Tracy Schroeder Swartz, OD, MS, FAAO, Dipl ABO and Michael Duplessie, MD
It is not surprising that once surgeons enjoyed the stability and accuracy of keratorefractive procedures for distance correction, they would desire algorithms for presbyopia as well. Presbyopic corneal refractive treatments such as conductive keratoplasty have proven less successful and less stable compared to LASIK.1
Myopes enjoy a marked loss of uncorrected near vision with distance correction. One fear of treating myopic presbyopic patients with keratorefractive procedures for distance correction is exposing an accommodative reserve inadequate for presbyopic demands and, subsequently, dissatisfied patients. This is especially true in low myopes who are accustomed to reading without their glasses. One study reported increase in add power of 0.5 diopters (D) or more in 60.4% of patients post-LASIK, and significant correlations between increased add powers, age, and the amount of power corrected by LASIK.2
Unlike myopes, hyperopes enjoy an improvement in near vision after keratorefractive distance correction. One study of aspheric wavefront-guided LASIK in hyperopic presbyopes reported 100% of patients had achieved binocular simultaneous uncorrected vision of 20/25 or better and J3 (Jaeger) at 12 months. Negative spherical aberration highly correlated with postoperative improvement of distance-corrected near visual acuity.3
Several presbyopic treatments were developed utilizing the excimer and/or femtosecond lasers. These include monovision, multifocal, and multizone treatments.
Monovision is the correction of the dominant eye for emmetropia, while the nondominant is left myopic, allowing the eyes to see both distance and near. The brain must learn to alternate dominance and suppress the blur at each distance for this to be successful.4
Not all patients adapt well to monovision. Monovision is an imperfect solution due to loss of binocularity. A social and work history is vital in patient selection. For rigid personality traits or careers such as engineering, surgeons should proceed with caution, if at all. Reduced low-contrast visual acuity, overall contrast sensitivity, stereopsis, small-angle esotropic shift, and problems performing daily tasks that require good depth perception have been reported with monovision correction.5
Monovision should be demonstrated using a trial frame or contact lenses in patients without a history of monovision. If the patient can function comfortably wearing the trial frame demonstration, he or she typically will adapt well to monovision after surgery. Successful monovision contact lens wearers do not require an in-office trial, and the refractive endpoint of their habitual monovision is typically the target with refractive surgery (Figure 3-1).
The myopic target for the near eye is typically -1.5 D, depending on age, history of monovision, and typical working distance. Barisic et al6 reported LASIK monovision with a 0.5 D to -1.25 D target in the nondominant eye in patients under 50 years old yielded good vision at all distances without affecting stereovision. The greater the degree of image size disparity (aniseikonia), the more difficult the transition. Barisic et al6 found better distance and intermediate visual acuity with less glare and halos than the refractive lens exchange with a multifocal intraocular lens (IOL) group.
Modified monovision, a myopic correction of -0.75 D, is sometimes used for those working primarily on a computer. These patients require less accommodative power compared to a patient with a working distance typical of a conventional book and inability to change the font.
Patients are typically satisfied with monovision as long as the distance eye has surgical correction within 0.5 D of emmetropia.5 Due to the reliance on one eye for each distance, the enhancement rate is higher with monovision correction, typically 10% to 17%.5
Success rates reported in the literature vary, as the definition of success and methodology of testing visual acuity is not consistent. Rates range from 85% to 97% (Table 3-1). Advanced algorithms have been used to combine under-correction, now termed mini-monovision, with multifocal ablations to increase patient satisfaction.
