81 ECG changes in Fabry disease women (both control subjects and patients) at all ages (figure 2A). Compared to control subjects, FD patients had a significantly higher Cornell index from a young age onwards (from 18-29 years in men with FD and 30-39 years in women with FD) (supplemental table 2, figure 4A). Spatial QRS-T angle increased with ageing in all subgroups, but the increases were much greater in patients with FD as compared to control subjects (men: FD minus controls β=25.1° per decade; 95%-CI 21.7-28.5, females: FD-controls: β= 24.0° per decade; 95%-CI 21.4-26.6) (table 2B). There was overlap between the sexes in FD, both in absolute value and increment with ageing, in spatial QRS-T angle (figure 2B). Compared to controls, both men and women with FD had a higher spatial QRS-T angle from 40 years onwards coinciding with this parameter exceeding the upper range of normal (105°) [31] (supplemental table 2, figure 4B). With ageing, the frontal QRS-axis became progressively more negative in all subgroups (i.e. left ward axis deviation), with progression being more pronounced in FD patients as compared to control subjects (men: FD minus controls: β= -6.1° per decade; CI -10.5 - -1.6, women: FD minus controls: β= -4.0° per decade; CI -7.4 - -0.6). There was no significant difference between men and women with FD with respect to the absolute value or change in QRS-axis deviation (table 2B, figure 2C). The absolute values of the frontal QRS-axis tended to be horizontal in patients with FD compared to a more normal QRS- axis in controls, but nonetheless remained within normal limits (figure 4C). Frontal T- axis The majority of control subjects remained within the normal range of 15°-75° regardless of age [32]. On the other hand, patients with FD developed a divergent frontal T-axis from approximately 30 years (men) and 40 years (women) onwards (supplemental figure 5). Electrocardiographic and CMR imaging properties in FD patients For the 133 included FD patients, a total of 119 CMRs (90%) were available that could be linked to the last obtained ECG during follow-up. LVMi and LGE data were reported for 101 (76%) and 118 (99%) of the 119 CMRs, respectively. See table 1 for detailed descriptive statistics on LVMi and LGE. Based on the Spearman analyses, we found statistically significant correlations between the seven main ECG parameters (P-wave duration, PR-interval, QRS-duration, QTc, Cornell index, spatial QRS-T angle and frontal QRS axis) and LVMi (supplemental figure 8). The absolute values of P- wave duration, QRS-duration, QTc, Cornell index and spatial QRS-T angle were significantly divergent in patients with LGE vs patients without LGE. The ECG parameter that best 3
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