Presbyopia / SPO - 3.2.5.3. Contrast sensitivity and circadian photoreception

3.2.5.3. Contrast sensitivity and circadian photoreception

João Paulo Cunha e Joana Tavares Ferreira

Centro Central del Hospital de Lisboa, Portugal

Facultad de Ciencias Médicas, Nueva Universidad de Lisboa, Portugal

CONTRAST SENSITIVITY

Considering the threshold contrast as the minimum necessary to view the visual target reliably and at the reciprocal of the sensitivity threshold, it can easily be concluded that the contrast sensitivity can be defined as the threshold between the visible and the unseen, which makes this subject of interest for both basic and clinical research, whose main objective is to know the fundamental visual capacity, regardless of the subjective criterion of the observer.

Contrast sensitivity may be reduced even when visual acuity is normal. Thus, in recent years, the contrast sensitivity function has been shown to be more indicative of visual performance than visual acuity and is accepted as an important method of assessing visual function. There are several ophthalmological pathologies in which contrast sensitivity is reduced, such as myopia1, cataracts2, amblyopia3, age-related macular degeneration4, glaucoma5, ocular hypertension6, and dry eye7.

There are also scientific publications on brain lesions8 and certain neurological pathologies that are also associated with a decrease in contrast sensitivity such as multiple sclerosis9, Parkinson's disease10 and schizophrenia11. In addition, loss of contrast sensitivity is a common side effect of many drugs12,13, with the most common being: topiramate, vigabatrin, isotretinoin, amiodarone, ethambutol, chloroquine and hydroxychloroquine, tamoxifen, among others.

Throughout life the contrast sensitivity decreases, with some optical and neural explanations for this change with age:

Senile pupillary miosis and increased light absorption by the crystalline lens that diminish the retinal illuminance. Weale estimated that a 20-year-old eye transmits about three times the amount of light f a 60-year-old eye does14. Kelly observed that by decreasing retinal illuminance, contrast sensitivity decreases mainly at high spatial frequencies15;
The greater light scattering in the aged eye16. This factor is expected to reduce contrast sensitivity at all spatial frequencies;
Retinal and neural changes: (a) age-related retinal neuronal loss17, (b) increased lipofuscin pigmentation in retinal pigmented epithelial cells with drusen formation, (c) vascular sclerosis of the retinal and neural circulation, altering the homeostasis of the different hemato-retinal and hemato-encephalic barriers, and (d) the cellular loss with increase of lipofuscin in the lateral geniculate bodies and cortex.

Also, increased optical aberrations of the aged eye18,19 may reduce image contrast, color vision, circadian photoreception and quality of life. Because it is known that optical characteristics are largely responsible for the previously mentioned deficits even in photopic environments, cataract surgery with or without presbyopia correction may improve this visual function depending on the techniques used and the implanted lenses.

Despite the contradictions, it is believed that loss of spatial contrast sensitivity in older adults tends, in a direct association, to be more severe at higher spatial frequencies (i.e. bars with narrow widths)15.

In addition, the commitment of the elderly increases in magnitude with the decrease of ambient light level, which implies that their contrast deficits are exacerbated in poorly lit environments and at night. Weale14,20 also argued that during the aging process some of the loss of this visual function may be the result of neural deterioration along the visual pathway, although it is highly probable that cataracts may partially contribute to this.

Some deficits of contrast sensitivity can be corrected by optic, surgical, pharmacological or rehabilitative intervention. Even when reduced contrast sensitivity cannot be fully corrected, patients may express satisfaction with residual visual function or the justification for its loss.

CIRCADIAN PHOTORECEPTION

The human crystalline changes throughout life, increasing in size and weight, with an estimated increase of 0.02 mm per year, and the fibers that are formed move towards the center, densifying the nucleus and reducing its transparency21. Absorption of ultraviolet (UV) light and visible light by the lens increases with age. Amino acids, fluorophores, yellow pigment and some endogenous compounds (such as riboflavin) are responsible for the absorptive properties of the crystalline22. With aging, the human lens acquires a yellow, brown or black color. These color changes are limited to the nucleus and are thought to be the result of the binding of 3-hydroxyquinurenine glycoside and its derivatives to proteins23, reducing the transmission of visible light and mainly absorbing the blue band24.

