Adopting a multidiagnostic method for characterising the anterior segment

Technologic advances in the field of vision care have led to the development of multidiagnostic platforms that integrate, in the same device, several technologies to measure different anatomical and optical parameters of the eye.¹ This type of diagnostic platform allows a complete characterisation of the corneal structure, including the analysis of the shape and optical aberrations of the two corneal surfaces, distribution of thickness and even a volumetric analysis of the cornea.¹

One of the main features of these devices is the clinical characterisation of the ocular optics, providing measurement of objective refraction and ocular aberrations.^2,3 Likewise, these instruments provide a complete geometric analysis of the cornea.

The combination of Scheimpflug imaging and Placido disk technologies has been shown to be a good option to obtain consistent curvature and elevation data in normal healthy and even in keratoconus eyes.^4-8 Finally, multidiagnostic platforms allow the clinician to obtain different anatomical dimensions of anterior segment structures, which are crucial for ocular pathology screening and to perform a comprehensive monitoring of ocular diseases.

Recently, a new multidiagnostic platform (Figure 1) has been developed that enables tangential and axial curvature data of the anterior corneal surface to be obtained, as well as a biometric estimation of various anterior segment structures; measurement of corneal, internal and ocular wavefront aberrations; visual quality simulations; corneal pachymetry maps; and IOP measurements.

Specifically, this system (VX120, Visionix-Luneau Technologies) (Figure 2) combines a Hartmann-Shack aberrometer; a Placido disk corneal topographer; a Scheimpflug imaging-based system; and an air tonometer. The Hartmann-Shack aberrometer measures 1,500 points in 0.2 seconds in an area ranging from 2 to 7 mm in diameter.

The Scheimpflug imaging-based system uses monochromatic blue light of 455 nm to obtain pachymetric measurements with a resolution of ± 1 µm and iridocorneal angle measurements with a resolution of ± 1º. The Placido disk system projects 24 rings on the corneal surface, measuring more than 100,000 points.

Recently, some improvements have been introduced in the Scheimpflug photography system included in the VX120 system that allows, additionally, the characterisation of the posterior corneal topographic profile and the generation of corneal pachymetric maps, which has given rise to the VX130 system.

Our validation

We recently conducted a study to evaluate the consistency of measurements provided by the VX120 system.^9,10 In this study, we evaluated a total of 107 healthy eyes of 107 patients with ages ranging from 23 to 65 years at the Optometric Clinic of the University of Alicante.

Inclusion criteria were: age of more than 18 years, refraction error between +5.00 and -10.00 D and eyes without pathology. Exclusion criteria were previous ocular surgery; glaucoma; fewer than 18 complete consecutive Placido rings projected on the cornea and therefore considered for the corneal analysis pseudophakia; corneal ectatic diseases; and any other type of pathologic condition of the eye.

A complete eye examination was performed in all cases, using the device to measure uncorrected visual acuity, and to obtain objective refraction, air tonometry and corneal topographic and anterior segment readings. In all cases, three consecutive measurements of objective refraction and ocular aberrations were obtained to assess intrasession consistency.

The within-subject standard deviation (S_w) and intrasubject precision (1.96xS_w) for refractive data were below 0.12 D and 0.20 D in all cases, respectively. Concerning the aberrometric measurements, S_w and intrasubject precision values were below 0.05 µm and 0.10 µm, respectively. No statistically significant correlations of refractive measurements with their S_w associated were found (-0.033 ≤ r ≤0.053, P ≥ 0.064).⁹

Regarding topographic parameters of the anterior corneal surface, all S_w for curvature measurements were below 0.05 mm. The S_w for the magnitude of corneal astigmatism calculated for different areas of analysis was below 0.21 D.

