IOL Calculation Quiz: Ultra-High Myopia

Happy New Year! We are introducing a new feature to CataractCoach.com for 2019: critical evaluation of biometry, IOL calculations, and surgical planning.

We will start with a reader submission. A subscriber to the daily posts at CataractCoach.com asks for help in calculating the best power IOL for this patient. And it is not a simple case.

The patient is 58 years old and has a life-long history of ultra-high myopia and right eye dominance. He wears contact lenses with a goal of plano OD and -1.5 OS. Both eyes started developing a mild posterior subcapsular cataract when he was last seen about a year ago. Shortly after, he suffered from a rhegmatogenous retinal detachment in the right eye and underwent a pars plana vitrectomy and scleral buckle combination surgery. The retina is now attached and stable. The right eye developed a nuclear cataract and that has also caused a myopic shift. The scleral buckle increased the axial length of the right eye due to scleral compression and the nuclear sclerosis induced a myopic shift from the change in refractive index of the crystalline lens.

Today the refraction is:

• OD: -23.00 +2.75 x 87 giving 20/100 vision (3+ NS, 2+ PSC cataract)
• OS: -14.00 +3.50 x 100 giving 20/50 vision (1+ND, 2+ PSC cataract)

The biometry shows:

• OD: Axial Length: 31.82 mm
• OD: Keratometry: 44.35 x 174 and 46.49 x 84 (astigmatism 2.14 D at 84)
• OD: corneal topography and tomography consistent with keratometry
• OS: Axial Length: 28.46 mm
• OS: Keratometry: 43.77 x 14 and 46.55 x 104 (astigmatism 2.78 D at 104)
• OD: corneal topography and tomography consistent with keratometry

The critical questions are:

1. What method do we use to do the IOL calculations? Which formula(e)?
2. What is the post-op goal for each eye? The patient wants plano OU.
3. What type of IOL for each eye? (patient is in USA, so only FDA-approved IOLs)
4. How do you treat the corneal astigmatism?

For the IOL power calculations: Do not use any of the formulae listed on the biometer print-out. These older methods will almost certainly lead to post-op hyperopia and that is something that we want to avoid in a myopic patient. We have previously discussed the best way of doing IOL calcs in patients with axial lengths of 30 mm or greater. In this case, we will use the Ladas Super Formula 2.0 with Artificial Intelligence which is available free to all ophthalmologists at www.IOLcalc.com

Post-Op Target: From this calculation you can see that we have chosen a target of -1.50 for both eyes to ensure that the patient will end up on the myopic side for the post-op refraction. Taking a patient from -14 (or more) to -1 or -2 is an amazing improvement for the patient. Do not aim for plano. In addition, if the patient truly desires a plano outcome we can do LASIK, PRK, or another corneal laser procedure like SMILE. In addition, we want the patient to end up myopic so that if we do an ablation for the astigmatism, we are doing a pure myopic cylinder and not a mixed ablation. Look at this example:

• If the post-op Rx is: -1.00 +2.00 x 90 (spherical equivalent of plano), this is the same as +1.00 -2.00 x 180. So the excimer laser will try to steepen one axis and then flatten the other axis which is an ugly ablation and less accurate.
• If the post-op Rx is -2.00 +2.00 x 90 (spherical equivalent of -1.00), this is the same as 0.00 -2.00 x 180. So now the excimer laser has an easy job of ablating a myopic cylinder which is quite accurate.
• Even if the post-op Rx is more myopic like -3.00 +2.00 x 90 (spherical equivalent of -2.00), this is the same as -1.00 -2.00 x 180. This is also an easy ablation for the excimer laser: it first treats the -1.00 sphere by flattening the corneal in all directions, then it ablates the myopic cylinder and the results are accurate.

The best IOL choice would be a monofocal IOL and one made of acrylic. The patient has a risk of another retinal detachment in the future given the prior detachment and the axial myopia. In a case like this, even after a perfectly performed cataract surgery, the patient should be seen regularly by a retinal specialist due to the risk of another detachment .

For the right eye, I would choose an IOL power of -1.0 which should give a post-op refraction of about -2 spherical equivalent. There is no toric IOL in the USA that comes in negative powers, so I would choose a monofocal, non-toric, three-piece acrylic IOL in this power.

To treat the astigmatism: After healing from the cataract surgery, we would do LASIK as described above to deliver the plano result desired by the patient by treating the myopia and astigmatism.

For the left eye, I would learn from the patient’s healing response and further hone the IOL calcs for this eye. Using a toric IOL in a power of +6.0 would give us a post-op refraction of about -1.25 in this left eye. This would give the patient about the same degree of monovision that he was used to in his contact lenses. Doing a second procedure such as LASIK to fine-tune the refractive result would not likely be needed.

What are your thoughts on this case? How would you proceed?