Sunday, June 4, 2023

Effect of pterygium on corneal astigmatism, irregularity and higher-order aberrations: a comparative study with normal fellow eyes – Scientific Reports

Considering that the pterygium primarily involves the corneal surface, in this study, we focused on pterygium-induced changes in corneal optical parameters compared with normal fellow eyes. Pterygium significantly increased the magnitude of corneal astigmatism and induced J0/J45 corneal astigmatism. Irregularities of the 3- and 5-mm zones were also significantly increased by pterygium. Moreover, pterygium significantly induced some 3rd- and 4th-order HOAs (trefoils, horizontal coma, and quatrefoils), which may affect the quality of vision. The area of the pterygium was an independent contributing factor to changes in most of astigmatic and irregularity parameters, horizontal trefoil/quatrefoil, and RMS of the 3rd order aberrations, while the length of the pterygium was independently related to an increase in oblique trefoil/quatrefoil and RMS of the 4th order aberrations. Δ Horizontal coma was significantly induced by both the length and width of the pterygium. The thickness of the pterygium was not associated with any induced corneal optical parameters.

It has been well documented that the corneal topographic data and HOAs of the anterior cornea, especially for 3rd- and 4th-order terms, of right and left eyes have mirror-image symmetry13. Therefore, in this study, we designated normal fellow eyes of the same patients as controls to more exactly investigate the effect of the pterygium itself on the corneal optical parameters. Considering that there is tremendous variability in eye aberrations from person to person, HOAs significantly change with age, and surgery itself would induce unexpected aberrations, the present study might suggest more reasonable data than the results described by other studies in which the pterygium-induced changes were estimated by the effect of pterygium excision7 or in comparisons with age- and sex-matched controls8.

Several studies have established a relationship between the size of the pterygium and corneal astigmatism14,15. Mohammad-Salih and Sharif14 reported that the total area, extension, and width of the pterygium showed a strong correlation with the magnitude of corneal astigmatism measured by keratometry. They suggested that the total area and length have a stronger correlation with corneal astigmatism than width, which is similar to our findings. In the present study, the magnitude and J0 corneal astigmatism were increased in eyes with pterygium, which indicates that nasal pterygium induces WTR astigmatism. On the other hand, J45 corneal astigmatism was decreased in eyes with pterygium, which means that nasal pterygium in the right eye leads to increased counterclockwise oblique astigmatism16.

It has also been well documented that corneal irregularity and distortion caused by pterygium induce HOAs, which may be responsible for the deterioration in the quality of the vision6,7,10. In the same manner, this study demonstrated that pterygium significantly increases corneal irregularities of the 3- and 5-mm zones and thus induces some HOAs. Considering that all pterygia included in this study were located in the nasal quadrant, resulting in asymmetric corneal distortion, we could expect that pterygia mainly affect corneal wavefront aberrations that are not symmetric across the eye. As expected, we found that absolute values of most of horizontally asymmetric aberrations (horizontal coma, horizontal trefoil, and oblique quatrefoil) were significantly higher in eyes with pterygium, while some horizontally symmetric aberrations, including vertical coma and spherical aberrations, were not affected by pterygium. Regarding spherical aberration, pterygium had no effect, as reported elsewhere5,6. This implies that surgeons may choose the asphericity of the intraocular lens that reduces postoperative ocular spherical aberrations according to the current topographic data, regardless of whether cataract surgery is performed simultaneously with excision of the pterygium or scheduled alone in cases with a small pterygium. In this study, pterygium-induced 3rd-order HOAs were greater than the 4th-order HOAs, and the first two HOAs showing the greatest change in absolute value were horizontal and oblique trefoil, which are consistent with the results reported in previous studies5,7.

