Ординатура / Офтальмология / Английские материалы / Phakic Intraocular Lenses_Hardten, Lindstrom, Davis_2004

refractive power (ie, k-readings), axial length (ie, biometry), horizontal corneal diameter (ie, horizontal white-to- white measurement), anterior chamber depth, lens thickness, preoperative refraction, and age. The accuracy of predicting the necessary power of an IOL is directly related to the accuracy of these measurements.22,23 The more unusual the eye, the greater the requirement for these additional measurements.

Fyodorov first estimated the optical power of an IOL using vergence formulas in 1967.24 Between 1972 and 1975, when accurate ultrasonic A-scan units became commercially available, several investigators derived and published the theoretical vergence formula.25-30 All of these formulas were identical31 except for the form in which they were written and the choice of various constants, such as retinal thickness, optical plane of the cornea, and optical plane of the IOL. These slightly different constants accounted for less than 0.50 D in the predicted refraction. The variation in these constants was a result of differences in lens styles, A-scan units, keratometers, and surgical techniques among the investigators.

In 1995, Olsen et al published a four variable predictor that used axial length, keratometry, preoperative anterior chamber depth, and lens thickness.32 His results did show improvement over the current two variable prediction formulas. The explanation is very simple: The more information we have about the anterior segment, the better we can

predict the effective lens position (ELP). This explanation is a well-known theorem in prediction theory in which the more variables that can be measured describing an event, the more precisely one can predict the outcome.

A recent study33 discovered that the anterior segment and posterior segment of the human eye are often not proportional in size, causing significant error in the prediction of the ELP in extremely short eyes (<20 mm). We found that even in eyes shorter than 20 mm, the anterior segment was completely normal in the majority of cases. Because the axial lengths were so short, the two variable prediction formulas severely underestimated the ELP, explaining part of the large hyperopic prediction errors with current two variable prediction formulas. After recognizing this problem, we began to take additional measurements on extremely short and extremely long eyes to determine if the prediction of ELP could be improved by knowing more about the anterior segment. Table 5-1 shows the clinical conditions that illustrate the independence of the anterior segment and the axial length.

Although several additional measurements of the eye can be taken, it is our opinion that only seven preoperative variables (ie, axial length, corneal power, horizontal corneal diameter, anterior chamber depth, lens thickness, preoperative refraction, and age) have been found to be useful for significantly improving the prediction of ELP in eyes ranging from 15 to 35 mm.

PHAKIC INTRAOCULAR LENS

POWER CALCULATION FORMULA

Formula and Rationale for

Using Preoperative Refraction

In a standard cataract removal with IOL implantation, the preoperative refraction is not very helpful in calculating the power of the implant because the crystalline lens will be removed so dioptric power is being removed and then replaced. In cases in which no power is being removed from the eye, such as secondary implant in aphakia, piggy-back IOL in pseudophakia, or a minus IOL in the anterior chamber of a phakic patient, the necessary IOL power for a desired postoperative refraction can be calculated from the corneal power and preoperative refraction; the axial length is not necessary. The formula for calculating the necessary IOL power is given below.34

Definition of Variables:

ELP = expected lens position in mm (distance from corneal vertex to principal plane of intraocular lens) IOL = intraocular lens power in D

Kref = net corneal power in diopters (0.996885 x Kkeratometric)

PreRx = preoperative refraction in D

DPostRx = desired postoperative refraction in D V = vertex distance in mm of refraction

The standardized 72-year-old phakic schematic eye is shown in Figure 5-3. Although axial length, horizontal corneal diameter, anatomic anterior chamber depth, lens thickness, and age do not appear in the primary formula, they are implicit in that they are used in the calculation of the phakic ELP and would be referred to as secondary variables. These secondary variables, along with the primary, are valuable in predicting the “vault” and final position of phakic lenses in the eye.

