Cataract Surgery in Patients with Keratoconus

Key Points at a Glance

Key Points of This Disease

• Keratoconus is an ectatic disorder characterized by progressive thinning and irregular anterior protrusion of the corneal stroma, with prevalence varying by region, ethnicity, and diagnostic criteria.
• In cataract patients with keratoconus, the accuracy of IOL power calculation is significantly reduced, and postoperative hyperopic surprise is the major challenge.
• Preoperatively, corneal topography and tomography should be performed after discontinuation of contact lens wear (soft lenses for at least 2 weeks, RGP for at least 5 weeks).
• Use of the Barrett True-K or Kane formula, which are specialized for keratoconus, is recommended.
• For patients with K values ≤55 D, a monofocal spherical IOL with myopia as the target refraction is the standard option.
• Multifocal IOLs are usually contraindicated due to the risk of increased higher-order aberrations.

1. Cataract Surgery with Keratoconus

Keratoconus (KCN) is an ectatic disorder characterized by progressive thinning and anterior protrusion of the corneal stroma. Corneal deformation causes irregular astigmatism, leading to visual impairment. According to a Dutch national registry study, the prevalence is estimated at 1:375 (265 per 100,000 population), with a mean age at diagnosis of 28.3 years¹⁾. Prevalence varies widely by region, ethnicity, and diagnostic criteria.

Patients with keratoconus have a higher risk of developing cataracts at a younger age. Moreover, many patients with severe keratoconus have already undergone corneal transplantation, and long-term steroid eye drops after transplantation as well as intraoperative manipulation accelerate cataract formation.

When cataracts significantly impair vision, cataract surgery is necessary, but the presence of keratoconus complicates IOL power calculation, surgical technique, and postoperative outcomes.

Q Does cataract surgery always become difficult in patients with keratoconus?

A

The difficulty is proportional to the severity of keratoconus. For mild cases (stage I), phacoemulsification (PEA) is reported to be safe and effective with appropriate preoperative planning. As severity increases, IOL power calculation errors become larger, and surgical technique becomes more challenging.

2. Main symptoms and clinical findings

Subjective symptoms

• Decreased visual acuity: Caused by a combination of irregular astigmatism, corneal opacity, and cataract.
• Glare and photophobia: Enhanced by light scattering due to irregular corneal shape.
• Polyopia and distorted vision: Resulting from irregular astigmatism. After a period when correction with contact lenses is possible, if it becomes uncorrectable, corneal transplantation is indicated.

Clinical findings

The following findings are observed on slit-lamp examination.

• Fleischer ring: an iron deposit ring at the base of the cone, beneath the epithelium.
• Vogt’s striae: vertical white lines in the stroma and endothelium at the corneal apex.
• Corneal stromal thinning and anterior protrusion: central to paracentral thinning and conical deformation.
• Acute corneal hydrops: a condition in which rupture of Descemet’s membrane allows aqueous humor to flow into the stroma, causing edema.
• Scarring after Bowman’s layer rupture: in advanced cases, reticular superficial stromal scars appear beneath the epithelium.

Corneal topography is characterized by localized steepening, pattern asymmetry, island-like anterior protrusion on elevation maps, and eccentric thinning on corneal thickness maps. Wavefront analysis shows a marked increase in vertical coma aberration.

3. Causes and Risk Factors

The pathogenesis of keratoconus is unknown, but the following factors are associated.

• Allergic diseases and eye rubbing habit: Frequently associated with atopic diseases, asthma, and allergic conjunctivitis. Eye rubbing is strongly correlated with progression, and avoidance is important²⁾.
• Genetic predisposition: Most cases are sporadic, but familial occurrence also exists. It is associated with connective tissue diseases such as Down syndrome, Ehlers-Danlos syndrome, and Marfan syndrome.
• Sex and age: More common in males, often onset during adolescence, and progression often stops around age 30.
• Contraindication for refractive surgery such as LASIK: Keratoconus (including suspected cases) is an absolute contraindication for LASIK and similar procedures.

Prevention and daily care

• Avoid the habit of rubbing eyes. For itchy eyes due to allergic conjunctivitis, active use of anti-allergic eye drops is recommended.
• Use contact lenses appropriately according to your doctor’s instructions. Long-term use of RGP lenses can cause corneal warpage (deformation), which may reduce the accuracy of preoperative measurements.

