Surgical Approach to Cataract Surgery with Keratoconus
Keratoconus (KCN) is an ectatic disease of the cornea, characterized by progressive thinning and irregularities in the corneal stroma and breaks in Bowman’s layer. As a result, the shape of the cornea is distorted into a more conical and asymmetrical formation. Clinically, this can lead to changes in visual acuity, irregular corneal astigmatism, corneal scarring, and increased sensitivity to glare and photophobia. Treatment options, depending on the stage of the disease, can include spectacles, contact lenses, intrastromal corneal ring segment, or corneal transplant. Corneal crosslinking is performed in patients that show disease progression. Those at higher risk of having KCN include patients with Marfan’s syndrome or other connective tissue disorders, atopic disease, and Down syndrome. Patients with KCN are at higher risk of developing cataracts, and of developing cataracts at a younger age. Cataract surgery is necessary when visual acuity is significantly impacted by the cataract, independent from the corneal changes. Therefore, in order for patients to achieve maximal visual outcomes, thorough preoperative, intraoperative, and postoperative care is required.
The surgeon should first determine the staging and stability of the disease. Amsler-Krumeich is a commonly used system for grading and staging KCN, ranging from stage 1 to 4:
● Stage 1 is early KCN, with corneal eccentric steepening < 48 D and astigmatism < 5 D.
● Stage 2 is mild KCN, with astigmatism from 5 D to 8 D and a mean central K of > 53 D. There is no scarring present, and the corneal thickness is > 400 μm.
● Stage 3 is moderate KCN with astigmatism from 8 D to 10 D and a mean central K of > 53 D. There is no scarring, and the corneal thickness is between at least 300 and 400 μm.
● Stage 4 is severe KCN, with corneal scarring and a mean K of > 55 D. The corneal thickness is at least 200 μm.
Similar to routine cataract surgery, an important aspect of preoperative planning for a KCN patient requiring cataract surgery is the target manifest refraction and determination of an accurate intraocular lens (IOL) power. Classically, keratometry [K] measurements, corneal topography or tomography, and optical biometry are the basis for this calculation. However, in patients with KCN, biometry can be less predictable and even more so in advanced disease, making IOL power selection difficult.
Keratometry measurements can be affected in keratoconic eyes in several ways. Keratoconic eyes often have an irregular tear film, which negatively impacts the reliability and repeatability of the K measurements. Keratoconic eyes often have a greater difficulty in fixating at the target that corneal topography or tomography requires. This results in a peripherally displaced visual axis with K measurements that do not account for the steepest part of the cornea. Simulated K values that only consider the 3 mm central zone of the cornea can be inaccurate in keratoconic eyes. Additionally, as the corneal surface is steepened between the central conical protrusion and peripheral cornea, an increased K value is seen which can lead to a falsely low power IOL calculation resulting in a postoperative hyperopic surprise.
Axial length (AL) is another important factor required for accurate IOL power calculation. Keratoconic eyes are more myopic on average because of both steeper corneal curvature and posterior segment elongation. As a result, keratoconic patients typically have longer axial lengths. Measurement of axial length in keratoconic eyes has been found to have similar reliability as when measured in normal eyes.
There are several approaches to minimize refractive surprise following cataract surgery. First and foremost is to operate on patients with stable, non-progressive, keratoconus. As with the natural progression of the disease, older patients with KCN tend to have more stable disease than younger patients with KCN. To improve corneal stability preoperatively in mild to moderate KCN, corneal crosslinking (CXL) and intrastromal corneal ring segments (ICRS) are two treatments surgeons can consider. However, CXL and ICRS do not eliminate the possibility of patients needing a corneal transplant in the future.
Many patients with severe KCN will have already undergone a corneal transplantation. Keratoplasty alone already places these patients at increased risk of accelerated cataract formation due to the use of a long-term topical steroid and from surgical manipulation during the procedure. Patients with keratoplasty should be aware about the possibility of graft rejection with any subsequent surgery, potentially requiring repeat corneal transplantation.
Patients with advanced KCN and a concomitant cataract may be good candidates for combined cataract surgery with intraocular lens implantation and keratoplasty, otherwise known as a triple procedure. While the triple procedure is advantageous, the surgeon should anticipate possible refractive surprise postoperatively.
