instrumental strategies

The Path to Diagnosis

New software helps simplify corneal topography analysis.

DIANNE M. ANDERSON, O.D., F.A.A.O.

Corneal topography presents a wealth of useful information regarding diagnosis, refractive surgery candidacy and contact lens fits, but it can often be difficult to interpret, as certain corneal conditions, such as forme fruste keratoconus and regular astigmatism, are difficult to differentiate from one another.

The good news: PathFinder II Corneal Analysis Software, which is part of the new Atlas Model 9000 Corneal Topography System, from Carl Zeiss Meditec Inc., aids in analysis. This is because it uses a classification algorithm to provide you with probabilities for five different anterior corneal conditions based on a comparison to hundreds of diagnosed eyes. (Carl Zeiss Meditec Inc. has culled this patient data from six clinical study sites for the Atlas Model 9000.)

A particular helpful feature: The software analyzes the mean curvature map — a new feature in the Atlas Model 9000. Mean curvature suppresses regular corneal astigmatism, which can mask corneal pathology, to reveal any underlying curvature anomalies. Research and early PathFinder II clinical feedback suggest that the mean curvature map is particularly sensitive in revealing topographic signs of early stage keratoconus. In fact, research reveals that the PathFinder II has a greater than 90% sensitivity, specificity and accuracy in detecting normal vs. abnormal corneas.¹

How it works

As with any diagnostic instrument, the quality of the output is directly related to the quality of the acquired images. You can be confident that you're obtaining accurate corneal curvature and elevation measurements with the Atlas Model 9000, as it features a proven, patented Placido disk design with SmartCapture technology that automatically selects the best image. This helps ensure clear photographs, even in the case of movement.

The key to the PathFinder II's ability to classify corneal conditions: an algorithm called a Support Vector Machine (SVM), which researchers have implemented in numerous applications, including image analysis and decryption.² The PathFinder II SVM algorithm evaluates twelve different corneal parameters, such as shape factor (asphericity of the cornea from the center to the periphery), against PathFinder II's aforementioned clinical database to provide you with a probability that the topography falls into (it will report up to two category matches) the following five categories:

ATLAS Corneal Topographer with PathFinder II Software: HEIGHT: 19.7 inches WIDTH: 14.6 inches LENGTH: 20.5 inches WEIGHT: 39 lbs. COST: $18,950

PathFinder II reveals a patient with suspect keratoconus.

► Normal. This includes corneas that have no corneal pathologies and no history of refractive surgery or other corneal surgery. The reason the device identifies surgeries: If it wasn't able to classify these conditions, it would incorrectly flag them as pathology, such as suspect keratoconus. This creates medico-legal issues for the doctor.

► Suspect keratoconus (KCN). This includes forme fruste keratoconus (also known as subclinical keratoconus), long-standing keratoconus (patients in whom you've already established the condition) and pellucid marginal degeneration (PMD).

► Myopic laser vision correction (LVC). This includes patients who've undergone myopic laser vision correction, such as LASIK, photorefractive keratectomy (PRK) or laser epithelial keratomileusis (LASEK).

► Hyperopic laser vision correction (LVC). This includes patients who've undergone hyperopic LVC, such as LASIK, PRK or LASEK.

► Other. This includes corneal pathologies, such as corneal scars and degenerations, and surgeries, such as post-penetrating keratoplasty (PKP) and LASIK, that affect the anterior corneal surface.

The PathFinder II Display

The twelve analyzed corneal parameters are all color-coded to indicate which parameters are beyond normal limits and may be contributing to the particular classification. The software considers green "normal" (within 90% of the normal study population), whereas it considers yellow "borderline" (from 90% to 98% of the normal study population) and red "abnormal" (beyond 98% of the normal study population plus all others). In addition, the software allows you to display the results of OD and OS side by side.

PathFinder II's OD/OS summary

Finally, since PathFinder II results are quantitative, you can track patients through time to determine any changes in probabilities that may indicate alterations in corneal health.

I've discovered that one of the paths to great patient care and a successful practice is ATLAS 9000's PathFinder II Corneal Analysis Software for four reasons:

1. It's assisted me in the diagnosis of irregular corneas.

2. It's aided me in my decision-making concerning treatment.

3. It's helped me to educate my patients regarding their corneal condition and the importance of my prescribed treatments. (The software provides a colored illustration of their anomaly. Seeing is believing!)

4. Its technology has fostered patient loyalty to my practice.

Since excellent patient care is integral to building a successful practice, you should consider adding the new ATLAS Corneal Topographer with PathFinder II to your arsenal of diagnostic devices. OM

1. 1023/D879. Bagherinia H, Chen X, Flachenecker C, et al. Support vector machine (SVM)-based classification of corneal topography. Poster presented at: Association for Research in Vision and Ophthalmology (ARVO) Annual Meeting; April 27 - May 1; 2008 Ft. Lauderdale, Fla.

2. Noble, WS. What is a support vector machine? Nature Biotech 2006 Dec;24 (12):1565-67.