High Myopia and Cataract Surgery
Myopia affects approximately 25% of the general population. It affects a larger proportion of Asians and a smaller proportion of African Americans. High myopia refers to a spherical equivalent of -6.0 D or more or an axial length of 26.5 mm or more. High myopia affects about 2% of the population. Pathologic myopia refers to a spherical equivalent of -8.0 D or more or an axial length of 32.5 mm or more.
According to the Beaver Dam Eye Study and the Blue Mountains Eye Study, there is an association between myopia and nuclear cataract. The Blue Mountains Eye Study also found that moderate and high myopia, especially with onset prior to age 20, are associated with posterior subcapsular cataract formation. The Singapore Malay Eye Study further determined patients with high myopia have 3 to 5 times increased risk of nuclear cataracts and a 30% increased risk of having posterior subcapsular cataracts.
Detailed Past Ocular History
Assessing the past ocular history in high myopes is of great importance as high myopes have been found to be 10 times more likely to have had excimer laser and previous refractive surgery and phakic intraocular lenses or retinal problems. These will determine which IOL formula to choose as well as the prognosis.
It is important to have a thorough discussion with the highly myopic patient about setting realistic goals and expectations regarding cataract surgery outcomes. If one eye undergoes surgery, the patient may experience significant anisometropia, or difference in refraction between the two eyes, prior to the second cataract surgery. If corrected for distance bilaterally, patients should be advised that they will experience more difficulty with near vision. Some may opt for monovision in order to maintain the ability to see up close.  If the patient has undergone prior refractive surgery, it is important to evaluate the prior refractive status and obtain previous records.Furthermore, increased age and axial length have both been associated with a negative effect on best-corrected visual acuity. An estimated 62% of myopic eyes have some degree of myopic or age-related retinal degeneration.
Risks and Informed Consent
Two of the most commonly discussed cataract surgery risks for highly myopic patients are increased risk of retinal detachment and variable postoperative refractive error. The cataract surgeon may choose to include an evaluation by a vitreoretinal specialist prior to cataract surgery, but this practice is controversial and not universally adopted. Please see the section entitled Late Complications for a more in-depth discussion of these risks.
One of the difficulties with preoperative calculations in highly myopic patients is the determination of axial length. As axial length increases, measurements may become less reliable. An estimated 70% of eyes with axial length greater than 33.5 mm are estimated to have posterior staphylomata, or localized ectasia of the sclera, choroid, and retinal pigment epithelium. However, almost all eyes with pathologic myopia are thought to have some degree of posterior staphylomata.
Not all experts agree on the best method to measure axial length. According to some sources, if a patient is able to fixate on a target, automated biometry such as the IOL Master (Carl Zeiss Meditec) may be able to estimate the patient's refractive axial length, from the corneal vertex to the fovea, with fairly high accuracy. In other studies, the IOL Master was found to underestimate the power of the IOL for eyes with axial length > 27.0 mm and eyes receiving a negative power IOL. In addition, A-scan contact and immersion biometry measures the anatomical axial length, from the corneal vertex to the posterior pole, and may overestimate axial length in the presence of staphylomata, leading to unexpected hyperopia.
There is also controversy over which formula is the best for calculating IOL power. Traditionally, the SRK/T, a third-generation formula, is thought to be an accurate formula for patients with high axial length. However, a study in 2012 demonstrated that the Haigis formula was superior to the SRK/T, SRK II, and Holladay I. 81% of eyes had refractive error within 1.0 D of predicted, and 54% were within 0.5 D of predicted using the Haigis formula. In contrast, 59.5% of eyes were within 1.0 D of predicted, and 29.7% were within 0.5 D of predicted using the SRK/T formula.  The problem with the third-generation IOL formulas is they select IOLs with insufficient power, thus hyperopic refractive outcomes are commonly seen.
Newer formulas have therefore been proposed to calculate the IOL power. Fourth-generation formulas include the Barrett Universal II, Holladay 2 and Olsen, of which Barrett Universal II has the best value of refractive prediction in myopic eyes and low IOL (<6 D).  Most recently, the Hill-Radial Basis Function (Hill-RBF) formula was introduced. This formula uses pattern recognition and data interpolation to calculate the IOL power by using information about central corneal power, and anterior chamber depth (ACD) to predict the desired IOL power. The Hill-RBF version 2.0 (Hill-RBF 2) replaced the Hill-RBF 1 as more data was collected and calculations were optimized. A study of 127 eyes with axial length more than 26mm, demonstrated that both Hill-RBF 2 and Barrett were more precise than Haigis, Hoffer, Holladay 1, and SRK/T in high axial myopia, and only Hill-RBF 2 formula was independent of the axial length.
When possible, it is advisable to place an IOL rather than leave a highly myopic patient aphakic. The IOL acts as a barrier to vitreous movement and subsequent retinal traction. If the patient may undergo future retinal surgery, an acrylic lens implant would be preferable to a silicone lens.
