Cataract Surgery in Small Eyes
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The first question to be addressed is what constitutes a small eye. This clinical spectrum is usually based on the assessment of ocular axial length (AL), anterior chamber depth (ACD), corneal diameter and concomitant anatomical malformations. Hence, this spectrum encopasses simple or complex microphthalmos, nanopthalmos and relative anterior microphthalmos.
Simple microphthalmos is an eye with an AL shorter than the age-adjusted mean by two standard deviations (typically <21.0 mm) with a normal ACD, normal scleral thickness and without anatomical malformations. If there are concomitant anatomical malformations such as anterior segment dysgenesis, iris or chorioretinal colobomas, retinal dysplasia or persistent fetal vasculature, there is a complex microphthalmos.
Nanopthalmos is a rare condition, characterized by short AL (<21.0 mm) with a shallow anterior chamber (<2.2 mm) and thickened choroid and sclera (>1.7 mm) but with no other anatomical malformations. Such eyes are particularly prone to angle-closure glaucoma during adulthood, as there is a disproportion between the normal increase in lens thickness and the shallow anterior chamber. Secondary to the increased scleral thickness, there may be resistance to the venous outflow from the vortex veins, leading to uveal effusions with or without exsudative retinal detachment which may even be present in the preoperative period.
On the other hand, eyes with relative anterior microphthalmos have a normal AL, shallow anterior chamber (<2.2 mm) and normal scleral thickness, without other anatomical malformations. Similarly to nanophthalmos, there is a high incidence of angle-closure glaucoma as well as pseudoexfoliation and corneal guttata.
As one would expect, performing cataract surgery in such small eyes is particularly challenging, not only because it is associated with a higher rate of intraoperative and postoperative complications but also due to poorer postoperative refractive predictability.
Hence, meticulous preoperative planning, surgical technique and postoperative management are key to improve visual outcomes.
In the preoperative period, a holistic assessment is preferable for adequate surgical planning, predicting possible complications and determining visual prognosis.
Comparing the best corrected visual acuity, refractive status (usually hyperopia) and the degree of cataract billaterally are valuable for surgical planning. Specifically, if there is a significant difference in visual acuity billaterally in the setting of similar degress of cataract density or if there is a large degree o anisometropia, not explained by any refractive shift due to the cataract (p.e myopic shift in the presence of advanced nuclear sclerosis), one has to consider the presence of amblyopia. Evaluating past prescribed refractions or inquiring the patient about whether they recall one eye having worse visual acuity since childhood is particularly important. This assessment may help identify pre-existing amblyopia to manage patient’s expectations for visual recovery and to determine whether they are suitable for premium intraocular lenses (IOLs).
In the setting of a shallow anterior chamber, it is necessary to determine intraocular pressure (IOP), ACD, peripheral anterior synechiae or posterior synechiae or anterior segment dysgenesis. In such cases, dynamic gonioscopy, anterior segment optical coherence tomography (AS-OCT) or ultrasound biomicroscopy (UBM) should be considered in addition to careful slitlamp biomicroscopy. If the angle is narrowed, specially in the setting of elevated IOP and glaucomatous neuropathy, the IOP should be treated preoperatively with topical medication, peripheral iridotomy and/or laser iridoplasty. In fact, laser iridotomy/iridoplasty preoperatively may open the angle and subsequently increase the ACD, in order to ease surgical maneuvering during cataract surgery. Fuchs endothelial dystrophy is more prevalent in shallowed anterior chamber eyes, for which endothelial cell counts and central corneal pachymetry are advised.
Ultrasonography, AS-OCT or UBM are useful in determining scleral thinckness, thought to increase the risk for uveal effusions. In fact, UBM should be considered to exclude subtle uveal effusions in the preoperative period that may worsen during or after cataract surgery. These preexisting effusions should be treated preoperatively with cycloplegics, and steroids, or even creating scleral windows several weeks before the cataract surgery.
The complex microphthalmic eye is associated with anatomical abnormalities, such as iris or retinochoroidal colobomas, which should be evaluated carefully. Indirect ophthalmoscopy, OCT and B-scan ultrasound are advised to identifying colobomatous microphthalmia. Also, macular and optic disc OCT are useful to exclude any concomitant retinal or optic disc condition (specifically glaucomatous optic neuropathy).
