<p>The&nbsp;lethal&nbsp;dose in pediatric patients is not known. The lethal dose in adults is estimated to be 2 to 5 grams. The initial symptoms are&nbsp;nystagmus,&nbsp;ataxia, and&nbsp;dysarthria. Other signs are&nbsp;tremor, hyperreflexia,&nbsp;lethargy, slurred speech, blurred vision, nausea, and vomiting. The patient may become comatose and&nbsp;hypotensive. Death is caused by respiratory and&nbsp;circulatory&nbsp;depression.<br />
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There are marked variations among individuals with respect to&nbsp;phenytoin&nbsp;serum levels where toxicity may occur. Nystagmus, on&nbsp;lateral&nbsp;gaze, usually appears at 20 mcg/mL, ataxia at 30 mcg/mL, dysarthria and lethargy appear when the serum 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 over 100 mcg/mL with complete recovery. Irreversible cerebellar dysfunction and&nbsp;atrophy&nbsp;have been reported.<br />
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<strong>Treatment</strong>: Treatment is nonspecific since there is no known&nbsp;antidote. The adequacy of the respiratory and circulatory systems should be carefully observed and appropriate supportive measures employed.&nbsp;Hemodialysis&nbsp;can be considered since phenytoin is not completely bound to plasma proteins. Total exchange&nbsp;transfusion&nbsp;has been used in the treatment of severe intoxication in pediatric patients. In acute overdosage the possibility of other CNS depressants, including alcohol, should be borne in mind.</p>