Cases Calculated From

Preoperative Refraction

As mentioned above, the appropriate cases for using the preoperative refraction and corneal power include a minus anterior (ACL) or posterior chamber (ICL) IOL in a high myopic phakic patient, secondary piggy-back IOL in pseudophakia, and secondary implant in aphakia. In each of these cases no dioptric power is being removed from the

eye, so the problem is simply to find the IOL at a given distance behind the cornea (ELP) that is equivalent to the spectacle lens at a given vertex distance in front of the cornea. If emmetropia is not desired, then an additional term, the desired postoperative refraction (DPostRx), must be included. The formulas for calculating the predicted refraction and the back-calculation of the effective lens position (ELP) are given in the reference and will not be repeated here.34

Example: Primary Minus Intraocular Lens in a High Myopic Phakic Patient

The calculation of a phakic IOLs in the anterior chamber is no different than the aphakic calculation of an anterior chamber lens, except the power of the lens is usually negative. In the past, these lenses have been reserved for high myopia that could not be corrected by radial keratotomy (RK), photorefractive keratectomy (PRK), or LASIK. Because most of these lenses fixate in the anterior chamber angle, concerns of iritis and glaucoma have been raised. Nevertheless, several successful cases have been performed with good refractive results. Because successful LASIK procedures have been performed in myopia up to ~ -12.00 D, phakic IOLs are usually reserved for myopia exceeding this power. Interestingly, the power of the negative anterior and posterior chamber implant is very close to 100% of the spectacle refraction for normal vertex distances (12 to 14 mm).

Mean keratometric corneal power (Kkeratometric) = 45.00 D

Phakic refraction (PreRx)= -20.00 sphere @ vertex

(V) of 14 mm

Manufacturers anterior chamber depth (ACD) lens constant (ELP) = 3.50 mm

Desired postoperative refraction (DPostRx) = -0.50 D Using an ELP of 3.50 and modifying the K-reading to net corneal power yields a -18.49 D powered IOL for a desired refraction of -0.50 D. If a -19.00 D lens is used, the patient would have a predicted postoperative refraction of -0.10 D. The -19.00 D IOL is very near the power of the

original spectacle refraction of -20.00 D.

Example: Primary Plus Intraocular Lens in a High Hyperopic Phakic Patient

The calculation of a plus phakic IOL in the anterior chamber is no different than the minus calculation of an anterior chamber lens, except the result is usually closer to 150% of the original refraction at the spectacle plane, rather than near 100% for the minus lens. The explanation relates to the vertexing of minus and plus lenses. When a minus lens is vertexed from the spectacle plane to the corneal plane, the power becomes less in magnitude (eg, -20.00 D @ spectacle to -15.00 D @ corneal plane). When

42 Chapter 5

Figure 5-5. No significant surprises have occurred in the backcalculated constants for the phakic anterior chamber IOLs in that the lens constants are no different than those obtained with secondary anterior chamber implants in aphakia or pseudophakia.

vertexing through the cornea to the ELP, the power must increase deeper in the anterior chamber (eg, -15.00 D @ cornea to -20.00 D @ the ELP). When a plus lens is vertexed from the spectacle plane to the corneal plane, the power must increase in magnitude (eg, +10.00 D @ spectacle to +12.50 D @ corneal plane) and then must increase again when vertexed through the cornea (eg, +12.50 @ corneal plane to +15.00 @ ELP). The result is that plus phakic IOLs are near 150% of the spectacle power for normal vertex distances (12 to 14 mm).

Mean keratometric corneal power (Kkeratometric) = 45.00 D

Phakic refraction (PreRx)= +10.00 sphere @ vertex

(V) of 14 mm

Manufacturers ACD lens constant (ELP) = 3.68 mm Desired postoperative refraction (DPostRx) = -0.50 D Using an ELP of 3.68 and modifying the K-reading to net corneal power yields a +17.30 D powered IOL for a desired refraction of -0.50 D. If a +17.50 D lens is used, the patient would have a predicted postoperative refrac-

tion of -0.65 D.

