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Phenytoin (Dilantin)

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Anaesthetics and Critical Care

Obstetrics Gynaecology

Trauma and Orthopaedics

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Urology

Version Jan 2023

Signs of toxicity include ataxia, diplopia, nystagmus, sedation, dysarthria.

About : Always check the BNF link here

Less popular AED used more IV rather than long term orally
Blood levels Target Trough (before next dose) 10-20 mg/l (40-80 micromol/L)

Mode of action

Inhibits Na⁺ influx into cells

Pharmacokinetics

Zero order kinetics - small dose rises soon becomes toxic
High plasma protein binding displaced by other drugs - Aspirin and Valproate
Induces liver enzymes

Indications

Epilepsy, Trigeminal neuralgia

Contraindications

Porphyria, Bradycardia, Heart Block

Side effects

Signs of toxicity: ataxia, diplopia, nystagmus, sedation, dysarthria.
Hirsutism, Coarsening of features, Gum hyperplasia, Greasy skin, Rashes
Macrocytic anaemia due to folate deficiency
Cerebellar symptoms, nystagmus, Lymphadenopathy
Vitamin D deficiency - osteomalacia, Teratogenic
High pH so extravasation will cause tissue damage.

Interactions

Reduced efficacy of OCP, Theophylline, TCAs, Thyroxine, Steroids

The following drugs reduce the breakdown of Phenytoin

Acute Alcohol, Erythromycin, Metronidazole, Allopurinol
Amiodarone, Omeprazole, Sulphonamides

Loading and Maintenance Dose

Loading dose in Status Epilepticus is 20 mg/kg. This assumes the patient is not on Phenytoin. This is given with 0.9% N-Saline. Infuse into a large vein. Flush before and after. Ideally use an in line filter. Patient should be closely cardiac monitored.
Infusion rates of more than 50 mg/minute. Use slower doses such as 25 mg/minute in elderly and with heart disease. Higher doses may be needed in heavier patients and the infusion time will be longer.

Emergency Loading Dose 20 mg/kg: Give over 30-60 mins with cardiac monitoring

Weight (kg)	Dose in N-Saline (Do not give in 5%Dextrose).
40-49 kg	750 mg in 100 ml
50-64 kg	1000 mg in 100ml
65-78kg	1250 mg in 250 ml
79-92 kg	1500 mg in 250 ml
> 92 kg	1750 mg in 250 ml

Maintenance Dose range

Name	Starting Dose	Frequency	Route
Phenytoin	200-300 mg	OD	PO
Phenytoin	100 mg	TDS	PO

References

PLEASE NOTE LEGAL ADVICE: The contents are under continuing development and improvements and may contain errors of omission or fact. The official launch will be at the end of 2018. Feedback vital and always welcome at drokane at gmail.com. This is not to be used for the assessment, diagnosis or management of patients. It should not be regarded as medical advice. It is only for educational purposes. Please adhere to your local protocols. If you are unwell please seek healthcare advice from your doctor. This does not replace senior or specialist advice. If you do not accept this then please do not use the website. If you need medical advice, please consult a doctor or other appropriate medical professional. If you are a medical professional and you need advice then speak to your senior or colleagues.

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