Ординатура / Офтальмология / Английские материалы / The Art and the Science of Cataract Surgery_Boyd, Barraquer_2000

Binocular Correction

In patients with Binocular Correction: one overriding rule when choosing an IOL power is that one should never aim for spectacles which give the patient a difference in the power between the right and left lens greater than three diopters. The reason for this is that even though the patient may have 20/20 vision in primary gaze, when the patient looks up or down, the induced vertical prism difference in the two eyes is so large that it will create double vision. Therefore, avoid anisometropia.

Good Vision in the Non-Operated Eye

In a patient who has good vision in the non-operative eye, one must target the intraocular lens power for a refraction within two diopters of his/her present prescription in the non-operative eye. This measurement should be two diopters, not three, due to our 1 diopter A-scan variability. For example, if we have a patient who is hyperopic and has +5 diopters correction in each eye, we cannot target the intraocular lens for a postoperative refraction of -1 diopter because this would produce a 6 diopter difference between the two lenses, resulting in double vision or confusion.

Holladay recommends selecting the intraocular lens power to obtain a refraction which is approximately two diopters less than the non-operative eye. Consequently, on our patient who is +5 diopters in both eyes, we shouldtargetthepostoperativerefractioninthe eye with the cataract for +3, so ther e is a 90% probability that there will be less than a 3 diopter difference.

In contrast, if the patient were highly myopicineacheye,forexample,-10.00inboth eyes, we should target the intraocular lens power to produce refraction of approximately

-8.00. We have limited the difference in the spectacle lenses to a two diopter difference in the final prescription. Again, we are advised to target for a two diopter difference, not a three diopter difference, because there is approximately a one diopter tolerance in the accuracy of the ultrasonic measurement.

When Cataracts in Both Eyes

If the operation on the second eye is to be done shortly after the first, the IOL calculation is made as if he were monocular, as in our previous discussion.

For example, with a patient +5 in both eyes, if the second eye is cataractous and it is planned that the patient would also need cataract surgery in that second eye within a short period of time, it would be wise to target for -1 in the first eye. When the vision in the operated eye exceeds the vision in the other eye, -1lensesinbothshouldbeprescribeduntil the second eye is operated. Soon. This is not only true with intraocular lens surgery; it is true in all forms of refraction. The patient needs to understand what we are doing and to be a part of the decision process. Oneshouldnevergive a patient more than a three diopter difference in his/her spectacles unless he has previously worn such a prescription. One exception is a child who is under five or six years of age and who can adjust to this difference by turning his head rather than moving his eyes. Another is the patient with an alternating strabismus.

We must continue in our efforts to avoid creating astigmatism by our surgery. If the patient is already astigmatic, try to avoid too much astigmatic imbalance (high plus at 90º in oneeyeandhighminusat90ºintheother).This results in a vertical prism effect in reading and the need for prescribing a slab off prism. This problem has fortunately been significantly diminished with small incision surgery, particu-

larly phaco, and with the application of refractive cataract surgery by placing the incision in the correct axis at the time of cataract surgery. Thiswewilldiscussunderthemajorheadingof

"The Incision."

IOL POWER CALCULATION IN COMPLEX CASES

Specific Methods to Use in Complex Cases

Considering that there are no specific methods on which there is full agreement as to what to do in these patients, and after consultingdifferentauthoritiesinthisfield,wehereby recommend the use of third generation formulas, preferably more than one and that the highest resulting IOL power should be used for the implant. These formulas are preferably the Holladay 2, the SRK/T or the Hoffer formulas. Do not use a regression formula (e.g., SRK I or SRK II). We also recommend that you use central topography's flattest curve as a keratometric method unless you are fortunate to have all the information needed in order to use the "historical method." This reading is fed to the computer utilizing the selected formulas. The computer will then provide you with the power of the IOL to use.

The modern formulas hereby recommended are already available in most of the computers available today to calculate IOL power. You just select the formulas you believe adequate which should be present within your equipment.