Blue band that has been identified in experimental assays as the portion of the light spectrum responsible for retinal pigment epithelial lesions25 and for the maximum sensitivity of the retinal-hypothalamic pathway (RHP) (λmax 447-480 nm)26. Some studies have correlated the increased incidence of sleep disorders with ophthalmological diseases27 and have reported a high prevalence of depression, anxiety and poor quality of life in patients with cataracts28.

Throughout life, the iris and the pupil undergo age-related changes, in which functional changes precede structural changes29. The contraction velocity of the pupil decreases with age30, the shape of the pupil is altered, possibly due to structural alterations of muscle fibers, stromal atrophy with decreased connective tissue and hyaline degeneration31.

Age-related pupil miosis, demonstrated in physiological alert studies in the elderly, has been suggested as a possible contributory factor for the disorganization of the sleep-wake rhythm, by reducing retinal illumination32. It should be noted that RHP also contributes to the pupil reflex to light and to other behavioral and psychological responses to environmental illumination33.

Hood et al identified a relationship between exposure to natural light and the quality of nocturnal sleep. However, it was not possible to determine whether it was the light that gave a good quality of sleep or whether it was the quality of sleep that predisposed the elderly to a lifestyle with greater exposure to natural light34. In most studies, self-rated sleep disorders are underestimated. About ¼ of people with sleep-wake rhythm disorders do not identify this problem as a pathology that affects their life, with possible increased morbidity and mortality. The elderly have a higher frequency of delayed sleep induction, nocturnal wakefulness, daytime somnolence and early waking up. In contrast, these elderly people complain less of insufficient duration of nocturnal sleep and of periods of daytime sleepiness35.

Several ophthalmological diseases may be accompanied by changes in the rhythm of melatonin secretion. In patients with uveitis, the plasma melatonin peak was decreased in the studies of Touitou et al36 and it is believed that in glaucoma and pigmentary retinopathy the reduction of melatonin peak is related to poor light perception37. The increased incidence of sleep disturbances described in blind individuals is due to the inability to synchronize the endogenous pacemaker with ambient light27. Another study associated visual impairment with "bad" sleep, with more nocturnal wakes and more difficulty falling asleep in individuals of both sexes38. Since an endogenous pacemaker generates the circadian sleep-wake rhythm, dependent on the periodic expression of genes39, and light is the main synchronizer of the RHP, cataract surgery by allowing greater transmission of visible light could improve the synchronization of this system.

The first studies that related cataract surgery to recovery from sleep-wake rhythm date from 200240, having found a higher percentage of sleep-wake rhythm disturbances in cataract patients than in other individuals of the same age without this pathology. Individuals with bilateral cataracts often have disturbances of the sleep-wake rhythm, either by underlying depression or by RHP defect, with consequent disturbance of the correct intrinsic clock synchronization to the 24-hour day-night cycle41. Cataract surgery has proven to improve visual function and quality of life42-46.

Currently, in both cataract and presbyopia surgery, both intraocular lens (IOL) filters with UV filters and IOLs with UV and yellow chromophore (UV+A) filter improve visual function including visual tasks for near and far, peripheral vision, driving, and mental health47 (Figure 1). However, older patients may report lower satisfaction in self-assessment of improved quality of life despite improved visual function42. Cataract surgery with implantation of UV IOL had two main justifications: to mimic the human lens to protect the retina from UV light and to reduce the incidence of cystoid macular edema48. The implantation of an IOL with a UV absorber and a yellow chromophore (such as natural AcrySof® SN60AT), which partially filters blue light, could act as a protective retinal factor49, but also as a modulator of the circadian rhythm pacemaker. According to Mainster50, blue light accounts for more than 50% of melanopsin sensitivity, so UV+A IOLs reduce melatonin suppression by about 27-38% (depending on the dioptric power) compared to UV IOLs.

