INTRODUCTION

Some patients over the age of 50 are looking for a solution to increase their independence from glasses or contact lenses for distance vision and reading. These “baby boomers” who find it difficult to see far (due to refractive error) and close (due to presbyopia) usually have "Dysfunctional Crystalline Lens Syndrome" (DLS). This concept was popularized by George O. Waring IV in 2013 and represents a neglected and previously inadequately characterized clinical entity. The term DLS was selected because, by definition, it is a syndrome that encompasses multiple signs and / or symptoms, related to progressive and degenerative lens dysfunction. Clinical findings typically include: (1) opacities in the lens, whether cortical or nuclear or both; (2) inability to accommodate due to presbyopia; and (3) a profile of high-order aberrations that has changed, notably with increased spherical aberration and coma^1,2.

DLS STAGES

DLS presents 3 stages. In stage 1, the patient has complaints of progressive loss of near vision derived from the loss of elasticity of the lens. This phase begins at the age of 40 years, and the lens remains transparent. The reason for the increase in crystalline stiffness derives from the accumulation of disulfide bridges between the crystalline proteins, due to the reduction in the concentration of reduced glutathione available in the central part of the nucleus³. Stage 2 occurs between 50 and 60 years of age, when the lens begins to turn yellowish and slightly opacified.

Patients may complain of loss of quality of vision during this phase, induced by the increase in magnitude of internal high-order aberrations. Stage 3 represents the cataract phase, with the patient having a visual acuity of less than 20/50¹.

WHERE DOES DLS FIT IN THE SUBSPECIALITY OF REFRATIVE SURGERY?

The surgical correction of presbyopia remains the Holy Grail of this area of ​​Ophthalmology. Surgical procedures on the cornea or sclera failed to achieve the same level of outcomes as LASIK. Depending on clinical and surgical experience, some refractive surgeons are reluctant to propose corneal refractive surgery in an individual over 45 years of age for several reasons, including: insufficient correction of presbyopia with monovision, problems with dry eye syndrome, and more often, refractive error instability and increased visual symptoms due to lens aging.

Thus, phacoemulsification surgery (refractive lensectomy) is also being performed earlier, as soon as the first signs of lens opacification appear, because patients require better quality of vision and greater independence of correction glasses. In this sense, instead of a patient presenting a "suboptimal" vision for 10 to 20 years before cataract surgery, we can help patients over 50 years to optimize their vision and reduce their dependence on glasses.

COMPLEMENTARY DIAGNOSTIC TESTS

These clinical points reinforce the need to use objective analysis methods that can assess the impact of degenerative lens changes on vision in order to improve our sensitivity in identifying patients whose quality of life can be markedly improved with surgery. Different complementary diagnostic equipment provides data on the performance of the lens, based on different forms of analysis. For example, the Scheimpflug chambers provide information regarding the density of the lens.

Aberrometry devices can analyze high-order aberrations or scattering that interfere with the visual quality of the patient. As previously reported, an altered internal profile of high-order aberrations and slight opacification of the lens may explain some symptoms, even with imperceptible findings in visual acuity measurements.

The cornea has a spherical aberration of +0.28 μm, considering a pupil of 6.0 mm. In young adults, this aberration is compensated by the negative spherical aberration (-0.27 μm) derived from the lens. With aging, the lens presents an increase in the magnitude of its spherical aberration, neutralizing it at around 40 years and becoming positive after 60 years of age^4,5. Thus, in the presence of normal corneal aberrations, the modification of the profile of internal aberrations is indicative of changes at the lens level, suggesting an inherent nuclear sclerosis process, although its transparency is not significantly altered. Previous studies have demonstrated these findings using wave-front devices based on the Hartmann-Shack principle^4,6,7.

The Optical Quality Analysis System (Visiometrics, Terrasa, Spain) is a clinical device that evaluates the combined effect of optical aberrations and the light scattering derived from the loss of ocular transparency. This double pass system provides the objective dispersion index (OSI) which represents the degree of dispersion induced by the deterioration of ocular transparency. Previous studies have shown that this parameter is useful for the quantification of nuclear cataracts⁸.

Artal et al, also described a significant correlation between the OSI and the nuclear opalescence score, based on the LOCS III classification system⁹. Lim et al reported a positive linear correlation between OSI and lens density, measured with a Scheimpflug camera¹⁰. The major disadvantage of this device is that the OSI parameter is representative of the general ocular system, not allowing a differentiation of the source of ocular dispersion (cornea or crystalline). The iTrace Visual Function Analyzer (Tracey Technologies, Houston, Texas, USA) is also a wave-front analyzer that integrates an aberrometer with a corneal topographer.

