[From CataractCoach.com – see below for full video] Cataract surgery is arguably the most powerful refractive surgery because the new lens implant can correct just about any degree of hyperopia, myopia, astigmatism and even presbyopia at the time of surgery. Highly myopic patients, with preoperative refractions of –10 D or more, are often the happiest because a lifetime of nearsightedness is finally cured with successful cataract surgery. However, these myopic eyes pose challenges and additional risks during surgery and in the peri-operative period.
Myopic patients often use their natural nearsightedness, and if they are corrected for plano they need to understand that their ability to see a few inches away from their face will be permanently lost. Because IOL calculations are less precise in eyes with extreme refractions, patients with high myopia should understand that while the cataract surgery can correct much of the myopia, its primary purpose is to correct the cataract, and the refractive effect is a secondary benefit. These patients may need a second surgical procedure to fine-tune the postoperative refractive result. In addition, there are increased risks of complications such as retinal lesions, which could limit the visual recovery.
(1) Be thorough in the pre-operative evaluation
Because of this increased risk for retinal complications, the preoperative exam should include a careful examination of the retina for any breaks, holes or weakness, as well as any macular pathology. The highly myopic patients may also have myopic macular degeneration, epi-retinal membranes or other significant changes. These may limit the postoperative vision achieved and may influence the development of postoperative complications such as cystoid macular edema. If any posterior segment issues are noted, referral to a vitreo-retinal colleague for treatment is recommended prior to cataract surgery.
In addition to the typical cataract evaluation, care must be taken to accurately assess the retinal status and measure the axial length of the eye. Highly myopic eyes often have a posterior staphyloma, which can generate an erroneously long axial length when measured with the standard A-scan ultrasound. This would cause an error in lens calculations and residual postop hyperopia, resulting in an unhappy patient. Using an optical method for measurement tends to be more accurate because it measures directly at the fovea.
(2) Be careful with the IOL power calculations
The IOL calculation methods, particularly the two-variable formulae, are less accurate at the extremes, and this is particularly true for very myopic eyes. Of the two-variable formulae, the SRK/T tends to perform somewhat better, as do more complex formulae such as the Haigis and Holladay 2.
My recommended IOL calculation method for highly myopic eyes is to use the www.IOLcalc.com website, which is free for all ophthalmologists, and aim for at least a little post-op myopia.
Certain IOLs have a significantly different design, and therefore A-constants at lower IOL powers and minus IOL powers. This variability in the A-constant of the IOL across these power ranges accounts for some of the difficulty in achieving accurate postoperative results. In general, the A-constant increases as the IOL power is lower, which leads to selection of a higher dioptric implant to lessen the odds of a postoperative hyperopic surprise. In other words, there is a strong tendency for a postoperative hyperopic surprise in these patients, so aim for some residual postoperative myopia.
A postoperative refractive goal of a mild amount of residual myopia, such as –0.5 D to –1 D, can be helpful to avoid a hyperopic surprise. In addition, some patients prefer to be left even more myopic such as -2 or -3 in order to emphasize the near vision. Remember that -1 has an optimal focal point of 1 meter, -2 is 50 cm, and -3 is 33cm. Taking someone from -14 D of myopia to -2 D and fixing the cataract and astigmatism is a magical result for these patients and may be preferred to plano.
(3) Fix the Reverse Pupillary Block during Cataract Surgery
The advantage of cataract surgery in myopic patients is the larger anterior chamber depth, which allows more working room during phacoemulsification. However, the infusion pressure from the phaco handpiece can cause overinflation of the anterior chamber and a tendency to push the entire lens-iris diaphragm posteriorly. With an overly deep anterior chamber, surgery becomes difficult and uncomfortable for both the surgeon and patient. To address this issue, the infusion pressure can be decreased by lowering the bottle height; however, this will result in less inflow of fluid and a higher tendency for surge.
A better solution that I learned from Professor Robert Osher MD is to break the reverse-pupillary block by making sure that there is fluid flow under the iris to equalize the anterior and posterior chamber pressures. By neutralizing this pressure gradient, the cataract will not be pushed so deeply within the eye and adequate infusion pressure can be used. I prefer to use the chopper to slightly tent up the iris at the pupillary margin to establish a channel for anterior-posterior fluid flow. Alternatively, a single nasal iris hook can be placed for the duration of the surgery.
Myopic patients are at a higher risk for post-op retinal detachment if there is tension or traction on the vitreous base during surgery. The primary culprit is allowing the anterior chamber to collapse when removing the phaco probe or irrigation and aspiration probe from the eye. Once the anterior chamber collapses from lack of infusion, the posterior capsule and vitreous have a tendency to move anteriorly, often quite abruptly and significantly. This can be avoided by one simple technique: Fully inflate the eye with viscoelastic via the paracentesis prior to removing the phaco probe or I&A probe from the eye. At the end of the procedure, once the IOL has been placed into the capsular bag, remove the viscoelastic completely and use balanced salt solution via the paracentesis to keep the eye pressurized as the I&A probe is withdrawn. These techniques will prevent collapse of the anterior chamber, increase patient comfort and lessen the risks.
The postop refraction in myopes can take time to stabilize due to the variation in effective lens position as the capsular bag shrink-wraps around the IOL. During this period, inflammation can be controlled using topical steroids and NSAIDs. During the postoperative period, a repeat dilated fundus examination is indicated to search for possible retinal breaks or weakness that may have been created during surgery.
Finally, keep in mind that there may be a large degree of anisometropia between the eyes, so performing timely surgery on the fellow eye will minimize the imbalance. While patients will be functionally emmetropic after bilateral cataract surgery, they will always have the elongated axial lengths and retinas that need to be followed on a regular basis and referred to a vitreoretinal colleague for any noted changes.
While cataract surgery in myopic patients can pose a variety of challenges, these patients tend to be among the happiest of all. In a safe, efficient surgery that takes just minutes, their cataract is removed, their myopia is treated and they can now enjoy a lifetime of excellent vision.
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