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Information about phenytoin

Published on February 6, 2014

Author: GghNeurology


EPILEPSY-brief introduction: EPILEPSY-brief introduction -These are a group of disorders of CNS characterized by paroxysmal cerebral dysrhythmia ,manifesting as brief episodes of loss or disturbance of consciousness,with or without characteristic body movements ,sensory or psychiatric phenomenon Nature of Epilepsy: Nature of Epilepsy Epilepsy affects about 0.5% of the population. The characteristic event is the seizure, which is often associated with convulsion, but may occur in many other forms. The seizure is caused by an abnormal high-frequency discharge of a group of neurons, starting locally and spreading to a varying extent to affect other parts of the brain. Nature of Epilepsy: Nature of Epilepsy Seizures may be partial or generalised depending on the location and spread of the abnormal neuronal discharge. The attack may involve mainly motor, sensory or behavioural phenomena. Unconsciousness occurs when the reticular formation is involved. Types of epilepsy: Types of epilepsy 1.generalised seizures: - generalised tonic-clonic seizures:- -grandmal seizures -commonest,last 1-2 min -aura-cry-unconsciousness-tonic spasm-clonic jerking followed by sleep and depression of all cns functions Absence seizures:- -petitmal epilepsy -children,last ½ min -pt apparently freezes and stares in one direction Types of epilepsy: Types of epilepsy No muscular component or little bilateral jerking.EEG shows charecteristic 3 cycles per sec spike and wave pattern Atonic seizures:- -akinetic epilepsy -relaxation of all muscles due to excessive inhibitory signals.pts may fall Myoclonic seizures:- -shock like momentary contraction of muscles of a limb or whole body Infantile spasms:- -hypsarrhythmia Types of epilepsy: Types of epilepsy -seen in infants -intermittant muscle spasm and progressive mental deterioration 2.partial seizures: simple partial seizures:- - confined to a group of muscles or localised sensory disturbance without loss of consciousness complex partial seizures:- -temporal lobe epilepsy -bizarre and confused behaviour,emotional changes with impaired conscious levels Nature of Epilepsy: Nature of Epilepsy Two common forms of generalised epilepsy are the tonic-clonic fit (grand mal) and the absence seizure (petit mal). Status epilepticus is a life- threatening condition in which seizure activity is uninterrupted. Nature of Epilepsy: Nature of Epilepsy The neurochemical basis of the abnormal discharge is not well understood. It may be associated with enhanced excitatory amino acid transmission, impaired inhibitory transmission, or abnormal electrical properties of the affected cells. The glutamate content in areas surrounding an epileptic focus is often raised. Nature of Epilepsy: Nature of Epilepsy Repeated epileptic discharge can cause neuronal death (excitotoxicity). Current drug therapy is effective in 70-80% of patients. Anti epileptics-classification: Anti epileptics-classification 1.barbiturate -phenobarbitone 2.deoxybarbiturate -primidone 3.bezodiazepines -diazepam -lorazepam -clonazepam -clobazam 4.hydantoins -phenytoin –fosphenytoin 5.iminostillbene -carbamazepine -oxcarbazepine 6.aliphatic carboxylic acid -valproic acid Anti epileptics-classification: Anti epileptics-classification -divalproex 7.succinimide -ethosuximide 8.phenyl triazine -lamotrigine 9.cyclic GABA analogues -gabapentin -pregabalin 10.newer drugs -topiramate -zonisamide -lacosamide -levetiracetam -vigabatrin -tiagabine -felbamate,rufinamide Chemical structure of classical anti convulsants: Chemical structure of classical anti convulsants Phenytoin(diphenylhydantoin): Phenytoin(diphenylhydantoin) -synthesized in 1908 as an analogue of phenobarbitone,but shelved due to poor sedative property.first tested in 1938 Not a dpressant It is effective against the tonic phase of maximal eletro shock seizures with no effect on clonic phase Mechanism of action:- Stabilising effect on neuronal membrane,prevents repetitive detonation of normal brain cells during “depolarization shift”that occurs in epileptic pts Prolongs the inactivated state of voltage gated na+channels that governs the refractory period of neurons Phenytoin(diphenylhydantoin): Phenytoin(diphenylhydantoin) High frequency discharges are inhibited with little effect on low frequency discharges Unlike phenobarb and valproate,doesn’t interfere with kindling But at higher conc-reduction in ca2+influx -inhibition of glutamate -facilitation of GABA shows efficacy in trigeminal neuralgia and cardiac arrhythmias Mechanism of action-antiepileptics: Mechanism of action-antiepileptics Mechanism of Action: Mechanism of Action Reducing electrical excitability of cell membranes, possibly through inhibition of sodium channel. Enhancing GABA-mediated synaptic inhibition. This may be achieved by an enhanced pre- or post- synaptic action of GABA, by inhibiting GABA-transaminase, or by drugs with direct GABA-agonist properties. Phenytoin(diphenylhydantoin): Phenytoin(diphenylhydantoin) Pharmacokinetics: Slow oral absorption due to poor aqueous solubility Widely distributed in body 80 to 90% plasma protein bound Metabololised in liver by CYP2C9 and CYP2C19 and by glucuronide conjugation Follow Capacity limited kinetics-changes from first order kinetics to zero order kinetics t1/2 is 12 to 24 hours in normal therapeutic conc,but upto 60 hours in high plasma conc only 5% is excreted unchanged in urine Phenytoin(diphenylhydantoin): Phenytoin(diphenylhydantoin) Adverse effects: @therapeutic levels(10-20mcg/ml): Gum hypertrophy-20%incidence -young pts -over growth of gingival collagen 2.hirsutism,coarsening of facial features,acne 3.megaloblastic anaemia 4.osteomalacia 5.hyperglycemia 6.hypersensitivity-rashes,lymphadenopathy,neutrupenia Phenytoin-adverse effects: Phenytoin-adverse effects Phenytoin-adverse effects: Phenytoin-adverse effects Phenytoin(diphenylhydantoin): Phenytoin(diphenylhydantoin) 7.teratogenicity-foetal hydantoin syndrome -hypoplastic phalanges -cleft palate,hare lip -microcephaly -caused by areneoxide metabolite at toxic levels: 1.cerebellar and vestibular manifestations 2.drowsiness,behavioral alterations,hallucinations,disorientation…. 3.mimic APD symtoms Phenytoin(diphenylhydantoin): Phenytoin(diphenylhydantoin) 4.iv injection rapid-thrombophlebitis.rate should be less than 50mg/min fall in bp and cardiac arrhythmias Drug interactions: Phenytoin is a potent inducer of CYP3A4/5 Carbamazepine and phenytoin induce each others metabolism Same with phenobarbitone Valproate may cause phenytoin toxicity Isoniazid,warfarin,chloramphenicol,cimetidine may induce phenytoin toxxicity Anti epileptics-therapeutic and toxic plasma levels: Anti epileptics-therapeutic and toxic plasma levels Phenytoin(diphenylhydantoin): Phenytoin(diphenylhydantoin) Phenytoin increases degradation of oc pills,theophylline,doxycycline Sucralfate decreases the absorption of phenytoin Uses: GTCS,SPS,CPS …max dose of 400mg/day..children5-8mg/kg/day Status epilepticus Trigeminal neuralgia Choice of antiepileptics: Choice of antiepileptics Attentions: Attentions Selection of an appropriate antiseizure agent Use of single drug Withdrawal Toxicity Fetal malformations

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