Corneal topography comes of age

Refractive surgery’s status is spurring its advance, aided by insights of a Viennese photographer from the early 1900s.

By M.L. Fox, MD, FACS

Topographic analysis of the cornea has come a long way since 1619 when Schiener used the reflection of glass marbles from the cornea as perhaps the attempt to understand and measure corneal shape.¹ Placido’s disk was the next major advancement in the late 19th century.^1,2 This manner of corneal reflection analysis has stood the test of time as many current topographers still work on the same principle of assessing the reflection of a concentric set of white rings on the convex anterior surface of the cornea.

But the explosive growth of corneal refractive surgery has fueled the rapid advance in the science of interpretation and understanding of anatomical features of both anterior and posterior surfaces of the cornea. The information acquired by these technologies has also become a mainstay in the office of the comprehensive ophthalmologist.

For sure, today’s topography has advanced remarkably, evolving to measure optical properties of the cornea while making use of sophisticated computerized systems — some of which incorporate the aerial photography discoveries of an Austrian army captain from the early 1900s.

Reflections and projections

Current topographers are generally categorized into two broad categories: reflection-based or projection-based systems. Projection-based systems generate tremendous amounts of data within seconds, reconstructing 3D images of the eye’s anterior segment, including data on curvature, shapes of the anterior and posterior surfaces, local surface changes, power and corneal thickness. The most recent advances have applied technologies of scanning slit, Scheimpflug (the Austrian captain) evaluation and ray tracing to improve anterior corneal assessment as well as adding essential posterior elevation information to corneal assessment.

Scanning slit designs make use of anterior corneal Placido imaging combined with high-resolution video to capture data points allowing for assessment of posterior corneal characteristics and pachymetry.³

Thanks, Capt. Scheimpflug

Ophthalmologists owe gratitude to Theodor Scheimpflug. It was this Viennese native with the curious mind for aerial photography who was convinced that angling the camera lens, from afar, brought into focus all that was hence blurred and indistinguishable.

Only a few topographer makers use the Scheimpflug methodology, one of them being Pentacam (Oculus). And, as he has done before, Jack T. Holladay, MD, wrote a guidance report (available at http://tinyurl.com/p8exraa) to improve calculations in patients with so-called off-label shaped corneas — those outside the norm — due to early keratoconus or past LASIK/PRK surgery.

“Measuring height data directly and knowing the back surface as well as the front is extremely important in detection of thinning disorders as well as determining the total power of the cornea,” says Dr. Holladay, clinical professor of ophthalmology, Baylor College of Medicine. “We have learned the importance and variation in the back of the cornea for both IOL power and astigmatism correction.”

New iterations

The Scheimpflug principle of photographic imaging of the cornea is based on Scheimpflug’s 1906 aerial photography work. The enhancement technique and angled photography improves imaging acuity.

Current iterations allow for resolution of 138,000 points to be processed in two seconds through a rotating Scheimpflug camera taking intersecting photographic cross-sections of the anterior segment that are illuminated by slit beams at different meridians. Sophisticated software processes the data from all points and reconstructs a 3D elevation representation of the anterior segment and generates readings of different parameters.

Surgeons then can assess corneas with severe irregularities as well as analyzing ocular wave fronts to detect higher-order aberrations.^4-7

Explains Dr. Holladay: “In refractive and cataract surgery it is essential to screen patients for corneal thinning disorders and irregular astigmatism. Using height data is extremely sensitive for detecting keratoconus and pellucid marginal degeneration.”

Corneal power display options include the sagittal (axial) and tangential curvature of the anterior and posterior corneal surfaces; the true net power; the keratometric power deviation; the refractive power; and the equivalent K-reading as refined by Holladay.⁸ Ray tracing topography makes use of multicolor red, yellow and green LED illumination that permits the surgeon to perform a sophisticated point-to-point ray tracing, which references anterior and posterior Purkinje reflections of the cornea in calculating total corneal astigmatism and axis. This variety of corneal topography is highly specific in estimating corneal keratometry and specific corneal irregularity indices. It is very useful in calculating astigmatic focusing disorders and their axes of influence.

Choosing refractive surgery candidates

Patients with normal placido disk imagery presenting for laser refractive surgery require careful evaluation of both corneal surfaces in order to identify masked structural issues that could lead to ectasia. In Figure 1, a healthy 23-year-old-male with normal corneal pachymetry and Placido disk imaging presented for refractive surgery. A subsequent elevation study revealed early posterior float abnormality. Such findings can avoid ectasia complications and allow for better patient counseling toward alternative treatments.

