16 | XXXII Congress of the ESCRS european ophthalmology news | 09.2014

How to avoid Emmetropia, the perfect imperfection misalignment A “perfect” visual function does not always coincide with the perfect static optical correction BELLAIRE; TX/USA The location of small aperture corneal devices and mul- HERAKLION/GR It is undisputed that which refrains the eye from any devi- signal to minimise the image blur on In addition, since accommodation tifocal intraocular lenses in the visual the human visual system could function ation from emmetropia. The opposite the retina, it would be expected that influences the shape and the position system is essential for their visual perfor- at its maximal potential efficiency only is found for short eyes (see figure 2). optimal accommodative performance, of the crystalline lens, it is not surpri- mance. Surgical techniques for achieving when a sharply focused image on the re- How such emmetropisation is achieved resulting in an in-focus retinal image, sing to find that, as the level of accom- the proper location of these devices in the tina is achieved. Thus, we might expect has long been a matter of dispute, would automatically be achieved for modation changes, alterations occur visual system will be discussed in a that the long process of evolution would with both genetic and environmental the full range of distances within the in both focus and higher-order ocular Clinical Research Symposium. have led an eye to ideally develop factors having their supporters. How- objective amplitude of accommoda- aberrations, the most prominent being towards emmetropia, the refractive state ever, much effort has been devoted tion. However, it is now well accepted spherical aberration. Although high he shape of the Stiles-Crawford ideally achieved with the crystalline lens recently to attempts that might aid that steady- state errors in focus are amounts of spherical aberration (posi- effect is a parabola centrally in a relaxed state. Furthermore, for near our understanding on refractive error an idiosyncratic feature of the accom- tive or negative) would be expected to T (2nd order within the 3 mm vision, the accommodation system development, suggesting that the state modation system3. The system is cha- produce larger errors in accommoda- diameter) and the radial image inten- should produce an appropriate increase of focus in both the central and the racterised by over-accommodation for tion, this type of ocular “imperfec- sity (x) for a point source is the in ocular power. peripheral part of the retina plays an far targets, known as “lead” of accom- tion” seems to provide a vital feed- standard circular diffraction pattern important role in the emmetropisation modation, and under-accommodation back signal to the accommodation (similar to the absolute value of the he term emmetropia is defined process. Similarly, because the accom- for near targets, known as “lag” of control system, while it is relevant to sinc(x) function). in several dictionaries as “the modative control system generates a accommodation. the changing second-order focus The result of combining these two optimal refractive condition of errors (ie, lags/leads of accommoda- T 3 functions (convolution) is a fourth the eye, in which the light rays are tion) with stimulus distance. order shape that is exquisitely sensi- focused on the retina“. It is obvious Considering the dynamic nature of tive to alignment. that definition is “inconsistent”, if the refraction, since accommodation someone wants to correlate optics response changes rapidly and con- With a 3 or 4 mm pupil, with optimal visual performance. tinuously (e.g. known as microfluctu- alignment is not critical Emmetropia (from the Greek ations), and a large amount of other With a typical 3 or 4 mm pupil and “εμμετρος”, “εν”+“μετρον” – well- optical and physiological factors that viewing an extended source (normal proportioned) literally denotes the possibly influence refraction, such as visual field outside fovea), normal “perfect” optical balance without pupil size, microsaccades, blood flow, aberrations in the human eye are so always facilitating best visual perfor- tear film stability etc., an eye with a significant that alignment of these mance. Although, in practice, these perfect refractive state does not exist. two is rarely critical for optimal visual “ideal” goals are not always achieved, Correspondingly, a particular method performance. the optics of the human eye approxi- of refractive correction is limited to mate reasonably close to the ideal, at static near and distance correction When alignment considerations least in younger eyes. Specifically, that cannot satisfy clear vision over become significant rather than refractive errors being the continuous range of distan-ces However, with small aperture normally distributed, in adulthood, Figure 1: The distribution of mean equivalent spherical refractive errors, quantised in 1D under any environmental condition. (~1.6 mm diameter) corneal