INTRODUCTION

Crystalline lens surgery is now more than a visual acuity repair surgery because of the technological advancement that allows safe surgery, with complication rates for experienced surgeons well below 1%, predictability of calculating lenses that are implanted and their ability to correct all ametropia. Thus, lens surgery is increasingly viewed as a refractive procedure.

Extraction of the transparent crystalline lens was, at the end of the last century, reserved for major ametropias, being associated in the myopes with an increased incidence of retinal detachment^1,2.

With the emergence of multifocal lenses, a paradigm shift has taken place and the clear lens extraction is beginning to be indicated in patients with ametropia-associated presbyopia³, in hyperopes from the age of fifty and in myopes from the age of fifty-five.

Today there is a plethora of multifocal (bifocal, trifocal, extended focus) and toric lenses that allow the presbyopia to be satisfactorily corrected in all patients who wish^4-8 and may undergo surgery. As a consequence of this progress, patients are becoming increasingly young and with lower refractive errors. In the USA, because of the need for insurers to share surgeries and for marketing, two concepts emerged: DLS9 and PRELEX.

The DLS (Dysfunctional Lens Syndrome) was so named for the first time by George Waring IV. According to this author the patient with DLS presents cataract-like symptoms with no evidence of it.

The choice of the DLS name stems from the fact that the syndrome shows signs and symptoms of a lens that has progressively become dysfunctional with visual quality degradation. It is characterized by the loss of transparency of the lens, the inability to accommodate from which presbyopia occurs and the alteration of the aberrometric profile with increased spherical aberration and coma.

Presbyopia is the progressive and irreversible inability to accommodate that, just like the loss of crystalline transparency, all the population will develop with age. Thus, as advocated by George Waring IV, the entire population will present DLS in the course of their lives.

Different diagnostic methods have been developed that evidence the syndrome and evaluated its correlation with visual acuity and phacodinamics¹⁰, such as the crystalline densitometry evaluated by Scheimpflug camera^11-12 or ray-tracing system aberrations such as iTrace Visual Function Analyzer (Tracey Technologies, Houston, TX)¹³. These changes are corrected by lens refractive surgery. PRELEX, first named in 2002 by R. Bruce Wallace III¹⁴, is presented as a concept in the treatment of presbyopia which emphasizes the fact that the procedure combines careful biometrics with astigmatism control with safe and predictable surgery and with constant analysis of outcomes to improve results. The PRELEX concept in the long run would lead, according to the author, surgeons to improve their results, with better rates of patient satisfaction, making the procedure increasingly popular.

PRELEX is the best surgical option for the correction of presbyopia since it is the only one that is involved in the main cause of loss of accommodation (aging of the lens) and allows a stable and definitive result to be obtained. Procedures for correction of corneal presbyopia (PresbiLASIK or intracorneal implants) have a limited efficacy over time and may compromise the future possibility of multifocal lens implantation. On the contrary in PRELEX the result is definitive only if you are anticipating a surgery that would be inevitable in the long run as the lens would eventually opacify.

For the success of this surgery a decisive contribution is made to the perfect combination of four factors:

1. Accuracy of the lens to be implanted,
2. Perfect surgery,
3. Astigmatism control,
4. Exhaustive preoperative evaluation, with careful patient selection with respect to the inclusion criteria.

Since the first three topics are covered in other chapters, this chapter will focus on patient selection.

PRELEX. DURA LEX SED LEX

Patient selection is a key step in the success of PRELEX (Table 1). Although the surgical procedure is identical, the approach of the surgeon may not be the same in cataract surgery and PRELEX. Here is an elective surgery motivated by the quest for a higher quality of life of the patient, so it should not be performed whenever there are conditions that may compromise safety, postoperative visual quality and patient satisfaction. It is therefore essential to inform the patient about the expected outcome for the procedure. This is the first step to avoid misunderstandings and dissatisfied patients. Always bearing in mind the "underpromise and overdeliver", they should be warned of the possible loss of contrast sensitivity as well as photic phenomena after surgery, which are however tolerable for most of those who undergo surgery. It is important to know the patient's lifestyle: their professional needs, their hobbies and other daily activities can condition the choice of lens to implant or, in the limit, contraindicate the surgery. Also, their personality that will be the determinant for their capacity of neuro-adaptation in cases of dysphotopsis, halos and glare in the postoperative period, should be considered. Patients with unrealistic expectations and those with hypercritical personality should be excluded.

Table 1 - Characteristics to choose the Ideal Patient

Positive

• Very motivated for independence of glasses
• Quiet personality with positive attitude
• Accepts commitment of small loss of contrast and vision

Negative

• Hypercritical personality with unrealistic expectations
• Values ​​perfect vision
• Values ​​vision in night driving
• Low myopic

In relation to age, the patient's refractive error must be taken into account. The hyperopic patient has a greater and earlier impairment of near visual acuity and a lower risk of postoperative complications in the posterior segment of the eye. Thus, it may be indicated to perform PRELEX in hyperopes above 45 years of age and the procedure should be reserved for myopia above 50 years of age¹⁵.

Some clinical comorbidities should be taken into account either in the choice of the lens to implant or in the exclusion of the patient. In glaucoma, the contrast sensitivity and mesopic vision are somehow compromised and therefore constitute a contraindication for PRELEX^16-17.

Also, the diabetic, myopic or age-related maculopathies constitute exclusion factors for surgery.

It is also important to exclude all retinal pathology that increases the risk of complications in the postoperative period. Therefore, an examination of the periphery of the retina as well as an OCT of the retina and optic nerve is mandatory (Figure 1).

