New Clinical Pathways for Keratoconus
New Clinical Pathways for Keratoconus
In the future, early intervention with CXL should greatly reduce the numbers of patients dependent on rigid contact lenses and corneal transplantation for visual rehabilitation in keratoconus. For the present, many patients are already past the point at which they still have good spectacle corrected or unaided acuity. Current strategies for contact lens fitting in keratoconus are comprehensively described by Barnett and Mannis, and summarised briefly below. While transplantation remains the principal treatment for contact lens intolerant patients with stage IV keratoconus, newer interventions can be combined for earlier stage disease with the aim of restoring good spectacle corrected or unaided vision.
Soft lenses and soft toric lenses can provide good visual rehabilitation in early keratoconus; but RGP lenses are generally required to neutralise significant corneal surface irregularity. RGP lenses provide good tear exchange, but lens stability deteriorates as the corneal profile steepens in more advanced keratoconus. Traditional corneal RGP lenses are between 8 and 10 mm in diameter. Newer intralimbal RGP lenses (10.5–12 mm diameter) can improve corneal coverage and centration in some patients, but the increased diameter may make application and removal more difficult. 'Piggyback' (soft beneath RGP) lens combinations and hybrid (RGP centre and a soft skirt) lenses can improve wearing time in patients with poor RGP lens tolerance. Where these options fail, RGP scleral (and semi-scleral) lenses, which vault clear of the corneal surface, are a solution applicable to almost any corneal shape. Scleral lenses can rescue vision in patients with late stage disease—particularly where technical obstacles such as a thin peripheral cornea limit options for corneal transplantation.
Despite advances in contact lens fitting for keratoconus, contact lens dependence may be associated with significant impairment of vision-related quality of life that continues to decline over time.
Intrastromal corneal ring segments (ICRS) are effective in flattening the corneal shape and improving vision for most recipients with keratoconus, but the magnitude of effect is highly variable; and inclusion of different implantation techniques, different types of ICRS, and patients with late stage disease in earlier studies makes the existing evidence base for ICRS harder to interpret.
Many surgeons now use smaller diameter ICRS designs ( Table 3 ) and femtosecond laser assisted implantation. Earlier case series reporting manual implantation of the original Intacs ICRS (designed for simple myopic rather than keratoconus correction) are therefore not considered here. Femtosecond laser channel cutting has, by ensuring a consistent cut depth, both facilitated and enhanced safety in ICRS implantation. Flattening of central curvature with ICRS implantation increases with smaller ring diameter and greater segment thickness. The penalty for using a smaller diameter ring is a greater potential for dysphotopsia. The triangular profile used in Ferrara and Keraring ICRS is designed to reduce forward light scatter by total internal reflection of incident light, but further clinical studies are required to quantify dysphotopsia-associated with ICRS implantation.
In a series with 12-month postoperative results of 50 consecutive femtosecond laser Keraring ICRS cases reported by Coskunseven et al, approximately two-thirds of patients gained spectacle CDVA compared with pre-implantation levels (1–4 Snellen lines). Only two patients lost two lines of spectacle CDVA, and neither wanted the implants removed because of gains in UDVA. Overall improvement in mean keratometry was ~3D (50.6±4D pre-ICRS; 47.6±4.5D post-ICRS) with larger gains for cases with a steeper initial shape (stage III, 3 mm zone steep K>53D) in which thicker ICRS was implanted. Asymmetric ICRS implantation (asymmetric arc length and thickness in superior and inferior ICRS) or single ring implantation can be used in patients with reduced spectacle CDVA associated with high pre-operative coma (cone decentration). Results in a smaller series of 21 cases of femtosecond laser Keraring ICRS implantation reported by Shabayek and Alio were similar, with significant gains in overall higher order aberration levels for patients with high starting levels of coma (>3 microns). A retrospective series of 173 femtosecond laser cut eyes comparing Intacs SK and Keraring SI6 (both with a 6 mm diameter optical zone) observed comparable improvements in vision and keratometry, with no statistically significant differences between the two ICRS models.