Given the relatively high incidence of postoperative hyperopia in patients receiving a negative power IOL, the surgeon may aim for a myopic target refraction around -2.0 D in this case.
Several lens options are available for the highly myopic patient, including the following:
- P574UV PMMA Non-Foldable IOL (Bausch & Lomb, Rochester, NY): down to -18.9
- Sensar AR40M Acrylic IOL (Abbott Medical Optics [AMO], Santa Ana, CA): -10.0 to +1.5 D
- AQ5010V Silicone IOL (STAAR, Monrovia, CA): down to -4.0 D
- Acrysof Acrylic IOL (Alcon, Fort Worth, TX): down to -5.0 D
- SofPort Silicone IOL (Bausch & Lomb, Rochester, NY): down to 0.0 D
- Crystalens Five-O (eyeonics, Aliso Viejo, CA): down to 3.0 D
Retrobulbar and peribulbar anesthesia carry the risk of perforation of the globe in a long, myopic eye. Topic anesthesia is safer, but the patient may experience more discomfort due to iris movements during surgery.
Various studies have focused on the rate of complications in high myopes. Cataract surgery in myopic eyes becomes challenging due to the increased depth of the anterior chamber, floppy and large capsular bag and zonular weakness in some cases. Some studies have reported that intraoperative complications are not significantly increased with high axial length. Several others have reported an increased risk. In a 2012 study, intraoperative complications increased 1.04-fold for each year of age increase, independent of refractive status. Complications increased 1.22-fold for every 1.0 mm increase in axial length. The following are estimated rates of intraoperative complications in high myopia:
- Posterior capsule tear: 2.3-9.3% for axial length > 27.0 mm
- Zonular dehiscence: 1.7% for axial length > 30.0 mm
- Anterior capsule tear: 1.1% for axial length > 30.0 mm
Highly myopic eyes are also at increased risk for anterior chamber depth fluctuations and lens-iris diaphragm retropulsion syndrome, characterized by 360 degrees of irido-capsular contact leading to reverse pupillary block, pupil dilation, and pain. Zonular weakness predisposes to this condition.
The American Academy of Ophthalmology recommends several techniques for minimizing complications in the operating room:
- Ensuring adequate pressurization of the anterior chamber during capsulorhexis with an ophthalmic viscosurgical device (OVD) to prevent capsule tears. 
- Minimizing incision leakage and repeated collapse of the anterior chamber to prevent excessive movement of the iris and vitreous.
- Decreasing irrigating bottle height and increasing flow rate to avoid deepening the anterior chamber, which would make lens manipulation difficult
- In the case of lens-iris diaphragm retropulsion syndrome and reverse pupillary block, using a spatula, cannula, or Sinskey hook to elevate the pupil margin to allow fluid to flow past the pupil.
Regardless of refractive status, surgeons usually prescribe topical steroids and NSAIDs to curb inflammation and decrease the risk of postoperative cystic macular edema (CME). Patients also usually use a topical fluoroquinolone antibiotic.
The highly myopic patient may be at increased risk of retinal detachment. Some studies have shown that the risk of retinal detachment increases with increasing axial length. In one study, an axial length > 26.0 mm was associated with a 0.9-3.8% risk of retinal detachment. Axial lengths of > 27.0 mm, > 29.0 mm, and 33.6-35.5 mm were associated with risks of 0.5-6.5%, 1.3-8.0%, and 11% respectively. Retinal breaks had an incidence of 1.1% for axial length greater than 30.0mm, and vitreous hemorrhage had an incidence of 0.6% for axial length greater than 30.0 mm. In another study, uncomplicated eyes with high myopia had a 0.8% risk, while emmetropic eyes had a 0.4% risk. In contrast, some studies have shown no significant difference in the rates of retinal detachment in myopia compared to emmetropia.
Refractive error is another potential complication, as hyperopic error appears to increase with axial length, especially in patients receiving a negative power lens. This phenomenon is most likely due to less accurate axial length measurement, especially in the presence of posterior staphyloma or poor fixation due to macular disease. One study identified myopic degenerative changes as the single independent factor limiting final visual acuity.
If a sulcus intraocular lens is inserted, it is also more likely to be unstable or decenter because of the larger sulcus size.
Clinically significant macular edema (CSME) was found to have an incidence of 0.6% for axial length > 30.0 mm.
The need for Nd:YAG capsulotomy for posterior capsular opacity (PCO) was demonstrated to be 31.6% for axial length > 30.0 mm versus 6.5-46.7% of the general population.
IOP usually decreases following cataract surgery. Nonetheless, lower speed of IOP reduction with an unstable IOP value in the first 30 days has been observed in highly myopic eyes.
Periodic dilated fundus exams to assess for retinal breaks are important in the postoperative management of the highly myopic patient.
If the other eye has a cataract, timely surgery would be important to reduce the duration of anisometropia for the patient.
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