The reason behind all these sophisticated and very careful IOL calculations in highly myopic patients with cataract is, of course, that although the cataract removal by itself can somewhat compensate for the high myopia, the

advantages of modern technology, the small incision extracapsulars and careful inspection of the peripheral retina allow us to perform a safe lens removal and provide an IOL implantation with a sufficiently desirable power to provide a specific patient with the very high quality of vision that we must demand of ourselves for the benefit of our patients.

Practical Method for Choosing Formulas in Complex Cases

From a practical standpoint, if several formulas are available to the clinician, the first choice as recommended by Zacharias and Centurion are as follows:

•short eyes:L<22.00mm:Holladay2or Hoffer Q. These constitute 8% of cases.

•L (axial length) between 22.00 and

24.50mm; 72% of the cases: mean of the three formulas: Hoffer, Holladay and SKR/T.

•L between 24.50 mm and 26.00 mm; 15% of the cases: Holladay 2 or SRK/T

•L higher than 26.00 mm; 5% of the cases: SRK/T

High Hyperopia

In eyes with short or very short axial lengths (Fig. 25) the third generation formulas such as Holladay 2 and Hoffer-Q seem to providethebestresults. Observinghighrefractive errors in extremely short eyes (<20.0 mm), Holladay has discovered that the size of the anterior and posterior segments is not proportional, and has devised certain measurements to be used to calculate the parameters in these eyes. Assembling data from 35 international researchersHolladayconcludedthatonly20% of short eyes present a small anterior segment (nanophthalmic eyes); 80% present a normal anterior segment and it is the posterior segment

that is abnormally short. This means that the formulas that predict a small anterior segment in a short eye provoke an 80% error margin, as they will predict an abnormally shallow anterior chamber which, in turn, can lead to hyperopic errors of up to 5 diopters. The Holladay 2 formula comprises the seven parameters previously described for IOL calculation: axial length, keratometry, ACD (anterior chamber depth),lenswidth,white-to-whitecornealhori- zontal diameter, preoperative refraction, and age. This new formula has reduced 5 D errors to less than 1 D in eyes with high hyperopia.

Although biometry is easy to perform, most errors in hyperopic patients occur because of probe compression. Zacharias and

Centurion emphasize that only the corneal epithelium should be touched, without any resulting indentation (Fig. 25-A).

The Use of Piggyback Lenses in

Very High Hyperopia

For very short eyes (<22.00 mm in length) even though the Holladay 2 or the Hoffer Q formulas are a significant advance in calculating the IOL power needed, we do not have IOLs easily available with a power higher than +34 diopters because a higher diopter lens would have a marked, almost spherical curvature, that would cause major optical aberrations. Such lenses can be customized but still may cause undesirable optical aberrations. In these cases the piggyback method is employed, i.e., the implantation of more than one IOL in a single eye, dividing the total power among the different lenses, placing 2/3 of the power in the posterior lens and 1/3 in the anterior lens (Fig. 25-B).

Gayton (1994) was the first to place two lenses in a single eye. He observed that placing multiple lenses in a single eye produces im-

proved optical quality because there are fewer spherical aberrations than with very high diopter lenses.

Measuring the position of piggyback lenses, Holladay observed that contrary to whathesupposed--thattheanteriorlenswould occupy a more anterior position -- what effectively happens is that the anterior lens preserves its normal position while the posterior lens moves backwards because of the distensible nature of the capsular bag. The latter may accommodate more than two IOLs and there arecasesofpatientswithfourpiggybacklenses in the same eye.

Holladay's recommendation for calculating the power of lenses with the piggyback procedure in high hyperopic patients is to add 3 diopters to the total value of the pre-op IOL power calculation and divide the total by 3, placing 2/3 of the power in the posterior lens and 1/3 in the anterior lens. This facilitates the replacement of the anterior lens, if necessary, as it is the thinnest lens. The 3 diopters added to the total value are meant to roughly compensate the hyperopic error resulting from the space behind the posterior lens. This is calculated more precisely with the Holladay 2 formula.

