Doctor X was much excited to implant his first presbyopia correction lens, and so was the patient. Together they chose one of the best lenses, that promised to give the opportunity to junk the glasses that the patient hates to wear. With the best optical biometry done with an experienced biometrician, and all necessary tests done to rule out any ocular pathology that may hinder the patient having a sub optimal outcome, nothing but the best outcome was predicted for the patient.
The post operative outcome surprisingly, turned out to be a dampner. The patient landed up with a myopic surprise of a -1.50 D spherical. **The doctor is now in a dilemna, should he go for the second eye targeing a refractive number that would balance the surprise in the first eye ? Or should he first try to find out what went wrong in the first eye, so that he is sure that a mistake, if there at all had been in biometry of first eye, would not be repeated again. Doctor X, decided to go for the second option, deciding to take the pain of investigating what went wrong.
To start with, and within the scope of this write up, there would be four broad aspects worth looking into. First, where is the IOL sitting in the eye or the Effective Lens Position (ELP). Second, was the axial length rightly measured ? Third, Was the right IOL power implanted? and last, the changes in corneal curvature post surgery.
Note, for a thorough examination, a dilated pupil may be necessary to check for any decentration/IOL position in bag, bag distension, retained viscoelastics that could lead to myopic shift, or retinal challenges like post operative macula edema that could lead to a hyperopic shift. In their article, Refractive changes and visual quality in patients with corneal edema after cataract surgery, Ajenjo et al shows a hyperopic shift post significant corneal edema. The authors hypothesize a decrease of corneal power that could be explained by a increase in central corneal thickness and a corresponding lowering of conreal refractive index. However, before dilatation drops are administered, pseudophakic measurement should be done to avoid dilatation drops interfering with corneal readings.
1. ELP - Biometry is not about measuring the right IOL power. It is at best, only half of the story. Even, a right IOL power implanted may give a wrong result if it is not sitting at the right place in the eye. How far an IOL sits from the cornea, is typically called the ELP of the IOL, which has a direct impact on the post operative outcome. So is the IOL implanted sitting at the right place. A pseudophakic anterior chamber depth can provide an idea. Without this information, it may be impossible to understand the right cause of the unexpected outcome. In one such study, and on an average eye, a .1mm of deviation in the ELP was found to impact the final outcome by .19 dioptres for an average Axial Length. Pseudophakic anterior chamber depth, though in some cases is difficult to obtain, may have an answer to the unexpected outcome.
Now that you have done a pseudophakic measurement, and got a value of post operative ACD or the lens position in eye, how do you understand if this lens position is average, or may have forward or backward shifted to a good extent. In their study, Ning et al, helps have a nomogram to relate the pre operative ACD with the pseudophakic ACD.
My own Biometry Validation Tool, incorporates this nomogram to help validate the lens position after surgery.
2. Axial Length - Was the Axial Length of the operated eye in this case rightly meseaured ? Can we re measure the AL and see if the pre and post surgery values matching ? Absolutely. We should match pseudophakic AL with the phakic AL measured pre surgery. To do that we have to measure the AL in the pseudophakic mode. The velocity of ultrasound in the pseudophakic mode is 1532 m/s. All biometers employ a correction factor for the material of the IOL implanted - PMMA/Silicone/Acrylic. However, if this measurement is being done in ultrasound biometry, chances are that it will account for a PMMA IOL only. To make this pseudophakic AL measurement work for acrylic IOLs, you have to know the correction factor for the IOL implanted. On an average the correction factor for acrylic IOL is .2, meaning you have to add .2mm to the AL that you have measured on the pseudophakic mode.
However, to be precise on the correction factor of the specific IOL implanted, we need to know - a) the velocity of sound in the IOL, b) the central optic thickness of the IOL dioptre implanted. Armed with these two information, we can then calculate the correction factor that needs to be added on the pseudophakic AL measured.
Here is an example of psedudophakic Axial length being calculated taking into account the specific lens characteristics of the IOL implanted.
If for some reasons you have not been able to measure the pseudophakic Axial length at 1532 m/s then you can convert the pre operative phakic axial length (1555 m/s) to psuedophakic axial length(1532 m/s). The equation provided by Jack Holladay* is:
Once you have theoretically derived the pseudophakic Axial length at 1532 m/s you can now calculate the true axial length taking into account the central optic thickness and velocity of ultrasound in the specific IOL material, or you can add the general correction factor of .2 mm if you have used an acrylic IOL.
When psedudophakic Axial Length is measured the IOP should be also measured. This is because uncontrolled IOP rise can increase sceral stretching and cause axial length increase. Most patients may see a decrease in IOP immediately after cataract surgery.
Kiss B et al. IOVS 1999; ARVO Abstract 2379 showed an AEL shortening of 46 μm (range: 15–70 μm) with a pharmacologically induced reduction of IOP of 5.7 mmHg (range: 3.2–8.3 mmHg). Thus the resulting AEL decrease per mmHg IOP decrease was 6.5 μm/mmHg.
3. Right IOL Power - This is the most common question that customers have,once they face a refractive surprise. Time and again I have come across customers raising their eyebrows and asking if the labelled IOL power was right enough. Possible, but rare. With all companies investing millions of dollars in the manufacturing process, proper checkpoints are created to reduce this possibility to extremely rare incidence. But it is possible to find out if the IOL implanted in the eye, was the power that was originally intended to be implanted. To that extent, the only information that is required is the IOL power and corresponding central optic thickness from the manufacturer to start with. A pseudophakic measurement of IOL optic thickness (Lens Thickness, LT) should then help relate to the IOL dioptric power. One challenge is that, measuring psedophakic Lens thickness may itself be a challenge. Add to this, and due to the sensitive nature of measurement, any eye movement will lead to wrong measuremnt of central optic thickness and thereby refer to the wrong IOL power.
Nevertheless, a thorough investigation of refractive surprise is important to help us not repeat the same mistake again. Biometry is said to be the art of applying mathematics to biology. With this art being mastered, the world of presbyopia and astigmatism correcting lenses would immensely benefit patients.
4. Last but not the least, a marked change in average corneal curvature post surgery may also induce a refractive surprise. Roughly there exists a 1:1 relationshiop between corneal curvature and IOL power in biometry (Ladi, J.S. Prevention and correction of residual refractive errors after cataract surgery. J. Clin. Ophthalmol. Res. 2017, 5, 45). A steeper cornea owing to corneal incisions may result in a myopic surprise depending on the amount of steepening induced in surgery.
The purpose of this article is to step by step guide you in evaluating a patient post operatively to find out the cause of refractive surprise, if any. At the centre of it all, lies the need for a psedudophakic measurement after the eye has settled down. Preferably, one month after the surgery, or as deemed fit by the physician, a pseudophakic measurement may help understand the reason of refractie surprise and help plan the other eye.
(This article is updated in December 2022 with pictures from Anterion, Heidelberg Engineering incorporated)
**Note: My own second eye IOL calculator based on first eye refractive surprise helps you to target the second eye based on refractive error of the first eye. It is also availabe as app on request https://www.quickguide.org/post/second-eye-iol-power-calculator-based-on-first-eye-prediction-error
1 Anterior chamber depth — a predictor of refractive outcomes after age-related cataract surgery, Ning et al, BMC Ophthalmology (2019) 19:134
2. Standardizing constants for ultrasonic biometry, keratometry, and intra ocular lens power calculations - Jack Holladay, J Cataract Refractive Surery- Vol 23, Nov 1997
3. Refractive changes and visual quality in patients with corneal edema after cataract surgery
Mª Amparo Díez-Ajenjo, BMC Ophthalmology volume 22, Article number: 242 (2022)