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Patient candidates for SMILE: Punt or treat
A variety of factors must be weighed to determine if a patient should be considered for this new corneal refractive surgery.
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by John F. Doane, MD
Whenever a surgeon identifies whether a patient is the right candidate for a refractive procedure, he or she must ensure that the entry criteria by which the patient has been evaluated has successful results. The goal is to identify prospective patients who will yield equivalent results in their surgery to their testing. The most obvious standard by which I decide to punt or treat patients with small incision lenticule extraction is determining whether the patient has had any pre-existing anatomy or physiologic conditions that may preclude them from achieving optimal results, not only in the short term but also in the long term. As with other refractive procedures, a detailed eye examination determines whether a patient is a suitable candidate, and there are many conditions that may present themselves. Here are the most common, along with my take on whether I punt or treat.
Spherical myopia: Treat
If a patient has mild (1 D to 3 D), moderate (3 D to 6 D) or marked (6 D to 10 D) myopia, SMILE may be an option. Presently in the U.S., only spherical myopia is approved. Compound myopic astigmatism clinical trial cases are complete, including 1-year postoperative examinations. The clinical trial module has been submitted to the FDA for consideration for approval to treat patients commercially. My unofficial best guess for commercial availability would be later 2018 or early 2019 if prior timelines are used as reference.
Presbyopia: Treat
Corneal refractive surgery is ideal for anyone who is pre-presbyopia, barring any anatomic or corneal physiology concerns. For anyone who is currently presbyopic, it is important to determine what the patient hopes to achieve. Does the patient want both eyes for distance? Will they be happy with glasses for near tasks? Conversely, can the patient tolerate monovision? If so, then SMILE-inducing monovision can be an acceptable option, much like it is with excimer laser-based procedures.
Hyperopia: Punt
SMILE is currently not a treatment option for hyperopia in the U.S. Ongoing investigations outside the U.S. are encouraging, and I believe we will have a much better idea on the efficacy of these treatments in the next 1 to 2 years.
Astigmatism: Treat
SMILE is approved in the U.S. to correct no more than 0.5 D of astigmatism. If a patient has 0.25 D of astigmatism, I feel comfortable treating the spherical equivalent. If a patient has 0.5 D of astigmatism and the steep axis is somewhere between 50° and 130°, I feel completely comfortable treating the spherical equivalent of this patient’s refraction. If the patient has 0.5 D of against-the-rule (ATR) or oblique astigmatism, I do not treat with SMILE and will perform traditional LASIK or PRK. The rationale behind this is with-the-rule astigmatism tends to decrease with time, so the patient should do well with SMILE. On the other hand, ATR astigmatism tends to increase with time, so leaving 0.5 D ATR untreated means that it will likely increase over time, and the patient will seek an enhancement. Using excimer laser treatments to correct 0.5 D of ATR astigmatism is the better choice in my opinion.
Thin, thick or steep corneas: Treat and punt
When it comes to thin, thick or steep corneas, my rule of thumb is that anybody that is a candidate for LASIK in terms of anatomy and physiology is a candidate for SMILE. However, there are certain cornea instances in which I will not treat a patient with SMILE. First, if there is an inferior steepening of greater than 1.4 D, I exclude the patient from both LASIK and SMILE. Second, if the patient’s corneal thickness is less than 500 µm, I will not perform a LASIK procedure, but I will perform SMILE down to 480 µm. For corneas thinner than 480 µm, I will lean toward PRK or ICL (Staar Surgical). This approach may change as we gather more information on corneal stability with SMILE. Third, if the cornea is steeper than 46.5 D, I will not perform SMILE or LASIK and instead will perform PRK or ICL.
Old age: Treat
I have treated patients older than age 50 years with SMILE. If a patient older than 50 years wants distance vision for both eyes and is happy with reading glasses, they are a candidate for SMILE. Lower corrections are more challenging because the lenticule is so thin, yet, as above, SMILE interrupts fewer anterior corneal nerve endings, so it may in fact be a better option with regard to dry eye postoperatively. With more experience, I believe this may be proven to be the case, and the literature to date certainly points to this conclusion.
FFKC topography suspects: Punt
I do not perform SMILE on patients with forme fruste keratoconus (FFKC) topography patterns. The SMILE procedure is not a guarantee to prevent ectasia as compared with LASIK, and it should not be seen as such. If a patient is a LASIK candidate based upon topography, he or she is likely a candidate for SMILE. SMILE as a lamellar procedure is a weakening procedure, and like LASIK it should be avoided in FFKC topography cases. There have been numerous published reports on biomechanical superiority of SMILE to LASIK, and I might sleep better knowing I performed SMILE for biomechanical concerns, yet it is not fail-safe in the face of FFKC. Interestingly, surgeons outside the U.S. have combined LASIK and cross-linking simultaneously for suspect cases. The same has been done with SMILE. Surgeons outside the U.S. have for the past 4 to 5 years performed simultaneous SMILE procedures, soaked the SMILE interface with vitamin B2, and then completed epithelium-on cross-linking. In the future, this may be a possibility in the U.S., and we will continue to learn from our international colleagues.
Corneal scar: Punt
I am cautious to treat any patient who has a corneal scar with SMILE. If the scar is inside the 6.5-mm optical zone, which is where the lenticule is extracted, the scar can cause optical breakdown to efficient photodisruption. A patient with a central scar, such as a corneal ulcer, also provides an environment in which photodisruption is highly variable for both the refractive and cap photodisruption steps. Flap creation with LASIK is difficult and discouraged by many, let alone a double pass of the femtosecond laser to create a lenticule. Additionally, if a patient had previous LASIK, I do not perform SMILE as creating a lenticule may pass pre-existing interface, and fragments of tissue may be left behind, leading to irregular astigmatism. In most instances, PRK or no surgery at all may be the better course.
Conclusion
SMILE is the new kid on the block in the corneal refractive surgery arena. Even considering its reported advantages of being minimally invasive and maintaining the biomechanical stability of the cornea, there are still pathologies that may discount SMILE as the best option for a patient seeking refractive surgery. As with all refractive surgeries, patients should undergo a detailed eye examination to determine whether they are suitable candidates for SMILE or whether they should consider other options.
References:
Denoyer A, et al. Ophthalmology. 2015;doi:10.1016/j.ophtha.2014.10.004.
Schallhorn S. Keynote: The year in big data. American-European Congress of Ophthalmic Surgeons 2017 Winter Symposium; Feb. 26, 2017.
For more information:
John F. Doane , MD, can be reached at Discover Vision Centers, 4741 S. Cochise Drive, Independence, MO 64055; email: jdoane@discovervision.com.
Disclosure: Doane reports he is a consultant to Carl Zeiss and the recipient of payment for research overhead for the FDA clinical trial of SMILE for myopia and myopic astigmatism.