Authors: Soraya M R Jonker, Tos T J M Berendschot, Isabelle E Y Saelens, Noël J C Bauer, Rudy M M A Nuijts
Categories: Review Article, High Myopia, Phakic Intraocular Lens, Refractive Surgery
Source: Indian Journal of Ophthalmology
Phakic intraocular lenses (pIOLs) are a common solution for the surgical correction of high myopia and myopia in thin corneas. Global trends result in increasing rates of patients with high myopia which will result in increased rates of pIOL implantation. Three types of lenses can be anterior chamber angle-supported, anterior chamber iris-fixated, and posterior chamber phakic IOLs. The efficacy of phakic intraocular lenses is generally very good, but pIOLs have undergone many changes over the years to improve the safety profile and decrease pIOL-related complications such as endothelial cell loss, corneal decompensation and cataract formation. This article describes the efficacy and safety profiles of the most recent pIOLs, as well as suggests gaps of knowledge that are deserve additional research to optimize the results of pIOLs.
Keywords: Phakic Intraocular Lens, High Myopia, Refractive Surgery
Prevalence of high myopia has increased over the past years, resulting in a so-called epidemic of myopia that is especially prevalent in East and Southeast Asia.[1,2] In these regions, myopia is seen in up to 90% of adolescents, a vast increase when compared to an incidence of maximum 30% in adolescents 60 years ago.[1,2,3] Next to genetic factors, environmental factors are the main reason for this increased prevalence of myopia. This is mainly caused by intensive education resulting in children both spending more time indoor and doing near work.[1,2,3,4,5] If children and adolescents are required to perform near work, their eyes require constant accommodation to focus incoming light onto, rather than behind the retina. This combination of accommodation and lack of natural light falling on the retina causes the eye to grow, and the axial length to increase to create a situation where constant accommodation is no longer required during near work. However, this longer axial length also results in a distant image that is focused in front of the retina, introducing (high) myopia. Studies have shown that the onset, and progression of myopia can be reduced significantly by increasing the time a child spends in daylight. A vast amount of research focuses on stopping the myopic progression in the (very) young child in order to prevent increasing incidence of high myopia, defined as -5 to -6 D or higher.[2,6] Prevention of high myopia is important since it is associated with potentially blinding complications such as myopic macular degeneration, retinal detachment, staphyloma formation, or macular retinoschizis.[1,3,5,7] Current treatments are based on using cycloplegic drops, orthokeratology (nighttime contact lenses) to temporarily change the shape of the cornea, or multifocal contact lenses.[4,5,8,9] Results seem promising in halting axial elongation, but long-term data and mechanisms have yet to be determined.[4,5,8,9] Furthermore, contact lens wear introduces a risk of corneal infection, which can result in corneal scarring requiring corneal transplantation.[10,11]
In a myopic patient non-surgical refractive correction can be obtained using contact lenses or glasses. It should be noted that refractive correction of high myopia will result in thicker contact lenses and glasses. Contact lenses in these patients are expensive, might cause problems with contact lens fitting and introduce the risk of contact lens-related infections.[10,11] Glasses are the best option to safeguard corneal health, but are aesthetically not preferred due to a misrepresentation of the contour of the face, and cause visual distortion in the peripheral visual field.
