Blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentration in essential tremor cases in Spain

► Environmental correlates for essential tremor (ET) are largely unexplored. ► Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing toxin. ► Blood harmane levels were quantified by high performance liquid chromatography. ► Blood harmane levels were elevated in familial ET cases vs. controls in Spain. ► These data extend observations from New York to a second cohort of cases in Spain. Environmental correlates for essential tremor (ET) are largely unexplored. The search for such environmental factors has involved the study of a number of neurotoxins. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing toxin. In two prior case–control studies in New York, we demonstrated that blood harmane concentration was elevated in ET patients vs. controls, and especially in familial ET cases. These findings, however, have been derived from a study of cases ascertained through a single tertiary referral center in New York. Our objective was to determine whether blood harmane concentrations are elevated in familial and sporadic ET cases, ascertained from central Spain, compared to controls without ET. Blood harmane concentrations were quantified by a well-established high performance liquid chromatography method. The median harmane concentrations were: 2.09g−10/ml (138 controls), 2.41g−10/ml (68 sporadic ET), and 2.90g−10/ml (62 familial ET). In an unadjusted logistic regression analysis, log blood harmane concentration was not significantly associated with diagnosis (familial ET vs. control): odds ratio=1.56, p=0.26. In a logistic regression analysis that adjusted for evaluation start time, which was an important confounding variable, the odds ratio increased to 2.35, p=0.049. Blood harmane levels were slightly elevated in a group of familial ET cases compared to a group of controls in Spain. These data seem to further extend our observations from New York to a second cohort of ET cases in Spain. This neurotoxin continues to be a source of interest for future confirmatory research.