JoaquinBarraquer,M.D.,inBarcelona, who often attends very complex anterior segment diseases referred to him from different parts of the world, has observed a substantial increase in depth of focus with the piggyback procedure as compared to the implantation of a single custom made lens. He has done both procedures. Barraquer as well as I. H. Fine, M.D., another master surgeon, are still cautious about the piggyback method. They feel that it is not yet clear how Elschnig pearls between the lenses will behave in the postoperative period if there is progressive capsular fibrosis. Recently, John Gayton, David

Apple et al described the presence of interlenticular opacification in two pairs of piggyback lenses that had to be explanted from 2 patients with significant visual loss related to opacification between the optics. They were submitted for pathological analysis. Gross and histopathological examinations were performed,andphotomicroscopywasusedtodocument the results.

Gross examination showed accumulation of a membrane-like white material between the lenses. Histopathological examination revealed that the tissue consisted of retained/proliferative lens epithelial cells (bladder cells or pearls) mixed with lens cortical material.

Theyrecommendedthreesurgicalmeans that may help prevent this complication: meticulouscorticalcleanup,especiallyintheequatorial region; creation of a relatively large continuous curvilinear capsulorhexis to sequester retainedcellsperipheraltotheIOLopticwithin the equatorial fornix; insertion of the posterior IOL in the capsular bag and the anterior IOL, in the ciliary sulcus to isolate retained cells from the interlenticular space.

Echobiometry in highly hyperopic eyes, especially microphthalmic and nanophthalmic eyes, is still far from desirable.

High Myopia

AccordingtoZachariasandCenturion's experience, resultsofcataractsurgeryinhighly myopic eyes with axial lengths higher than 31.0 mm with implantation of low or negative power IOLs may be successful, without any more operative or postoperative complications than normal eyes. The use of the SRK II formula with an individual surgeon's factor showed good predictability of the refractive target (Fig. 26). However, better formulas

without the use of a personalized correction factor have yet to be developed. Zacharias and Centurion emphasize that there are technical difficulties in performing the echobiometry of patients with high myopia, especially when they have a posterior staphyloma. In those cases they obtain extremely irregular retinal echoes that cannot provide certainty in terms of really correct results of the IOL calculation. In addition, a posterior staphyloma may not always coincide with the macula, so the higher measurement is not necessarily the correct one, as is the case with normal eyes.

In these patients it is useful to perform B type ultrasound to identify the existence of a staphyloma and its relation with the macula. Equally important is to have an ultrasound probe with a fixation light. The patient is asked to fixate at the light -- which he will do with the macula -- facilitating the measurement.

LacavaandCenturion studied 27myopic eyes with an axial length of more than 26.50 mm, and found that 88% of the patients with whom they used the SRK/T formula were within the emmetropic criteria established by George Waring.

DETERMINING IOL POWER IN PATIENTS WITH PREVIOUSREFRACTIVESURGERY

Patients who have undergone excimer laserprocedures,radialkeratotomyorINTACS have had modifications to their corneal curvatures (Figs. 27, 28, 29). Accurate keratometric readings are fundamental in calculating IOL power. IOL power calculation for cataract surgery in patients previously submitted to refractive surgery by modification of the corneal curvature is a new challenge for the cataract surgeon basically because of two

features. 1) Patients who previously decided to undergo refractive surgery are more phychologically resistant to using spectacles to correctresidualametropia. Consequently,their expectations for cataract surgery are unusually high. 2) So far there is no universally accepted formula to calculate these patients' IOL power accurately. Routine keratometry readings do not accurately reflect the true corneal curvature in these cases and may result in

errors if used for IOL calculations. Therefore, standard keratometry readings should not be used for IOL calculations in these patients. If done, the standard IOL power-predictive formulas based on such readings commonly result in substantial undercorrection with postoperativehyperopicrefractionoranisometropiaboth of which are very undesirable.

Jack Holladay, M.D., a recognized authority on IOL power calculations and in all

optical-refractive subjects in ophthalmology, considers that accurate determination of the corneal power in these patients is difficult and is usually the determining factor in the accuracy of the predicted refraction following cataract surgery. Providing this group of patients with the same accuracy of intraocular lens power calculations as we have provided our standard cataract patients presents an especially difficult challenge.