4. Diagnosis and Examination Methods

Preoperative Evaluation Procedure

When planning cataract surgery, the following stepwise evaluation is necessary.

Step 1: Confirmation of disease stage and stability

Evaluate the stage using the Amsler-Krumeich grading system.

Stage	Characteristics
Stage 1	Corneal steepening <48D, astigmatism <5D
Stage 2	Astigmatism 5–8 D, mean central K >53 D, corneal thickness >400 μm
Stage 3	Astigmatism 8–10 D, mean central K >53 D, corneal thickness 300–400 μm
Stage 4	Corneal scarring present, mean K >55 D, corneal thickness ≥200 μm

Step 2: Contact lens wear cessation (contact lens holiday)

• Soft contact lenses: Discontinue wear for at least 2 weeks. Additional time may be needed for stabilization.
• RGP (rigid gas permeable contact lenses): Long-term discontinuation of 5 weeks or more is necessary for corneal stabilization.

Surgery should not proceed until stable biometric measurements are obtained multiple times.

Step 3: Various examinations

• Corneal topography/tomography (e.g., Pentacam): Pentacam shows the best measurement reproducibility in stages 1–3.
• Evaluation of anterior and posterior corneal astigmatism and total corneal astigmatism: Evaluation of anterior, posterior, and total corneal astigmatism is essential to reduce postoperative refractive error³⁾.
• Optical axial length measurement: The reliability of axial length measurement in keratoconus eyes is reported to be equivalent to that in normal eyes.

Q What happens if contact lenses are not discontinued early?

A

Contact lenses cause corneal warpage, reducing the reliability of K values, corneal topography, and corneal thickness measurements. Determining IOL power based on inaccurate biometry leads to large postoperative refractive errors. It is important to postpone surgery until measurements stabilize.

5. Standard Treatment

Consideration of Preoperative Stabilization Procedures

If keratoconus has a risk of progression, consider stabilization procedures before cataract surgery ³⁾.

• Corneal cross-linking (CXL): Slows or stops progression and improves stability for IOL calculation. However, it does not preclude future corneal transplantation ³⁾.
• Intracorneal ring segments (ICRS): A refractive correction device; no effect on stabilizing disease progression.

IOL Power Calculation

In keratoconic eyes, standard formulas tend to cause postoperative hyperopia ³⁾.

Formula selection:

• Third-generation formula (SRK/T): Slightly less hyperopic shift compared to other third-generation formulas, but still low accuracy³⁾. Use of third-generation formulas other than SRK/T is recommended to be avoided³⁾.
• Barrett True-K and Kane keratoconus formulas: More accurate in advanced cases. Their use is recommended³⁾. Some studies have shown Barrett True-K to be more accurate than Barrett Universal II or Kane’s newer formulas³⁾.
• EVO 2.0 (with TK): Improved accuracy reported in moderate keratoconus³⁾.

In stage III eyes, all formulas show significantly reduced accuracy with MedAE >2.50D, posing a high risk of postoperative refractive error³⁾.

Refractive target: For patients with K values ≤55D, targeting myopia is recommended to anticipate postoperative hyperopic shift³⁾.

IOL Selection

Monofocal Spherical IOL

First choice: Standard option for keratoconus patients.

Myopic target setting: Set a myopic target considering postoperative hyperopic shift. For K>55D, it is recommended to use standard K value instead of measured K value.

Multifocal IOL

Usually contraindicated: Multifocal IOL implantation is not recommended for keratoconus patients due to increased higher-order aberrations and risk of residual refractive error.

There are few reports of successful outcomes.

Indications for toric IOL:

Toric IOL implantation via MICS is considered a safe and effective procedure for keratoconus patients with stable keratometry ³⁾. However, note the following:

• Toric IOL is not recommended for patients who achieve good vision with RGP or scleral lenses (as it may complicate hard contact lens fitting).
• It may be considered in mild to moderate (stage I/II) cases with regular astigmatism to mild irregular astigmatism.
• It is necessary to discuss thoroughly with the patient preoperatively, including the possibility of future progression and corneal transplantation.