Biometry and Intraocular Lens (IOL) calculation
The most accurate method of obtaining AL is partial coherence interferometry (IOLMaster, Carl Zeiss Meditec AG) or optical low coherence reflectometry (OLCR) (Lenstar, Haag-Streit AG), instead of using ultrasound biometry.
The main challenges about selecting the appropriate IOL in eyes with short AL are mainly because these eyes are subjected to more frequent systematic errors in the axial length measurement; high IOL powers have a wider range of acceptable manufacturers’ tolerance (for an IOL power >30 diopter, variabilitiy for true dioptric power can be up to +/-1 diopter); and even a slight change in the effective lens position (ELP) can have a significant effect on the refractive results due to the short AL.
Hence, the best adapted IOL power calculation formulas for such eyes are those that most accurately predict better the ELP.
In recent years, several studies have reported the refractive outcomes in short AL eyes for IOL calculation formulas, with no definitive conclusion as to which formula is most suitable.
In a study with 8108 eyes with different AL, Aristodemou et al. compared Hoffer Q, Holladay 1, and SRK/T formulas and concluded that for eyes with an AL < 21.50mm (134 eyes), the Hoffer Q formula was the most accurate. However, Gokce et al. showed no significant difference between the Hoffer Q, Haigis, Barrett, Hill-RBF 2.0, Holladay 1, Holladay 2, and Olsen formulas for eyes with an AL of ≤22.0 mm. In addition the authors found that Hoffer Q and Holladay 2 formulas produced a slight myopic refractive prediction errors (about -0.22 diopter and -0.23 diopter, respectively), which may not be so undesirable for postoperative visual outcome. A recent meta-analysis from 2018 that included 10 studies of IOL formula calculation for eyes with an AL ≤22.0 mm showed that the Haigis formula was superior to the Hoffer Q, Holladay 1, Holladay 2, SRK/T and SRK II formulas. It seems reasonable that, in addition to Hoffer Q formula, the fourth-generation formulas Haigis and Holladay 2 are appropriate for short AL eyes, since they include multiple input variables in addition to keratometry and AL measurement, such as ACD, WTW, lens thickness, preoperative refraction, or age.
The relatively new Kane IOL calculation formula uses AL, keratometry, ACD, lens thickness, central corneal thickness and gender. It was created using several large data sets from selected high-volume surgeons and is based in both theoretical optics and regression/artificial intelligence components to further refine its predictions. A recent study with 10930 eyes compared the new Kane formula with Holladay 2 AL-adjusted, Olsen, Hill-RBF 1.0 and 2.0, Barrett 1 and 2, and third-generation formulas in patients implantated with 4 different IOL types. In all AL subgroups, including for AL ≤22.0 mm, the Kane formula was found to be the most accurate, with the lowest mean absolute prediction error. Furthermore, another study included 270 eyes that underwent uneventful cataract surgery with implantation of an SA60AT IOL with a spherical equivalent power of ≥30 diopter, with a mean AL 20.82 +/- 0.63 mm. The optimized lens constants were then used to calculate the predicted refraction for several IOL calculation formulas, which was compared with the actual refractive outcome to give the prediction errors. The authors concluded that the Kane formula was more accurate when compared to Barrett Universal II, Haigis, Hill-RBF 2.0, Holladay 1 and 2-AL adjusted, Hoffer Q, Olsen, and SRK/T formulas. Approximately 20% more patients achieving an outcome within ±0.50 D of the prediction using the Kane formula when compared with either the Barrett or Hoffer Q formula. Hence, the new Kane formula shows promise for IOL calculation in short AL eyes.
Regardless of the IOL calculation formula used, as long as both eyes have similar preoperative biometric readings, they are likely to have the same ELP. Therefore, IOL power calculation for the second eye may be refined by the postoperative refraction from the first eye.