Contact Lens Holiday
Many KCN patients wear contact lenses to achieve best corrected visual acuity. Depending on the type of contact lens the patient uses, a contact lens holiday should be implemented prior to obtaining K values, corneal topography, corneal pachymetry, and manifest refraction. Contact lenses can contribute to corneal warpage, which leads to inaccurate biometry. The purpose of the contact lens holiday is to ensure that the cornea returns to its baseline configuration and allow for more accurate and consistent measurements. It has been found that corneal topography and pachymetry take the longest to stabilize. It has been proposed that a two-week holiday for soft contact lenses is adequate for most patients, although some patients may require an even longer period. In patients using rigid gas-permeable (RGP) lenses, a longer holiday of five weeks was found to provide corneal stability.  Because the cornea will continue to stabilize throughout the lens holiday, especially for a rigid lens user, the surgeon should not proceed with surgery until biometric measurements have been measured multiple times and are stable, which could take longer than the standard holiday dependent upon the contact lens type. Patients may be tempted to be non-adherent to the lens holiday, as the decrease in visual acuity with lens abstinence can be significant and incompatible with their daily living. This of course requires a thorough discussion with the patient to ensure complete understanding of the situation. Patients should also be counseled preoperatively, regardless of whether they currently wear spectacles or contact lenses, on the possibility of postoperative contact lens use for best visual outcomes. In cases where a refractive surprise is found and the patient is unable to tolerate conservative therapy with spectacles or contact lenses, a piggyback lens or IOL exchange may be considered.
IOL Power Calculation
There are a multitude of imaging modalities to evaluate the patient’s corneal power, including the Pentacam, IOL Master, EyeSys, Orbscan, and Javal manual keratometer. Hashemi et al. found that in stage 1 to stage 3 KCN, the Pentacam had the highest repeatability of measurement. In stage 4 KCN, no individual device showed superior repeatability. Additionally, the AL-Scan by Nidek was found to have high repeatability in obtaining flat and steep K values, but the steep K values were less accurate in eyes with KCN.
After stable keratometric, topographic, and refractive measurements have been obtained, and the array of KCN treatment options have been weighed, the surgeon can proceed with biometry. There are multiple formulas that surgeons can use in determining IOL power. However, with more severe KCN, all formulas will have some degree of inaccuracy. There is conflicting data as to which intraocular lens formula is most accurate in keratoconic eyes, and further research is needed. Two studies looking at second and third generation IOL formulas suggest that SRK II formula led to less postoperative refractive error as compared to SRK and SRK/T formulas in mild KCN . SRK II formula was found to be more accurate regardless of KCN stage of progression as compared to Haigis, Holladay I and II, Hoffer Q and SRK/T by Zare Mehrjerdi et al.
Fourth generation formulas may be more accurate in determining effective lens position (ELP), which is an important variable in calculating IOL power in eyes with abnormal anterior chamber depth (ACD). Keratoconic eyes are more likely to have steeper K’s, larger ACD and longer AL. Since ACD is an important factor in determining ELP, it is hypothesized that fourth generation formulas (Holladay II, Olsen, Barrett) may do better than second (SRK II) or third (SRK/T) generation formulas in KCN patients to minimize postoperative hyperopic surprise.
Overall, it is important to acknowledge two principles when deciding on which formula to use. First, consider the individual patient and his/her unique presentation. For example, if an eye is not myopic, has mild KCN, and has a normal anterior chamber depth, the SRK II formula may be appropriate. If a patient has a deep anterior chamber, making ELP less predictable, a fourth-generation formula, Barrett II Universal formula, may be a better choice. Second, recognize that there is no unanimous “best” choice in selecting a formula and that postoperative refractive error correlates highly with the severity of disease; it is vital the surgeon uses their best clinical judgement. 
Selection of a proper IOL implant is a crucial step in planning for cataract surgery in KCN patients. The key is to consider the severity of KCN in patients, along with their degree of astigmatism.
Monofocal spherical lenses have been the standard choice for IOLs in patients with KCN. Due to common postoperative hyperopic error in KCN patients, many surgeons will designate a myopic target that also accounts for some residual astigmatism postoperatively when selecting a monofocal spherical lens. According to Watson et al., a spherical lens with myopic target is safe for patients with a K of less than 55D. However, in patients with more severe KCN (K > 55), standard K should be used in IOL power calculation rather than the patient’s actual measured K, due to the risk of hyperopic error postoperatively that was observed in the study.
Another approach is to use the equivalent K readings (EKR) from a topographer/tomographer and using the K's reported in the central 3-4mm for the Ks to input into a modern formula.
In terms of multifocal lenses, these are usually not recommended in KCN patients due to the increased risk of higher order aberrations. There is a paucity of literature describing successful implantation of multifocal lenses in KCN patients. It is recommended that increased caution be undertaken when considering implanting a multifocal IOL in a KCN patient.
