug-induced extrapyramidal side effects (EPS). EPS lasted 25 hours and did not cease after administration of antiepileptic drugs, but rapidly resolved after administration of chlorphenamine, an H1 antihistamine with anticholinergic activity. This report provides proposed mechanisms for the pathogenesis of these neuroanaesthetic EPS, frequently termed excitatory phenomena, and reports a successful treatment of prolonged EPS in a dog by blockade of muscarinic acetylcholine receptors.

  • dogs
  • neurology
  • anaesthesia
  • drug administration
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  • Contributors JMF: initial manuscript generation, minor manuscript editing and manuscript submission. JM: patient assessment, general anaesthesia planning, supervision, treatment, assisted with initial manuscript generation and major manuscript editing. AF: patient assessment, diagnostic testing, interpretation, treatment, assisted with the initial manuscript generation and major manuscript editing.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement All data relevant to the study are included in the article.

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