This technique combines nonlinear aspheric ablation profiles with micro-monovision, using depth of focus to extend the range of functional vision at near. The MEL 80 excimer laser (Carl Zeiss Meditec) software uses algorithms to increase the depth of focus by increasing a specific corneal aberration Z (4.0). An increased Z (4.0) combined with small amounts of myopia reduce near myopic blur at distance and promote pseudoaccommodation with small pupils due to accommodation.7
The dominant eye is targeted for exactly plano, and the nondominant eye is targeted for slight myopia. The ideal target is -1.5 D, although this can be varied between -0.75 D to -1.75 D depending on age, degree of presbyopia, and patient tolerance. The increase in depth of field is such that the ranges of clear vision achieved by the distance and near eyes overlap at intermediate distances, unlike the traditional monovision approach in which there is a gap in the range of clear vision.
Figure 3-2 illustrates the treatment. This 54-year-old male patient presented with manifest refraction and corrected distance visual acuity (CDVA) of +1.00 -0.25 x 75 (20/16) in the right eye and +0.75 D (20/16) in the left eye. Presbyond Laser Blended Vision with the MEL 90 excimer laser and VisuMax femtosecond laser (both by Carl Zeiss Meditec) was performed using an intended target of -1.5 D for the right eye and plano for the left eye. One year after surgery, the patient’s binocular uncorrected visual acuity was 20/16 + 1 at distance, J2 at intermediate, and J1 at near. Monocularly, uncorrected distance visual acuity (UDVA) was 20/63 – 1 in the right eye, slightly better than expected for the manifest refraction of -1.50 -0.25 x 166 (20/16). In the left eye, UDVA was 20/16 – 1 with manifest refraction of 0.00 -0.25 x 64 (20/16). Subjectively, the patient had adapted to the cross-blur by 3 months and was spectacle independent. Contrast sensitivity was unchanged from before surgery.
A study of micro-monovision LASIK in myopic presbyopes evaluated 80 eyes of 40 patients, with a mean age of 43.4 years old, treated bilaterally using an aspheric micromonovision. Distance eye target was plano while the near target ranged from -0.75 D to -2.25 D. Ninety-five percent of patients achieved simultaneous UDVA of 20/20 and 20/25 or better.8
A larger retrospective study by Reinstein et al9 included 296 eyes from 148 presbyopes treated using this technique, aiming for a postoperative refraction between -1.0 D and -1.88 D. Binocularly, 98% of patients achieved 20/20 or better UDVA. Ninety-nine percent achieved uncorrected near visual acuity (UNVA) of J3 or better. No eyes lost 2 or more lines of best correction and no significant change in contrast was noted at 6, 12, or 18 cycles per degree.9
MULTIFOCAL ABLATION PROFILES
Multifocal ablation has the intention of correcting both distance and near vision in presbyopic patients. Various profiles may be employed. Central presbyopic ablations create an elevated central zone to induce reading ability, leaving the periphery for distance vision. In peripheral presbyopic ablations, the central cornea is treated for distance, whereas in the periphery a negative asphericity is created to increase the depth of field.10 This method is more difficult, as it requires an excimer laser beam profile capable of compensating for the loss of energy found with peripheral ablation.10 These techniques include PresbyMAX (Schwind Eye-Tech-Solutions GmbH and Co. KG), Supracor, multifocal LASIK, and Intracor.
PresbyMAX uses a central presbyopic ablation approach to create simultaneous distance and near vision correction. This uses a bi-aspheric technique employing a central center area for reading and pericentral distance correction in hyperopic, emmetropic, and myopic patients. The ablation profile is shown in Figure 3-3.
Six-month data on 60 eyes, 20 each of hyperopic, emmetropic, and myopic patients, demonstrated that presbyopic myopes on average lost mean binocular UNVA, but other groups maintained satisfactory distance and near vision.11
Luger et al12 reported 1-year results in 31 patients. Seventy percent of patients achieved uncorrected distance vision of 0.1 logMAR or better, and 84% obtained uncorrected near vision of 0.1 logRAD or better. Despite impressive results, authors reported some patients reported difficulty with adaption to the compromise in vision or were dissatisfied by the minor loss of distance visual acuity. They recommended a multifocal contact lens trial or trial frame with a mild defocus to simulate postoperative visual impression and evaluate patient acceptance.12