Special consideration must be taken when considering implantation of an astigmatism correcting (toric) IOL. Toric IOLs are considered for and found to be successful in patients with cataracts and regular corneal astigmatism. However, keratoconic patients tend to have more irregular corneal astigmatism. For keratoconic patients that have good vision in RGP and are happy wearing RGP pre-operatively, toric IOL would not be recommended and consideration of a monofocal IOL with a near aim should be considered. In the situation where the patient has stable disease and has good vision without use of RGP lens or is RGP intolerant, a toric IOL may be considered. More recent literature is showing that in patients with mild to moderate (stage I or II) KCN and regular to slightly irregular astigmatism, a toric lens can be effective and successful in achieving suitable postoperative visual outcomes.  Kamiya et al. examined toric IOL implantation in those with mild to moderate KCN and found that postoperative spherical equivalent correction fell within ±1D of target in 95% of eyes, with a significant decrease in refractive astigmatism, a nonsignificant decrease in corneal astigmatism and HOA. Further research is needed into the efficacy of toric IOL implantation in KCN patients with cataracts, especially those with more irregular astigmatism and more advanced disease.
While there is currently limited data in the United States on Pinhole Intraocular Lenses, they may show promise in eyes with advanced disease causing severe irregular astigmatism.
Once preoperative counseling, IOL power calculation, and IOL selection have been completed, the surgeon may proceed with cataract surgery. While the actual steps of cataract removal and IOL implantation remain the same, some technical variations may lead to better outcomes for patients. As a general rule-of-thumb, increased severity of KCN in the patient will cause increased surgical difficulty. Corneal instability and poor intraocular visibility/depth perception are the primary impediments that surgeons should be aware of.
The main corneal incision is a critical step in cataract surgery, but especially in keratoconic eyes. With a keratoconic cornea, using the same incision technique as for a healthy eye can lead to unwanted changes in K postoperatively. It is crucial that when making the initial corneal incision, the patient’s degree of corneal scarring is accounted for. This is because the incision on a scarred cornea can induce a significant amount of astigmatism, as compared to a slight amount of induced astigmatism in a healthy cornea. Therefore, surgeons should make the incision at a 90-degree angle relative to the location of the scar. Care should also be taken to avoid premature entrance into the anterior chamber, as the peripheral corneal steepening caused by KCN can result in a shorter incision. Along with corneal scarring, corneal steepness and thinner pachymetry can lead to a wound leak if a clear corneal incision is made. To avoid this, a sclero-corneal tunnel incision or scleral tunneled incisions can lead to greater corneal stability and less astigmatism, making it a better choice for cataract surgery in KCN patients  Corneal glue or corneal suture over the main incision can also help to decrease the risk of wound leak.
Scarring and irregularities in the keratoconic cornea can also lead to light scattering during cataract surgery. This causes poor intraocular visualization for the surgeon and especially increases capsulorrhexis difficulty as there is a loss of contrast between tissue and decrease in clarity of depth perception. Three options can help the surgeon. Usage of trypan blue to stain the anterior capsule may aid in visualization for the surgeon. Another option is usage of a dispersive ophthalmic viscoelastic device (OVD). While a dispersive OVD is usually injected intracamerally, a study by Chen et al. proposes injecting a dispersive OVD such as hydroxypropyl methylcellulose (HPMC) 2% gel onto the corneal surface of the cornea. This has a dual purpose, as it leads to greater hydration of the epithelium and provides magnification of the intraocular contents for the surgeon. The last option is newer and more experimental, as it has only been described in a study by Oie et al. on two patients with severe keratoconus. The authors first injected a cohesive OVD onto the corneal surface, and then placed an RGP contact lens on top of it. They found that this provided greater visibility and less image distortion of the anterior and posterior capsules with no complications.
The techniques of phacoemulsification do not change for a keratoconic eye. Considerations include adjusting the phacoemulsification settings to better control ACD (minimize deep anterior chamber) by lowering inflow pressure and allow for decreased intraocular pressure during the surgery. Decreasing the intraocular pressure will help lessen stress on the cornea. Extra care should be given during hydrodissection, OVD injection, and IOL implantation, which are periods during cataract surgery when IOP is highest.
The postoperative treatment plan is similar for KCN and non-KCN patients undergoing cataract surgery. Monitoring for complications such as endophthalmitis and retinal detachment is crucial. There is no current literature that describes complications specifically associated with cataract surgery in the setting of keratoconus. Due to KCN patients’ corneal instability, it may take longer for them to heal fully, and so an extended period of non-activity following surgery is recommended. Surgeons should also evaluate their patients for increased astigmatism postoperatively, with their contact lens or spectacle prescription updated as needed. Resuming contact lens wear postoperatively is encouraged for patients who used them preoperatively, as this will allow for maximum visual acuity. Patients that wore spectacles before surgery may need to use contact lens after surgery, depending on their satisfaction with their visual acuity. It is imperative that patients be monitored regularly for keratoconus progression following cataract surgery, as this can affect visual acuity. If visual acuity is poor due to refractive error postoperatively, an IOL exchange or placement of piggyback lens may be considered.
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