Figure 1. Intraocular lens models. On the left, MA60AC model, three-piece with UV filter, in the middle a single-piece with UV filter, and on the right, SN60AT model, single-piece lens with UV filter and yellow chromophore (Kindly provided by Alcon™).

Several studies have compared the contrast sensitivity of patients with implanted UV+A IOLs51-62. A meta-analysis by Xiao-feng Zhu et al demonstrated that the visual function with IOL UV+A is approximately similar in terms of contrast sensitivity, but the chromatic vision in the blue spectrum is somewhat compromised in mesopic conditions when compared to UV IOL63.

Regarding asphericity, several studies have shown that the implantation of aspherical IOLs with prolate surfaces is associated with fewer (high-order spherical) aberrations and higher contrast sensitivities under mesopic and photopic conditions, compared to spherical lenses64-67. One study showed that the correction of astigmatism with toric lenses positively influenced visual function with lower glare and greater road safety and found no differences for the filters68.

For multifocality versus monofocality, statistically and clinically significant differences were found between the groups of patients with monofocal and multifocal lenses, at all spatial frequencies, and in different lighting conditions, and near and far contrast sensitivity, offering the monofocal IOL a better performance in all cases. Distant contrast sensitivity was similarly compromised in all multifocal IOL models, although multifocal IOLs with diffractive optics and aspherical profiles showed a non-statistically significant trend of better performance under mesopic conditions. As for near contrast sensitivity, it was lower for refractive lens models, particularly at medium to high spatial frequencies69.

In the literature, articles on the influence of filters on the depressive state in the geriatric age were also presented, with no statistically significant differences between the UV and UV+A lens groups70.

Brondsted et al71 reported a reduction in melatonin stimulation and melatonin suppression from 0.6 to 0.7% for each year of life, while Kessel et al72 reported an inverse relationship between the risk of sleep-wake rhythm disorders and the transmission of blue light. Despite these data, the findings remain inconclusive in some respects. The sleep-wake circadian rhythm regulation can be studied by a variety of methods, which may include sleep quality questionnaires or insomnia scales, serum melatonin measurements, and actigraphic recordings. Studies based on the “Pittsburgh Sleep Quality Index” (PSQI) showed a subjective improvement in sleep quality at the first month38,73 and at the 6th and 12th postoperative months73. Schmoll et al74 obtained a reduction in daytime naps after cataract surgery using the “Epworth Sleepiness Score” (ESS). However, there are studies that do not show improvement in sleep quality after cataract surgery75,76. There are also contradictory results regarding melatonin concentrations. Brondsted et al71 observed higher melatonin peaks at three weeks postoperatively, irrespective of IOL filter type, whereas the Tanaka study group77 was unable to demonstrate differences in either maximal melatonin concentrations or in the post-operative interval in which they could be found. As for the studies with actigraphy, Cunha et al published results that showed improvement in 75% of patients in at least one of the studied parameters (sleep regularity, insomnia and daytime sleepiness)78,79 (Figure 2).

Figure 2 A and B. Actigraphic records of the same patient preoperatively and postoperatively. Fewer night waking, less episodes of daytime sleepiness and greater postoperative activity are observed.

Despite the discordant results, the results of most studies show that filters for blue light do not disturb the circadian sleep-wake rhythm, an important element since light is crucial for human health and the eye important for the timing of hormonal rhythms, cognitive functions and emotional stability. Even in studies with long follow-ups, no statistically significant differences in contrast sensitivity have been observed in scotopic and photopic conditions80 and circadian photoreception71 between UV and UV+A IOLs. The potential macular protective factor of the yellow chromophore remains to be proved.

According to the US Air Force: "any IOLs with plate designs, tints in the visual spectrum including blue-blocking chromophores and positioning holes are not approved."

CONCLUSIONS

With the ideal and universal IOL still not in existence, it can be said that:

aspherical and toric IOLs appear to improve contrast sensitivity;
multifocal lenses improve the independence from glasses but worsen contrast sensitivity;
filter lenses for UV rays and the blue band of the spectrum do not significantly impair contrast sensitivity or circadian photoreception but have not been shown to have macular protective effect.

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