The mantra of modern keratoconus management is: diagnose early, stop progression, rehabilitate vision.
There are a number of diagnostic instruments that are used to diagnose and monitor keratoconus. The corneal topographer is one such instrument and essentially it maps out the contours of the anterior corneal structure. I, as well as many others, was taught in school that corneal topography, aka videokeratography, is the gold standard for diagnosing keratoconus. There are a number of different corneal topography systems, each of which provides slightly different information about the cornea surface shape, but all the principles are the same. These instruments have come to exist due in part to the principals of the keratometer and the placido disk.
Keeler
Placido Disk Keratoscope
Together, these reflection based techniques indirectly measure the anterior corneal curvature by assessing reflections on the tear coated cornea to extrapolate the contour of the cornea surface. In a normal, spherical cornea the perfect circle shaped light source is mirrored on the precorneal tear film. In keratoconus, these reflections become less circular and more distorted and warped as the disease becomes more severe.
B+L Keratometry
Regular Mires on a Normal Cornea
Irregular Mires on Severe Keratoconus
Corneal Topography (Placido Ring Based)
Mild WTR Astigmatism, note the smooth and regular mires
Severe Keratoconus, note the smooth but distorted and warped mires
The first fully automated corneal topographer was the Corneal Modeling System (CMS-1), developed by Computed Anatomy Inc. in New York during the early 1990’s. At the time, the cost to purchase the instrument was an astronomical $75,000 making it prohibitively expensive and prevented mass adoption. Over the years numerous corneal topographers have been developed utilizing various light sources, number of light sources, and cone sizes in an attempt to measure as much of the corneal surface as accurately as possible. Many modern corneal topographers have been combined with other useful instruments such as aberrometers to hasten diagnostic data acquisition and workflow. Thankfully, with the advancements in the technology, and keeping with the trends of technology, these diagnostic instruments have become more economical. A very capable corneal topographer can now be purchased for under $15,000. At this point in time, every single eyecare practitioner should own and utilize this instrument.
Klyce and Rabinowitz developed a definitive set of corneal topography metrics for the diagnosis of keratoconus. The metrics are maximum keratometry > 47.00 D, inferior-superior keratometry difference >1.40 D at a 6 mm optical zone, and astigmatic axis skew >20°. However, a corneal topography only tells part of the story in keratoconus, especially when it comes to early diagnosis.
One of the weaknesses of corneal topography is that it only measures anterior corneal curvature. There are many other corneal metrics that are relevant in diagnosis of keratoconus which are not measured by topography, these include posterior corneal elevation, global corneal pachymetry specifically thinnest point, visual quality as measured by aberrometry, and in-vivo corneal biomechanical strength. Newer technology exists to collect this data and by aggregating and analyzing this data in conjunction with corneal topography, it creates a more comprehensive and complete picture of keratoconus allowing for the earliest diagnosis. This process is much like the analysis of various diagnostic data utilized to diagnose glaucoma at its earliest onset.
Another weakness of placido disk corneal topography is its reliance on the precorneal tear film. These are reflection based devices and the reflective quality is a direct result of the tear film on the corneal surface. When ocular surface disease is present it affects the quality of the reflection and disrupts the mires resulting in inaccurate and/or incomplete topographical data. This can be frustrating, but this weakness can be exploited and actually turned into a strength. Many corneal topography systems utilize reflection quality and mire stability as a method to measure the quality of the tear film which can be valuable in dry eye diagnostics and monitoring.
Keratoconus, note the mires are distorted but are not smooth, rather they are broken and incomplete, this is caused by tear break up and leads to incomplete scan data in the topographic map.
Lastly, accuracy of placido disc corneal topography is dependent on alignment. Misalignment of placido rings on corneal surface can cause normal corneal shapes to look suspicious for Keratoconus and other ectatic corneal disease.
These topographic maps come from the same normal cornea and were captured only moments apart, the differences in the maps are a result of lateral misalignment. In the top image the patient is well aligned, but in the bottom image, the patient’s gaze is off axis.
Corneal topography, though tremendously useful, provides only a portion of the data needed in early diagnosis and monitoring of keratoconus, just as a visual field provides only a portion of the relevant data in diagnosing and monitoring glaucoma. As we delve further into these blogs we will explore other technologies, such as corneal tomography, aberrometry, in-vivo biomechanical meters, and the role they play in multifactorial diagnosis and monitoring of keratoconus.
