-Topiramate infrequently induces anion gap metabolic acidosis through carbonic anhydrase inhibition on the distal tubule of the nephron -a type 2 renal tubular acidosis. We report on a 40 years old woman previously healthy that developed significant asymptomatic metabolic acidosis during topiramate therapy at a dosage of 100mg/day for three months. Stopping medication was followed by normalization of the acid-base status within five weeks. This infrequent side effect appears unpredictable and should be given careful attention.KEY WORDS: topiramate, metabolic acidosis, carbonic anhydrase inhibitor.Acidose metabólica grave por topiramato: relato de caso RESUMO -Topiramato pode produzir raramente uma acidose metabólica através da inibição da anidrase carbônica no túbulo distal do néfron -acidose tubular renal do tipo 2. Relatamos o caso de mulher de 40 anos previamente saudável que desenvolveu quadro de acidose metabólica assintomática grave, sem outra etiologia identificável, durante uso de topiramato na dose de 100mg/dia por três meses. Este efeito colateral, embora infrequente, parece ser imprevisível e requer atenção cuidadosa.PALAVRAS-CHAVE: topiramato, acidose metabólica, inibidor da anidrase carbônica.Topiramate was approved by the FDA (US -F o o d and Drug Administration) in 1996 for clinical use as an anticonvulsant drug. Since then, other clinical i ndications have been studied, such as the tre a tment of bipolar disord e r, or neuropathic pain syndro m e s re l i e f 1 , 2 , and migraine and cluster headache 3 -5 . As a consequence, it is likely that clinical use will tend t o expand progressively. Initially, significant adverse reactions to topiramate were related to the central nervous system (somnolence, nervousness, psychomotor slowing, memory problems, fatigue), or w e re gastrointestinal (nausea), ocular (diplopia, nystagmus) and neuromuscular (tre m o r, pare s t h e s i a ) . N e v e rtheless, since 1996, when it was approved by FDA, at least twelve reports were made relating metabolic acidosis to the use of topiramate, both in adults and in childre n 6 -1 7. Finally, in December 20 03, FDA released a note about the risk of metabolic acidosis caused by topiramate usage 1 8. Severe m etabolic acidosis can occur in patients under topiramate treatment, even when the daily dosage is low. The absence of relevant clinical symptoms re n d e r s it more difficult to recognize.We present a case of severe asymptomatic metabolic acidosis due to topiramate in an adult woman. CASEA 46-year old previously healthy white woman came to our hospital with a complaint of intense chest pain for at least 5 days with tenderness on the site of the pain i n d icating traumatic origin. There were no other symptoms, and physical examination plus X-ray and laboratory evaluation excluded pleural or pulmonary disease. Later, a radiograph of the ribs, a chest CT scan and a radionuc l e a r bone scan showed a small traumatic lesion in the area of the pain. At the hospital entrance, her arterial blood gas analysis...