Are different diagnostic instruments interchangeable when it comes measure corneal HOAs in eyes with keratoconus?

Why do stars sometimes “twinkle” in the sky? Atmospheric turbulence: the light that comes from the star gets distorted by particles in the atmosphere. It is the same with light entering the eye: it passes through the cornea and the lens (the “optics”) and is focussed on the retina (the neurological part) where the light is converted into nerve impulses that are transmitted up into the brain. Imperfections – particularly in the cornea, which is responsible of 80% of the eye’s focusing power – can then distort the light in many different ways. Some wholesale imperfections in the system (like myopia, hyperopia and astigmatism) are quite obvious, and are termed “lower-order aberrations”, whereas others tend to be only noticeable at night, and have terms like “trefoil, coma and spherical aberration”, and are referred to as “higher order aberrations” – HOAs. The more complicated the shape that the light gets distorted into, the higher the order of aberration (Figure 1).

Figure 1. Common aberration shapes created when a “wavefront” of light passes through eyes with imperfect vision. A theoretically perfect eye (top) is represented by an aberration-free flat plane known as a “piston”. Each row further down from the top represents an increasingly “higher order” (i.e. more complex) aberration.

But when glare and halos around street-lamps and car headlights at night (two examples of HOAs) are unpleasant for most people who experience them, generally people can get by. However, when eyes have a large amount of HOA, then visual performance begins to be seriously compromised – and this is especially the case in people with keratoconus. Keratoconic eyes gave significantly more HOAs than “normal” corneas – so much so that some researchers are advocating using HOA assessments (as well as the usual corneal shape assessments made using corneal topographers) to screen for the early stages of keratoconus. Unfortunately, the more irregular the cornea, the more difficult it is for aberrometers to work.

Many clinics have corneal topographers that can also perform a corneal aberration analysis, and they work by determining anterior and posterior corneal elevation data (the shape of the top and bottom of the cornea). This is combined with a technique called ray-tracing, and the resulting data is represented as something called the “Zernike polynomial” – as seen above in Figure 1.

But these topographers take different approaches to make these measurements – one popular instrument uses “a rotating Scheimpflug camera” (Oculus Pentacam HR), whereas another popular instrument uses “dual Scheimpflug technology combined with Placido disk-based reflection imaging” (Ziemer Galilei G4). While both approaches are known to result in broadly similar corneal topography maps, when it comes to HOA assessments, do these instruments produce equivalent results – especially in keratoconic eyes? In essence: are the results each instrument interchangeable? It is important to know this information – if an ophthalmologist with one instrument refers a patient to a corneal specialist with a different instrument, can the specialist assume that the patient’s previous new measurements would be consistent with the old ones?

A group of researchers, including the ELZA Institute’s Prof. Farhad Hafezi and Dr. Emilio Torres-Netto, set out to answer those questions by examining 50 eyes from 50 patients with clear evidence of keratoconus. Each eye was examined in the same patient visit by the same technician.

The investigators compared the third- and fourth-order Zernike polynomial data each instrument produced with a 6.0 mm pupil, including:

  • trefoil at 0°,
  • trefoil at 30°,
  • vertical coma,
  • horizontal coma,
  • spherical aberration, and
  • total cornea root mean square (RMS),

In other words, they picked six parameters that describe HOAs that can significantly affect vision, and compared the values that each instrument produced for them. The way the researchers assessed whether the two approaches “agreed” with each other was to perform a “Limits of Agreement” analysis, which compares the likely differences between individual results measured by two methods, and the Pearson’s r coefficient (a measure of how correlated two variables are). They then performed a subgroup analysis and analysed these results by how steep the cornea is (a measurement termed Kmax, measured in dioptres (D), with a higher number equalling a steeper cornea). Three groups were defined: eyes with Kmax < 50 D, 50–55 D and > 55 D. So what did the investigators find?

They found that for most of the parameters assessed, the Pearson r correlations were generally quite low, and the Limits of Agreement were wide. To make a long story short, there were significant differences between measurements of corneal HOAs generated by Scheimpflug and dual Scheimpflug–Placido devices in patients with keratoconus. Stronger correlations with less significant differences were found in keratoconic eyes with less steep Kmax values – in other words, the steeper the cornea (which usually means the more progressed keratoconus is), the worse the agreement between the instruments.

The researchers concluded with “We do not recommend using measurements from these technologies interchangeably when assessing HOAs in eyes with keratoconus.”

Reference

Piccinini AL, Golan O, Torres-Netto EA, Hafezi F, Randleman JB. Corneal higher-order aberrations measurements: Comparison between Scheimpflug and dual Scheimpflug-Placido technology in keratoconic eyes. J Cataract Refract Surg. 2019;45(7):985-991.

