Sodium Phenytoin

 Trade names: Dilantin,Dephenylhydantoin

Translated and edited by:  Joe Hing Kwok Chu

Possible Benefits:

Effective control of tonic-clonic (grand mal), psychomotor

(temporal lobe), myoclonic, and focal seizures.

Possible Risks:

Very narrow treatment margin of dosage

Possible birth defects

Overgrowth of gums (especially in children)

Blood cell disorders: impaired production of all blood cells

Drug-induced hepatitis or nephritis

Characteristics and Uses:

This drug is effective for serious seizure attacks and not effective for mild attacks. It takes several days for it to reach the effective concentration in the brain tissue, so the therapeutic effect is not immediate. It is necessary to take it for several days before the effectiveness shows. To control symptoms, phenobarbital is being used as the main drug, but to prevent attacks and for maintenance, sodium phenytoin (Dilantin) is a better selection.

(1) Anti-Seizures:

(2) Trigeminal Neuralgia and Sciatica Therapy

(3) Anti-Arrhythmia

(4) Anti-Hypertension

Applications:

(1) Anti-Seizures

Oral dosage: 50~100 mg each time, 2 ~3 times a day, after meal. Maximum usage: 300 mg each time, with 600 mg maximum per day. Start with low dosage and increase gradually according to requirement. Do not overdose. For children with body weight under 30 kgs: the dosage should be 5~10 mg per kg per day to be taken 2~3 times per day. Muscle or venous injection:  During continuous seizure attacks, a muscle injection of 100 ~ 250 mg can be used. If the patient has never used sodium phenytoin (Dilantin) before,  150 ~250 mg added to a 5% glucose solution of 20~40 ml should be used in a slow venous injection of 6 ~10 minutes.  If necessary, after 30 minutes apply another 100~150 mg.

(2) Trigeminal Neuralgia and Sciatica Therapy

100~100 mg each time, 2~3 times per day.

(3) Anti-Arrhythmia

Oral dosage: 100~200 mg each time, 2~3 times per day. Muscle or venous injection:  200~400 mg per day. Or 125~250 mg added to 5% glucose solution of 20~40 mg can be used in 5~15 minutes slow venous injection, (not to exceed 50 mg per minute). When necessary,  every 5~10 minutes repeat with 100 mg, but not to exceed 50 mg within two hours.  An intravenous feed can be applied with the same dosage in 5% glucose solution of 100 ml. The dosage should not  exceed 1000 mg per day.

Warning:

There are not many serious side effects to this drug; however, these could include obstruction of vision, a confused emotional state, or purpura.   Long- term effects include headache, nausea, vomiting, dizziness, lack of appetite, skin rash, and overgrowth of gums in children (which can be corrected with calcium salt.)  Occasionally there is also the possibility of lack of coordination, decrease of white cell count, or nervous tremors. Long -term use should not be abruptly discontinued, as this could cause a more serious or prolonged epileptic attack.

Intravenous injections should not be given too fast, or it could obstruct the transmission of signals between the atria and ventricles of the heart causing slow heart rate, collapsed blood vessels, or  inhibition of respiration.

Long- term use by children can create retardation of bone growth (soft bones), as sodium phenytoin inhibits the metabolism of vitamin D.  (This

 problem can be prevented by increasing the amount of vitamin D.)  It also can cause  enlarged lymph nodes, which can lead to malignant tumors.  In this case the patient should stop taking the drug.  It can cause macrocytic anemia because it disrupts folic acid.  To correct this, take folic acid and vitamin B12. It can impair the production of all blood cells, especially the white cells.  It can cause aplastic anemia. During long-term usage, patients should have blood tests regularly.  Pregnant women should not use this drug as it could cause birth defects.  People with liver problems should not  use this drug.

Use of phenobarbital, carbamazepine, or folic acid can decrease the concentration of the drug in the blood; or if when taken orally the absorption is not good, the concentration also will go down.  It is necessary to pay attention and adjust the dosage.

Phenytoin may increase the metabolism of digoxin and may decrease the levels of digoxin and possibly decrease its clinical effectiveness. Digoxin levels should be monitored in patients receiving concurrent therapy or when concurrent therapy is discontinued.

Overdosage:¹

The lethal dose in pediatric patients is not known. The lethal dose in adults is estimated to be 2 to 5 grams. The initial symptoms are nystagmus, ataxia (lack of movement coordination), and dysarthria (speech difficulty). Other signs are tremors, hyperflexia (exaggerated deep tendon reflexes), lethargy, slurred speech, nausea, and vomiting. The patient may become comatose and/or hypotensive. Death is due to respiratory and circulatory depression.

There are marked variations among individuals with respect to phenytoin plasma levels where toxicity may occur. Nystagmus or lateral gaze usually appears at 20 mcg/ml, ataxia at 30 mcg/ml. Dysarthria (speech difficulty) and lethargy appear when the plasma concentration is over 40 mcg/ml, but as high a concentration as 50 mcg/ml has been reported without evidence of toxicity. As much as 25 times the therapeutic dose has been taken to result in a serum concentration of over 100 mcg/ml with complete recovery.

Treatment of overdosage:

Treatment is nonspecific since there is no known antidote. The adequacy of the respiratory and circulatory systems

should be carefully observed and appropriate supportive measures employed. Hemodialysis can be considered since phenytoin is not completely bound to plasma proteins.

Total exchange transfusion has been used in the treatment of severe intoxication in pediatric patients.

In acute overdose the possibility of other CNS depressants, including alcohol, should be borne in mind.

The manufactured dosage: in pills is 50 ml or 100 mg.  The vial for injections is 100 mg or 250 mg.

Storage:  Sodium phenytoin  absorbs moisture and disintegrates easily, so it should be stored in a dark,

dry place.

Source:  New Edition of

Pharmacology, by Chen Xin Qian, Jin You Yu, 1996 (in Chinese)  

(1) Overdosage and treatment: PDR 1988 

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