MRCOG Phenytoin

Phenytoin

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Pharmacokinetics
Phenytoin is poorly absorbed from the intestine.[1]
It is metabolised in the liver.[2]


The half life of phenytoin at low doses (giving subtheraputic concentrations) is 6-24 hours.[3]It increases to 20-60 hours at higher doses.


Phenytoin is administered orally or by intravenous injection.[4]
Phenytoin sodium may be given by slow intravenous injection, with ECG monitoring, followed by the maintenance dosage. Intramuscular use of phenytoin is not recommended (absorption is slow and erratic).[5]
Fosphenytoin is a water soluble pro-drug of phenytoin which can be administered by intramuscular injection.


Magnesium sulphate is superior to phenytoin for the prevention of eclampsia in hypertensive pregnant women.[6]


Phenytoin can cause fetal hydantoin syndrome. The following are the features of fetal hydantoin syndrome[7]

(a) mild to moderate growth deficiency
(b) mild mental deficiency
(c) craniofacial abnormalities 

(i) ocular hypertelorism
(ii)broad nasal bridge
(iii) low set abnormal ears
(iv)cleft lip and palate
(v) hypoplasia of distal phalanges
(vi)micro - brachycephaly

Hirsutism is side effect of phenytoin. Phenytoin could also cause folate deficiency and this is why epileptic patients needs folic acid in a higher dose(5 mg).

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[1] http://www.emedicine.com/emerg/topic421.htm
[2] http://www.emedicine.com/emerg/topic421.htm
[3] Clinical Pharmacology,9th edition, P.N.Bennet & M.J.Brown
[4] http://findarticles.com/p/articles/mi_m0FSS/is_6_14/ai_n17211622/pg_3
[5] BNF 53 section 4.8.2
[6] http://content.nejm.org/cgi/content/abstract/333/4/201
[7] http://www.ulg.upol.cz/www/lectures/Teratogen%ED%20faktoryAng.ppt#262,9,Slide 9