In primary gaze, the mean horizontal fissure length (HEF) was 27.13 ± 1.5 mm with a mean palpebral fissure angle (Theta_HEF) of −3.71 ± 3.9° indicating, on average, a slight upslanting of the palpebral fissure in primary gaze. Upslanting means that the temporal canthus is higher than the nasal canthus. In downgaze, the horizontal fissure length (HEF) gradually shortens to a mean of 25.62 ± 1.8 mm and the angle of the fissure changes to a mean of 8.91 ± 3.9° downslanted for 40° downgaze. As would be expected from these results, the vertical distance between the nasal and temporal canthus (NC_TC) changes from a mean of −1.75 ± 1.8 mm in primary gaze (i.e., temporal canthus is 1.75 mm higher than the nasal canthus in primary gaze) to a mean of 4.01 ± 1.8 mm (i.e., temporal canthus is 4.01 mm lower than the nasal canthus) in 40° downgaze. The horizontal distance from the pupil center to the temporal canthus (PC_TC) also slightly shortened from 12.08 ± 0.6 mm in primary gaze to 10.96 ± 0.8 mm in 40° downgaze.
The mean horizontal head tilt (Theta_Head) was found to be 0.51 ± 2.1° (i.e., on average, subjects tilted their heads very slightly toward the right). In downgaze, the group mean Theta_Head changed slightly to be 1.1 ± 2.8° in 40° downgaze. The average horizontal head tilt was small but will lead to a slight underestimation of an upward slanting palpebral fissure and a slight overestimation of a downward slanting palpebral fissure. In primary gaze, the average corrected palpebral fissure angle (i.e., taking into account the slight head tilt) is −4.2° upward slanted. The change in horizontal head tilt in downward gaze accounts for only 5% of the total change in the palpebral fissure angle (Theta_HEF) with downgaze.
The vertical palpebral aperture also underwent significant change. In primary gaze, the mean vertical distance from pupil center to upper lid (PC_UL) measured at 3.54 ± 0.8 mm and the mean distance from pupil center to lower lid (PC_LL) measured at −6.13 ± 0.6 mm (total PA of 9.67 ± 1.2 mm). In downgaze, the upper lid moved downward slightly with mean PC_UL measuring at 3.22 ± 0.9 mm and 3.03 ± 0.8 mm for the 20° and 40° downgaze positions, respectively. A larger movement was found to occur in the lower lid in downgaze with respect to pupil center with mean PC_LL moving to be −4.71 ± 0.6 mm and −3.39 ± 0.8 mm for the 20° and 40° downgaze positions, respectively. This is a change in upper lid position of 0.5 mm and a change in lower lid position of 2.7 mm from the primary gaze to the 40° downgaze position. This equates to a total vertical PA reduction from 9.67 mm in primary gaze to 6.41 mm in 40° downgaze.
The inferior nasal aperture (INA) measurement significantly increased from 2.47 ± 1.1 mm in primary gaze to 4.17 ± 1.2 mm in 40° downgaze. This indicates a lowering of the bottom eyelid in relation to the nasal canthus in downgaze. The superior temporal aperture (STA) measurement also significantly increased in downgaze (from a mean of 5.46 ± 1.1 mm in primary gaze to a mean of 6.25 ± 1.2 mm in 40° downgaze). This suggests that the temporal canthus is moving downward in relation to the upper lid in downgaze.
The contour of both the upper and lower eyelid was found to undergo significant changes in downgaze. The curvature of the eyelid (term “A”) was found to slightly flatten (reduction in magnitude of term “A”) in downgaze for both the upper and lower eyelid. The angle of the upper lid also changed from being close to horizontal in primary gaze (term B = 0.001 ± 0.07) to being angled slightly downward (term B = 0.19 ± 0.07 in 40° downgaze) in the downgaze positions. The angle of the lower lid changed similarly, being close to horizontal (B = 0.062 ± 0.06) in primary gaze and slightly downslanted in downgaze (B = 0.166 ± 0.07).
Male and female subjects were found to exhibit a significant difference in only one of the biometric anterior eye measures. The horizontal palpebral fissure length (HEF) was significantly larger in male subjects (mean across all conditions of 27.00 ± 1.8 mm) compared with female (26.21 ± 1.6 mm) subjects across all conditions (p = 0.02). Significant angle of gaze and gender interactions were found for the PC_LL (p = 0.01) and lower eyelid contour term C (p = 0.01) measures. These interactions were brought about by the female lower eyelid exhibiting slightly less change (narrowing) than the male lower eyelid with downward gaze.