Concerning the axis of corneal astigmatism, its S_w was below 11.27º. Corneal eccentricity showed S_w of 0.067. The analysis of the consistency of corneal aberrometric measurements showed a S_w of 0.048 µm for the root mean square associated to higher-order aberrations (HOA). For the aberrometric parameters of the third and fourth order, all S_w were below 0.037 µm.¹⁰

Concerning the anatomical measurements of the anterior segment, the results were excellent, with S_w within acceptable ranges. The S_w for anterior chamber depth (ACD) was 0.03 mm and remained below 9 µm for central and peripheral pachymetry.

For iridocorneal angle (IA) measurements, S_W values of 0.84 or lower were found. The S_W for white-to-white corneal diameter was 0.24 mm. No statistically significant correlations were found between any anatomical parameter evaluated and their associated S_w values (-0.220 ≤ r ≤ 0.204, P ≥ 0.125).

Clinical cases

We have reported some clinical cases to show the clinical applicability of this new multidiagnostic platform as follows.

Case 1

A 73-year-old patient with a diagnosis of Fuchs’ dystrophy had undergone cataract surgery recently and since then the endothelial corneal dystrophy had worsened. The analysis obtained with the VX120 system revealed the following information in one shot:

·      Significant narrowing of the anterior chamber in both eyes, especially in the left eye, as well as a central corneal thickening (Scheimpflug images, Figures 3 and 4);

·      Measurement of central corneal thickness in the right eye not obtained, suggesting the presence of a significant intraocular opacity that was confirmed with retroillumination images (Figure 4);

·      Identification of the location of the opacity at endothelial level in the central part of the cornea with the analysis of the Scheimflug image (Figure 4);

·      Presence of moderate amounts of astigmatism that could also be observed in the corneal topographic map, confirming that the main source of astigmatism in this patient is the cornea (Figure 3);

·      Great relevance of the aberrations of the anterior corneal surface in the total aberrometric pattern, although the internal aberrometric pattern in this specific case also showed a high level of influence over the ocular pattern, possibly due to the alterations of the posterior corneal surface as a consequence of the Fuchs’ dystrophy (Figure 5);

·      Low levels of IOP that could be biased due to the significant corneal oedema that was present. For this reason, the system always displays the measurement of central corneal thickness and IOP together, providing also a compensated IOP value considering this corneal thickness (Figure 3).

Case 2

In an asymptomatic 60-year-old man, the technology enabled an incipient corneal ectasia to be detected. Several years prior to this, the patient had undergone cataract surgery in both eyes with implantation of monofocal IOLs.

The system allowed the following information to be obtained:

·      Presence of a low refractive error in both eyes (Figure 6);

·      In corneal topographic analysis, presence of an inferior-superior corneal asymmetry that was more marked in the left eye (Figure 6);

·      Keratoconus prediction index of 32% and 46% in the right and left eyes, respectively (Figure 6);

·      Reduced central corneal thickness, with some asymmetry in the distribution of the peripheral corneal thickness, especially in the left eye (Figure 7);

·      The corneal elevation map of the left eye allowed us to define clearly the location and dimension of the incipient corneal ectasia detected in the left eye (Figure 8);

·      Anomalous level of HOAs in the left eye (Figure 8) and for this reason the red alert sign HOA appeared on the initial screen once the measurement was taken (Figure 6). This is an alert to inform the clinician that the eye explored has an abnormal level of HOAs;

·      Some level of posterior capsular opacification detected in the analysis of retroillumination images in left eye, which can contribute further to the visual degradation that may be experienced in this eye (Figure 9).

All this information, along with other data provided by the system, confirmed the presence of an incipient atypical keratoconus in the form of temporal-inferior ectasia. The system is therefore an excellent tool for the analysis and detection of ectatic corneal diseases, even when they are presented in atypical forms.

Case 3

A 42-year-man complained of a significant worsening of the vision of his right eye, especially at night, during the past 3 months. No refractive analysis was provided by the instrument in the right eye, although the corneal topographic, pachymetric and tonometric evaluation was obtained without problems (Figure 10).

Considering that a measurement of ocular aberrations could not be obtained in the right eye, we suspected that a significant opacity was present in the central part of the crystalline lens (Figure 10). This was confirmed with the analysis of the retroillumination images, which showed a significant central posterior subcapsular opacification in the right eye and a mild level of opacification in the left eye (Figure 11).