Femtosecond ICRS implantation has a good safety profile. A recent multicenter retrospective review of 850 eyes with femtosecond Keraring ICRS implantation using an Intralase femtosecond laser for ring channel creation reported a 5.7% overall complication rate. Intraoperative complications did not result in failure to complete ICRS implantation. Incomplete ring channel formation was the commonest complication (2.7%). In these cases, ring channels can be completed manually allowing ICRS implantation to proceed. Endothelial perforation was evident in 0.6% cases. In this circumstance, channel creation 90 μm superficial to the initial attempted depth can be completed 1 month after surgery. The target depth for femtosecond Keraring implantation is 80% of corneal thickness at the location of the ring channel (5 mm diameter). Ultrasound pachymetry was used in this study. Availability of more accurate regional pachymetry methods, OCT in particular, may eliminate the risk of perforation. Postoperative complications occurred in 1.6% of cases. Within-channel migration of ring segments was noted in seven eyes and treated successfully with repositioning and a sutured vertical incision closure. Four cases of stromal thinning over the ICRS and two cases of corneal melting required implant removal. As with endothelial perforation, melting was attributed to inaccurate regional pachymetry and superficial channel creation in relatively thin corneas. Infection (an infiltrate at the entry site) was seen in one case in this series and treated without ICRS explantation. Problems for this study are lack of acuity, refractive, and keratometric outcome data. The authors also do not state the follow-up period studied or whether these were consecutive cases. Nonetheless, data from this and smaller prospective series indicate that sight-threatening complications are rare.
Very little long-term data are available for ICRS, and any effect of ICRS implantation on disease progression remains uncertain. In a case series of Intacs ICRS with 3-year follow-up in 13 eyes, significant increases in average K values were observed between 6 months and 3 years, indicating that disease stabilisation was not achieved by ICRS alone.
ICRS implantation can be combined with CXL, but the treatment sequence is important: ICRS appears more effective in improving corneal shape before the cornea is stiffened with CXL. Coskunseven et al, in an RCT comparing CXL first and ICRS later (group 1) with ICRS first and later CXL (group 2), showed significantly greater improvements (mean 7 months follow-up) in spectacle CDVA (three line gain (group 1); two line gain (group 2), P<0.01) and manifest astigmatism (2.48D mean absolute cylinder reduction (group 1); 1.76D reduction (group 2), P<0.05) where ICRS was performed first. Simultaneous CXL and ICRS may be equally effective. El-Raggal showed a trend towards greater improvement at 12 months in mean K values where simultaneous ICRS and CXL was performed vs CXL 6 months after ICRS (50.2±3.8D to 44.9±2.9D simultaneous treatment; 50.4±3.8D to 47.3±3.5D delayed CXL, P=0.046), although there were no differences in UDVA, CDVA, or refractive error. But this study was underpowered (n=8 each arm). More evidence is required to determine the optimum interval between ICRS and CXL in combined treatment; but the practical advantages to combining ICRS with transepithelial treatment in particular are clear. An interesting variation here has been described by Saelens et al who augmented stromal riboflavin penetration by injection into the ring channels before transepithelial CXL.
With increasing availability of femtosecond laser technology, OCT regional pachymetry, and intraoperative guidance systems to help reduce axial misplacement during implantation, ICRS implantation should become safer and more predictable. Several important gaps in the evidence base for this intervention exist, but there are already strong arguments for ICRS implantation for gross corneal shape correction in keratoconus patients with reduced CDVA—particularly if they are contact lens intolerant. Disease progression after standard, epithelium-off CXL is more likely for patients with later stage III disease (3 mm zone steep K >53D), and most patients with stage III keratometric changes already have reduced CDVA. Because contemporary ICRS implantation has a good safety profile and results are better if implantation is performed simultaneously with or before CXL, the balance of existing evidence suggests a role for ICRS implantation in combination with transepithelial CXL at presentation for patients with keratometric stage III keratoconus, or for earlier stage disease if vision is reduced (spectacle CDVA ≤20/25) in association with high levels of coma (>3 μm) (Figure 2).
