READ, SCOTT A. PhD; COLLINS, MICHAEL J. PhD, FAAO; CARNEY, LEO G. DSc, FAAO; ISKANDER, D ROBERT PhD
Accurate knowledge of the biometric parameters of the anterior eye and adnexae is important in a variety of clinical and research applications. Some examples of these applications include the diagnosis and management of various ocular pathologies and abnormalities, ocular surgery, the design and fitting of contact lenses, and the measurement of eye growth.
The natural appearance of the palpebral fissure is influenced by various factors, including age, ethnicity, and gender. Paiva et al.1 found rapid changes to occur in palpebral fissure measurements from birth to 48 months of age, with many measurements reaching close to adult levels within the first 1 to 2 years of life. A general increase in the laxity of the eyelids has been found in subjects over the age of 50.2–4 This increased laxity leads to a shortening of the horizontal eyelid fissure2,5 and sagging of the lower eyelid.4,5 Van den Bosch et al.5 reported their male subjects to exhibit wider horizontal eyelid fissures (by approximately 0.7 mm) than their female subjects. Subjects of Asian ethnic origin have been shown to exhibit smaller vertical palpebral apertures and more angled palpebral fissures than whites.6,7
Palpebral fissure biometry can be important in certain aspects of contact lens practice. Carney et al.8 found that the position of the eyelids can influence rigid contact lens centration and stability. The position and angle of the eyelids have also been shown to effect soft toric contact lens orientation and stability.9,10 Alternating vision bifocal contact lenses rely on the movement of the lens upward (in relation to the pupil) in downgaze so that the line of sight is directed through the near segment of the contact lens.11,12 Success with these lenses will be influenced by the position of the eyelids and their interaction with the contact lens in downgaze.
Accurate biometry of the anterior eye is also important for the diagnosis and surgical management of anterior eye malformations13–15 and eyelid abnormalities.16–21 Attaining a “normal” eyelid position, smooth contour, and symmetry between the two eyes after surgery are important factors for achieving a good surgical result.16,22
Although there is a range of information about the biometric measures of the eye and adnexae in primary gaze, there is limited information about many of these dimensions in downward gaze. Such data are important because a large proportion of visual tasks occur in varying degrees of downward gaze (e.g., reading, walking, and computer work). We have made a series of biometric eye measurements of 76 young white subjects in three different directions of vertical gaze (primary gaze, 20° downgaze, and 40° downgaze) and discuss the implications of these findings.
Subjects and Procedures
Digital images of the anterior eye and adnexae from 80 young white adult subjects were captured. The images from four subjects were excluded completely from analysis as a result of poor image quality, leaving a total of 76 subjects. These subjects ranged in age from 18 to 35 years with a mean age of 24 years. Forty-four of the 76 subjects were female. All subjects had normal ocular health, were free of any ocular or systemic disease that may alter anterior eye appearance, and had no history of any ocular or eyelid surgery. The subjects had a range of refractive errors with the mean best sphere refractive error being −0.66 D (range, +0.63 D to −4.38 D). No rigid gas-permeable contact lens wearers were included in the study because there is evidence that long-term rigid contact lens wear can lead to a slight ptosis.23–26 Three of the subjects were occasional soft contact lens wearers, but they were instructed not to wear their contact lenses on the day of testing. Approval from the university human research ethics committee was obtained before beginning the study and informed consent was obtained from all of the subjects.
High-resolution (3072 × 2048 pixels) digital images of the right eye were captured using a Canon 300D 6.3 megapixel digital SLR camera (Canon Inc., Tokyo, Japan) with a 100-mm macrolens. Because a subject's level of fatigue or alertness may cause variations in eyelid position,27 all digital images were captured in the morning. The digital images were all taken in the same room with the same ambient room lighting conditions for each subject. The room lighting was provided by standard fluorescent lights, which provided an approximate illuminance at the plane of the subject's eye of 300 lux. The camera's built-in flash was used for all images and the camera was positioned at a distance of approximately 500 mm (distance from the camera lens to the subject's eye) from the subject. The camera's flash intensity is rated as guide number 13 and is triggered on a first curtain flash sync. The camera's shutter speed for all images was 0.017 seconds. It is therefore unlikely that the images would be confounded by flash-induced reflex blinking, which has typically been found to have a latency of onset of approximately 0.05 seconds.28,29
A camera mount was constructed for capturing the images of the eye in three vertical directions of gaze (0°, 20°, and 40° below horizontal). The camera mount consisted of a counter-weighted metal arm attached to a tripod (Fig. 1). A calibration cross (to allow later scaling of the digital images) was attached to the tripod and positioned at the center of rotation of the tripod head.
