RESUMEN
BACKGROUND: Isoniazid is the most widely used anti-tuberculosis agent, yet it may lead to life-threatening complications. CASE PRESENTATION: Here we report the case of a chronic hemodialysis patient who developed severe encephalopathy after the start of isoniazid. Blood levels of isoniazid were elevated, and acetyl-isoniazid over isoniazid ratio was decreased 3 h after intake of the medication, suggesting that a slow acetylator phenotype may have contributed to drug toxicity, in addition to pyridoxal phosphate removal by dialysis. This hypothesis was confirmed by sequencing of NAT2, the gene responsible for isoniazid elimination, and identification of NAT2 polymorphisms compatible with a slow acetylator phenotype. Isoniazid withdrawal along with supplementation using high doses of pyridoxine successfully reversed the drug toxicity. Isoniazid toxicity occurs in populations at risk, including patients with chronic kidney failure or NAT2 polymorphisms, who have a disturbed metabolism of pyridoxine or isoniazid, respectively, and those on renal replacement therapies, in whom pyridoxal phosphate - the active metabolite of pyridoxine - is inadvertently removed by dialysis. CONCLUSIONS: Physicians should be aware of the increased risk of isoniazid toxicity in patients on dialysis and in those with a slow acetylator phenotype conferred by NAT2 polymorphisms. Adaptation of prescription - either with higher doses of pyridoxine or decreased doses of isoniazid, respectively - has been suggested to reduce the risk of potentially life-threatening toxicity of isoniazid.