Inclusion criteria in Coskunseven's recent prospective series of femtosecond Keraring ICRS implantation include a minimum central corneal thickness of 350 and 450 μm at the incision site. No upper limit for keratometry values was specified. Neovascularisation has not been reported after femtosecond Keraring ICRS implantation and 5 mm diameter ICRS are within the block of tissue normally removed in any subsequent transplantation procedure. Where corneal thickness is adequate and there is no central scar, it is probably reasonable to consider ICRS implantation with CXL in patients with advanced stage III disease since there should be no increased risk of failure in any subsequent corneal transplant. But a stratified analysis of patients with contact lens fitting problems associated with steep keratometry is required to determine whether ICRS is a viable alternative to corneal transplantation for this group or whether there is an upper limit to the Kmax for ICRS. For the present, limits for ICRS implantation are probably best defined by Coskunseven's pachymetric inclusion criteria above.
Microwave thermal keratoplasty is being explored as a possible alternative to ICRS and transplantation for gross corneal shape correction in advanced keratoconus. Although large initial shape improvements can be obtained, almost complete regression is seen within a year. Kato et al treated 21 advanced keratoconic eyes with topography guided conductive keratoplasty (with intraoperative keratometric monitoring): mean keratometry values were 55±8D at baseline, 45±9D at 1 week, 50±7D at 1 month, and 54±7D at 3 months. Studies are in progress to determine the extent to which shape regression after thermokeratoplasty can be modulated by combination with CXL.
Combination with CXL may allow safe application of surface excimer laser ablation techniques to fine tune corneal shape in keratoconus. To date, topography-guided PRK has been studied in combined therapy. But wavefront ablation would be a viable alternative in cases with good aberrometry data.
For simultaneous combined treatment, in which CXL is applied immediately after PRK, the aim is to improve CDVA by reducing irregular astigmatism rather than to correct spherocylindrical error fully. Reasons for this are the requirement for a 400-μm residual stromal bed after ablation to allow safe CXL, and the danger of hypermetropic overcorrection if spherical equivalent reductions induced by CXL are not anticipated. Simultaneous combined treatment has clear advantages for patient comfort (the epithelium is only removed once), but takes no account of improvements in corneal regularity and spectacle CDVA that might normally be produced by CXL alone.
In a large comparative case series with minimum 2-year postoperative follow-up, Kanellopoulos compared outcomes of 198 eyes treated with topography-guided PRK followed immediately by CXL (simultaneous combined treatment) with an earlier series of 127 eyes treated with topography-guided PRK a minimum of 6 months after CXL (sequential combined treatment). Simultaneous combined treatment produced greater improvement across a range of measures: LogMAR CDVA improved from 0.39±0.3D to 0.11±0.2D, with a reduction in spherical equivalent of −3.2±1.4D and mean keratometry of −3.5±1.3D. This compares with the sequential group's CDVA improvement from 0.41±0.3D to 0.16±0.2D, spherical equivalent reduction of −2.5±1.2D and mean keratometry reduction of −2.75±1.3D. Haze scores were also significantly better for simultaneous combined treatment. In all, 20 s intraoperative applications of mitomycin C were used throughout, and the maximum ablation depth was limited to 50 μm. The essential problem for this comparison is that the time interval between CXL and PRK for sequential treatment was not specified, and may have been considerably shorter than the 2-year period in which refractive results typically continue to improve after CXL alone. Further study is required to quantify any gain in spectacle CDVA over CXL alone for simultaneous combined CXL and PRK, and to determine whether increased haze scores for sequential combined treatment observed by Kanellopoulos are still evident where PRK is performed later in the post-CXL wound healing cycle, a minimum of 2 years after treatment.
The current evidence base does not allow a clear recommendation with regard to the place for simultaneous combined CXL and PRK. But topography-guided or wavefront-driven PRK is a reasonable option to choose once refraction is stable after CXL if spectacle CDVA remains poor (Figure 4).
(Enlarge Image)
Figure 4.