As expected in small eyes, it is preferrable to insert a single IOL than implanting two IOLs with the same combined dioptric power. Depending on the country, however, the existing IOL models may differ and some IOL manufacturers may be able to customize very high-powered IOLs as needed by physician's orders. If that is not possible and a very high powered IOL is needed (>40 diopters), the refractive error can be corrected through piggyback IOLs, laser refractive surgery (if there is a small degree of residual hyperopia), contact lenses or spectacles. Considerations about piggyback IOLs are described in the Surgical Technique section.
Due to the fact that small eyes may appear deeper in the orbit and that corneal horizontal diameter is wider than the vertical diameter, a temporal approach can ease instrument positioning and maneuvering.
In order to prevent uveal effusions, previous literature suggested creating scleral windows in the beggining of the surgery. Rajendrababu et al, through multivariate models, concluded that prophylactic sclerostomy decreased the risk for uveal effusions in a study that included 60 nanophthalmic eyes. However, in the highly pressurized globe during phacoemulsification, having a portion of the globe only contained by uveal tissue can be unwise if performed by an unexperienced surgeon. In order to reduce the risk of uveal effusion or suprachoroidal hemorrhage, pressure fluctuations during the surgery should be limited as much as possible.
If uveal effusions develop during phacoemulsification, as indicated by a sudden increase in IOP often accompanied by shallowing of the anterior chamber, the surgeon can create inferior sclerectomies to lower the IOP and complete the procedure or close all wounds and complete the case later on, after resolution of the effusion.
Performing the paracenteses and the primary corneal wound should be adjusted for the thicker corneal pachymetry often encountered, in order to avoid excessive tunnel length, which would difficult maneuvering and accessing the lens.
Small eyes may have suboptimal dilation, small pupillary aberture or even iris coloboma, in which case, and particularly in the presence of a shallow anterior chamber, iris hooks are a better option than most iris expansion devices.
In small eyes with a shallow anterior chamber, posterior pressure can make the surgery particularly challenging and increase the risk for complications. Preoperatively intravenous mannitol dehydrates the vitreous, thereby reducing its volume and the likelyhood of significant posterior pressure. In some cases, especially when ACD did not increase with mannitol, a vitreous tap can be performed, using a trocar system (example transconjunctival 25-gauge trocar), which may limit complications from pars plana vitrectomy in small eyes (with altered anatomy) such as peripheral retinal tear, puncture of the lens capsule, entering suprachoroidal space and choroidal detachment or hemorrhage. In the postoperative period, eyes submitted to pars plana vitrectomy should have a thorough evaluation of the peripheral retina.
Trypan blue staining of the anterior capsule reduces its elasticity and improves visualization, thus being particularly useful in eyes with an inadequate pupil aperture.
During cataract surgery, it is of utmost importance to maintain the anterior chamber pressurized and deepened. During capsulorrhexis, using a highly cohesive ophthalmic viscosurgical device (OVD), a microincisional capsulorhexis forceps or a bent cystotome though the paracentesis can prevent anterior chamber shallowing and run-out rhexis.
In shallow anterior chamber eyes there is an increased risk of iris prolapse with subsequent iatrogenic damage to the iris and loss of dilation, which often occurs during hydrodissection, mainly through the main incision. Therefore, having only the paracentesis performed, injecting a judicious amount of balanced salt solution (BSS) while allowing excess OVD and BSS to egress through gentle pressure on the posterior lip of the corneal wound can minimize iris prolapse. In addition, upon removing instruments from the eye, the surgeon should stop irrigation (position 0 on the foot pedal) just before the phaco handpiece exits the main incision.
Immediately before performing phacoemulsification, injecting a layer of dispersive OVD in the posterior surface of the cornea protects the endothelium from ultrasonic energy and fluid turbulence. This can be repeated throughout phacoemulsification as needed, based on the degree of cataract density and duration of this step. Prolapsing the cataract nucleus, as performed in the “Flip and Chip” technique, should be avoided in order to protect the corneal endothelium.
Insertion of the IOL in the capsular bag filled with cohesive OVD should be performed carefully, as some authors report increased zonular weakness in these small eyes. In fact, its correct placement is vital for subsequent piggyback IOL.
