EPILEPSY: PHENYTOIN Flashcards
Category
Category 1
High risk drug
Maintain on a specific manufacturers product (same brand or generic drug manufacturer)
Different oral formulations vary in bioavailability
Phenytoin sodium vs Phenytoin base
Phenytoin sodium is NOT bioequivalent to Phenytoin base
100 mg of Phenytoin sodium = 92 mg of Phenytoin base
What type of seizures is it used for?
Focal
Generalised tonic-clonic
What type of seizures should it NOT be used for?
Absence
Myoclonic
Exacerbates seizures
What are the other uses of phenytoin?
- Status epilepticus
- Prevention and treatment of seizures during or following neurosurgery or severe head injury
Contraindications
- Acute porphyrias
- Sino-atrial and heat block if given IV
Therapeutic range (ADULTS)
10-20 mg/L OR
40-80 micromol/L
Therapeutic range (NEONATES)
6-15 mg/L OR
25-60 micromoles/L
Is phenytoin a CYP450 inducer or inhibitor
inducer
Relationship between dose + plasma concentration (Cp)
Non-linear
Small changes in dose/missed dose/changes in drug absorption = large changes in Cp
Phenytoin + protein binding
Phenytoin is a highly protein-bound drug
Reduced protein binding = monitor plasma free-drug concentration.
The patient groups show early signs of toxicity
What groups of people have reduced protein-binding?
Pregnancy
Children (neonates)
Elderly
Liver failure
Signs + symptoms of toxicity
A
SNAtCHeD
* Slurred speech
* Nystagmus (uncontrolled repetitive eye movements e.g. eye rolling)
* Ataxia (involuntary co-ordination of muscle movement)
* Confusion
* Hyperglycaemia
* Diplopia (double vision), blurred vision
Side effects
- Skin disorder/acne
- Coarsening of facial appearnace/ hirsuitism
- Constipation
- Drowsiness
- Gingival hypertrophy
- Hepatotoxicity
- Blood disorders
- Suicidal thoughts
- Low vitamin D levels
- Paraesthesia
Hypersensitivity reactions (AHS)
Fever
Rash
Swollen lymph nodes
Blood disorders
Occurs as phenytoin is also an antifolate
Report signs of infection
* Fever
* Sore throat
* Mouth ulcers
* Unexplained bruising or bleeding
Leucopoenia that is severe, progressive or associated with clinical symptoms requires withdrawal.
Monitor FBC
Skin disorders
Rash
* Reintroduce if mild
* Discontinue if recurrence
* Han chinese and Thai patients with HLA-B*1502 Allele = increased risk of Steven-Johnson syndrome
Low vitamin D
Vit D def
Leads to osteomalacia and rickets
Vit D supplements:
- Immobilised patients
- Inadequate vit D exposure
- Inadequate dietary intake of calcium
Hepatotoxicity
Discontinue immediately
Monitor LFTs
Do not restart
* Dark urine
* N + V
* Abdominal pain
* Jaundice
* Itching
Suicidal ideation
Small increased risk
Can occur within 1 week of starting treatment
Seek help
IV phenytoin - Most common SE
Bradycardia
Hypotension
If these SE occur, reduce administration rate.
IV phenytoin - Other common SE
Arrhythmias
Cardiovascular collapse
Respiratory arrest
Rapid administration = CVS/CNS depression
Monitor = ECG/BP
IV fosphenytoin - SE
Severe cardiovascular reactions.
* Asystole
* Ventricular fibrillation
* Cardiac arrest
* Heart block
* Hypotension
* Bradycardia
IV fosphenytoin - monitoring
Observe patient for 30 minutes after infusion
* HR
* BP
* Respiratory function during infusion