Figure 1. Pentacam elevation study showing posterior corneal elevation suspicious for early keratoconus. Corneal crosslinking was advised for the patient with topography-based custom PRK, a conceivable possibility in the future.
COURTESY JACK HOLLADAY

Assessing visual symptomatology

The evaluation of surface corneal topography is essential in the explanation of visual symptomatology. In the case of a 48-year-old-female (Figure 2) presenting with complaints of blurry vision, a mild nasal pterygium was not suspected as the source of complaints until a Scheimpflug-based tomography was performed.

Figure 2. Scheimpflug-based tomography reveals with-the-rule astigmatism with nasal corneal flattening, a patient with visually significant nasal pterygium.
COURTESY JACK HOLLADAY

Clinical corneal surface evaluation belied the topographic appearance that confirmed nasal corneal topographic flattening explaning symptoms.

Symptomatology is likely due to what clinically appears to be a visually insignificant pterygium.

Intacs surgical planning in kerataconus

Essential in successful Intacs (Addition Technology, Inc.) outcomes in the surgical treatment of keratoconus is an understanding of the specific keratonic shape. In this case of asymmetric corneal ectasia, (Figure 3) a single ring implantation is indicated in order to address the ectasia in an appropriate manner.

Figure 3. Keratoconus patient presenting for Intacs. Pentacam study indicates asymmetric pathology best treated with single ring, inferior placement approach.
COURTESY JACK HOLLADAY

Figure 4. Normal exam displaying regular astigmatism with low levels of spherical aberration.
COURTESY JACK HOLLADAY

Figure 5. Page 2 of the Holladay report displaying tight consistency over the full range of pupillary sizes in a normal cornea.
COURTESY JACK HOLLADAY

IOL calculations and selection in irregular corneas

Perhaps the greatest contribution of modern topography to the ophthalmologist is that it can now evaluate anterior and posterior surface curvature to determine accurate corneal power. Antiquated Placido disk evaluations that solely measure the anterior corneal surface to decide corneal power are fraught with inaccuracies and can lead to postoperative surprises, especially when dealing with patients with any corneal irregularity.

Figure 6. Bimodal corneal power estimate in keratoconus making intraoperative biometry essential in determining IOL power.
COURTESY JACK HOLLADAY

Figure 7. Post-LASIK Pentacam evaluation with highly segmented axial and sagittal maps indicative of irregular astigmatism.
COURTESY JACK HOLLADAY

The Holladay Report

The Holladay Report includes calculations for an Equivalent K65 metric, which can lead the surgeon to make more informed decisions regarding lens power and its characteristics. The report allows for improvement in calculation of IOLs for spherical and toric IOLs and is especially critical when evaluating patients with prior refractive surgery or abnormal corneas.

The Holladay equivalent keratometry reading (EKR65) or the true net corneal power can be accessed in the Pentacam Holladay Report display and the Holladay EKR Detail Report, which displays values for 1.0 mm to 7.0 mm zones.

An important part of the detailed display is the EKR distribution graph, which can be a predictor of the accuracy of anticipated lens power calculation.⁸ Not only does this report allow for a more refined corneal power calculation, but it also alerts the surgeon to the total spherical aberration (summing 4th-, 6th- and 8th-order Zernike spherical aberration terms) of the cornea allowing for the selection of better matching IOL asphericity to avoid lens miscalculations. The surgeon is best advised to select a lens that will allow for small amounts of negative spherical aberration to allow for better visual function at near. When there is good agreement, the surgeon can rely on the power calculation without modification.

In the case of a cornea with an ectatic disorder, the EK65 shows a bimodal power calculation indicating that the precise lens power calculation and toricity would be best determined with intraoperative biometry rather than attempting to estimate requisite lens power. In this post-LASIK evaluation the axis display on sagittal and tangential maps is segmented making axis determination for a toric IOL difficult. In such post-LASIK cases, it is best to match the EKR65 calculation to the best estimate of the patient’s pupil size as determined by pupillometry.

Figure 8. Page 2 of post-LASIK evaluation reveals a wide variation of corneal power. Lens selection would best make use of intraoperative aberrometry to identify the best choice in corneal power to select in calculating IOL power.
COURTESY JACK HOLLADAY

As corneal topography technology advances, the general ophthalmologist will find the information generated by his office topographer as an increasingly indespensable aide in patient management. The cornea is the greatest focusing component of the eye, and a better understanding of its shape and power will be essential in our understanding of vision and it rehabilitation. OM

Contributing Editor Vanessa Caceres assisted with the reporting for this article.

REFERENCES

About the Author
	Dr. Fox is medical director at the Cornea and Refractive Surgery Practice of New York and at Clarity Refractive Services of West Orange, N.J. E-mail him at foxmd@laserfox.com or by phone 917-207-3147. His website is www.laserfox.com.