Phenytoin, anti-epileptic /Seizures Drug: Mechanism of Action and Side Effects

Phenytoin, anti-epileptic /Seizures Drug: Mechanism of Action and Side Effects – Phenytoin is a drug for epileptic seizures. Epilepsy is a disorder of the brain characterized by repeated seizures. A seizure is usually defined as a sudden alteration of behaviour due to a temporary change in the electrical functioning of the brain. Normally, the brain continuously generates tiny electrical impulses in an orderly pattern.

What is phenytoin?

Phenytoin is an anti-epileptic drug, also called an anticonvulsant. Phenytoin works by slowing down impulses in the brain that cause seizures.

Mechanism of action of phenytoin

Phenytoin is believed to protect against seizures by causing voltage-dependent block of voltage gated sodium channels. This blocks sustained high frequency repetitive firing of action potentials.

What to do before taking this phenytoin medicine

You should not use phenytoin if you are allergic to it, or if you have ever had:

• liver problems caused by phenytoin;
• an allergy to similar medicines such as ethotoin, fosphenytoin, or mephenytoin; or
• if you currently take delavirdine (Rescriptor).

To make sure phenytoin is safe for you, tell your doctor if you have ever had:

• heart problems;
• liver disease;
• diabetes;
• depression;
• suicidal thoughts or actions;
• a vitamin D deficiency or any other condition that causes thinning of the bones;
• porphyria (a genetic enzyme disorder that causes symptoms affecting the skin or nervous system); or
• if you are of Asian ancestry (you may need a special blood test to determine your risk for having a skin reaction to this medicine).

Some people have thoughts about suicide while taking this medicine. Your doctor will need to check your progress at regular visits. Your family or other caregivers should also be alert to changes in your mood or symptoms.

Phenytoin can cause swelling in your gums. Brush and floss your teeth and visit your dentist regularly to help prevent this problem.

Store at room temperature away from moisture, light, and heat.

Dosing information

Usual Adult Dose for Seizures:

Oral (except suspension) Loading dose: Only when indicated for inpatients.

1 g orally divided in 3 doses (400 mg, 300 mg, 300 mg) given at 2 hour intervals. Then normal maintenance dosage started 24 hours after loading dose.

Initial dose: 100 mg extended release orally 3 times a day.

Maintenance dose: 100 mg orally 3 to 4 times a day. If seizure control is established with divided doses of three 100 mg capsules daily, once-a-day dosage with 300 mg of extended release phenytoin sodium may be considered. Alternatively, the dosage may need to be increased up to 200 mg orally 3 times a day, if necessary.

Suspension: Patients who have received no previous treatment may be started on 125 mg (one teaspoonful) of the suspension three times daily, and the dose is then adjusted to suit individual requirements. An increase to five teaspoonfuls daily may be made, if necessary.

IV: Do not exceed the infusion rate of 50 mg/min.

Loading dose: 10 to 15 mg/kg IV slowly.

Maintenance dose: 100 mg IV every 6 to 8 hours.

IM: Avoid the IM route due to erratic absorption.

Usual Adult Dose for Arrhythmias:

Loading Dose:

1.25 mg/kg IV every 5 minutes. May repeat up to a loading dose of 15 mg/kg, or

250 mg orally 4 times a day for 1 day, then 250 mg twice daily for 2 days

Maintenance Dose:

300 to 400 mg/day orally in divided doses 1 to 4 times a day

Usual Adult Dose for Status Epilepticus:

IV:

Loading dose: Manufacturer recommends 10 to 15 mg/kg by slow IV administration (at a rate not exceeding 50 mg/minute). Alternatively, generally accepted guidelines suggest 15 to 20 mg/kg by slow IV administration (at a rate not exceeding 50 mg/minute).

Maintenance rate: 100 mg orally or IV every 6 to 8 hours

Maximum rate: 50 mg/minute

Maintenance dose: IV or Oral: 100 mg every 6 to 8 hours

Usual Adult Dose for Neurosurgery:

Neurosurgery (prophylactic): 100 to 200 mg IM at about 4 hour intervals during surgery and the immediate postoperative period. (Note: While the manufacturer recommends IM administration, this route may cause severe local tissue destruction and necrosis. Some clinicians recommend the use of fosphenytoin if IM administration is necessary.) If IM administration is not necessary, accepted protocol has been 100 to 200 mg IV at about 4 hour intervals during surgery and the immediate postoperative period.

Usual Pediatric Dose for Seizures:

Status Epilepticus: Loading Dose:

Infants, Children: 15 to 20 mg/kg IV in a single or divided doses

Anticonvulsant: Loading Dose:

All ages: 15 to 20 mg/kg orally (based on phenytoin serum concentrations and recent dosing history). The oral loading dose should be given in 3 divided doses administered every 2 to 4 hours.

Anticonvulsant: Maintenance Dose:

(IV or oral) (Note: May initially divided daily dose into 3 doses/day, then adjust to suit individual requirements.)

Less than or equal to 4 weeks: Initial: 5 mg/kg/day in 2 divided doses

Usual: 5 to 8 mg/kg/day IV in 2 divided doses (may require dosing every 8 hours).

Greater than or equal to 4 weeks: Initial: 5 mg/kg/day in 2 to 3 divided doses

Usual: (may require up to every 8 hour dosing)

6 months to 3 years: 8 to 10 mg/kg/day

4 to 6 years: 7.5 to 9 mg/kg/day

7 to 9 years: 7 to 8 mg/kg/day

10 to 16 years: 6 to 7 mg/kg/day

Phenytoin side effects

Get emergency medical help if you have signs of an allergic reaction to phenytoin (hives, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning in your eyes, skin pain, red or purple skin rash that spreads and causes blistering and peeling).

Seek medical treatment if you have a serious drug reaction that can affect many parts of your body. Symptoms may include: skin rash, fever, swollen glands, muscle aches, severe weakness, unusual bruising, or yellowing of your skin or eyes.

Call your doctor at once if you have:

• slow or uneven heartbeats, chest pain, fluttering in your chest, and dizziness (like you might pass out);
• any skin rash, no matter how mild;
• fever, chills, sore throat, swollen glands;
• red or swollen gums, mouth sores;
• easy bruising, unusual bleeding, purple or red spots under your skin; or
• liver problems – loss of appetite, upper stomach pain, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).