Long-term treatment of stable myopia can be obtained by three different types of laser refractive surgery, phakic intraocular lens (pIOL) implantation, and lens surgery for refractive purposes (refractive lens exchange [RLE]) [Fig. 1]. Both laser refractive surgery and refractive lens exchange are permanent, whereas refractive surgery using pIOL implantation is a reversible procedure.[12,13] For safety reasons, laser refractive surgery is performed in cases up to -8 D of myopia, depending on corneal thickness, thus excluding a large group of high myopes from treatment.[13] The relative risk of developing a retinal detachment increases four times after cataract surgery, with patients under 50 years old and patients with a long axial length reported as especially at risk for developing a retinal detachment.[14] Taking into account that cataract surgery also removes all accommodative capacity, refractive lens exchange is rarely performed in non-presbyopic patients. As a result, pIOL implantation is regularly performed as a treatment for non-presbyopic high myopes.[13,15,16]
Phakic IOLs can be divided into (1) anterior chamber angle supported pIOLs that function with haptics positioned in the angle where the iris and cornea meet; (2) anterior chamber iris-fixated pIOLs that use small “lobster claws” to enclavate iris-tissue and position the pIOL in front of the pupil; and (3) posterior chamber pIOLs that use plate haptics to support and position the lens in the posterior chamber [Fig. 2].[15,17]
Figure 2 Three different phakic intraocular angle supported (left), iris-fixated (center), posterior chamber (right)
Implantation of pIOLs for the correction of myopia started in 1953 when Strampelli implanted the first rigid angle-supported pIOL. Although refractive results were promising, the high rate of corneal complications, and excessive pressure on the iris root causing inflammation and pupil malformation, resulted in withdrawal of the pIOL from the market. From the 1970s until the early 2010s multiple new angle-supported pIOLs were available, using different materials and different optic designs to try and cause less corneal complications and induce less pressure on the iris-root. Despite rendering good visual and refractive results, the complication rate of angle-supported pIOLs resulted in the withdrawal of all angle-supported pIOLs from the market [Tables 1a, 2a and 3a].[15,16,18]
First designed in 1953 by the Dutch ophthalmologist Cornelis Binkhorst, iris-fixated IOLs were originally created for implantation after crystalline lens removal. The Binkhorst IOL, using a paper-clip design to stabilize the lens, was associated with lens instability resulting in lens luxation, corneal complications and chronic inflammation causing retinal edema and problems with intraocular pressure.[29] In 1978 the iris-fixated IOL design was updated by a second Dutch ophthalmologist, Jan Worst. He discovered that fixation of midperipheral iris tissue with small “lobster-claws” provided good stabilization of the IOL, without inducing the complications associated with chronic inflammation in the paper-clip design. Over time the Worst IOL was also implemented as a pIOL for the correction of refractive errors. Current iris-fixated pIOLs are still largely based on the Worst lens, with different materials and designs for the correction of myopia, hyperopia and astigmatism, producing excellent visual and refractive results, and low rates of complications [Tables 1a, 2a and 3a].[15,16,30]
Posterior pIOLs were launched in the 1980s, driven by the complications associated with angle-supported pIOLs. By positioning the pIOL further back and away from the cornea, it was hypothesized that the risk of corneal complications would diminish. The specific downside of this design is its proximity to the crystalline lens, and zonular fibers. Early designs of posterior pIOLs frequently touched the anterior part of the crystalline lens causing a significant anterior subcapsular cataract, whereas friction between the pIOL and iris caused pigmentary dispersion and inflammation resulting in raised intraocular pressure. Modern posterior pIOLs have evolved to use plate haptics to support them in the sulcus between the iris base and ciliary muscle, and apertures in the optic to facilitate aqueous humor flow, providing good visual and refractive results [Table 1c]. They rely on correct sizing to prevent both iris chafing or pupillary block glaucoma caused by an oversized plate pushing the iris forward, and cataract formation caused by an undersized plate resulting in insufficient support and touch between the pIOL and crystalline lens [Table 3c]. Although the altered aqueous humor flow in posterior pIOLs with apertures reduces the rate of cataract formation after implantation, correct sizing remains the main determinant in the prevention of complications. Unfortunately, there is no perfect way yet to measure the sulcus, introducing a higher risk of sizing problems.