A pathway for visual rehabilitation in stage II and III keratoconus. Initial intervention in keratoconus (Figure 2) may include collagen CXL±intracorneal ring segment implantation (ICRS). Neither intervention provides a predictable shape change. After a 2-year period to allow shape stabilisation post-CXL, further fine shape correction with topographic PRK may therefore be required to achieve good spectacle CDVA. If CDVA is good but uncorrected distance vision remains poor (UDVA) remains poor, then patients may opt for pIOL implantation to complete visual rehabilitation.
For patients with a stable corneal shape and good CDVA, pIOL implantation can be used to complete functional visual rehabilitation. A number of recent papers have reported effective refractive correction using pIOLs in keratoconus (post-operative spherical equivalent range −0.08±0.4 to +0.1±0.4; 64–67% within 0.5D and 84–100% within 1D of target refraction) and for post-keratoplasty ametropia. Of the pIOLs available, the Visian ICL (Staar Surgical, Monrovia, CA, USA) offers the longest safety track record for an injectable pIOL and is available in a wide range of powers (including toric correction up to 6D). Pesando et al reported retrospective 10-year follow-up data in 59 ICL-implanted hypermetropic eyes, showing a mean endothelial cell loss of 4.7%, mostly occurring within the first few weeks of implantation and remaining almost unchanged thereafter. Edelhauser et al reported prospective, multicenter, 4-year follow-up data on 526 eyes with a 2–3% cell loss rate over the first 3 years, followed by a cell increase of 0.1% between years 3 and 4, suggesting that endothelial remodelling and stability may have been achieved.
Combined ICRS and pIOL implantation has been described, but refractive correction with pIOLs is highly predictable, whereas the refractive effect of ICRS implantation is highly variable. We would therefore argue for gross shape correction with ICRS implantation followed by CXL, fine shape correction with PRK if necessary, then finally pIOL implantation as a logical pathway to visual rehabilitation in grade II-III keratoconus (Figure 4).
Visual Rehabilitation
In the future, early intervention with CXL should greatly reduce the numbers of patients dependent on rigid contact lenses and corneal transplantation for visual rehabilitation in keratoconus. For the present, many patients are already past the point at which they still have good spectacle corrected or unaided acuity. Current strategies for contact lens fitting in keratoconus are comprehensively described by Barnett and Mannis, and summarised briefly below. While transplantation remains the principal treatment for contact lens intolerant patients with stage IV keratoconus, newer interventions can be combined for earlier stage disease with the aim of restoring good spectacle corrected or unaided vision.
Contact Lens Fitting
Soft lenses and soft toric lenses can provide good visual rehabilitation in early keratoconus; but RGP lenses are generally required to neutralise significant corneal surface irregularity. RGP lenses provide good tear exchange, but lens stability deteriorates as the corneal profile steepens in more advanced keratoconus. Traditional corneal RGP lenses are between 8 and 10 mm in diameter. Newer intralimbal RGP lenses (10.5–12 mm diameter) can improve corneal coverage and centration in some patients, but the increased diameter may make application and removal more difficult. 'Piggyback' (soft beneath RGP) lens combinations and hybrid (RGP centre and a soft skirt) lenses can improve wearing time in patients with poor RGP lens tolerance. Where these options fail, RGP scleral (and semi-scleral) lenses, which vault clear of the corneal surface, are a solution applicable to almost any corneal shape. Scleral lenses can rescue vision in patients with late stage disease—particularly where technical obstacles such as a thin peripheral cornea limit options for corneal transplantation.
Despite advances in contact lens fitting for keratoconus, contact lens dependence may be associated with significant impairment of vision-related quality of life that continues to decline over time.
Gross Corneal Shape Adjustment—Intracorneal Ring Segments
Intrastromal corneal ring segments (ICRS) are effective in flattening the corneal shape and improving vision for most recipients with keratoconus, but the magnitude of effect is highly variable; and inclusion of different implantation techniques, different types of ICRS, and patients with late stage disease in earlier studies makes the existing evidence base for ICRS harder to interpret.