CLINICAL RESULTS

We have had the opportunity to evaluate several data sets for both anterior and posterior chamber IOLs. No significant surprises have occurred in the back-calculated constants for the phakic anterior chamber IOLs in that the lens constants are no different than those obtained with secondary anterior chamber implants in aphakia or pseudophakia (Figure 5-5). The accuracy of the predicted refractions are very similar to standard IOL calculations

from axial length, with more than 50% of the cases resulting in a refraction that is within ± 0.50 D. The number of cases with greater than a 2 D prediction error are virtually zero, as with calculations from axial length.

ICLs are different. Unlike anterior chamber phakic IOLs that have primarily biconcave optics, ICLs can be both biconcave and meniscus in shape like contact lenses (see Figure 5-5). The current predictive accuracy of these lenses is less than anterior chamber phakic IOLs. The exact reasons are unknown at this time, but most include parameters such as the meniscus shape, new index of refraction, possible interaction with the power of the anterior crystalline lens, and variation in the lens power from room temperature (21°C) to anterior chamber eye temperature (35°C).

In all of the data sets we have analyzed, the ICLs appear to consistently perform with 25% to 33% less effective power than the labeled power (ie, a lens labeled -20 D performs as if its power were -15 D). Although there are many plausible explanations for this finding, as mentioned above, the exact cause is unknown at this time and the labeling issue is unique to this manufacturer.

Whatever the cause of the mislabeled power, back-cal- culated constants for the ICLs, using the phakic IOL formula above results in lens constant ELPs that are 5.47 mm to 13.86 mm (mean value ~9.0 mm), even though the average measured ELP is 3.6 mm. In the data sets that we have analyzed, when the optimized back-calculated ELP is used, the mean absolute error is approximately 0.67 D, indicating that 50% of the cases are within ± 0.67 D. This value is higher than the ± 0.50 D typically found with standard IOL calculations following cataract surgery. The ICLs should be better than ACLs since the exact location of the lens can be predicted from the anatomic anterior chamber depth preoperatively. This difference is puzzling, not only because of the better prediction of the ELP but also because any errors in the measurement of the axial length are irrelevant because it is not used in the phakic IOL formula. Because of the labeling issue with ICLs, power calculations should be confirmed with the company or some other reliable source familiar with this problem.

BIOPTICS (LASER IN-SITU

KERATOMILEUSIS AND ACL

OR IMPLANTABLE CONTACT LENS)

When patients have greater than 15 D of myopia, a combination of LASIK and phakic IOLs have been used to correct these large refractive errors. Pioneered by Roberto Zaldivar, the procedure has results that are remarkably good. The surgeon performs the phakic IOL as the first stage, leaving the patient with low compound myopic astigmatism. Then LASIK or PRK is performed to correct

the residual sphere and astigmatism. These patients are especially grateful because glasses and contact lenses do not provide adequate correction and the tremendous minification of these corrections causes a significant reduction in preoperative visual acuity. Changing a 20 D myopic patient from spectacles to emmetropia with a phakic IOL and LASIK or PRK can increase the image size by approximately 50%. This would improve the visual acuity by two lines due to magnification alone (one line improvement in visual acuity for each 25% increase in magnification).

In contrast, phakic IOLs for hyperopia result in minification compared to spectacles and would have the reverse affect. However, problems with “ring scotoma,” “jack-in-the- box phenomenon,” and pincushion distortion with high plus spectacles are much worse than the loss of magnification.