Methods Most Often Used

There are three methods to determine the effective power of the cornea in these complex cases: 1) the clinical history method, also termed by Holladay "the calculation method"; 2) the contact lens method; and 3) the topog-

raphy method. Holladay believes that the calculation or "clinical history" method and the hard contact lens trial are the two more reliable of the three, because the corneal topography instruments presently available do not provide accuratecentralcornealpowerfollowingPRK, LASIK and RKs with optical zones of 3 mm or less. In RKs with larger optical zones, the topography instruments become more reliable. The great majority of cases, however, have had RK with an optimal zone larger than 3 mm, so they should also qualify for this method.

The Clinical History Method

The "clinical history" method is the most often used. In the "historical or calculation method", however, the keratometry reading

and refraction before refractive surgery must be known along with an accurate postoperative refraction which is not often the case. It is also important to keep in mind that at present, far more patients have had RK than PRK and LASIK combined. Also, our long-term fol- low-up of RK patients is much greater. The long-term studies of RK patients reveal that some have hyperopic shifts in their refraction anddevelopprogressiveagainst-the-ruleastig- matism which may complicate the final vision of the patient operated for cataract, unless detected at the time of preoperative evaluation andcorrected. Thelong-termrefractivechanges in PRK and LASIK are unknown, except for the regression effect following attempted PRK corrections exceeding 8 D. Whichever procedure the patient has had, the stability or instability of the refraction must be determined. When using the "clinical history or calculation method" a subtraction of the spherical equivalent(SEQ)changeafterrefractivesurgeryfrom the original K-reading is done to determine the new "accurate" corneal curve. This, however, is not information easily found. It is useful and can be applied whenever refraction and the K- reading before the keratorefractive procedure are available to cataract surgeons. If this information is not available, which is not unusual, we recommend that the keratometry be measured with corneal topography and use the flattest curve of this reading as the new corneal curve to feed the computer that will then automatically provide us with the IOL power to use.

Another downfall of the history method is that cataracts frequently cause induced myopia. This method, however, requires an accurate and stabilized refraction after the keratorefractive procedure and at the time we are contemplating cataract surgery. In many

cases, calculation is complicated by the progressive flattening that occurs in about 25% of RK patients. It is nearly impossible to separate these two factors and determine the impact of each on the refraction before cataract surgery.

The Trial Hard Contact Lens

Method

The second method often used, which is the trial hard contact lens method, requires a plano hard contact lens with a known base curve and is limited to patients whose cataract does not prevent them from being refracted to approximately +0.50 D. This usually requires avisualacuityofbetterthan20/80. Thepatient's spheroequivalent refraction is determined by standard refraction. The refraction is then repeated with the hard contact lens in place. If thespheroequivalentrefractiondoesnotchange with the contact lens, then the patient`s cornea must have the same power as the base curve of the plano contact lens, since the base curve and front curve are the same in a plano contact lens. Ifthepatienthasamyopicshiftintherefraction with the contact lens, then the base curve of the contact lens is stronger than the cornea by the amount of the shift. If there is a hyperopic shift in the refraction with the contact lens, then the basecurveofthecontactlensisweakerthanthe cornea by the amount of the shift.

Example as Provided by Holladay

Thepatienthasacurrentspheroequivalent refraction of +0.25 D. When a plano hard contact lens with a base curve of 35.00 D is placed on the cornea, the spherical refraction changes to -2.00 D. Since the patient had a myopic shift with the contact lens, the cornea must be weaker than the base curve of the

contact by 2.25 D. Therefore, the cornea must be 32.75 D (35.00 - 2.25), which is slightly differentfromthevalueobtainedbythehistorical or calculation method. This method is limitedbytheaccuracyoftherefractions,which may be limited by the cataract.

The Corneal Topography

Method

Current corneal topography instruments provide greater accuracy, compared to keratometers, in determining the power of corneas with irregular astigmatism. The computer in topography instruments provides a very accurate determination of the anterior surface of the cornea. The limitation of this method is that the computer in corneal topography provides no information about the posterior surface of the cornea. In order to accurately determine the total power of the cornea, the power of both surfaces must be known.

