Correcting High Myopia at the Nodal Point

Patients with high myopia who are interested in spectacle or contact lens independence often visit our practice to inquire about LASIK. Many of them, however, are not interested in the procedure specifically, but instead in a specific result. That is, high-quality vision achieved safely and predictably. They do not want to choose from a list of procedures. They want us to employ our expertise and skills to recommend the procedure that is best suited to them and that provides them with their desired result.

An implantable collamer lens (ICL) is our first choice over other phakic IOLs because of the postoperative visual quality and low risk for complications, especially since the introduction of the EVO Visian ICL (STAAR Surgical), which was approved by the FDA in March.

A REDESIGNED PHAKIC LENS

ICLs have been implanted in more than 2 million eyes in 75 countries since the technology’s introduction in the 1990s.¹ Widespread acceptance of early ICLs was limited by a risk of pupillary block and anterior subcapsular cataract formation. These challenges have been addressed in the design of the EVO Visian ICL. A similar version of the EVO lens, the Visian V4c (STAAR Surgical), has been available worldwide since 2011.

A centrally placed 360-µm port maintains the normal physiology of the anterior segment of the eye and facilitates aqueous flow from the posterior chamber to the anterior chamber. This creates a protective cushion of aqueous between the phakic IOL and the crystalline lens, which increases safety and substantially reduces the implant’s risk profile.^2,3 The central port obviates the need for a preoperative peripheral iridotomy and all but eliminates adverse events related to vault, including anterior subcapsular cataract formation, pupillary block, and the development of angle- closure glaucoma.⁴

Based on data from 27 peer-reviewed articles drawing on effectiveness outcomes from retrospective and prospective case series that included 1,905 eyes with a weighted average follow-up period of 12.5 months, the weighted average uncorrected distance visual acuity was -0.02 logMAR after EVO implantation.^5,6 In 38 peer-reviewed articles that presented safety outcomes from retrospective and prospective studies that included 4,196 eyes with up to 5 years of follow-up, no patient developed a visually significant anterior subcapsular cataract. Only one eye experienced pupillary block, and this was associated with a retained ophthalmic viscosurgical device.⁴

Benefits

The EVO Visian ICL is approved to treat 3.00 D to 20.00 D of myopia and 1.00 D to 4.00 D of astigmatism. Based on the technology’s safety profile and optical quality, our threshold for recommending ICL surgery rather than LASIK has dropped significantly. We routinely recommend the EVO to patients with -5.00 D or -6.00 D of myopia.

One advantage of the lens is that it can be removed and exchanged, if necessary, although none of our patients to date has been dissatisfied with their result. Removability is also a benefit with respect to future cataract surgery, and keeping the cornea intact gives patients more IOL options.

Unlike with corneal refractive procedures, corneal thickness is not an issue with the EVO Visian ICL.⁷ In addition, there is minimal concern regarding dry eye disease and no potential for corneal ectasia.⁸

Another critical advantage of the lens is that it corrects vision at the nodal point of the eye, whereas LASIK and photorefractive keratectomy correct vision at the corneal plane. Taking into consideration the differences between contact lens wear and spectacle wear can foster an appreciation for the optical principle involved. Correcting high degrees of myopia with a contact lens is optically advantageous compared to correcting it at the spectacle plane because of vertex distance and, more specifically, the distance between the refractive correction and the ocular nodal point. When this concept is applied to refractive surgery, retinal image minification can be reduced even further because the nodal point of the eye sits near the crystalline lens—roughly 17 mm anterior to the retina in an average eye.^9,10

COLLABORATIVE CARE

Eliminating the requirement for a peripheral iridotomy has streamlined our clinical and surgical process for the EVO Visian ICL. First, a prescreening evaluation is performed that includes a dilated examination and refraction. A key element of the initial evaluation is measuring the anterior chamber depth, which must be at least 3 mm for the patient to be a candidate for ICL surgery. OCT imaging, topography, and axial measurements are obtained to determine the appropriate ICL size and rule out subclinical pathology (see Patient Management Tips).

On the day of surgery, the patient receives intravenous twilight sedation. ICL implantation takes about 10 minutes, and the patient is seen 4 to 6 hours postoperatively. If IOP elevation is detected, it is relieved by manually burping the wound with a sterile instrument, by initiating therapy with a topical hypotensive medication, or both.

Patients return for a routine evaluation 1 day after surgery. The vault of the lens is assessed and compared to the thickness of the cornea. Assuming the cornea is about 500 µm thick, the vault space should be about 100% of the thickness of the cornea or between 250 µm and 750 µm (Figure). A vault greater than 750 µm raises the risk of pupillary block, and a vault less than 250 µm raises the risk of cataract formation.

Figure. Right (A) and left (B) eyes show excellent vault of the EVO Visian ICL over the natural crystalline lens. The ratio of the vaulted space to the corneal thickness should be 1:1, which is visible in each scan. This indicates approximately 500 µm of clearance, which is ideal.

Patients are seen again at 1 week and at 1 month. Minor fluctuations in the refraction may continue for a few weeks after surgery, but the refraction should be stable at the 1-month postoperative visit. During this visit, anterior segment OCT imaging is obtained to confirm that the angles are not at risk of occlusion. If a toric model of the EVO was implanted, its orientation is checked.

1. STAAR Surgical celebrates two million lens milestone for implantable collamer lens (ICL). Business Wire. June 7, 2022. www.businesswire.com/news/home/20220607005138/en/. Accessed September 28, 2022.

2. Fujisawa K, Shimizu K, Uga S, et al. Changes in the crystalline lens resulting from insertion of a phakic IOL (ICL) into the porcine eye. Graefes Arch Clin Exp Ophthalmol. 2007;245(1):114-122.

3. Shiratani T, Shimizu K, Fujisawa K, Uga S, Nagano K, Murakami Y. Crystalline lens changes in porcine eyes with implanted phakic IOL (ICL) with a central hole. Graefes Arch Clin Exp Ophthalmol. 2008;246(5):719-728.

4. Packer M. The Implantable Collamer Lens with a central port: review of the literature. Clin Ophthalmol. 2018:12:2427-2438.

5. Liu T, Linghu S, Pan L, Shi R. Effects of V4c-ICL implantation on myopic patients’ vision-related daily activities. J Ophthalmol. 2016;2016:5717932.

6. Shimizu K, Kamiya K, Igarashi A, Shiratani T. Intraindividual comparison of visual performance after posterior chamber phakic intraocular lens with and without a central hole implantation for moderate to high myopia. Am J Ophthalmol. 2012;154(3):486-494.e1

7. Parkhurst GD, Psolka M, Kezirian GM. Phakic intraocular lens implantation in United States military warfighters: a retrospective analysis of early clinical outcomes of the Visian ICL. J Refract Surg. 2011;27(7):473-481.

8. Vargas V, Alió JL, Barraquer RI, Antin JC, et al. Safety and visual outcomes following posterior chamber phakic intraocular lens bilensectomy. Eye Vis (Lond). 2020;7:34.

9. Keating MP. Geometric, Physical, and Visual Optics. 2nd ed. Butterworth Heinemann; 2002.

10. Milder B. Optics of human eyes. Ocular Surgery News. March 23, 2011. Accessed September 13, 2022. www.healio.com/news/ophthalmology/20120331/optics-of-human-eyes#:~:text=The%20average%20eye%20has%20an,in%20front%20of%20the%20retina