CONCLUSIONS

Phakic IOLs are still in their adolescence. Power labeling issues and temperature dependent index of refractions, changes in the meniscus shape, and actual lens locations are being experimentally evaluated and are similar to the evolution of IOLs used following cataract surgery in the early 1980s. There is no question that our ability to predict the necessary phakic IOL power to correct ametropia will improve, possibly exceeding the results with standard IOLs as they should because of the more accurate prediction of the lens location axially. Determining the optimal vaulting and overall diameter to minimize crystalline lens contact, posterior iris contact and zonular, ciliary processes or sulcus contact are all being investigated for ICLs at this time. These refinements are no different than the evolution in location from the iris, to the sulcus, and, finally, the bag for standard IOLs. Because of our improved instrumentation with high resolution A- and B-scans, confocal microscopes, and anterior segment laser imaging and slit scanning systems, these refinements should and will occur much more rapidly. The use of phakic IOLs will become more widespread as the current problems are solved and will begin to cause a decline in the percentage of patients who have LASIK because of the potential for better overall optical performance of the eye.

REFERENCES

1.Thibos LN, Applegate RA, Schwiegerling JT, et al. Standards for reporting the optical aberrations of eyes.

Vision Science and its Applications: OSA Trends in Optics and Photonics. 2000;35:110-130.

2.Howland HC, Howland B. A subjective method for the measurement of the monochromatic aberrations of the eye. J Opt Soc Am. 1977;67:1508-1518.

3.Walsh G, Charman WN. Measurement of the axial wavefront aberration of the human eye. Ophthalmic Physiol Opt. 1985;5:23-31.

4.Liang J, Grimm B, Goelz S, Bille JF. Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor. J Opt Soc Am. 1994; 11:1949-1957.

5.Guirao A, Artal P. Corneal wave aberration from videokeratography: accuracy and limitations of the procedure. J Opt Soc Am. 2000;17:955-965.

6.Wang JY, Silva DE. Wave-front interpretation with Zernike polynomials. Appl Opt. 1980;19:1510-1518.

7.Artal P, Berrio E, Guirao A, Piers P. Contribution of the cornea and internal surfaces to the change of ocular aberration with age. J Opt Soc Am A. 2002;19:137-143.

8.Oshika, T, Klyce SD, Applegate RA, Howland HC. Changes in corneal wavefront aberrations with aging. Invest Ophthalmol Vis Sci. 1999;40:1351-1355.

9.Glasser A, Campbell MC. Biometric, optical and physical changes in the isolated human crystalline lens with age in relation to presbyopia. Vision Res. 1999;39:1991-2015.

10.Smith G, Atchison DA, Pierscionek BK. Modeling the power of the aging eye. J Opt Soc Am A. 1992;9:21112117.

11.Guirao A, Gonzalez C, Redondo M, et al. Average optical performance of the human eye as a function of age in a normal population. Invest Ophthalmol Vis Sci. 1999;40:203213.

12.Nio YK, Jansonius NM, Fidler V, et al. Age-related changes of defocus-specific contrast sensitivity in healthy subjects.

Ophthalmic Physiol Opt. 2000;20:323-334.

13.McLellan JS, Marcos S, Burns SA. Age-related changes in monochromatic wave aberrations of the human eye. Invest Ophthalmol Vis Sci. 2001;42:1390-1395.

14.El Hage SG, Berny F. Contribution of the crystalline lens to the spherical aberration of the eye. J Opt Soc Am. 1973;63:205-211.

15.Artal P, Guirao A, Berrio E, Williams D. Compensation of corneal aberrations by the internal optics in the human eye.

Journal of Vision. 2001;1:1-8.

16.Smith G, Cox MJ, Calver R, Garner LF. The spherical aberration of the crystalline lens of the human eye. Vision Res. 2001;41:235-243.

17.Dubbelman M, Van der Heijde GL. The shape of the aging human lens: curvature, equivalent refractive index and the lens paradox. Vision Res. 2001;41:1867-1877.

18.Glasser A, Campbell MC. Presbyopia and the optical changes in the human crystalline lens with age. Vision Res. 1998;38:209-229.

19.Kiely PM, Smith G, Carney LG. The mean shape of the human cornea. Optica Acta. 1982;29:1027-1040.

