210 An unusual case of Mahaim fibers in hypertrophic cardiomyopathy

Introduction The coexistence of Mahaim fibbers (MF) and hypertrophic cardiomyopathy (HCM) is exceptional. We report an unusual case of a young female patient presenting with HCM and MF. Case-report A 17 y/o woman was referred in 1998 for a familial HCM screening. She had no symptoms; her surface EKG revealed a preexcitation suggesting a right anteroseptal WPW syndrome. The transthoracic echocardiogram (TTE) demonstrated an asymmetric septal HCM without LV outflow tract (LVOT) obstruction. The electrophysiological study (EPS) demonstrated the presence of MF (normal AH interval, a short and fixed HV interval and atrial pacing produced AH prolongation without change of preexcitation degree, indicating fasciculo-ventricular MF). Eight years later (25 y/o) while the patient was doing well, the ECG was similar and TEE demonstrated the same aspect of asymmetric septal HCM without obstruction. Three months later the patient was pregnant and gave birth to a healthy baby. (During pregnancy: Delta wave was present and there was no LVOT obstruction). After delivery, Surprisingly there was no more preexcitation on her ECG and the TEE demonstrated now a significant LVOT obstruction with a systolic anterior motion of the mitral valve and an LVOT gradient of 36 mmHg. A second EPS was performed in 2008 and showed no more preexcitation. We speculate the hypothesis that FV fiber played in this case a similar pathogenic role as does the apical right ventricular pacing when it is used to decrease LVOT gradient in patients with HCM. Conclusion This case deserves to be reported because the association of MF and HCM is rare (only 3 cases reported in english litterature), the disappearance of MF during pregnancy has never been reported and the hypothesis that the RV preexcitation via FV Mahaim fiber played a role in decreasing LVOT gradient in this patient with HCM.