Physician discusses preoperative assessments before procedure focusing on cornea.
Reviewed by Joshua Teichman, MD, MPH, FRCSC
Cataract surgeons should become well acquainted with technologies that capture topography and tomography of the cornea, according to Joshua Teichman, MD, MPH, FRCSC, codirector of the cornea, external disease and refractive surgery fellowship and assistant professor in the Department of Ophthalmology and Vision Sciences at the University of Toronto, Trillium Health Partners, Prism Eye Institute, and the Laser Centre Oakville in Ontario, Canada.
Teichman recently spoke at the Toronto Cataract Course, where he discussed preoperative assessments prior to cataract surgery that focus on the cornea and underlined that different technologies serve different purposes in these assessments.
Teichman differentiated between the function of topography and tomography as they apply to cataract surgery. He explained that topography uses reflection-based technology to calculate keratometry values, whereas tomography scans the cornea (similar to how an imaging modality would) with technology like Scheimpflug photography and optical coherence tomography. “Tomographers measure elevation and then calculate the corneal information from this data” Teichman said.
When clinicians see a patient, they have to make sure things line up in terms of patient information, the date of the test, the eye tested, quality indicators, as well as ensuring maps and scales are correct, according to Teichman. He also stressed the importance of examining the rings image, stating “garbage in, garbage out.”
“You have to have a systematic approach,” he said. “If you change the scales so [they’re] a quarter of a diopter, all your patients will appear to have keratoconus. If you change it to 2 diopters, everyone is going to appear to have a normal cornea.”
Clinicians also need to be aware of whether they are using an absolute or relative scale, Teichman added. It is imperative that clinicians verify their scales and step sizes, review the values, and compare results with manual or automated keratometry, biometry, the contralateral eye, and with previous maps, Teichman said.
Artifacts such as eye dryness and contact lens wear may influence the topography and tomography measurements, Teichman explained. The presence of dryness can manifest as irregular mires, data gaps, and/or present as flat areas. “Make sure [contact lenses] are out [of the eyes] at the time of testing, which may sound obvious but is not always,” he said.
In terms of when patients should remove their contact lenses before having their corneas evaluated prior to cataract surgery, that decision must be tailored to the patient and consider factors such as the duration of contact lens wear and whether a patient wears soft or rigid contact lenses, Teichman said. “You have to decide on how long [contact lenses] should be out [of the eyes] before testing,” he said, noting a general guide for soft contact lenses is from a few days to 2 weeks, and for rigid lenses is anywhere from 2 weeks in total to 1 month per decade of wear.
“If you have a 20-year-old [patient with keratoconus, they] won’t have to have [contact lenses] out very long. [However], if you have a 70-year-old patient who has been wearing rigid contact lenses for 50 years, that is a long time,” he said.
One of the key maps that should be employed in preoperative assessment is the axial or sagittal map, which measures the corneal curvature and relates corneal power to corneal shape. Another is the instantaneous or tangential map, which helps to better localize lesions, Teichman explained.
Clinicians need to reflect on the aim of the assessment, such as whether they want to detect keratoconus, Teichman said. “If you are screening for keratoconus, you might want to use an elevation-based device,” he said.
The presence of epithelial basement membrane dystrophy or Salzmann nodules will also affect results with topography, tomography, and ultimately outcomes of cataract surgery, according to Teichman. “Pretreat with a superficial keratectomy,” Teichman said.
It is critical to be aware of the various lesions or presentations that can affect topography display, according to Teichman. Pterygium is another common finding prior to cataract surgery and should be removed prior to surgery, with sufficient time al- located to permit the cornea to normalize prior to repeat testing, he added.
Teichman also mentioned the importance of imaging patients with corneal scars or trauma before cataract surgery. Although many patients may have irregular corneas, some may have decent regularity in the central zone and sufficiently regular astigmatism, such that a toric IOL may improve their postoperative outcomes.
Moreover, Teichman discussed the various challenges in IOL selection in patients with keratoconus and the various methods one can use to aim for the best outcomes in these challenging eyes.
“Patients may also forget to mention they have had previous refractive surgery, and the topography can be your safety net,” he said. “Moreover, they may not remember [whether] they were myopic or hyperopic prior. The maps can assist with this.”
Clinicians should take the opportunity to customize their printouts for their own clinical uses, according to Teichman. “Do not let the manufacturer decide [what is displayed on your overview],” Teichman said. “Be aware of the artifacts. Really know your scans, your maps, your scales, and control the technology—not the other way around. Customize your display and read the manuals.”