Many surgeons now use smaller diameter ICRS designs ( Table 3 ) and femtosecond laser assisted implantation. Earlier case series reporting manual implantation of the original Intacs ICRS (designed for simple myopic rather than keratoconus correction) are therefore not considered here. Femtosecond laser channel cutting has, by ensuring a consistent cut depth, both facilitated and enhanced safety in ICRS implantation. Flattening of central curvature with ICRS implantation increases with smaller ring diameter and greater segment thickness. The penalty for using a smaller diameter ring is a greater potential for dysphotopsia. The triangular profile used in Ferrara and Keraring ICRS is designed to reduce forward light scatter by total internal reflection of incident light, but further clinical studies are required to quantify dysphotopsia-associated with ICRS implantation.
In a series with 12-month postoperative results of 50 consecutive femtosecond laser Keraring ICRS cases reported by Coskunseven et al, approximately two-thirds of patients gained spectacle CDVA compared with pre-implantation levels (1–4 Snellen lines). Only two patients lost two lines of spectacle CDVA, and neither wanted the implants removed because of gains in UDVA. Overall improvement in mean keratometry was ~3D (50.6±4D pre-ICRS; 47.6±4.5D post-ICRS) with larger gains for cases with a steeper initial shape (stage III, 3 mm zone steep K>53D) in which thicker ICRS was implanted. Asymmetric ICRS implantation (asymmetric arc length and thickness in superior and inferior ICRS) or single ring implantation can be used in patients with reduced spectacle CDVA associated with high pre-operative coma (cone decentration). Results in a smaller series of 21 cases of femtosecond laser Keraring ICRS implantation reported by Shabayek and Alio were similar, with significant gains in overall higher order aberration levels for patients with high starting levels of coma (>3 microns). A retrospective series of 173 femtosecond laser cut eyes comparing Intacs SK and Keraring SI6 (both with a 6 mm diameter optical zone) observed comparable improvements in vision and keratometry, with no statistically significant differences between the two ICRS models.
Femtosecond ICRS implantation has a good safety profile. A recent multicenter retrospective review of 850 eyes with femtosecond Keraring ICRS implantation using an Intralase femtosecond laser for ring channel creation reported a 5.7% overall complication rate. Intraoperative complications did not result in failure to complete ICRS implantation. Incomplete ring channel formation was the commonest complication (2.7%). In these cases, ring channels can be completed manually allowing ICRS implantation to proceed. Endothelial perforation was evident in 0.6% cases. In this circumstance, channel creation 90 μm superficial to the initial attempted depth can be completed 1 month after surgery. The target depth for femtosecond Keraring implantation is 80% of corneal thickness at the location of the ring channel (5 mm diameter). Ultrasound pachymetry was used in this study. Availability of more accurate regional pachymetry methods, OCT in particular, may eliminate the risk of perforation. Postoperative complications occurred in 1.6% of cases. Within-channel migration of ring segments was noted in seven eyes and treated successfully with repositioning and a sutured vertical incision closure. Four cases of stromal thinning over the ICRS and two cases of corneal melting required implant removal. As with endothelial perforation, melting was attributed to inaccurate regional pachymetry and superficial channel creation in relatively thin corneas. Infection (an infiltrate at the entry site) was seen in one case in this series and treated without ICRS explantation. Problems for this study are lack of acuity, refractive, and keratometric outcome data. The authors also do not state the follow-up period studied or whether these were consecutive cases. Nonetheless, data from this and smaller prospective series indicate that sight-threatening complications are rare.
Very little long-term data are available for ICRS, and any effect of ICRS implantation on disease progression remains uncertain. In a case series of Intacs ICRS with 3-year follow-up in 13 eyes, significant increases in average K values were observed between 6 months and 3 years, indicating that disease stabilisation was not achieved by ICRS alone.
