children constitute approximately 40% of india 's population but information on adverse drug reactions occurring in them is limited . phenytoin sodium ( pht ) is one of the commonest antiepileptic drug ( aed ) used in children in india . the drug has wide pharmacokinetic variability and has a narrow therapeutic range that leads to toxicity . there is some evidence of the association of long - term use of pht and toxicity like cerebellar atrophy . such cerebellar changes have been reported even with the long - term use of nontoxic levels of phenytoin . , we report a case of reversible cerebellar atrophy induced by pht in a 10-year - old boy . after initiating therapy with pht 10 years back , he was seizure free for 2 years . patient was also occasionally prescribed clobazam , 5 mg , oral , as and when required . the episodes used to last for a brief period of over 6 - 7 days . seizures used to occur at every 3 - 4 months interval . the boy had history of birth asphyxia and delay in developmental milestones . after the patient recovered , he was evaluated again due to complaint of difficulty in walking . cns examination revealed normal mentation with cerebellar signs including gaze - evoked nystagamus , truncal , and appendicular ataxia . mri scan of the brain was advised and it showed cerebellar atrophy as shown in figure 1 . serum phenytoin level was high ( 30 mcg / ml ) and pht was withdrawn immediately . patient was started on valproic acid and followed up . however , a sizeable proportion of patients may develop drug induced adverse effects that include sedation , gum hypertrophy , lymphadenopathy , chorea , ataxia , etc . there are a few reports on cerebellar atrophy after long - term use of pth . these adverse effects are usually reported if the drug serum levels are above the therapeutic range . our patient presented with severe cerebellar disorder and he was on the drug for 10 years . hence , a regular monitoring for adverse drug reaction should be considered in patients who are on drugs on long term basis . these reactions could be idiosyncratic or dose - dependent which again may be acute or chronic in nature . since most of these effects are reversible , it is important to identify the clinical manifestations related to drug toxicity and to manage them appropriately . also , it warns the need for regular monitoring of plasma concentration , accurate dosing , and identification of adherence issues in patients on phenytoin .
epilepsy is an important health problem due to its high prevalence and potential for causing long - term morbidity . 
 it is commonly treated in children with phenytoin sodium . 
 it has wide pharmacokinetic variability and a narrow therapeutic range that leads to toxicity . here 
 , we report a case of phenytoin - induced cerebellar atrophy in a 16-year - old epileptic boy who presented to the hospital with a viral infection .