3.2.1.6. Multifocal lenses and endothelium
3.2.1.6. Multifocal lenses and endothelium
Instituto de Microcirurgia Ocular (IMO), Lisboa, Portugal
The objective of implantation of multifocal or pseudo-accommodative lenses is the reduction of eyeglass dependence. The technologies used in the construction of multifocal lenses are designed to create at least two foci in order to generate simultaneous vision1.
To focus light from different distances, multifocal lenses use optics of different types: refractive or diffractive. The multifocal refractive optics have variable curvatures to produce different focal points, while the diffractive optics divide the light, obtaining a focal plane for distance and another focal plane for near2.
Multifocal lenses form two images of an object located at a certain distance, with only one of these images focused on the retina. So, while one image is focused, the other one is overlapped and blurred.
This phenomenon results in decreased contrast and sharpness of the image on the retina and may contribute to the appearance of dysphotopsies, such as halos and glare3.
Multifocal lenses cause about 30% of loss of contrast sensitivity and also a slight decrease in corrected visual acuity, usually less than one line4. Diffractive multifocal lenses, when compared with refractive lenses, present in the majority of studies better results with regard to near visual acuity and contrast sensitivity, causing less dysphotopsies2.
The careful selection of patients who are candidates for multifocal lens implantation is essential to obtain a good functional result: independence of glasses for far and near, and simultaneously good quality of vision.
On one hand, a careful assessment of the patient’s personality, expectations and occupational needs should be made4. On the other, and in addition to this subjective evaluation, it is also fundamental to perform a complete ophthalmologic examination in order to identify any alterations that may compromise the quality of vision after the implantation of a multifocal lens, particularly the transparency of the cornea and the quality of the tear film, corneal aberrometry, K angle and macular function4.
The corneal endothelium is a monolayer of cells that covers the posterior surface of the cornea and whose main function is to transport water from the stroma to the anterior chamber. It is thanks to this mechanism that the cornea remains transparent. In the human species, the endothelial cells do not have the capacity to replicate, so endothelial regeneration, after cell loss, is done only at the expense of enlargement (megalocytosis) and cell migration5.
Clinical evaluation of endothelial cell density, morphology and endothelial function is done by specular microscopy and pachymetry.
Endothelial changes may be congenital, dystrophic or acquired.
Congenital abnormalities consist of anterior segment malformations, hereditary or sporadic – Axenfeld-Rieger syndrome and Peter's anomaly.
Among the Endothelial Dystrophies we encounter are: Congenital Hereditary Endothelial Dystrophy, Polymorphic Posterior Dystrophy and Fuchs Endothelial Dystrophy. The latter is particularly of interest in this chapter, both because it is the most frequent and because of its slowly progressive evolution.
Fuchs' dystrophy is an autosomal dominant hereditary disease of incomplete penetrance, although there are also sporadic cases. It is bilateral and asymmetric and more frequent in women than in men (2.5/1). It usually begins in the 4th-5th decade of life, presenting in its first stage by the appearance of central Guttate – focal excrescences of the Descemet’s Membrane and geographic deposition of pigment. It then evolves slowly into confluent Guttate, progressive loss of endothelial cells and consequent stroma and epithelial edema, initially microcystic and finally edema of the posterior stroma, with the appearance of Descemet's membrane folds (stage 2). In stage 3, subepithelial scarring occurs with corneal opacification and peripheral vascularization. Its incidence is not well known, but in a large series, Guttate was found to be confluent in 3.7% of individuals over 40 years of age5.
The endothelium may also undergo acquired changes as a result of trauma, hypoxia (contact lenses, acute glaucoma), uveitis, viral infections, pseudo-exfoliative glaucoma, and diabetes.
Considering that multifocal lenses, regardless of the type of technology they employ, are associated with a loss of quality of vision, with decreased contrast sensitivity and appearance of dysphotopsies, it becomes evident that good optical quality of the cornea is a key factor for the success of these implants.
The average of high order aberrations (HOAs) in normal corneas is 0.38 ± 0.14 μm in the optical zone of 6.0 mm6.
The additional loss of optical quality caused by the multifocal lens suggests that patients with HOAs greater than 0.50 μm in the 6.0 mm optical zone are not good candidates for the implantation of this type of lenses4.
Thus, in patients who are candidates for multifocal lens implantation, it is essential to evaluate the optical quality of the cornea.
Several studies have shown that the morphological and functional changes of the corneal endothelium, particularly in the context of Fuchs' dystrophy, compromise the quality of vision7,8.
In the initial phase of Fuchs' dystrophy, the presence of guttate, still in the absence of edema, already causes changes in the quality of vision, with decreased visual acuity and contrast sensitivity. There is a strong correlation between the guttate area and the deterioration of vision, which is attributed to the anterior light scattering (Straylight)7,8.
With the evolution of the disease, the increase in corneal thickness caused by edema, which is more pronounced in its central area, is observed. The consequent corneal irregularity leads to an increase in HOAs7.
In addition, Fuchs' dystrophy is a progressive disease, although this progression is slow after its onset between the 4th and 5th decades of life.
Kim et al studied the natural evolution of the disease long term, in non-operated eyes and the effect of cataract surgery on the progression of the disease. In non-operated eyes, in a 4-year follow-up period, significant progression was observed only in the increase of pachymetry, and the variations in cell density and coefficient of variation (CV) were not significant.
In the eyes submitted to cataract phacoemulsification, there was an increase in mean annual loss of endothelial cells compared to non-operated eyes of 20.39%/year and 0.82%/year respectively9.
In patients who are candidates for multifocal lens implantation, the preoperative assessment of the transparency and optical quality of the cornea, as well as the tear film, is essential. In order to guarantee a good functional result with the implantation of these lenses, all cases with alterations of corneal transparency, high aberrometry, and poor quality of the tear film, should be disregarded, although the latter may in many cases be amenable to correction with appropriate treatment. Endothelial changes affect the quality of vision, either by decreasing the transparency of the endothelium itself and Descemet's membrane, or by the potential progressive corneal edema caused by endothelial dysfunction.
In particular, Fuchs' dystrophy should be considered an absolute contraindication for the implantation of multifocal lenses, especially because it impairs the quality of vision from the beginning with the appearance of guttate. Also despite being slowly progressive, its progression may be accelerated by phacoemulsification surgery.