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Augeninstitut ELZA
4.8
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mike
mike
09:02 04 Jan 23
Overall as a 50 year old, who wanted to continue to avoid wearing glasses and/or contact lenses, my goal was 100%... achieved with ELZA and Dr. Hafezi. I greatly appreciated the overall approach by Dr. Hafezi and his friendly and competent team in reviewing my vision at the start and following a comprehensive eye exam- the explanations of my options. For me I chose PRK in one eye for a monovision solution to correct short sightedness which was becoming more prevalent in night driving and sports such as skiing and cycling. PRK heals slowly but the outcome is worth the wait- and the cost- you use your eyes every waking moment- and for me I cannot see any reason not to pursue better vision with the ELZA team! A great quality of life enhancement!ler mais
Zuzana Fecko
Zuzana Fecko
21:48 31 Oct 22
Thank you so much, Dr. Torres and prof. Hafezi. 26 years I was only “dreaming” about the eye surgery due to possible... complications. I was worried ..Thank you for encouraging me to make a decision and thank you for your professionalism, explanations and great work!I wish you all the best, lots of success and only happy patients!ler mais
Serena Fazio
Serena Fazio
11:15 06 Mar 22
I have been to the ELZA clinic to have surgery for my miopy, as my eyes were rejecting the contacts and I was tired of... wearing glasses. From the first preliminary visit, to the actual surgery and post-surgery care Dr. Hafezi and all his collaborators have been extremely professional, considerate and helpful. All the steps of the (very short!) surgery were thoroughly explained to me in order to be as calm as possible, together with the medication plan and precautions to be aware of so that my eyes could heal in the best way possible. By following their exact instructions, the healing process has been very successful and smooth, and after about one week I was able to see perfectly. I absolutely recommend this team and their expertise!ler mais
Robin A
Robin A
11:43 16 Feb 22
Top experts and professional team. Highly recommended
Thunder Shiviah
Thunder Shiviah
14:05 23 Nov 21
After some research on local options I decided to go with ELZA since it stood out as the best. The operation and the... post-operation recovery went extremely smooth (something I was worried about with PRK). Now my vision is much better than I had hoped and I have no side effects such as dry eye or night vision problems. Thank you!ler mais
Thomas S.
Thomas S.
09:37 07 Jul 21
My left eye had to be operated on because of a cataract. As I had an Artiflex lens implanted a few years ago, it had to... be removed first before a modern lens could be implanted to correct the cataract. Prof. Hafezi treated me in an very competent and friendly manner. I am very happy with the result.ler mais
Selma A. Rahim
Selma A. Rahim
19:45 02 Jul 21
Dr. Hafezi.. the KING OF CXL🤴
Gianluca Ricci
Gianluca Ricci
12:46 10 Apr 21
Great team and service!
rehaneyecare
rehaneyecare
05:36 08 Apr 21
Excellent surgeon
Steven H.A
Steven H.A
16:13 11 Jan 21
Prof. F. Hafezi s expertise is enormous and he has a very pleasant and calm nature which gives the patient... security.Thank youler mais
Online Commande
Online Commande
09:19 05 Dec 20
Totally creepy, very arrogant Prof.He just want your money.It may explain, why this Prof. is not in the center of the... city of Zürich and why he has been thrown away from University of Geneva.ler mais
Elena Churilova
Elena Churilova
10:06 17 Nov 20
At the moment, 3 months have passed since the moment I made laser vision correction in this clinic. and I can say that... I am happy that I chose this particular clinic and Prof. F. Hafezi. The entire staff of the clinic is very friendly, treats patients with sympathy and patience if they are nervous before surgery (my big gratitude to Aida Alili for all support and patience). The doctor Hafezi and other doctors of the clinic are very professional and ready to explain to you as many times as you need. I can only recommend!ler mais
Michele Aggiato
Michele Aggiato
18:17 10 Jan 20
The day I had Prof Hafezi checking my keratoconus I knew I was finally in the right place after years of searching for... a good ophthalmologist. Prof Hafezi recommended a surgery due to a drastic degeneration of my cornea. The surgery was performed by himself with the assistance of Dr Rodriguez, one more collaborator of Prof Hafezi’s team who got my full trust.ler mais
Ashley Faulkes
Ashley Faulkes
15:30 21 Mar 16
Professor Hafezi and the team at the ELZA Institute have done an amazing job of creating a first class eye clinic, with... cutting edge technology and support. I went there for a dry eye issue and other issues with my prescription, and they were easily able to help me and put me on the road to better vision! High recommend them to anyone in the Zurich/Aargau area.ler mais
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