Abstract: Objective To reveal the pathogenesis of congenital blephroptosis by comparing the thickness and cross sectional area of the levator palpebrae superioris (LPS) on magnetic resonance imaging (MRI) between patients suffered from moderate to severe congenital blephroptosis (aged from 14 to 22) and healthy volunteers (aged from 14 to 23). Design Cross-sectional study. Participants Twenty-seven sick eyelids and 13 normal eyelids of 20 patients and 40 healthy eyelids of 20 volunteers were enrolled. 13 normal eyelids of the unilateral congenital blephroptosis patients were also included. Methods Twenty-seven sick eyelids and 13 normal eyelids of 20 patients who had unilateral or bilateral blephroptosis and 40 healthy eyelids of 20 volunteers were evaluated by MRI. MRI was taken when the eyes were naturally closed. The origin of the measurement was chosen at the point where the optic nerve left the eyeball. 2 points with 0.5 mm in space forward and 2 points with 0.5 mm in space backward from the origin were set. Based on each setting points, the cross-sectional area was measured on coronal MRI, while the height of the LPS was measured on sagittal MRI. Main Outcome Measures The cross-section area of LPS measured on coronal MRI and the height of LPS measured on sagittal MRI. Results The data of the patients was in normal distribution, while the data of the volunteer was not. So the nonparameter test was used in data analysis of the two groups. The cross-sectional area（S2=2.66±1.25，S1=2.51±1.29，S0=2.39±1.28，S-1=1.78±0.96，S-2=1.09±0.42） and the height （T2=1.05±0.27，T1=1.04±0.26，T0=1.00±0.24，T-1=0.89±0.28，T-2=0.79±0.26） of LPS in patients’ sick eyes decreased significantly compared with volunteers’ （T2=1.48±0.08，T1=1.40±0.09，T0=1.32±0.10，T-1=1.21±0.07，T-2=1.10±0.06， S2=4.83±0.71，S1=4.53±0.75，S0=4.50±0.68，S-1=3.13±0.56，S-2=1.85±0.48）（all P<0.001）. As for the unilateral patients, the cross-sectional area and the height of LPS of the sick eyes showed significant difference when compared with the patients’ normal eyes（all P＜0.05）. However, there was no statistical difference between the patients’ normal eyes and the volunteers’ eyes （all P＞0.05）. Moreover, as far as the cross-sectional area and the height of LPS was concerned, there was no difference between the right and left eyes of the volunteers（all P＞0.05）. Conclusion LPS of congenital blephroptosis patients was significantly thinner than those of the healthy volunteers. Dysgenesis of the LPS may be the main cause of the simple congenital blephroptosis.

Key words: blephroptosis, levator palpebrae superioris, dysgenesis, MRI