Conclusions

In conclusion, the new multidiagnostic device is able to provide consistent measurements of refraction and ocular aberrations in healthy eyes. The consistency of refractive measurements is not dependent on the magnitude of the refractive error, with the same precision ability for moderate to high myopia and for hyperopia.

Likewise, the system is able to provide consistent measurements of corneal curvature at different areas, eccentricity and aberrations in healthy eyes. Corneal curvature and eccentricity measured were found not to be dependent on the magnitude of the measurement, with the same precision ability for flat and steep corneas within the normal range.

Finally, our study has also confirmed that the multidiagnostic system is able to provide consistent measurements of ACD, central and peripheral pachymetry, WTW and iridocorneal angle in healthy eyes.

The consistency of the measurements of these anatomical parameters was not dependent on the magnitude of such parameters, with the same precision ability for short and long eyes, small and large corneas, and eyes with deep and shallow anterior chambers within the normal range.

In clinical practice, the technology can be very useful for detecting different types of pathologies, including incipient forms of corneal ectasia; for evaluating the visual and optical quality outcomes of different refractive treatments; and to evaluate the performance of intraocular implants. Future studies must be performed to evaluate further the possibilities of this new multidiagnostic platform.

References

1.     Piñero DP. Technologies for anatomical and geometric characterization of the corneal structure and anterior segment: a review. Semin Ophthalmol. 2015;30:161-170.

2.     Asgari S, Hashemi H, Jafarzadehpur E, Mohamadi A, Rezvan F, Fotouhi A. OPD-Scan III: a repeatability and inter-device agreement study of a multifunctional device in emmetropia, ametropia, and keratoconus. Int Ophthalmol. 2016;36:697-705.

3.     Piñero DP, Juan JT, Alió JL. Intrasubject repeatability of internal aberrometry obtained with a new integrated aberrometer. J Refract Surg. 2011;27:509-517.

4.     Cerviño A, et al. Intrasubject repeatability of corneal power, thickness, and wavefront aberrations with a new version of a dual rotating Scheimpflug-Placido system. J Cataract Refract Surg. 2015;41:186-192.

5.     Hernández-Camarena JC, et al. Repeatability, reproducibility, and agreement between three different Scheimpflug systems in measuring corneal and anterior segment biometry. J Refract Surg. 2014;30:616-621.

6.     Shetty R, et al. Repeatability and agreement of three Scheimpflug-based imaging systems for measuring anterior segment parameters in keratoconus. Invest Ophthalmol Vis Sci. 2014;55:5263-5268.

7.     Montalbán R, Alió JL, Javaloy J, Piñero DP. Intrasubject repeatability in keratoconus-eye measurements obtained with a new Scheimpflug photography-based system. J Cataract Refract Surg. 2013;39:211-218.

8.     Montalbán R, Piñero DP, Javaloy J, Alió JL. Intrasubject repeatability of corneal morphology measurements obtained with a new Scheimpflug photography-based system. J Cataract Refract Surg. 2012;38:971-977.

9.     Piñero DP, et al. Intrasession repeatability of refractive and ocular aberrometric measurements obtained using a multidiagnostic device in healthy eyes. Clin Optom. 2017;9:91-96.

10.  Piñero DP, et al. Corneal topographic and aberrometric measurements obtained with a multidiagnostic device in healthy eyes: intrasession repeatability. J Ophthalmol. 2017; Article ID 2149145.

David P. Piñero, PhD

Alberto López-Navarro, OD, MSc

Inmaculada Cabezos, OD

Dolores de Fez, PhD

María T Caballero, PhD

Vicent J Camps, PhD

Dr Piñero is a lecturer and senior researcher at the Department of Optics, Pharmacology and Anatomy, University of Alicante, Spain. He has no financial interests in the subject matter. He can be contacted at: david.pinyero@ua.es