Combined ICRS and CXL
ICRS implantation can be combined with CXL, but the treatment sequence is important: ICRS appears more effective in improving corneal shape before the cornea is stiffened with CXL. Coskunseven et al, in an RCT comparing CXL first and ICRS later (group 1) with ICRS first and later CXL (group 2), showed significantly greater improvements (mean 7 months follow-up) in spectacle CDVA (three line gain (group 1); two line gain (group 2), P<0.01) and manifest astigmatism (2.48D mean absolute cylinder reduction (group 1); 1.76D reduction (group 2), P<0.05) where ICRS was performed first. Simultaneous CXL and ICRS may be equally effective. El-Raggal showed a trend towards greater improvement at 12 months in mean K values where simultaneous ICRS and CXL was performed vs CXL 6 months after ICRS (50.2±3.8D to 44.9±2.9D simultaneous treatment; 50.4±3.8D to 47.3±3.5D delayed CXL, P=0.046), although there were no differences in UDVA, CDVA, or refractive error. But this study was underpowered (n=8 each arm). More evidence is required to determine the optimum interval between ICRS and CXL in combined treatment; but the practical advantages to combining ICRS with transepithelial treatment in particular are clear. An interesting variation here has been described by Saelens et al who augmented stromal riboflavin penetration by injection into the ring channels before transepithelial CXL.
ICRS Implantation Protocols
With increasing availability of femtosecond laser technology, OCT regional pachymetry, and intraoperative guidance systems to help reduce axial misplacement during implantation, ICRS implantation should become safer and more predictable. Several important gaps in the evidence base for this intervention exist, but there are already strong arguments for ICRS implantation for gross corneal shape correction in keratoconus patients with reduced CDVA—particularly if they are contact lens intolerant. Disease progression after standard, epithelium-off CXL is more likely for patients with later stage III disease (3 mm zone steep K >53D), and most patients with stage III keratometric changes already have reduced CDVA. Because contemporary ICRS implantation has a good safety profile and results are better if implantation is performed simultaneously with or before CXL, the balance of existing evidence suggests a role for ICRS implantation in combination with transepithelial CXL at presentation for patients with keratometric stage III keratoconus, or for earlier stage disease if vision is reduced (spectacle CDVA ≤20/25) in association with high levels of coma (>3 μm) (Figure 2).
Inclusion criteria in Coskunseven's recent prospective series of femtosecond Keraring ICRS implantation include a minimum central corneal thickness of 350 and 450 μm at the incision site. No upper limit for keratometry values was specified. Neovascularisation has not been reported after femtosecond Keraring ICRS implantation and 5 mm diameter ICRS are within the block of tissue normally removed in any subsequent transplantation procedure. Where corneal thickness is adequate and there is no central scar, it is probably reasonable to consider ICRS implantation with CXL in patients with advanced stage III disease since there should be no increased risk of failure in any subsequent corneal transplant. But a stratified analysis of patients with contact lens fitting problems associated with steep keratometry is required to determine whether ICRS is a viable alternative to corneal transplantation for this group or whether there is an upper limit to the Kmax for ICRS. For the present, limits for ICRS implantation are probably best defined by Coskunseven's pachymetric inclusion criteria above.
Thermal Keratoplasty
Microwave thermal keratoplasty is being explored as a possible alternative to ICRS and transplantation for gross corneal shape correction in advanced keratoconus. Although large initial shape improvements can be obtained, almost complete regression is seen within a year. Kato et al treated 21 advanced keratoconic eyes with topography guided conductive keratoplasty (with intraoperative keratometric monitoring): mean keratometry values were 55±8D at baseline, 45±9D at 1 week, 50±7D at 1 month, and 54±7D at 3 months. Studies are in progress to determine the extent to which shape regression after thermokeratoplasty can be modulated by combination with CXL.
Fine Corneal Shape Adjustment—PRK
Combination with CXL may allow safe application of surface excimer laser ablation techniques to fine tune corneal shape in keratoconus. To date, topography-guided PRK has been studied in combined therapy. But wavefront ablation would be a viable alternative in cases with good aberrometry data.