For each subject, digital images were taken in the frontal plane. Images were captured in primary gaze (0°), in 20° downgaze, and in 40° downgaze. The subject was positioned such that their right eye was directly adjacent to the calibration cross. For all images, the subject was instructed to maintain natural posture and to direct their angle of gaze through the center of the camera's lens (i.e., the alignment target for all images was the center of the lens). We aimed to capture the natural position of the eye and eyelids in a variety of downgaze visual tasks. Each image was therefore captured in “free space” with the subjects instructed to assume a natural posture when viewing the three target positions (i.e., we did not use a chin- or headrest). In a pilot study on five subjects, we found that on average, the subjects used approximately a 15° downward head tilt to view an object 40° below the horizontal, which is consistent with previous studies that have measured the natural head tilt used in downward gaze.30,31
To help to account for any normal variations occurring in the palpebral aperture of each subject, three images were taken for each angle of gaze. A total of nine images were therefore captured for each subject. Whenever possible, the three images from each subject from each angle of gaze were used in analysis; however, any image displaying poor focus or captured midblink was discarded. This left a total of 608 images from the 76 subjects included in the analysis (i.e., an average of 2.7 images per angle of gaze for each subject). The images for each subject in each angle of gaze were generally found to be well correlated. When the vertical palpebral aperture width (PA) of the first image was plotted against the PA of the second image for each subject (and for all angles of gaze), the correlation (r2) was 0.93.
Each digital image was analyzed using custom-written software that provides a range of biometric measures of the anterior eye and adnexae.32 This program enables the user to locate the Cartesian coordinates of a number of different anterior eye landmarks, including the limbus (16 points), pupil (eight points), temporal and nasal canthi (one point at each canthus), and the upper and lower eyelid contour33 (eight points along each lid margin). Based on the coordinates of these locations, the program then calculates a range of palpebral fissure biometric parameters (Fig. 2) and these measures are all relative to the subjects line of sight (i.e., along 0°, 20°, and 40°). The software program also allows the user to digitally zoom in on the image, thus making fine features of each image more easily identified and allowing individual pixels in the image to be distinguished. The average scaling factor from all images analyzed was 28.1 pixels per millimeter (i.e., each pixel was 35.6 μm in width).
All of the digital images were analyzed by two masked observers. When the difference between the results of the two observers was outside of three standard deviations of the mean difference, the result was identified as an outlier. All outlier results were then reanalyzed by a third observer and the two closest results (of the three observers) were then used for subsequent analysis (the third observer was used to analyze 9% of the total images). The results from the two primary observers generally correlated very closely (e.g., an average correlation coefficient [r2] of 0.91).
The group mean data were analyzed using a repeated-measures analysis of variance with one within-subject effect (angle of gaze) and one between-subject effect (gender).
The palpebral fissure undergoes a number of significant changes in downgaze (Table 1). All of the biometric measures showed highly significant change with angle of gaze (p < 0.001), except for the Theta_Head measure (p = 0.04). Figure 3 illustrates diagrammatically, the mean biometric palpebral fissure, and anterior eye measures for the 76 subjects in primary gaze, 20° downgaze, and 40° downgaze. The palpebral fissure outline in this figure was derived from the eyelid contour polynomial. Figure 3 also displays the primary gaze, 20° downgaze, and 40° downgaze images from a typical subject (subject no. 43) whose individual data were very similar to those of the population mean.
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.
Evinger et al.43 investigated the mechanisms of upper eyelid movement occurring in blinking and with downward gaze. They found that the downward movement of the upper lid during a blink is caused by contraction of the orbicularis oculi muscle and relaxation of the levator palpebrae muscle. However, the movement of the upper lid in downward gaze (referred to as a lid saccade) was caused primarily by a relaxation of the levator muscle and passive downward force (i.e., the orbicularis muscle has very little involvement in the movement of the eyelids in downward gaze). It is therefore likely that the changes occurring in eyelid position, angle, and contour during blinking may be slightly different from the changes accompanying downward shifts in gaze.
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).
Young et al.10 showed that the vertical palpebral aperture width and the angle of the central eyelid in primary gaze were associated with soft toric contact lens orientation and stability. We found that the angle of the central upper eyelid (term “B”) in primary gaze is very close to horizontal, and the central lower lid is slightly downslanted in primary gaze. However, both upper and lower eyelid angles become significantly more downslanted in downward gaze. It seems likely therefore that the interaction between the eyelid angle during blinking and a soft toric lens will differ depending on the degree of downward gaze of the lens wearer. The location and orientation of stabilization zones used in soft toric lenses (i.e., regional increases in lens thickness) must therefore be a compromise between the eyelid angles in various angles of gaze. In practical terms, a patient wearing a soft toric lens may have significantly different lens orientation in primary gaze compared with downgaze with an associated error of cylinder axis. This may be an area worthy of further investigation.
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).
Cook et al.44 demonstrated that significant changes to the position of the temporal and nasal canthi occur with horizontal gaze shifts (i.e., they showed that the medial and temporal canthi can move). We have now shown that significant changes in position of the canthi (and numerous other palpebral landmarks) occur with vertical shifts in gaze. This highlights the fact that the canthi, and in fact the entire palpebral fissure, is dynamic and changes significantly with changes in gaze.
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
School of Optometry
Queensland University of Technology
Room B547, O Block
Victoria Park Road
Kelvin Grove 4059
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