One posterior pIOL type that was taken of the market did not require measurement of the sulcus. This lens relied on the natural flow of intraocular fluid to keep the pIOL from touching the crystalline lens. However, its instability increased the rate of cataract formation, the rate of pIOL luxation behind the crystalline lens in the vitreous, and made it unsuitable for the correction of astigmatism.[15,16,108]
Large numbers of studies have shown that all three types of pIOLs render excellent results with regard to uncorrected visual acuity, distance corrected visual acuity, and residual refractive error [Tables 1a-1c]. Other outcome measures reported after pIOL implantation are mainly related to the safety of the pIOL in the eye and report complications associated with the cornea, crystalline lens, or retina.[16]
The cornea is essential in refracting light – functioning as a positive lens – and its clarity is of vital importance to procure a clear image on the retina. Corneal clarity is maintained by the innermost cells of the cornea, called the endothelial cells. Endothelial cells function as an active pump that transports water from the corneal stroma, essential because excessive intracorneal water causes corneal edema and compromises its clarity. Although the number of endothelial cells decreases with advancing age, it remains sufficiently high to retain adequate pump function.[12,109,110,111] Any intraocular surgery causes a surgery-related, acute peak in endothelial cell loss that varies per person as well as per type of surgery. Additionally, the intraocular presence of a pIOL increases chronic endothelial cell loss. Therefore, endothelial cell density should be checked with regular intervals, both to monitor individual safety as well as safety of the specific pIOL in general.[112] On a patient-level these measurements are performed to prevent corneal transplantation due to endothelial cell loss and corneal decompensation. On a population level, regular measurements and standardized reporting can detect trends to remove high-risk pIOLs from the market. Recent evidence have led to withdrawal of angle-supported pIOLs because of increased levels of endothelial cell loss [Tables 2a-2c].[15,16,18,112,113]
Although high myopes are known to develop cataract at a younger age, pIOLs may also lead to accelerated cataract formation, causing loss of accommodation and decreased visual acuity, requiring pIOL explantation combined with cataract surgery [Tables 3a-3c].[12,16] Cataract formation after uneventful pIOL implantation is thought to be associated with inflammation in all pIOL types, but patients with posterior chamber pIOLs are especially at risk for cataract formation due to the design of the pIOL [Tables 3a-3c].[12,16,116] As highlighted previously, the position of the posterior pIOL in proximity to the crystalline lens, combined with known preoperative sizing difficulties, frequently induces contact between the pIOL and crystalline lens. As a result of this friction, these patients are at a high risk to develop anterior subcapsular cataract, which can require cataract surgery at a much younger age, whilst adhesions between the crystalline lens and its surrounding capsule increase the difficulty of the surgery. Fortunately, recent modifications with additional apertures in the optic have presumably changed the aqeous humor flow and resulted in a decreased rate of cataract formation [Table 3c].[117]
High myopia is associated with retinal complications because the retina is stretched out and becomes thinner, increasing the risk of developing weak spots in the retina. Ultimately these weak spots can result in complications such as myopic macular degeneration, retinal detachment, macular retinoschizis or choroidal neovascularization [Tables 3a-3c].[6] Even though these complications can occur regardless of intraocular surgery, it is known that cataract surgery significantly increases the risk of developing a retinal detachment, especially when surgery is performed in younger patients.[118,119] The causative mechanism might be that cataract surgery requires the removal of the crystalline lens, changing the dynamics of the posterior part of the eye, whereas in pIOL implantation changes are only applied in the anterior segment of the eye, which is unlikely to result in a similar risk.
As reported in Tables 1a-2a-2c, and 3a-3c previous publications show large variations in the duration of follow-up, and reported outcome measures. This is especially true when reporting safety outcomes as endothelial cell loss, complications, and number and reason of pIOL explantations Tables 2a-2c, and 3a-3c.
Cataract formation in patients with pIOLs can be caused by (A) aging, (B) crystalline lens damage, or possibly by (C) insufficient aqueous humor circulation.[16,105,120,121]
Endothelial cell loss is the second most common complication related to pIOL implantation. Three hypotheses exist, attributing EC loss to either (i) the proximity of the pIOL to the corneal endothelium, (ii) the pIOL-related change in aqueous humor flow, or (iii) the chronic subclinical inflammatory response to the pIOL.[48,60,64]
