The corneal surface is vulnerable to eyelid pressure [2]. Using the Pentacam system, Zhu et al. [14] indicated that congenital blepharoptosis not only induced corneal asymmetry and irregularity, but also affected corneal tomography, such as increased corneal elevation in blepharoptosis eyes with more than moderate severity and even focalized corneal thinning in severe cases. Changes in corneal topography and corneal astigmatism induced by eyelid surgeries were also observed after gold-weight implant, ptosis, ectropion, and eyelid mass surgeries [16, 17].Such changes may be explained by the anatomical proximity between the eyelids and the cornea and by the pressure exerted by the eyelids on the corneal surface, resulting in corneal deformation [17].
There have been relatively few reports in the literature on the effect of blepharospasm on corneal curvatures. Using corneal topography, Osaki has demonstrated that patients treated with BTX-A for hemifacial spasm developed significant eyelid and corneal changes on the affected eyes during the toxin’s period of action. The results showed a statistically significant decrease in steep K and astigmatism 2 months after BTX-A treatment [18]. In the present study, a significant difference in corneal astigmatism magnitude was found between the control and blepharospasm groups, including front K1, front K2, front Km, back K1, back K2, and back Km, and the curvature change is positively proportional to the severity of the disease. Additionally, we found the radii minimum were significantly decreased in the moderate and severe groups, indicating the decrease of the smallest radius of curvature in the entire field of measurements, further confirming corneal curvature change in the blepharospasm groups.
The Pentacam system could also provide several topometric indices that only consider the anterior corneal surface. These changes may be related to either the restoration of symmetry in the upper and lower lid apposition on the cornea or the rearrangement of the tear film [17]. We found a significant difference between ISV and IVA in the severe group. These results indicate that blepharospasm may increase the asymmetry and irregularity of the anterior corneal surface in cases of severe blepharospasm.
Among the properties of the corneal surface, elevation provides the most accurate representation of its shape. However, the Pentacam system have two reference database in Belin/Ambrósio Enhanced Ectasia Display, including Myopia/Normal and Hyperopia/Mixed Cyl, previous clinical observations indicated that there is an increased variability in the posterior elevation in hyperopic eyes on tomographic evaluation [19]. If this is true, it would lead to false positives when compared against a myopic biased normative database. Therefore, we exclude patients with significant hyperopia (> + 1 diopter). Corneal topographers can be categorized into two groups based on whether they can measure the elevation of the anterior and posterior surfaces of the cornea (front BFS and back BFS). However, to our knowledge, no study has reported on changes in corneal elevation in blepharospasm eyes. In our study, no significant difference in most corneal elevation parameters was found between the control and blepharospasm groups, except for front BFS, which was significantly different in the severe group. The diagnostic value of front BFS was suggested by Lim et al. [20], who found that front BFS was significantly higher among cases of keratoconus. However, to our knowledge of the seindices, their variability, as well as their value in reflecting the shape of the cornea are limited, and much research is still required before they can be applied with confidence and certainty.
An increased frequency of blinking was one of the main symptoms of blepharospasm. The normal blink rate is about 12 blinks/min [21], but a mean blink rate of 24.8 blinks/min has also been reported [22]. In the present study, the normal blink rate was about 17.56 ± 3.62blinks/min, while blepharospasm patients had rates of 30.64 ± 5.41blinks/min or higher. Rapid blinking is associated with worse ocular surface disease and tear stability [23], and we found a correlation between blink rate and several corneal parameters, including front K1, front K2, front Km, back K1, back K2, back Km, final D, and radii minimum, indicating that rapid blinking also increases the mechanical pressure on the surface of the cornea and changes the corneal curvature.
The final D index from the Belin/Ambrósio Enhanced Ectasia Display is amultimetric combination parameter composed of keratometric, pachymetric, pachymetric progression, and back elevation parameters. It is suggested that the final D index could be used as the sole parameter to identify early corneal ectasia [24]. Using a final D value greater than 2.61 as a cut off value may help to identify the majority of keratoconus suspects who truly have the disease [25]. In the present study, we found that final D values were significantly higher in the severe blepharospasm group than among controls, which indicated a high risk of subclinical keratoconus-like changes in severely blepharospastic eyes.
The limitations of this study should be noted as well. Firstly, all included subjects were Chinese. Because the anatomy of the eyelids and orbits differ between Asians and other races, the accurate definition and classification of blepharospasm also differ between these groups [26, 27]. Secondly, the sample size of our study was small, and the parameters must be investigated in a larger patient group. Third, it is a single-center study, which may make our results prone to a hospital-based bias. Finally, it is important to be aware of the predictive limitations of our cross-sectional study. Although the cross-sectional design allowed us to provide evidence of corneal tomography differences between blepharospasm patients and the control group, longitudinal design studies are necessary to establish a true cause-and-effect relationship.