The Importance of Detecting

Irregular Astigmatism

Holladayhasstronglyrecommendedthat biomicroscopy, retinoscopy, corneal topography and endothelial cell counts be performed in all of these complex cases. The first three tests are primarily directed at evaluating the amount of irregular astigmatism. This determination is extremely important preoperatively because the irregular astigmatism may be contributing to the reduced vision as well as the cataract. The irregular astigmatism may also be the limiting factor in the patient'svisionfollowingcataractsurgery. The endothelial cell count is necessary to recog-

nize any patients with low cell counts from the previous surgery who may be at higher risk for corneal decompensation or prolonged visual recovery.

The potential acuity meter (PAM), super pinhole and hard contact lens trial are often helpful as secondary tests in determining the respective contribution to reduced vision by the cataract and the corneal irregular astigmatism. The patient should be informed that only the glare from the cataract will be eliminated. Any glare from the keratorefractive procedure will essentially remain unchanged.

IOL Power Calculation in

Pediatric Cataracts

How to optically correct patients with bilateral congenital cataracts and monocular congenital cataract has been a major subject of controversy for many years. Some distinguishedophthalmicsurgeons20yearsagowere strongly against performing surgery in monocular congenital cataract followed by treatment of amblyopia with a contact lens. Visual results were so bad that children with this problem must be amblyopic by nature, they thought, and the psychological damage to the children and the parents by forcing such treatment was to be condemned.

Surgery of bilateral congenital cataracts at a very early age followed by correction with spectacles and sometimes with contact lenses usually ended with no better than 20/60 vision bilaterally. This was again a source for belief that congenital cataracts either unilateral or bilateral were by nature associated with amblyopia, profound in cases of monocular cases and fairly strong in bilateral cataracts.

When posterior chamber IOL implantation in adults became established as the procedure of choice, strong influences within ophthalmology were adamantly opposed to their use in children for the following reasons: 1) the eye grows in length with consequent significant change in refraction. It was considered impossible to predict such change and consequently, the accurate IOL power adequate for each child. 2) There was opacification of posterior capsule in most cases. This required a second operation for posterior capsulotomy and the presence of an IOL would impede proper surgical maneuvers.

You will not find this concise history in any other book. I lived through it and therefore share it with you.

The situation has now significantly changed. The previous failures with spectacles and contact lenses, the new developments in technology and surgical techniques and the fresh insight of surgeons of a new generation has led us to discard the previous thinking and very definitely implant posterior chamber IOL's in children. This has been made possible from the surgical point of view by the followingdevelopments: newmedicationsthat effectively prevent and/or control inflammation; the introduction of posterior capsule capsulorhexis introduced by Gimbel in North America promptly followed by Everardo Barojas in Mexico and Latin America (Fig. 30); high viscosity viscoelastics to facilitate intraocular surgery in smaller eyes; new, more appropriate IOL's for children and implantation in their capsular bag; more refined technology that leads to a less difficult calculation of the IOL power.

Different Alternatives

Thelimitationsincalculatingtheselenses powers (Fig. 31) is due to the fact that the eye grows after cataract surgery and therefore refraction will change. Two main methods of choosing an IOL power for pediatric patients are available: 1) Make the eye emmetropic at the time of surgery and thereby treat amblyopia immediately taking advantage of a much better visual acuity. This is followed later by an IOL exchange because of increasing myopia (growth of the eye).

Even though there are more practical and efficient techniques for IOL exchange, as devised by Jack Dodick, M.D., this alternative is second choice.

2) Proceed with incomplete overcorrection of the eye at the time of surgery (treated with glasses or contact lenses) taking advantage of the trend toward emmetropization which will occur as the eye grows. By "incomplete" we mean leaving the eyes hyperopic. Astheeyegrowsinlengthwithage(axial growth), the myopization that takes place in an eye artificially rendered hyperopic will lead to emmetropia or close to normal refraction. This measureavoidsmyopicanisometropiathatmay lead to an undesirable change of IOL surgically. In the meantime, the temporary hyperopia is managed with standard spectacles or contact lenses.

Alternatives of Choice

In the IOL power calculation in children younger than 1 year, keratometry is difficult and fortunately less important because the