Theories around the causes of the corneal distortion and flattening induced by pterygium include the tractional force of contractile elements, the localized pooling of tears at the pterygium apex, and stromal scarring14,17,18. Interestingly, in the present study, pterygium-induced changes in corneal optical parameters were mostly associated with the length and area of the pterygium, whereas the thickness and grading of the pterygium was not related to any induced corneal optical parameters. This implies the morphology of the head rather than the body or tail of the pterygium may be the key to determining aforementioned factors affecting the corneal distortion and flattening. The facts that pterygia usually exhibit firm adhesion to the anterior corneal stroma, while spanning the limbal region without adherence18, pterygium with a flat corneal scleral transition zone induced more corneal scarring and astigmatism than pterygium with a nodular appearance18, and traction by body or tail evoked by temporal gaze is not an important factor in the change of astigmatism, 19 may support the importance of the head morphology. Meanwhile, the thickness of the pterygium was closely linked to the grading of the pterygium in this study, which has been known to be predictive of recurrence after pterygium excision. 20. Therefore, we think above mentioned negative result also has clinical significance. That is, the thickness of the pterygium itself may have clinical significance as one of predictive parameters for the recurrence after pterygium excision.

As HOAs have been known to be associated with glare, halos, and other various visual symptoms, pterygium-induced HOAs can also affect the visual quality of the patient. In particular, total RMS and coma have been known to be responsible for night vision disturbances21. Horizontal coma, but not vertical coma, has been reported to be associated with double vision in patients who undergo refractive surgery22. In the present study, we found that horizontal coma was significantly induced by pterygium, while vertical coma was not different between eyes with pterygium and normal fellow eyes. Moreover, RMSs of the 3rd and 4th order, trefoils, and quatrefoils were also significantly increased in eyes with pterygium, which would explain the cause of the visual symptoms in patients with pterygium who suffer from visual disturbance and low visual quality despite glasses correction.

With regard to determining the optimal time of pterygium excision, information about HOAs may provide surgeons with a valuable tool. In fact, Pesudovs and Figueiredo7 suggested that surgeons consider the removal of pterygia before they grow to 4 mm in size to avoid residual aberrations. Considering that patients with significant subjective visual complaints after corneal refractive surgery had an increase in RMS of the 3rd-order aberrations of 0.63 μm compared to those without symptoms, 23 surgical excision of the pterygium would be expected to improve the visual function when the area of the pterygium is greater than 5.76 mm2 according to the regression formula described above. This might provide additional valuable insight to surgeons during their decision-making process.

There are some limitations of this study. First, the measurement of HOAs for 6 mm diameter with a Placido disc-based topography is improbable to be accurate especially when the pterygium is large. These may be fundamental limitations of all studies which deal with the effect of the pterygium on astigmatism or corneal/ocular HOAs using a Placido disc-based topography5,7,8,19,24. However, we think corneal wavefront data in this study could be interpretable and have clinical significance as a subgroup analysis (the small versus large pterygium group according to the involvement of central 6 mm zone) showed almost the same results between two groups, even allowing for exaggeration in the eye with large pterygium (Supplementary Fig. 1). Second, slit lamp photography-based measurement of the size of the pterygium might not be accurate, considering that Fuchs flecks and the pterygium cap may not be evident on slit lamp photographs and the position of the limbus under the pterygium is judged by projecting the limbal position from where it is located above and below the pterygium. In line with this, in vivo confocal microscopy (IVCM) may be a valuable tool for accurately measuring the size of the pterygium as it has been well known that IVCM identifies Fuchs flecks sensitively and measurements of the pterygium are generally larger in confocal images than in anterior segment photographs25,26. Third, we could not evaluate whole ocular aberrations because the aberrometer (iDesign aberrometer; Johnson & Johnson Vision Care, Inc., Santa Ana, CA) in our clinic repeatedly showed acquisition errors in most patients, especially those with large pterygia. Therefore, the direct effect of pterygium-induced corneal HOAs on the whole ocular optical system could not be investigated. Last, we only included patients with unilateral nasal pterygium. Lesions located at other sites may show different results from those of this study.

In conclusion, through a comparative study with normal fellow eyes, pterygium had a significant effect on corneal astigmatism, irregularity and HOAs. Most of the differences in astigmatism, irregularity and HOAs are related to the length or area of the pterygium, while the thickness of the pterygium has no effect. These findings may be helpful in explaining the visual symptoms in patients with pterygium and for deciding about the timing of the surgical excision of the pterygium.

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