The control of astigmatism is vital for the result obtained with the implantation of a multifocal lens being accepted that patients are not tolerant of residual astigmatism greater than 0.50 D¹⁸. Thus, in addition to the possibility of resorting to toric lenses, evaluation of the cornea is fundamental. The astigmatism to be corrected and its regularity should be determined and assessed, to establish if the patient can undergo correction of residual astigmatism with excimer laser. The aberrometric profile of the cornea should also be evaluated and high-order aberrations observed, especially those associated with keratoconus.

Studies have reported that coma values ​​above 0.32 may result in intolerable dysphotopsia in the presence of a multifocal lens¹⁹. Also, despite the fact that some authors report good results in multifocal lens implantation in patients with forms of keratoconus frustra, this pathology is a contraindication for PRELEX because of the greater possibility of errors in the biometry and the impairment of visual quality and intolerance to photic phenomena.

Asphericity influences the refractive result after intraocular lens implantation and should be taken into account in the formulas used to select the lenses, this factor being more relevant in patients already undergoing corneal refractive surgery^20-21. It is also known that the ideal values ​​of asphericity oscillate between Q = -0.15 and Q = -0.4522, so this fact must be taken into account in the preoperative evaluation and in the choice of lens to be implanted.

In patients with Fuchs' dystrophy the visual quality is compromised especially in low light and glare conditions²³. On the other hand, there is an increased risk of endothelial decompensation after surgery in this group of patients. Thus, this pathology should be disregarded in the preoperative study with specular biomicroscopy and the use of PRELEX in this group of patients is contraindicated.

Dry eye is an important factor in preoperative assessment. A good ocular surface plays a key role in the outcome of surgery and dry eye is, with residual refractive error, the main cause of patient dissatisfaction after PRELEX. The tear film is the first refractive plane of the eye whereby its irregularities interfere with the patients' visual quality. Thus, the identification and categorization of dry eye is mandatory in the preoperative consultation and must be treated before surgery.

Patients with larger pupils, in addition to being associated with a worse performance of near visual acuity²⁴, have an increased risk of glare after surgery which may vary with the lens type, with apodized diffractive lenses being more dependent on the pupil diameter. However, even considering that the size of the pupil can be altered by surgery, the pupillary diameter should be evaluated and taken into account in the preoperative evaluation, and pupils with mesopic conditions greater than 6.5 mm should not be considered for PRELEX²⁵.

There is a correlation such as the deviation between the visual axis and the pupillary axis (kappa angle) and the photic phenomena²⁶ and the increase of high-order coma aberrations^27-28. As such, the evaluation of the kappa angle must be part of the preoperative study, usually increased in patients with hyperopia, and the implantation of lenses with a deviation greater than 40 is not recommended.

Refractive surgery in the cornea is a safe procedure with good refractive results and may correct myopia, astigmatism or hyperopia. Patients undergoing this surgery, when they become presbyopes, maintain the same motivation for the independence of the glasses that led them in the past to be operated, and therefore also seek correction of presbyopia. As previously mentioned, they are conditioning factors for the indication of PRELEX the conditions of the cornea as its curvature, asphericity, spherical aberration and coma. These parameters are altered with refractive surgery on the cornea^29,30, which, in addition to the difficulty in calculating the correct power of the lens to be implanted, may be a contraindication for surgery. Previous Radial Keratotomy is always contraindicated for multifocal lens implantation due to corneal topographic irregularity. On the other hand, LASIK for low myopia is not, as a rule, an impediment to multifocal lens implantation, provided that the aforementioned asphericity and aberrometry parameters are respected³¹.

The implantation of aspheric multifocal lenses allows better visual acuity in glare and mesopic conditions in patients previously having undergone LASIK for correction of myopia, because they present a negative spherical aberration that compensates for the induced aberration affected in the cornea by previous surgery³².

LASIK for hyperopia correction induces spherical negative aberration and increases coma, resulting in a hyperprolated cornea with more negative values ​​of asphericity. Thus, in eyes submitted to LASIK for correction of hyperopia, opting for a spherical multifocal lens will have an advantage to compensate for corneal asphericity³³. In these cases, there is a greater risk of refractive surprise because of biometrics errors and there is a possibility of an increase in loss of contrast sensitivity.

MONOFOCAL IMPLANT AND MIX AND MATCH

The monofocal implant, despite the greater difficulty in the neuroadaptation due to the different quality of the images perceived by both eyes, is now acceptable in cataract surgery due to the greater independence of glasses and the functional capacity it provides. In PRELEX, the monofocal implant does not make any sense.

The implantation of mixed multifocal lenses (mix and match) that aims to provide the best of both lenses to the patient is advocated by some authors, by the theoretical better visual quality that the extended focus lenses provide and by the better near vision of the bifocal or trifocal lenses³⁴. Critics of this option refer to the difficulty in defining the dominance of one eye and the satisfactory results obtained with the current generation of trifocal lenses, superior to those achieved with mix and match.

NEUROADAPTATION

The diffraction of light caused by the optical design of multifocal lenses induces different focal points whereby the brain simultaneously receives different images that have to be processed to focus objects at different distances³⁵.

This abrupt change in relation to the visual function that the patient had before the surgery, in which he had a focal point that changed, with the accommodation, to adapt to the distance to which the object was, is what makes necessary the neuroadaptation, so that the brain properly uses the different images provided by multifocal lenses. Failure of neuroadaptation can cause confusion, blurred vision, glare, and poor vision.

Neuroadaptation is an acquired process through which the brain learns to correct the image so that the final perception is as real as possible.

This process was evidenced in a study that demonstrated the association between the patients' subjective complaints and the results of functional magnetic resonances to which they were submitted³⁶. In most cases, this process takes place in a minimum period of three months, until a significant reduction of the photic phenomena is seen, with the best possible adaptation achieved up to a year after surgery³⁷.