Introduction
By Erik L. Mertens, MD, FEBOphth
I joined Cataract and Refractive Surgery Today Europe's Editorial Board in June 2003 and now have the opportunity and privilege to become more involved in this exciting publication.
In the early stages of cataract formation, visual disturbances may appear while visual acuity in standard conditions is still preserved.1 These patients typically complain that headlights, lamps, or sunlight may appear too bright, with halos appearing around objects. Poor night vision, glare, double vision, or multiple images in one eye are possible signs of early cataract, which can be compatible with 20/20 BCVA or UCVA. In these situations, visual disturbances are caused by crystalline lens opacities, which randomly refract and diffract the incident light focused toward the retina.2 This effect, called intraocular scattering, reduces the patient's quality of vision. In young, healthy eyes, its impact is usually minimal for most visual tasks; however, when paired with another ocular condition, such as cataract, patients experience noticeable visual symptoms.
The parallel between the anatomic and clinical degree of opacity (estimated by slit-lamp examination) and the repercussion of these opacities on optical quality is not always relevant, particularly for early cataracts. However, because permanent visual discomfort may be compatible with moderate visual acuity loss, clinicians may want to objectively confirm and quantitatively assess ocular light scattering. Different levels of scatter are caused by differences in the degree of lenticular opacification.
Wavefront evaluation techniques measure optical aberrations in the human eye. Psychophysical techniques, such the Scheimpflug camera or Hartmann-Shack aberrometry, estimate ocular scatter but do not image the retina. More recently, double-pass aberrometry was introduced.3,4 This technique does not depend on the accuracy of the wavefront reconstruction; it integrates higher-order aberrations and light scatter into a prediction of retinal image quality.
Diaz-Douton et al5 compared retinal image quality with a Hartmann-Shack wavefront sensor and a double-pass aberrometer. In young patients with clear lenses, no difference was found between the devices in the estimation of image quality. However, in older eyes with early stages of cataract, retinal image quality was estimated to be significantly better with the Hartmann-Shack instrument than with the direct double-pass technique. Thus, wavefront measurements may fail to produce an objective assessment or overestimate image quality in eyes with scatter symptoms, such as cataractous eyes.
Some anterior segment imaging devices, including the acquisition system of the Scheimplflug camera, allow quantification of the degree of lens opacification via densitometry. However, Scheimpflug does not directly measure the effect of the opacification on light transmission. Ortiz et al6 established a correlation between subjective clinical classification and other objective methods of quantifying nuclear lens opacification. They evaluated changes in density and optical properties in eyes with lower grades of nuclear cataract. Modulation transfer function (MTF) calculations, including the effect of scatter, were based on the double-pass technique. They found that very early stages of cataract are difficult to differentiate using clinical classification (ie, Lens Opacities Classification System III; LOCS III), highlighting a statistically significant difference between the cataract grade groups in MTF values but not in optical aberrations values. Therefore, only the MTF should be used as an indicative parameter of the optical degradation of the eye, including scattering and aberrations.
This article reports my clinical experience with the Optical Quality Analysis System (OQAS) double-pass instrument (Visiometrics, Castelldefels, Spain; Figure 1) to investigate patients with visual symptoms related to early cataract.
MEASUREMENT SEQUENCE
The OQAS provides an objective measurement of the combined effects of optical aberrations and the loss of ocular transparency on optical quality. Data are established by a study of the retinal image obtained after the eye focuses on an infrared light beam.
Standard data, including the patient's name and age, are entered, followed by the value of his refractive error. The OQAS measures the natural pupil diameter; however, measurements are taken at a fixed pupillary diameter (3–6 mm) chosen by the operator. In patients presenting with strong ocular astigmatism, compensation for cylindrical error can be achieved by placing a corrective spectacle cylindrical lens in the machine's mounting frame. Alternatively, measurements can be performed while the patient wears spectacles.
Pupil diameter is measured once the pupil is aligned on the light reticules. With the patient staring at a landscape test card, the retinal foveal plan is completed, and several aerial images of the retinal point spread function (PSF) are captured (Figure 2) and averaged. To initiate accommodative exploration, repeated PSF measurements are taken. Plans located on increasing vergence optically simulate forward movement of the fixed object. The depth of field is estimated by collecting a series of measurements for points located at successive distances.
MEASUREMENT PRINCIPLES
Studying ocular astigmatism provides the overall data set. The dimensions and spatial distribution of light energy onto the retina, after focusing on a source point through the ocular refractive interfaces, determine the PSF (represented as a 2- or 3-D diagram). The diameter of the PSF conditions the eye's separating power and contrast sensitivity. The maximum visual acuity is theoretically related to the circular diameter of the light intensity spread and the foveal photoreceptors.