devices they show a strong excess near emme- intervals, found by Sorsby et al. (1960)1 and Stenstöm (1948)2. In other words a “perfect” visual and intraocular lenses with diffractive tropia, even though in infancy this is function does not always coincide optics (multifocal intraocular lenses) much less well marked. with the perfect static optical correc- alignment considerations become sig- Figure 1, for example, illustrates tion, which can be achieved today nificant. The location of the small the cycloplegic refractive distributions using adaptive optics-based corrective aperture devices must be nearly coin- from two historical datasets. First, options. Thus, targeting emmetropia, cident with the axis (ray) of the peak those found by Sorsby et al.1, for a may not always provide the optimal of the Stiles-Crawford Effect (SCE), single eye of each of 1033 unselected correction, as described in the litera- which is near the visual axis and young male military recruits from the ture, but forms a customized proce- 1st Purkinje-Sanson Image. United Kingdom (mostly aged 19 to dure which should be driven by the The location of the optical center 21 years inclusive, total range 17 to personal and professional needs of of the diffractive intraocular lens 27 years). Second, those measured by each individual. must be between the peak of the SCE Stenström,2 for the right eye of Finally, it seems that the word and the center of the pupil to 983 Caucasian subjects, with an age emmetropia and its meaning of “well optimize the diffractive and refrac- range between 20 and 35 years old, proportioned” could be better tive optics and reduce higher order who were recruited from his clinic and explained through a philosophical diffractive light scatter. a local air force academy. rather than a scientific discussion. As is now well established, refrac- Protagoras, the pre-Socratic Greek Reduced visual performance tive errors in an adult population are philosopher, postulated that “παντων in low light levels always not normally distributed, χρηματων μετρον ανθρωπος“ which The result of misalignment is reduced showing a characteristic leptokurtic means that “the measure (criterion) is 1 visual performance in low light levels distributional profile. Sorby et al.’s Figure 2: Correlation between the corneal power and the axial length for a group of defined by the human being“. W with the small aperture device data, show a pronounced “right” tail, emmetropic boys (aged range: 9 to 14 years). (decreased best-corrected distance with most subjects being low hyper- Symposium: Targeting emmetropia visual acuity, BCDVA, and contrast opes. Stenström’s2 data show a pro- Wed, 17.09.2014 11.00 - 13.00 sensitivity function, CSF) and with the nounced “left” tail, which is consistent Boulevard A multifocal intraocular lenses subjec- with the refractive error distribution tive complaints of “waxy vision” and found in more recent studies. ( Authors: Ioannis Pallikaris, Sotiris Plainis increased glare and halos due to Refraction is influenced by the Institute of Vision and Optics (IVO), increased light scatter also causing following ocular parameters: University of Crete, Heraklion, Greece reduced CSF and BCDVA. • Positions and thicknesses of ocu- E-mail: [email protected] Surgical techniques for achieving lar elements (e.g. anterior chamber the proper location of these devices in depth, vitreous body length, corneal References the visual system will be discussed. W and lenticular thickness) 1. Sorsby A, Sheridan M, Leary GA, Benja- • Refractive indices of media (which min B. Vision, visual acuity, and ocular refraction of young men: findings in a Clinical Research Symposium: may have gradients of index, e.g. lens) sample of 1,033 subjects. Br Med J. Optimising Refractive Procedures: • Curvatures of corneal and crys- 1960;1: 1394-8 Centration, Alignment and More talline lens surfaces 2. Stenstrom S. Investigation of the varia- Sat, 13.09.2014 8.30 - 10.30 Ideally, these must all combine to tion and the correlation of the optical Boulevard B give emmetropia, although a wide elements of human eyes. Am J Optom Arch Am Acad Optom. 1948;25: 496- range of axial lengths and corneal 504. ( Author: Jack T. Holladay, curvatures is found among healthy 3. Charman WN, Plainis S. Physiology and MD, MSEE, FACS emmetropic eyes. For example, the behavior of the accommodation system.

5108 Braeburn Drive myopic “tendency” of eyes with long (3) Pallikaris In: Pallikaris IG, Plainis S, Charman WN, Bellaire, axial lengths (an unwilling optical Figure 3: Accommodation response/stimulus curve from 13 young adult patients (age eds. Presbyopia: Origins, Effects, and Treatment. Thorofare NJ: SLACK Incor- TX, USA 77401 range: 23 to 33 years) under constant photopic conditions. The dashed line represents the imperfection) is frequently counter- porated; 2012. E-mail: [email protected] balanced by less powerful corneas, ideal one-to-one relationship required for “perfect” focus. (Charman and Plainis, 2012).3