Corneal topography and tomography have an important role as screening tools in patients seeking laser vision correction surgey and to diagnose and follow keratoconus and other corneal ectatic diseases.
This article was reviewed by Vishal Jhangji, MD
Advances in hardware and software make corneal topography and tomography useful tools for ectasia risk screening and for the diagnosis and follow-up of keratoconus and post-LASIK ectasia, but other factors and clinical correlation are important when interpreting the findings, according to Vishal Jhanji, MD.
“Corneal topography and tomography allow precise measurement of anterior and posterior corneal curvatures and corneal thickness and enable the diagnosis, classification, and monitoring of progression of corneal diseases, but there can be more to the picture,” said Dr. Jhanji, professor of ophthalmology, University of Pittsburgh, Pittsburgh. “Hopefully, these processes will be improved in the future through a combination of biomechanical factors, topographic indices, and surgery-induced risk stratification.”
Corneal topographers analyze the pattern of light rays reflected off the cornea and tear film-air interface and reconstruct the corneal shape. Corneal tomographers evaluate the whole cornea by obtaining information from the anterior and posterior corneal surfaces to reconstruct three-dimensional images of the anterior segment.
The corneal imaging technology has continued to evolve. Initial instruments based on Placido disk technology only analyzed the central anterior surface of the cornea and only provided anterior corneal shape-based indices. Subsequently, scanning slit-based technology was introduced that also imaged the posterior cornea.
Dr. Jhanji pointed out that with information about posterior surface elevation, keratoconus began to be diagnosed more efficiently.
Keratoconus affects 86 in 100, 000 people, resulting in visual loss due to increasing irregular corneal astigmatism, and the quality of life declines in patients. Ophthalmologists can stabilize the disease or improve vision by utilizing corneal cross-linking (CXL) and grafting, but these carry risks.
Scheimpflug camera-based devices were developed that could image the central anterior and posterior cornea as well as the peripheral cornea. Software for these systems was developed to generate comprehensive diagnostic indices for identifying subclinical and frank keratoconus.
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Because abnormal topography is arguably the most important risk factor for post-LASIK ectasia, one of the most common indications for topography/tomography in an anterior segment practice is screening for ectasia risk.
In post-LASIK ectasia, the cornea starts to bulge forwards at a variable time after LASIK, PRK, or SMILE corneal laser eye surgery, much like kerataconus.
Red flags for poor surgical candidates include image-based identification of abnormalities such as an asymmetric bow tie, skewed radial axis, or inferior/paraxial steepening or thinning. In addition, different devices generate calculation-based scores (eg., BCV index, Sirius; BAD-D score, Pentacam; KISA%, Orbscan).
“The reliability of these scores depends on the reliability of the individual factors that are used to calculate them,” Dr. Jhanji said. “However, interchange between devices can lead to extrapolation errors. For that reason, it is preferable to do serial follow-up using the same device.”
LASIK involves removing corneal tissue and reshaping the corneal wall. This procedure is effective for correcting issues such as nearsightedness, farsightedness, and astigmatism, but it also inherently results in a thinner cornea.
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Limitations of device-based screening
Dr. Jhanji pointed out that the risk of post-LASIK ectasia is also influenced by surgical and postsurgical factors that are not accounted for by corneal topography/tomography.
These include corneal biomechanics, flap thickness variation, ablation volume and decentration, and possibly eye rubbing or IOP spikes postoperatively.
In addition, although serial imaging showing changes in individual parameters or calculated indices could be interpreted as providing a clear indication of ectasia development and progression, there are other factors may need to be considered for decisions about patient management.
According to the Global Consensus on Keratoconus and Ectatic Disease, consistent change in at least two of the following parameters can be used to identify progression: steepening of the anterior corneal surface, steepening of the posterior corneal surface, thinning and/or changes in the pachymetric rate of change.
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The group also recognized, however, the importance of considering patient age and change in refraction.
Best-corrected distance visual acuity is incorporated in the ABCD Grading System for keratoconus (available on Pentacam) along with the radius of curvature of the anterior and posterior corneal surfaces and corneal thickness at the thinnest point.
“In all cases, clinical correlation is important,” Dr. Jhanji concluded. “What looks like keratoconus on a topographic map may be a change caused by other corneal pathology.”
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Vishal Jhanji, MD
This article is based on a presentation by Dr. Jhanji at the American Academy of Ophthalmology annual meeting. He has no relevant financial interests to disclose.