The PSF is obtained outside the eye after a double pass of the light hits the eye (ie, aerial PSF). Although it is not the exact retinal PSF (ie, the PSF that would be perceived at the retinal level after a single pass), this measurement estimates the optical ability to focus light on the retina. OQAS provides a 3-D aerial image of the PSF, which is in the form of peak light intensity from an image translated into colored levels (Figure 2).
In transparent and aberration-free eyes, the focal image formed on the retina is not a point but, due to diffraction, is a luminous spot whose diameter depends on that of the pupil size. Diffraction is an essential limitation of absolute astigmatism; it widens the formed PSF. To achieve good optical quality, spreading and scattering of light beyond diffraction must be minimized. Higher-degree optical aberrations (eg, coma, spherical aberration) are detrimental to optical quality because they reduce stigmatism, which is added to that imposed by the diffraction. The spatial distribution of this spread is usually restrained within an angular diameter with less than 10° of arc. When ocular transparency is reduced (eg, diffuse or localized micro-opacities), the light intensity spread scatters and is distributed onto a larger zone (Figure 3).
DERIVED FROM DOUBLE PASS
The OQAS software includes the following indices:
- The maximum visual acuity is predicted for objects with 100%, 50%, 20%, and 9% contrast. Acuity is calculated by accounting for optical characteristics of the analyzed eye, including aberrations and ocular scatter.
- The MTF curve represents the attenuation percentage of retinal image contrast at various resolutions (spatial frequencies). It also includes the combined effects of scatter and high-degree optical aberrations.
- An optical scattering index (OSI) quantifies the degree of scattering caused by the loss of transparency in one or more ocular structures, such as haze, corneal opacities, cataract, and vitreous opacities (Figures 3).
DOUBLE PASS AND CATARACT ASSESSMENT
The OQAS measurement confirms or repudiates the responsibility of diffuse opalescence or discrete biomicroscopic opacities in the genesis of visual symptoms. In our experience, and confirmed by others,6 correlation between the LOCS III scale and scattering is inconsistent. The direct visualization of deterioration of the retinal PSF and the OSI value confirm the responsibility of a lenticular opacification in case of diagnostic anatomic uncertainty (ie, when the crystalline lens opacification is moderate at slit-lamp examination). The OSI is proportional to the scattering of lenticular origin (Figure 4). Light scatter at any ocular level may cause an increased OSI (Figure 5); it is important to eliminate other causes of scatter, including corneal haze and vitreous opacification.
In Figure 6, we see the preoperative map of a patient complaining of severe night vision reduction and glare. At the slit lamp, only mild lens opacification and discrete small central opacities were observed. Despite their limited size, the role of these small opacities may be particularly important in the light scattering.
In an otherwise healthy eye, a decrease in the transparency of the crystalline lens may increase the OSI value, which enables us to forecast a possible medicolegal role in crystalline lens surgery for OSI, provides an objective distinction between clear crystalline lens surgery and cataract surgery. Documenting the existence of scattering may help the surgeon to justify the indication of cataract surgery in patients complaining of reduced visual performance due to scattering and in whom the maximal contrast visual acuity is preserved or minimally affected. Alternatively, in older patients who do not have lenticular scattering, it may document the need for a corneal refractive procedure, such as hyperopic LASIK.
The OQAS can also be used to quantify the degree of posterior capsular opacification after lens implantation. Improved PSF and reduced OSI values after capsulotomy reflect the reduction of the scatter after capsular opening. Traditional aberrometry methods, such as Hartmann-Shack, do not accurately measure visiual quality after multifocal diffractive IOL implantation. The principle of rebuilding the wavefront requires a monofocal optic and cannot properly describe the variations caused by the diffractive network of the implant. OQAS measurements may be more relevant in this context because they incorporate the scatter generated by the diffractive element of the bifocal IOL.
CONCLUSION
There are many clinical circumstances where ocular media may become less transparent and generate visually significant light scatter. When confronted with the diagnosis of early cataract, the direct measurement of ocular light scatter is particularly relevant because it documents the objective effect of crystalline lens opacities on the incident light. It should confirm or repudiate the responsibility of a young cataract in the genesis of visual symptoms. Further studies should aim to establish new functional classification of cataract based on the quantification of scattering values.
Damien Gatinel, MD, PhD, is a cataract, corneal and refractive surgery specialist. He is an Assistant Professor and Head of the Anterior Segment and Refractive Surgery Department at the Rothschild Ophthalmology Foundation, Paris. Dr. Gatinel is a member of the CRST Europe Editorial Board. He may be reached at tel: +33 1 48 03 64 82; e-mail: gatinel@aol.com.
