Cataract Surgery in the Setting of Posterior Keratoconus
Posterior keratoconus is a rare, non-progressive ectatic disorder of the cornea demonstrated by increased posterior corneal curvature often associated with overlying stromal opacity. This condition is mostly congenital, unilateral, and sporadic, but acquired, bilateral, and familial cases have also been reported. The posterior corneal changes primarily cause corneal scarring and refractive changes, including astigmatism.
Patients with posterior keratoconus may be conservatively managed with glasses or contact lenses prior to the onset of cataracts. Specialty contact lenses, including rigid gas permeable and scleral lenses, may be required for patients with significant and/or irregular astigmatism.
Posterior keratoconus patients with visually significant cataracts may require additional considerations that are not frequently encountered in routine clinical practice.
When planning cataract surgery, a careful preoperative clinical exam is required to diagnose and assess the severity of corneal changes. Some posterior keratoconus patients may have light-blocking corneal scarring, so penetrating keratoplasty (PKP) may be required, either as a staged or combined approach. If corneal scarring prevents acceptable keratometry measurements for intraocular lens (IOL) calculation, a PKP may be considered prior to cataract surgery.
Additional considerations must be considered for posterior keratoconus patients without significant corneal scarring. Currently available biometers and topography devices primarily measure the anterior corneal radius; the total corneal power is extrapolated based on an assumed and fixed ratio between the anterior and posterior cornea using the Gullstrand model eye. Since the posterior curvature of the cornea is irregular in posterior keratoconus, using a fixed ratio will inevitably lead to measurement errors. Specifically, the posterior corneal changes will decrease the measured total corneal power; this will potentially cause a given IOL calculation formula to call for a lower-than-required IOL power and an unwanted potential hyperopic surprise. While some newer devices (such as Scheimpflug tomography) and measurement capabilities (such as the “Total K” measurement of the IOL Master 700 [Carl Zeiss Meditec, Jena, Germany]) offer some promise at more accurate measurements of the posterior cornea, extensive studies demonstrating efficacy or superiority of these approaches have not been fully elucidated in the literature at this time.
Therefore, all posterior keratoconus patients must be counseled preoperatively regarding these IOL calculation challenges and the possibility of undesirable postoperative refractive error. This may require correction with conservative measurements such as glasses or contact lenses or additional surgical procedures, including but not limited to IOL exchange and secondary piggyback IOL implantation. Excimer laser refractive surgery should be considered with extreme precaution in this patient population.
Given the inaccuracy in current technology and IOL calculation formulas, a monofocal IOL is the preferred choice for posterior keratoconus patients. Toric IOLs may be considered in select cases with measured regular astigmatism, ideally reproducible with multiple devices or exams. However, it should be kept in mind that specialty contact lenses, such as rigid gas permeable or scleral lenses, may not be an option postoperatively if toric IOLs were preferred. Diffractive multifocal and extended depth of focus IOLs should be avoided given the high concurrence of astigmatic and higher-order aberrations that may be encountered in these.
Devices for IOL power calculation
The Pentacam HR (Oculus, Wetzlar, Germany) uses a rotating Scheimpflug camera combined with partial coherence interferometry (PCI) to produce imaging of the anterior eye structures. Total corneal power and astigmatism are calculated by ray tracing of the anterior and posterior corneal surfaces.
The IOLMaster 700 (Carl Zeiss Meditec, Jena, Germany) is optical biometry that utilizes swept-source optical coherence tomography (SS-OCT) to produce total keratometry (TK), which is calculated with measurements from the direct posterior corneal surface in combination with telecentric keratometry. Standard keratometry measurements are derived from 15 telecentric measurements of three rings (1.5, 2.5, and 3.3mm). Posterior corneal curvature is measured via a toric anterior surface model from the three zones mentioned above combined with pachymetry using SS-OCT in six meridians. The anterior and posterior corneal curvatures, combined with pachymetry measurements, are then used in a thick-lens formula to calculate the TK values.
Intraoperative Approach and Surgical Technique
In posterior keratoconus patients, a surgeon’s standard cataract surgery method may be generally used with a few additional considerations. First, given that some posterior keratoconus patients may have corneal scarring, methods to improve the visibility of the anterior capsule, such as epithelial debridement and/or the use of trypan blue, may be helpful. Second, while the posterior cornea has an intact endothelium, care should be given to avoid iatrogenic trauma during cataract surgery to the localized area of posterior keratoconus, given the pre-existing thinning in this area. Finally, some surgeons have proposed using intraoperative aberrometry in patients with abnormal anterior-posterior corneas, which may be extrapolated to posterior keratoconus patients. However, the efficacy or superiority of this approach has not been demonstrated and should be kept in mind, given the additional financial cost incurred through this technology.
Postoperative refractive errors are common in posterior keratoconus patients. Glasses and/or contact lenses should be first attempted as a conservative means to correct postoperative refractive error. For hyperopic surprise, a secondary piggyback IOL may be considered. With this approach, a three-piece IOL of additional plus power of different material (e.g., silicone piggyback IOL in the presence of acrylic IOL in the capsular bag) can be used in a 1.5:1 ratio (IOL power:Refractive Error). Alternatively, an IOL exchange can be performed as well.
Long term, these patients should be monitored at regular intervals to monitor disease progression.
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