For simultaneous combined treatment, in which CXL is applied immediately after PRK, the aim is to improve CDVA by reducing irregular astigmatism rather than to correct spherocylindrical error fully. Reasons for this are the requirement for a 400-μm residual stromal bed after ablation to allow safe CXL, and the danger of hypermetropic overcorrection if spherical equivalent reductions induced by CXL are not anticipated. Simultaneous combined treatment has clear advantages for patient comfort (the epithelium is only removed once), but takes no account of improvements in corneal regularity and spectacle CDVA that might normally be produced by CXL alone.
In a large comparative case series with minimum 2-year postoperative follow-up, Kanellopoulos compared outcomes of 198 eyes treated with topography-guided PRK followed immediately by CXL (simultaneous combined treatment) with an earlier series of 127 eyes treated with topography-guided PRK a minimum of 6 months after CXL (sequential combined treatment). Simultaneous combined treatment produced greater improvement across a range of measures: LogMAR CDVA improved from 0.39±0.3D to 0.11±0.2D, with a reduction in spherical equivalent of −3.2±1.4D and mean keratometry of −3.5±1.3D. This compares with the sequential group's CDVA improvement from 0.41±0.3D to 0.16±0.2D, spherical equivalent reduction of −2.5±1.2D and mean keratometry reduction of −2.75±1.3D. Haze scores were also significantly better for simultaneous combined treatment. In all, 20 s intraoperative applications of mitomycin C were used throughout, and the maximum ablation depth was limited to 50 μm. The essential problem for this comparison is that the time interval between CXL and PRK for sequential treatment was not specified, and may have been considerably shorter than the 2-year period in which refractive results typically continue to improve after CXL alone. Further study is required to quantify any gain in spectacle CDVA over CXL alone for simultaneous combined CXL and PRK, and to determine whether increased haze scores for sequential combined treatment observed by Kanellopoulos are still evident where PRK is performed later in the post-CXL wound healing cycle, a minimum of 2 years after treatment.
The current evidence base does not allow a clear recommendation with regard to the place for simultaneous combined CXL and PRK. But topography-guided or wavefront-driven PRK is a reasonable option to choose once refraction is stable after CXL if spectacle CDVA remains poor (Figure 4).
(Enlarge Image)
Figure 4.
A pathway for visual rehabilitation in stage II and III keratoconus. Initial intervention in keratoconus (Figure 2) may include collagen CXL±intracorneal ring segment implantation (ICRS). Neither intervention provides a predictable shape change. After a 2-year period to allow shape stabilisation post-CXL, further fine shape correction with topographic PRK may therefore be required to achieve good spectacle CDVA. If CDVA is good but uncorrected distance vision remains poor (UDVA) remains poor, then patients may opt for pIOL implantation to complete visual rehabilitation.
Refractive Correction—pIOL Implantation
For patients with a stable corneal shape and good CDVA, pIOL implantation can be used to complete functional visual rehabilitation. A number of recent papers have reported effective refractive correction using pIOLs in keratoconus (post-operative spherical equivalent range −0.08±0.4 to +0.1±0.4; 64–67% within 0.5D and 84–100% within 1D of target refraction) and for post-keratoplasty ametropia. Of the pIOLs available, the Visian ICL (Staar Surgical, Monrovia, CA, USA) offers the longest safety track record for an injectable pIOL and is available in a wide range of powers (including toric correction up to 6D). Pesando et al reported retrospective 10-year follow-up data in 59 ICL-implanted hypermetropic eyes, showing a mean endothelial cell loss of 4.7%, mostly occurring within the first few weeks of implantation and remaining almost unchanged thereafter. Edelhauser et al reported prospective, multicenter, 4-year follow-up data on 526 eyes with a 2–3% cell loss rate over the first 3 years, followed by a cell increase of 0.1% between years 3 and 4, suggesting that endothelial remodelling and stability may have been achieved.
Combined ICRS and pIOL implantation has been described, but refractive correction with pIOLs is highly predictable, whereas the refractive effect of ICRS implantation is highly variable. We would therefore argue for gross shape correction with ICRS implantation followed by CXL, fine shape correction with PRK if necessary, then finally pIOL implantation as a logical pathway to visual rehabilitation in grade II-III keratoconus (Figure 4).
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