The aberrometer uses the principle of ray tracing, which sequentially projects 256 infrared laser beams in a specific scanning pattern. This system has some advantages compared to other technologies. First, the capture is fast and causes no confusion in the analysis of the original location of the points at the pupillary entry with the location reflected in the retina, since each point is analyzed sequentially and separately. Due to the rapid control of the laser dot profile projected on the pupil, the software can track pupil size and project all 256 dots into both small to large pupils (1 mm to 8 mm). In this device, aberrations of the cornea are calculated based on topography data, and internal aberrations are obtained by subtracting aberrations of the cornea from those measured by the aberrometer.

This system has the particularity of providing the Dysfunctional Lens Index (DLI), which is an objective parameter of lens performance and is calculated based on the data of internal high-order aberrations, pupil size and contrast sensitivity (Figure 1). This objective index ranks the overall lens performance from 0 (weak) to 10 (excellent) points¹¹.

OUR EXPERIENCE

In the Department of Ophthalmology of the Hospital of Braga a study was carried out, to analyze and describe the relationship of DLI with distance-corrected visual acuity (DCVA), the classification of nuclear opalescence of the crystalline lens based on LOCS III, and the density of the crystalline lens based on analysis with a Scheimpflug camera (Pentacam HR, Oculus, Wetzlar, Germany). In this last device, the Pentacam Nuclear Staging allows the objective determination of the densitometry of the lens. The software automatically generates a cylindrical model for density measurement. The three-dimensional model was placed in the center of the nucleus, excluding the anterior and posterior cortex, and had the following characteristics: 4 mm diameter, 2.4 mm height, 8.3 mm anterior curvature and 4.8 mm posterior curvature (Figure 2). This analysis allows the objective quantification of lens opacities within the model (mean density and maximum density) on a continuous scale of 0 to 100 points.

For the purpose of this study, only the parameter "mean density" was recorded. Forty eyes of 30 patients (15 women and 15 men) were included in this study. Mean age was 67.53 ± 10.70 years (range: 46 to 90 years) and the mean DCVA in logMAR units was 0.15 ± 0.13 (range: 0 to 0.4). DLI showed a high linear and negative correlation with the nuclear opalescence score (r = -0.616, P <0.01). The mean nuclear density showed a positive correlation with the nuclear opalescence score (r = 0.697, P <0.01). The mean density parameter based on the Scheimpflug principle showed a negative correlation with the DLI (r = -0.555; P <0.01). With respect to DCVA, this parameter showed the strongest correlation with DLI (r = -0.702, P <0.01) compared to the other cataract assessment methods (Figure 3)¹².

In another study, the relationship between objective parameters of quantification of lens dysfunction with reduction of DCVA, and the phacodynamics in patients with age-related nuclear cataract, was analyzed. The same characteristics described previously for the analysis of the nuclear region of the cataract by the Scheimpflug camera (Pentacam HR) were also used in this study. We included 51 eyes of 34 patients (20 women and 14 men). The mean age was 70.77 ± 9.19 years (range: 52 to 90 years) and mean preoperative DCVA was 0.24 ± 0.16 in logMAR units (range: 0 to 0.7). DLI demonstrated a high negative linear correlation with nuclear opalescence (r = -0.728, P <0.01). The mean nuclear density showed a positive correlation with nuclear opalescence (r = 0.680, P <0.01). The preoperative DCVA presented a statistically significant relationship with the different methods of cataract evaluation. However, DLI showed the strongest correlation with the DCVA parameter (r = -0.670, P <0.01). All patients underwent phacoemulsification with posterior chamber intraocular lens implantation under local anesthesia. The stop-and-chop phacoemulsification technique using the Infiniti system (Alcon Laboratories, Inc, Fort Worth, Texas, USA) was performed. At the end of the procedure, the Cumulative Dissipative Energy (CDE) parameter was recorded. The mean CDE was 8.92 ± 6.70 (range: 0.05 to 21.42). Figure 4 shows the relationships between the CDE and the different parameters of cataract analysis. CDE showed stronger relationships with nuclear lens density and DLI (r = -0.744 r = 0.700, respectively, both P <0.01)¹³.

The results demonstrate, that both the crystalline densitometry and the DLI presented a statistically significant relationship with the LOCS III classification system and the DCVA. Thus, these data suggest the usefulness of lens densitometry and DLI as essential tools for objectively assessing the severity of mild forms of age-related nuclear cataracts, as well as for determining the functional status of the lens. Both objective parameters were also helpful in predicting the phacodynamics in the eyes with nuclear cataracts. However, anterior segment imaging is considered a constantly evolving clinical and research area, so in the future, new technologies may provide more detailed data on the functional status of the lens, helping to select the best therapeutic method for the patient.