We have shown that highly significant changes occur in many of the dimensions of the palpebral fissure in downward gaze. These changes cause significant alterations to the vertical and horizontal palpebral fissure dimensions, the eyelid contour, and the angles of the upper and lower eyelids. In general, the palpebral fissure narrows, with the upper lid moving slightly downward (by 0.5 mm on average) and the lower eyelid moving upward (by 2.7 mm on average) in relation to the pupil center. The angle of the palpebral fissure changes from on average being slightly upslanted in primary gaze to being slightly downslanted in downward gaze positions.
Our results for the primary gaze palpebral fissure biometric measures generally correlate closely with previous investigations into the horizontal and vertical dimensions,2,5,6,10,23,34,35 the angle,7,10,36 and the contour34,36 of the palpebral fissure for white subjects in primary gaze for most measures. A possible reason for some intersubject variation in palpebral fissure measures in our study would be differences in vertical head tilt between subjects. The fact that our primary gaze measures generally correlate closely with previous studies that have controlled vertical head tilt suggests that any vertical head tilt adopted in primary gaze was relatively small and consistent between subjects. Recent research into head tilt in downward gaze has found that subjects may alter their head tilt depending on the complexity of the task37 (e.g., head tilt may be altered when reading smaller fonts). The near task performed by our subjects (i.e., directing their line of sight toward the camera lens) was not complex and remained constant between subjects, which would promote a consistent degree of vertical head tilt between subjects in downward gaze.
Previous investigators have also found significant narrowing of the vertical palpebral fissure width38,39 and reduction in the exposed ocular surface area with downward gaze.40–42 However, we have further shown that significant changes in palpebral fissure angle and contour also occur with downward gaze. It appears that in downgaze, the upper lid moves downward to maintain its location with the superior cornea, presumably to protect the ocular surface. The temporal canthus moves downward in conjunction with the upper lid, whereas the nasal canthus remains relatively immobile, thus causing the alterations in eyelid angle.
The palpebral fissure in the vertical dimension is an important measure in the diagnosis of eyelid malpositions such as ptosis and lid retraction. The average total vertical PA width from previous studies ranges from 9.1 mm to 10.8 mm,2,5,6,10,23,34,35,38,39 which compares closely to our average value of 9.7 ± 1.2 mm in primary gaze. Some previous studies have defined ptosis as when the upper eyelid is <2 mm from the center of the pupil,16,18,27 and this value matches closely with the upper 95% confidence interval for the pupil center to upper lid (PC_UL) measurement in our present study. The eyelid contour term “A” has previously been shown to be greater in cases of eyelid retraction and lower in blepharoptosis (i.e., the higher the upper lid, the more steeply curved is the eyelid contour and vice versa).36 Our results are consistent with this premise, because we found a significant flattening of the eyelid contour to occur in downward gaze (i.e., as the palpebral aperture narrowed, the contour of the eyelids flattened).
Translating rigid bifocal contact lenses rely on the upward movement of the lower lid with respect to the pupil center (PC_LL) to achieve “alternating vision” in downward gaze. Our data for young subjects suggests that, on average, this degree of movement is only 1.42 mm in 20° downward gaze and 2.74 mm in 40° downward gaze. In older subjects, it is conceivable that this degree of movement may not be achieved as a result of the reported increased laxity of the lids.2–4 These findings highlight the difficulty in achieving adequate translation of the near vision segment in alternating bifocal contact lenses (e.g., an upward movement of the lower lid of at least 3 mm is required to shift the near segment over a 3-mm pupil).
Various eyelid pathologies (such as chalazia and capillary hemangioma) and eyelid malpositions (such as ptosis) can cause significant changes in corneal astigmatism.45–51 There is also evidence to suggest that altering the position of the eyelids may cause changes in corneal astigmatism.52–54 Speculation also exists that pressure from the eyelids may be involved in the etiology of with-the-rule astigmatism.55,56 If the position of the eyelids can influence the shape of the cornea, then it remains a possibility that the normal angle and position of the eyelids (both in primary gaze and downward gaze) may play a role in the etiology of corneal astigmatism.
Many of our daily visual tasks involve downward gaze and our results have shown that most palpebral fissure dimensions change significantly in downward gaze. These changes in the palpebral fissure should be considered in various applications such as contact lens practice in which the eyelid morphology plays an important role in the biomechanics of lens centration and orientation.
We thank Brett Davis for his assistance in the construction and design of the camera apparatus. We also thank Claudia Hackl and Wiebke Rohloff for their assistance with the analysis of the digital images.
Scott A. Read
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