Corneal wavefront aberration measurements to detect keratoconus patients

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Abstract

Distortions of the cornea associated with keratoconus are the manifestations of increased level, compared to normal, of higher order aberrations. It is highly relevant to use corneal aberrations to describe the optical quality of the cornea. The aim of the current study was to develop a keratoconus detection scheme based on Zernike coefficients. The results showed that the best detector to differentiate between suspected keratoconus and normal corneas was vertical coma (Z3−1) (specificity 71.9%; sensitivity 89.3%). The results demonstrated an improved use of videokeratoscope as a diagnostic tool for keratoconus detection that can be used for all types of videokeratoscopes.

Introduction

Keratoconus is a corneal disorder that affects the shape, the structure and the transparency of the cornea and creates significant visual problems. The disease is progressive, normally bilateral and creates, what clinicians habitually describe, as irregular astigmatism that is difficult to correct with spectacle lenses. For moderate levels of keratoconus, a rigid gas permeable contact lens provides the best vision and for severe cases, keratoplasty needs to be performed. Techniques for detecting keratoconus and following the evolution of the disease are valuable to the clinicians, as they lead to better patient management. Screening early keratoconus patients is also essential in the field of refractive surgery as operating on an undetected keratoconic cornea (keratoconus fruste) is a major cause of post-Lasik complaints [1] due to poor surgical outcomes and at times to a rapid worsening of the condition.

The determination of quantitative indices of the anterior corneal surface, obtained from corneal topography, has been suggested by several workers as a mean of detection of early keratoconus [2], [3]. Cox and co-workers [4] has reported that the measurement of the total ocular wavefront aberration may provide a sensitive and reliable tool to detect early keratoconus and to follow its progression. The main disadvantages of Cox's method are that it measures the whole ocular aberrations, whereas in keratoconus it is the corneal component that is of interest and it uses a videoaberroscope, which is still a very specialised instrument mainly used in clinical research. The advantage of Cox's approach is that it incorporates a well-established optical analysis principle, based upon Zernike analysis, whereas other indices proposed are proprietary to the instruments. Barbero et al. [5] found that both videokeratoscopy and videoaberroscopy were useful techniques in diagnosing and quantifying optical degradation in keratoconus. However, preliminary data in our clinic on a population of keratoconus (unpublished data) indicates that for large pupil sizes, for which aberrations are most relevant, the videoaberroscope fails to produce reliable measurements ocular aberrations, whereas the videokeratoscope does so. Videokeratoscopes becoming more widely used in routine clinical practice, it is highly relevant to optimise the use of corneal data to establish an objective criterion for the diagnosis of keratoconus. Wavefront aberration of the cornea obtained from corneal height data is a mean to quantify the optical quality of the cornea using an established analytic technique that can be common to all videokeratoscopes and avoids proprietary analyses, which are not comparable across instruments.

The anterior surface of the cornea being the most important refracting element of the eye, higher order corneal aberrations are more significant in a keratoconus population than in a normal population with regular corneas. A percentage of corneal aberrations are masked by a rigid gas permeable contact lens, but some higher order aberrations remain uncorrected and contribute to the degradation of the retinal image quality and visual performance. The interest in investigating corneal aberrations has increased with the development of customised contact lenses [6] and customised surgical refractive corrections [7], [8] that aim to improve visual performance beyond the results achieved by solely correcting the sphere and cylinder elements. These customised techniques require the reliable determination of both specific higher order aberration terms and overall higher order corneal and ocular aberrations. Measurement of corneal aberrations in a keratoconus population is the first step towards the correction of the optical aberrations and improvement of visual performance. Further, knowledge of corneal aberrations might also help to screen early keratoconus.

The primary aim of the study was to develop a keratoconus detection and classification scheme based upon corneal aberrations. One mean of quantifying corneal aberrations is to use the set of orthogonal Zernike polynomials (Table 1) describing the corneal wavefront aberration obtained from corneal height data [9].

The rationale for the investigation was that values of Zernike coefficients would provide indices that enable the clinician to: (i) detect and screen out early keratoconus patients and (ii) determine the demographics of corneal aberrations in a population of keratoconus at different stages of the disease.

The hypotheses that were tested were that:

  • (i)

    Corneal aberrations being the most significant of ocular aberrations in a keratoconus population, their measurement using a videokeratoscope suffices to predict the potential visual performance achieved with spectacles and contact lenses in such population.

  • (ii)

    Corneal aberrations quantified in terms of Zernike coefficients and measured at different pupil sizes produce an objective mean of detecting early keratoconus and quantifying the severity of moderate to advanced keratoconus.

Section snippets

Nomenclature used to describe the aberrations in the human eye

Alternative representations of the aberrations of the eye can be used [10] such as the wavefront aberrations, transverse aberrations and longitudinal aberrations. In this study, aberrations were described in terms of wavefront aberrations. The wavefront is an imaginary surface joining all points in space that are in the same phase of motion [11]. The deviation of the wavefrom, the ideal spherical form is called the wavefront aberration [12]. It indicates the extent to which the intensity

Population

Forty-six subjects (n = 92 eyes) took part in the study. The population was made of 23 male and 23 female, with a mean age of the 37.5 years (Table 2). The population was sub-divided in two groups based upon their keratoconus status. The first group comprised subjects with suspected keratoconus. These subjects presented with an interest to be fitted with contact lenses and had neither been previously diagnosed with keratoconus and nor did they present with any symptoms or vision complaints

General remarks

With a view of detecting and classifying keratoconus, corneal aberrations were analysed using a set of fixed pupil diameters: 3.0, 4.5 and 6.0 mm. This allowed the comparison between patients, and also between groups in particular with the control group with regular corneas. In contrast, when testing for association between visual performance and the magnitude of corneal aberrations, it was crucial to reference the data to individual pupil sizes, representing the physiological conditions under

Discussion

Analysis of higher order corneal aberrations of normal control, suspected keratoconus and diagnosed keratoconus eyes revealed differences in the optical quality of the corneas of the three groups. Significant differences were recorded between the diagnosed keratoconus and the other two groups for all pupil sizes and between the suspected keratoconus and the normal control for large pupils. Trefoil (Z33 and Z33), vertical coma (Z31) and spherical aberration (Z40) were the individual Zernike

Conclusion

Corneal front surface aberrations obtained from corneal height data allow for the detailed description of the optical quality of the cornea. It is shown to be a reliable tool for the detection of early keratoconus and to follow up the progression of the condition.

  • Differences between normal corneas and diagnosed keratoconus were recorded at all pupil diameters, but were more marked at the largest pupil diameters. Differences between normal corneas and suspected keratoconus were significant only

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