Partial topography-guided surface ablation, combined with corneal collagen crosslinking when visual acuity is still quite good, optimises prognosis for the patient with keratoconus.
Corneal collagen crosslinking (CXL) to increase corneal biomechanical strength has become the standard of care for eyes with the progressive disease keratoconus. The procedure is very effective at halting the ectatic process and thus reducing the risk for eventual corneal transplantation.
Because of the corneal flattening that occurs with CXL, patients can also benefit from some improvement in uncorrected and best-corrected visual acuity (UCVA and BCVA). Nevertheless, irregularity of the anterior corneal surface persisting despite CXL limits the improvement to visual acuity (VA) and is also associated with higher order aberrations (HOAs) that degrade the quality of vision.
The efficacy of the Athens Protocol, combining partial topography-guidedphotorefractive keratectomy (PRK) and CXL for stabilising keratectasia and improving VA, was first reported in the literature in 2010 and its long-term benefits have been shown in eyes with 10 years of follow-up.1,2 Although many investigators have reported on their experience with combined topography-guided PRK and CXL to treat eyes with keratoconus, widespread knowledge about its effects on HOAs is more limited.3
We have been performing partial topography-guided surface ablation with CXL for visual rehabilitation and stabilisation of eyes with keratoconus. Our experience shows the benefits of using the surface ablation technique for regularising the anterior corneal surface, reducing HOAs and improving visual function.
The HOAs that are predominantly elevated in eyes with keratoconus are vertical and total coma.4,5 Coma is associated with a “comet-like” blur of the perceived image6 and it can be particularly disabling for patients. Spectacles and contact lenses correct errors in sphere and cylinder in eyes with keratoconus and thus improve VA, but these modalities do not address HOAs.
HOAs can be reduced with a wavefront-guided corneal ablation, but this technique is not the most appropriate for treating eyes with keratoconus because of limitations such as the challenge of obtaining a reliable wavefront map. Corneal topography, which is a static measurement, provides more reproducible data, and topography-guided PRK reduces HOAs by regularising the anterior corneal surface.
Ablation depth with excimer laser treatment is limited in eyes with keratoconus because of their thinner and inherently biomechanically weaker corneas; therefore the aim of the treatment is not to reach emmetropia but to normalise the anterior surface. The treatment results in improved BCVA secondary to the induced reduction in coma and reduction in spherical equivalent of manifest refractive error.7
We perform subjective refraction and cycloplegic refraction according to our standardised protocols, with a focus on measuring the complete astigmatic error. Because of the inability to obtain an accurate clinical refraction in eyes with keratoconus, especially for moderate to advanced disease, we rely greatly on topographic imaging for the data.
To fully characterise the corneal surface, we use both a Scheimpflug topographer (Pentacam HR, Oculus) and a Placido imaging modality (ATLAS 9000, Carl Zeiss). Dual imaging also allows us to analyse HOAs with both Zernike and Fourier reconstructions. To evaluate the axis of the vertical coma and correctly identify the axis of astigmatism with Scheimpflug-based imaging, we turn off all HOAs except those related to coma (N8, N9, N18 and N19).
The topography scan and the irregularity data map generated by the 22 Placido discs of the ATLAS 9000 are transferred to the dedicated treatment planning station (CRS Master, Carl Zeiss) for the excimer laser (MEL 90, Carl Zeiss). The optical zone, treatment refraction and Z axes are adjusted to keep the ablation depth to a maximum of 50 microns.
The treatment begins with alcohol-assisted epithelial removal using 20% alcohol applied for 40 seconds. The ablation excimer pattern is then applied by the MEL 90. Mitomycin C 0.02% is applied to the ablated corneal surface with a soaked sponge for 30 seconds followed by copious irrigation with chilled balanced salt solution.
This is followed by the cross-linking procedure. Riboflavin 0.1% saline, HPMC (VibeX Rapid, Glaukos) is applied every minute for a total of 10 minutes. Ultraviolet-A light (10 mW/cm2) is delivered for 4.5 minutes, corresponding to a total energy of 2.7 J/cm2.
Postoperative management includes a bandage contact lens, topical dexamethasone 0.1% and topical ofloxacin 0.3% for 2 weeks. The following case highlights our approach and how it improves refraction, UCVA and coma.
A 39-year-old man presented with an interest in having laser vision surgery to correct his low myopia. He reported that he was managing with contact lenses and spectacles, but he found those options inconvenient because he was in the military.
His refraction was –0.75/–1.75×155 logMAR 0.00 OD and –0.75/–0.50×20 logMAR 0.00 OS. Slit-lamp evaluation showed a few corneal guttata. There were no other abnormal findings on clinical examination.
On corneal topography, the front surface axial curvature maps for both eyes showed vertical asymmetry that strongly suggested keratoconus (Figure 1). Total HOA root mean square (RMS) was 0.617 μm OD and 0.601 μm OS (Figure 2).
Minimum corneal thickness was 545 μm OD and 543 μm OS. Treatment with topography-guided PRK plus CXL was recommended and the patient consented. Figure 3 shows the planned ablation patterns. The ablations were done with a 6.5 mm optical zone and expected depths of 37 μm OD and 25 μm OS.
Complete corneal re-epithelialisation was achieved at the 1-week follow-up visit. At 3 weeks, there was no evidence of corneal haze or other adverse sequelae, and the patient’s UCVA had improved to logMAR 0.22 OD and 0.24 OS.
At 3 months after the procedure, the patient’s UCVA was logMAR 0.00 OD and logMAR 0.50 OS. Visual acuity in the left eye improved to logMAR 0.00 with a refraction of –1.25/–0.50×113.
Corneal topography showed normalisation of the axial curvature maps OU, no evidence of ectasia and a small amount of regular astigmatism OS (Figure 4). Total HOA RMS had decreased to 0.445 μm OD and 0.447 μm OS (Figure 5), providing further evidence of the treatment’s benefit for regularising the anterior corneal surface.
The patient underwent an enhancement excimer laser procedure in the left eye at 12 months after the initial treatment, with an excellent result. At his last available follow-up, 6 weeks after the enhancement, UCVA in the left eye was logMAR 0.10. The patient was very satisfied with his visual outcome.
Early detection of keratoconus allows CXL to be performed when VA is still reasonably good and optimises prognosis for the patient. As in this case, patients with early keratoconus causing decreased vision often present for a refractive surgery consultation that enables diagnosis of their corneal disorder. The good outcomes that can be achieved for patients with keratoconus highlight the need to consider the implementation of screening programmes for this condition, as diagnosis and treatment at early stages can achieve excellent unaided acuities.
This case also highlights the efficacy of the topography-guided PRK procedure for normalising the anterior surface of the cornea and reducing coma in eyes with keratoconus. Recent reports substantiate its benefit for improving visual quality.3,7,8 We have also used topography-guided surface ablation techniques successfully for visual rehabilitation in eyes with irregular astigmatism and elevated HOAs related to conditions other than keratoconus, including in eyes with a history of trauma or infectious keratitis.
Regardless of the indication, patients need to be consented appropriately to establish realistic expectations for the outcome, depending on the severity of their condition and including the high probability of needing more than one laser treatment. In our experience, patients who elect to undergo the surgery are gratified by their improved visual quality.