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It is never a good sign when a technician says a patient is not happy when exiting the exam lane. I do not like to go in these rooms. Having worked in the refractive surgery world, I know how to calm most patients down.
In most cases, the surgery did not meet the patient’s expectations.
Reasons for this include:
• Post-operative pain
• Blurry vision
• Dry eyes
• Increased ocular surface disease
• Patients saying, “It was expensive, “My result was not perfect” and “I can’t read”
In this case, my patient was returning one week after his cataract surgery, and he had undergone LASIK 15 years ago. My patient reported he could read. Because distance vision was the goal, he was upset. The patient’s previous LASIK complicated his intraocular lens (IOL) calculations.
At one week after the second eye, he was 20/20 at near and computer but unable to see far away.
Surgeons strive to minimize postoperative refractive errors using modern biometry, and some patients pay extra for intraoperative aberrometry to reduce the risk of residual refractive error.
Patients with a history of refractive surgery are more likely to suffer from residual refractive error after cataract surgery.
Here are nine reasons patients may not receive the optimum IOL power.
1. Poor ocular surface affecting corneal measurement
Corneal curvature is required for IOL calculations. Unstable tear film, keratitis, dry eye, and epithelial basement membrane dystrophy (EBMD) are common causes of unstable and— therefore—erroneous corneal curvature measurements.
Ocular surface disease should be addressed prior to referral for cataract surgery.
2. Contact lens wear was not discontinued for enough time
If the cornea does not return to its natural shape prior to measurement, the corneal power will be inaccurate.
Note that contact lenses must be removed prior to measurement and not the actual surgery day because the corneal power measurement is typically performed prior to surgery.
Gas permeable contact lens patients should wear glasses for several weeks prior to measurement for best results. Repeated keratometry and/or topography measurements should be stable prior to being referred for IOL measurement testing.
3. Corneal measurements performed after cornea was touched with Goldmann tonometer
Goldmann tonometry changes the corneal power. All IOL measurements should be performed prior to administering drops or touching the cornea. See Table 1.
4. Extremely short or long axial lengths results in calculation errors
The average axial length is 23.30 mm. Errors may occur using IOL calculation formulas in short eyes (<22 mm) and long eyes (>25 mm).
Axial length errors are amplified in short eyes. Posterior staphyloma results in elongated axial lengths, leading to postoperative hyperopia. See Table 1.
|Factor||Error||Error in postoperative refraction (D)|
|IOL power||1.00 D||0.67D|
|Axial length||0.50 mm||1.25D|
|IOL position||0.50 mm||1.00D|
5. Shallow or deep anterior chambers are directly related to effective lens position (ELP)
ELP is the distance of the principal plane of the IOL from the cornea. A deep chamber in a hyperope will introduce error into the measurements. See Table 1.
6. Keratorefractive surgery (KRS, such as radial keratotomy, Lasik, or PRK) results in reduced precision of corneal power measurement
Biometry for IOL calculations assumes the natural spherical curvature of the cornea, which is directly altered by KRS.
Changing the central cornea shape, and possibly the posterior corneal curvature, alters this math. Most of our vision occurs within the central effective zone, which many devices do not measure.
The greater the amount of excimer treatment (and greater the change in corneal curvature), the greater than error.
Theoretically, myopic treatments require an increase in IOL power. Hyperopic treatments require a decrease in IOL power. RK incisions typically cause a hyperopic result, increasing with the amount of incisions.1 See Table 1.
6. The IOL is inserted upside down
Inadvertent insertion of a posterior chamber intraocular lens with a reversed front to back orientation will result in residual refractive error in a single-piece IOL.
7. Unpredictability of final effective lens position
ELP in the original IOL calculation formulae was a constant because most implants were anterior chamber lenses.
Posterior chamber lenses sit farther back, introducing more error into the calculations.
8. Even in a perfect situation, there are limitations to the precision and accuracy of biometric measurement
This testing is typically performed by technicians, who may not be detailed-oriented. The tests are performed on living, breathing patients in a clinical environment that is often busy.
1. Ladi JS. Prevention and correction of residual refractive errors after cataract surgery. J Clin Ophthalmol Res. 2017;5:45-50.
2. Amon M, Kahraman G. Enhancement of refractive results after intraocular lens implantation. European Ophthalmic Review. 2011,5(1):59-61.