Precautions in treatment

• If refractive error remains postoperatively and the patient cannot tolerate conservative therapy with glasses or contact lenses, consider piggyback lens or IOL exchange.
• Patients who have undergone corneal transplantation are at risk of graft rejection in subsequent surgeries.
• In patients with advanced keratoconus and cataract, a “triple procedure” combining cataract surgery and corneal transplantation may be indicated.

Surgical technique considerations

Corneal incision:

• Make an incision at a 90-degree angle to the location of the corneal scar to minimize induced astigmatism.
• Corneal steepening and thin stroma pose a risk of wound leakage. Sclerocorneal tunnel incision is recommended over clear corneal incision because it provides higher corneal stability and induces less astigmatism.
• Corneal sutures and corneal adhesives are also useful for reducing the risk of wound leakage.

Anterior capsulotomy (capsulorhexis): Intraocular visibility is reduced due to light scattering from corneal irregularity. Countermeasures include:

• Trypan blue staining: Stains the anterior capsule to ensure visibility.
• Application of dispersive viscoelastic material (e.g., HPMC) to the corneal surface: Has a dual effect of improving epithelial wettability and normalizing the intraoperative visual field.

Phacoemulsification (PEA): The procedure itself is unchanged, but reducing infusion pressure to control anterior chamber depth and lower intraocular pressure reduces the load on the cornea.

6. Pathophysiology and Detailed Mechanisms

Mechanisms of IOL Calculation Errors in Keratoconic Eyes

The reasons why IOL calculation is difficult in keratoconus eyes are as follows.

• Irregular tear film: Adversely affects the reliability and reproducibility of K values.
• Difficulty with fixation: K values are measured with the visual axis shifted peripherally.
• Inaccuracy of SimK (central 3 mm): Cannot adequately reflect the conical protrusion of the central cornea.
• Overestimation of K values: Steepening from the periphery to the center increases K values, leading to erroneous calculation of a low-power IOL.
• Change in anterior-posterior curvature ratio: Many IOL calculation formulas assume a normal anterior-posterior curvature ratio (condition of 1.33), but this is altered in keratoconus, resulting in estimation errors.

In a retrospective study of 71 cases (102 eyes), the postoperative refractive error using K values was significantly hyperopic (p<0.001) compared to the target, while using total corneal refractive power resulted in a significant myopic shift (p=0.013). Only 36% of eyes were within ±0.5 D of the target at 1 month postoperatively³⁾.

Complexity of Corneal Astigmatism Components

In keratoconus, irregular astigmatism components are predominant, which limits the corrective effect of toric IOLs. Toric IOLs that correct only regular astigmatism leave residual aberrations in advanced cases with strong irregular astigmatism. Increased higher-order aberrations (especially vertical coma) are the main cause of visual function decline²⁾.

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7. Latest Research and Future Prospects (Research Stage Reports)

For patients: Please read this

The following content is currently in the research stage or under clinical trial, and is not standard treatment available at general hospitals. It is reference information for professionals regarding future medical developments.

Pinhole IOL

For advanced keratoconus causing severe irregular astigmatism, there are studies showing the effectiveness of pinhole IOLs that reduce aberrations through the pinhole effect. However, data in the United States are limited, and large-scale studies are needed in the future.

Improved IOL Calculation Accuracy After Corneal Cross-Linking

Performing cataract surgery after corneal cross-linking may stabilize corneal shape and improve IOL calculation accuracy. It has been shown that performing cataract surgery after corneal cross-linking or intracorneal ring insertion may lead to better postoperative visual outcomes³⁾.

Artificial Intelligence (AI)-Based IOL Calculation Formulas

Next-generation IOL calculation formulas using machine learning are being developed, and are expected to improve accuracy in complex cases such as keratoconus.

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8. References

1. Godefrooij DA, de Wit GA, Uiterwaal CS, Imhof SM, Wisse RPL. Age-specific Incidence and Prevalence of Keratoconus: A Nationwide Registration Study. Am J Ophthalmol. 2017;175:169-172. doi:10.1016/j.ajo.2016.12.015. PMID:28039037.
2. American Academy of Ophthalmology. Corneal Ectasia Preferred Practice Pattern 2024. Ophthalmology. 2024.
3. European Society of Cataract and Refractive Surgeons. ESCRS Cataract Guideline 2023. ESCRS.