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Ama - Epilepsy > Phenytoin > Flashcards

Phenytoin Flashcards

1
Q

Category

A

Category 1

High risk drug

Maintain on a specific manufacturers product (same brand or generic drug manufacturer)

Different oral formulations vary in bioavailability

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2
Q

Phenytoin sodium vs Phenytoin base

A

Phenytoin sodium is NOT bioequivalent to Phenytoin base

100 mg of Phenytoin sodium = 92 mg of Phenytoin base

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3
Q

What type of seizures is it used for?

A

Focal

Generalised tonic-clonic

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4
Q

What type of seizures should it NOT be used for?

A

Absence

Myoclonic

Exacerbates seizures

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5
Q

Therapeutic range (ADULTS)

A

10-20 mg/L OR
40-80 micromol/L

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6
Q

Therapeutic range (NEONATES)

A

6-15 mg/L OR
25-60 micromoles/L

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7
Q

Relationship between dose + plasma concentration (Cp)

A

Non-linear
Small changes in dose/missed dose/changes in drug absorption = large changes in Cp

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8
Q

Phenytoin + protein binding

A

Phenytoin is a highly protein-bound drug

Reduced protein binding = monitor plasma free-drug concentration.

The patient groups show early signs of toxicity

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9
Q

What groups of people have reduced protein-binding?

A

Pregnancy
Children (neonates)
Elderly
Liver failure

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10
Q

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

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11
Q

Side effects

A
  • Skin disorder
  • Hepatotoxicity
  • Blood disorders
  • Suicidal thoughts
  • Low vitamin D levels
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12
Q

Hypersensitivity reactions (AHS)

A
  • Fever
  • Rash
  • Swollen lymph nodes
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13
Q

Blood disorders

A

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

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14
Q

Skin disorders

A

Rash
* Reintroduce if mild
* Discontinue if recurrence
* Han chinese and Thai patients with HLA-B*1502 Allele = increased risk of Steven-Johnson syndrome

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15
Q

Low vitamin D

A

Phenytoin induces Vit D metabolism
Leads to osteomalacia and rickets
Vit D supplements:
- Immobilised patients
- Inadequate vit D exposure
- Inadequate dietary intake of calcium

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16
Q

Hepatotoxicity

A

Discontinue immediately
Monitor LFTs
Do not restart
* Dark urine
* N + V
* Abdominal pain
* Jaundice
* Itching

17
Q

Suicidal ideation

A

Small increased risk
Can occur within 1 week of starting treatment
Seek help

18
Q

IV phenytoin - Most common SE

A
  • Bradycardia
  • Hypotension
    If these SE occur, reduce administration rate.
19
Q

IV phenytoin - Other common SE

A
  • Arrhythmias
  • Cardiovascular collapse
  • Respiratory arrest
  • Rapid administration = CVS/CNS depression
    Monitor = ECG/BP
20
Q

IV fosphenytoin - SE

A

Severe cardiovascular reactions.
* Asystole
* Ventricular fibrillation
* Cardiac arrest
* Heart block
* Hypotension
* Bradycardia

21
Q

IV fosphenytoin - monitoring

A

Observe patient for 30 minutes after infusion
* HR
* BP
* Respiratory function during infusion

22
Q

Fosphenytoin vs Phenytoin sodium

A

1.5 mg of Fosphenytoin = 1 mg of Phenytoin sodium

23
Q

Administration of Fosphenytoin

A

Prodrug of Phenytoin
IV or IM only.
IV = less injection site reactions and can be given more rapidly

24
Q

Drugs that increase phenytoin concentration

A

Increased concentrat
Enzyme inhibitors
* Amiodarone
* Chloramphenicol
* Cimetidine
* Clarithromycin
* Diltiazem
* Fluconazole
* Fluoxetine
* Metronidazole
* Miconazole
* Sertraline
* Trimethoprim
* Valproate

25
Q

Drugs that reduce phenytoin concentration

A

Reduced concentration = therapeutic failure
* Rifampicin
* St. Johns wort

26
Q

Increased antifolate effect

A

Increased antifolate effect = increased risk of blood dyscrasias
* Methotrexate
* Trimethoprim

27
Q

Drugs that antagonise the anticonvulsant effect of phenytoin

A

Lower the seizure threshold
* Quinolones
* Tramadol
* Mefloquine
* SSRIs
* Antipsychotics
* TCA/related antidepressants

28
Q

Phenytoin reduces the concentration of which drugs?

A

Phenytoin is an enzyme inducer = reduces drug concentrations
* Contraceptives/HRT
* Warfarin
* Corticosteroids/Levothyroxine/Liothyronine (increases risk of hypothyroidism)

Ama - Epilepsy (8 decks)

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