Primary placement of two IOLs in the capsular bag has been associated with interlenticular membranes and opacifications, reduced visual acuity, and late hyperopic shift, reasons why current recommendation is to place one IOL (usually acrylic) in the capsular bag and a three-piece IOL (usually silicone) in the cilliary sulcus. This piggyback IOL can be placed at the time of the cataract surgery or performed as a secondary procedure. The latter approach may be preferable, after implanting the maximum-powered IOL in the capsular bag. In the postoperative period, it is assessed if there is sufficient room for a sulcus-placed IOL and a more accurate calculation for the piggiback IOL can be made. Such calculation can be made using the refractive vergence calculation with the Holladay IOL Consultant or estimated using the Gills or Nichamin nomogram.
Some authors advocate performing a surgical iridectomy/laser iridotomy to facilitate postoperative zonulotomy or hyaloidotomy in the event of postoperative aqueous misdirection or even performing a preemptive iridectomy-hyaloido-zonulectomy after implanting the IOL.
Finally, the aspiration of OVD and gentle hydration of the paracentesis and corneal main wound should be performed slowly and carefully to avoid iris prolapse. In such cases however, where even gentle hydration of the corneal wounds incites iris prolapse, placing a 10-0 nylon suture may be advisable.
Complications and Postoperative Management
The most common complications described in the literature include: posterior capsule rupture, zonular dehiscence, iris prolapse, corneal endothelial/Descemet membrane trauma, transient severe corneal edema, cystoid macular edema (CME), severe anterior uveitis, uveal effusion, angle-closure glaucoma, retinal detachment or aqueous misdirection.
Severe anterior uveitis in the early postoperative period have reported rates ranging from 3.8% to 12.0%, although there is usually an adequate response to increased topical corticosteroids without the need for subconjunctival or oral supplementation.
Aqueous misdirection has been reported in 0.4% to 6.0% of cataract surgery in short eyes (particularly when the AL <20.0 mm). Medical management include cycloplegics-mydriatics, aqueous suppressants, and hyperosmotics. Cycloplegics inhibit contraction of the muscle fibers of the ciliary body, tightening the zonules, which in turn pulls the lens backward. Aqueous suppressants decrease production and consequent posterior flow of aqueous humor. Through osmotic gradient, hyperosmotic agents dehydrate the vitreous and draw fluid out of the posterior segment. In combination, these pharmacologic agents can potentially break an acute attack of aqueous misdirection. If not however, specially after one week, laser/surgical approach should be considered.
Laser treatment include Nd:YAG laser to disrupt the anterior vitreous surface and create a conduit between the posterior and anterior chamber (through an iridotomy or capsulotomy) (9) or transscleral diode laser cyclophotocoagulation (CPC) to decrease cilio-hyaloidal apposition and decrease posterior aqueous flow.
Surgical treatment include performing a iridectomy-hyaloido-zonulectomy via the iridectomy or via pars plana. In refractory cases, a complete pars plana vitrectomy or posterior chamber glaucoma drainage device should be considered.
The incidence of elevated IOP following phacoemulsification in small eyes have been reported between 4.7% and 46.0%, from an increased risk of angle-closure glaucoma or aqueous misdirection. In eyes with increased IOP preoperatively, phacoemulsification can reduce IOP proportionally to the increase in the ACD with greater IOP lowering effect in eyes with narrow angles. In addition to the previously described management strategies for aqueous misdirection, some of these patients eventually need filtering surgery.
In the setting of postoperative elevated IOP, UBM is particularly useful in differentiating aqueous misdirection from ACG secondary to peripheral uveal effusions, which can have significant implications in treatment decisions.
In previous studies including eyes that underwent phacoemulsification and extracapsular cataract extraction, uveal effusions have been reported with rates ranging from 2.9% and 9.3%, although in a recent study from 2021 that included 71 eyes reported no cases of uveal effusions.
Regarding the residual refractive error, study results are variable although most authors demonstrated a trend towards hyperopia in nanophthalmic and microphthalmic eyes, with less predictability in the former. In fact, the number of nanophthalmic eyes achieving the predicted postoperative refraction within 1.0 dioptre ranges from 42.9% to 66.6%, depending on the IOL formula used and AL. As stated before, this residual refractive error can be corrected with a piggyback IOL, laser refractive surgery, contact lenses or spectacles. In rare instances where there is a significant refractive error, an IOL exchange may be needed.
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