Acute primidone overdosage with massive crystalluria.

A patient admitted to the hospital in coma was found to have massive primidone crystalluria. Gas chromatographic analysis of blood and urine for primidone and phenobarbital showed high urine primidone levels and high blood phenobarbital levels. The primidone levels suggest that primidone is rapidly cleared into urine. The high blood phenobarbital levels within 12 hr of overdosage with a history of diphenylhydantoin therapy and without phenobarbital therapy or overdosage suggests that diphenylhydantoin may influence the metabolic conversion of primidone to phenobarbital. The relationship of clinical symptomatology to high levels of primidone and phenobarbital is unclear. Analysis of blood and urine for primidone and phenobarbital and urine for crystals is of value in establishing diagnosis and prognosis in cases of suspected primidone overdosage.