Regardless of the cause of EC loss, EC monitoring is essential whenever a pIOL is implanted. In 2006, the AFSSAPS published a guideline reporting an ECD of 1500 cells/mm^2^ or less as a reason for pIOL explantation, after the Vivarte angle-supported pIOL had to be taken of the market.[128] After consulting with large numbers of specialists from the field, the AAO published a second, more extensive guideline in 2018.[112] It describes the importance of correct ECD measurements and provides specific endpoints when reporting ECD (i.e., the proportion of eyes with ≥25% EC loss after 3 years). In addition, it also refers to clause D.4.2 of the ANSI standard Z80.13 Phakic Intraocular Lenses standard for recommendations on how to perform ECD measurements. Studies should report the mean of three acceptable measurements of the central cornea, identifying at least 100 cells per frame, and use the center-to-center method with the same non-contact specular microscope throughout the study.[112] Identifying 100 cells per image can be non-contact specular microscopes are capable of capturing 120 to 170 cells per image, depending on ECD and the quality of the image. Contact specular microscopes on the other hand can capture 700 to 3000 cells per image, depending on the skill of the technician.[129] Contact specular microscopy however is time-consuming and invasive, as well as a skill that requires a certain level of training and upkeep, making it more difficult to implement in a busy practice. Another option to increase the number of analyzed cells is to use the corner method instead of the center-to center-method.[130] A 2010 study confirmed that the corner method is likely to benefit representation of ECD and morphological characteristics in transplanted corneas, but did not find clinically relevant differences between these measurement methods in healthy corneas.[130] The corner method takes up significantly more time in the clinic and probably only has additional value in studies on transplanted or diseased corneal endothelium, or in studies focusing on morphometric data. Selection of 100 contiguous cells in a non-contact specular microscopic image is challenging, even in healthy corneas. For this reason, most studies reporting ECD data select 50 contiguous cells and report the mean of three ECD measurements to provide reliable results. The ANSI standard might press researchers to select cells that would not qualify as clearly identifiable, possibly resulting in misrepresentation of ECD and EC morphology. It is important to acknowledge that neither the AFSSAPS, nor the AAO criteria present the researcher with cut-off values as to what proportion of eyes is considered 'safe' at a predefined time point.[112,128] The observed rates of EC loss clearly indicate the need for higher preoperative ECD in each age group, in order to provide a safe number of ≥1500 cells/mm^2^, when cataract surgery becomes necessary.[60,64] Additional risk-factors for increased EC loss differed between rigid (PMMA) and foldable (silicone) iris-fixated pIOLs, prompting the need for research on different intraocular materials (i.e., differences in intraocular inflammation) as a cause for increased EC loss.[60,64]
Phakic IOL implantation yields excellent visual and refractive results. An analysis of the preoperative characteristics of patients implanted with pIOLs identify (high) myopia as the main reason for surgery, followed by (high) astigmatism or (high) hyperopia Tables 1a-1c. Multiple studies assessing patient satisfaction, spectacle independence and occurrence of bothersome side-effects have indeed reported excellent outcomes, and few bothersome side effects (glare, halo's) after implantation with different types of pIOLs.[36,131] Several questionnaires are available for the evaluation of refractive errors, with three questionnaires (i.e., Quality of Life Impact of Refractive Correction [QIRC][visual symptoms], Quality of Vision [QoV][quality of life], Near Activity Visual Questionnaire [NAVQ][activity limitations]) showing slightly superior results in the assessment of refractive surgery.[132]
Decreased visual acuity over time has been reported with pIOLs, and can be attributed to refractive changes or occurrence of complications. Recent publications by our group report significant changes in refractive error over time in a mainly myopic population.[48] Age-related myopisation due to cataract formation did not entirely account for these changes. Subgroup analyses imply a significant increase in axial length over time in a highly myopic – but small – patient population. Increasing axial length is known to occur in the growing, adolescent eye, but is assumed to stop at around the age of 21. These hypotheses are based on older epidemiological studies that have used cross-sectional analyses to report axial length, resulting in data implying a decrease in axial length with age.[133,134,135] The data presented in the abovementioned papers suggest that axial length keeps increasing after reaching adulthood. New studies are necessary to determine if this is indeed the case, and if so, if it only occurs in (high) myopes.[48]
It is hard to confirm an association between progressive cataract formation and myopisation based on research since there are no guidelines defining how to describe the rate and progression of cataract formation in patients with pIOLs. Three options can be identified to gather information on cataract formation in a study population.
Phakic IOL implantation in highly myopic patients is associated with excellent visual and refractive outcomes shortly after surgery. Due to age-related and morphometric changes, visual and refractive outcomes might change over time. The exact mechanisms causing this are yet unknown and require additional research to formulate definitive recommendations. Until then, surgeons implanting pIOLs in highly myopic patients should inform their patients of the possibility that their refractive error might change slightly over time. As shown by the large variability in the reported data, there are still large variations in the assessment and description of complications occurring in patients with pIOLs. Further standardization of outcomes is required to make sure that complications are reported reliably and results can be compared. Specific attention should be paid towards reporting EC loss, cataract formation, including developing methods to reliably assess cataract progression over time, and patient-reported outcomes using validated questionnaires.
Nil.
Soraya M.R. Jonker, Tos T.J.M. Berendschot, Annick E. Ronden, Isabelle E.Y. Saelens: none. Noël J.C. Bauer: Alcon (C, L, S), Bausch & Lomb (C, L), Ophtec (S). Rudy M.M.A. Nuijts: Abbott (S), Alcon (C, L, S), Asico (C), Bausch & Lomb (S), Carl-Zeiss (S), HumanOptics (S), Ophtec (S), TheaPharma (S, C).