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Pharmacology

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CNS Part-1

Question
51 out of 58
 

True about phenytoin is all except (LQ)



A Saturation kinetics

B Anti seizure activity closely resembles blood concentration

C Does not depress CNS

D Cerebellar atrophy occurs on long term administration

Ans. C Does not depress CNS

a. Merritt and Putnam described use of phenytoin in controlling seizures in 1938. The drug blocks depolarization shift and is useful in a number of disorders. The drug also has anti-glutamate properties and blocks sodium channels.

b. Given orally, the drug is well absorbed. It has a saturable metabolism and follows zero order kinetics. The drug has a half life of 24 hours and is a protein bound drug (90%).

c. Metabolism is inhibited by INH, which can precipitate its toxicity. Phenytoin is very effective for both generalized and partial seizures and has no value for absence seizures. It has special value in posttraumatic seizures, perhaps due to its antioxidant activity (CMDT’2010, pp-883).

d. The drug commonly causes enlargement of gums (Most common) and nystagmus, ataxia and diplopia. IV injection can cause hypotension and cardio depression.

e. On long term administration; the drug can cause osteomalacia (increase in vitamin D metabolism), Hodgkin lymphoma like illness, cerebellar atrophy and megaloblastic anemia (folic acid deficiency).

f. Rare side effects include: interstitial lung disease, gynecomastia, pure red cell aplasia (PRCA). A safer version of phenytoin is fosphenytoin and is a prodrug.

g. It does not cause ‘red glove’ syndrome upon local leakage when injected intravenously and is more cardiovascular stable.

h. An important clinical advantage is its ease of IM administration. Importantly, phenytoin can not be given intramuscularly.

i. Importantly, though uncommon, phenytoin is associated with toxic epidermal necrolysis and Steven-Johnson’s syndrome. Occurrence of hypersensitivity is an indication of stopping the drug.

CNS Part-1 Flashcard List

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