20.Holladay JT, Piers PA, Koranyi G, van der Mooren M, Norrby NE. A new intraocular lens design to reduce spherical aberration of pseudophakic eyes. J Refract Surg. 2002;18:683-691.

21.Packer M, Fine IH, Hoffman RS, Piers, PA. Prospective randomized trial of an anterior surface modified prolate intraocular lens. J Refract Surg. 2002;18:692-696.

22.Holladay JT, Prager TC, Ruiz RS, Lewis JL. Improving the predictability of intraocular lens calculations. Arch Ophthalmol. 1986;104:539-541.

23.Holladay JT, Prager TC, Chandler TY, Musgrove KH, Lewis JW, Ruiz RS. A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg. 1988;13:17-24.

24.Fyodorov SN, Kolinko AI, Kolinko AI. Estimation of optical power of the intraocular lens. Vestn Oftalmol. 1967;80:27-31.

25.Fyodorov SN, Galin MA, Linksz A. A calculation of the optical power of intraocular lenses. Invest Ophthalmol. 1975;14:625-628.

26.Binkhorst CD. Power of the prepupillary pseudophakos. Br J Ophthalmol. 1972;56:332-337.

27.Colenbrander MC. Calculation of the power of an iris clip lens for distant vision. Br J Ophthalmol. 1973;57:735-740.

28.Binkhorst RD. The optical design of intraocular lens implants. Ophthalmic Surg. 1975;6:17-31.

29.van der Heijde GL. The optical correction of unilateral aphakia. Trans Am Acad Ophthalmol Otolaryngol.

1976;81:80-88.

30.Thijssen JM. The emmetropic and the iseikonic implant lens: computer calculation of the refractive power and its accuracy. Ophthalmologica. 1975;171:467-486.

31.Fritz KJ. Intraocular lens power formulas. Am J Ophthalmol. 1981;91:414-415.

32.Olsen T, Corydon L, Gimbel H. Intraocular lens power calculation with an improved anterior chamber depth prediction algorithm. J Cataract Refract Surg. 1995;21:313-319.

33.Holladay JT, Gills JP, Leidlein J, Cherchio M. Achieving emmetropia in extremely short eyes with two piggy-back posterior chamber intraocular lenses. Ophthalmology. 1996;103:1118-1123.

34.Holladay, JT. Refractive power calculations for intraocular lenses in the phakic eye. Am J Ophthalmol. 1993;116:6366.

INTRODUCTION

Various techniques have evolved to improve the safety and efficacy of ocular anesthesia since its introduction in the late 1800s. General anesthesia and orbital blocks were the preferred methods of ocular anesthesia for most of the 20th century. However, with the advent of clear corneal phacoemulsification, surgical times have decreased, incisions have become smaller, and techniques have become more efficient. As a result, many surgeons now prefer topical anesthesia with or without intracameral lidocaine. The choice of anesthesia for an individual patient is determined by many factors, including the health of the patient, length of the eye, type of surgery, and surgical technique.

Phakic intraocular lenses (IOLs) were introduced in the 1950s, but were largely abandoned because of a high complication rate, most significantly corneal endothelial damage.1,2 Several modifications and improvements of lens designs ensued, resulting in a renewed interest in this modality of refractive correction.

Surgical techniques for the implantation of phakic IOLs most closely resemble techniques used for modern day cataract surgery. As a result, anesthetic techniques are virtually identical. There is no evidence to support one technique over the other for the implantation of phakic IOLs. In fact, all current anesthesia techniques used for phakic IOLs are based on studies in patients undergoing cataract extraction. However, there are many differences between cataract patients and those electing to undergo phakic IOL implantation. In general, phakic IOL patients are young and healthy. Heart disease, diabetes, hypertension, and

chronic obstructive pulmonary disease may not be as much of a concern in phakic IOL patients as it is in more elderly patients undergoing cataract surgery. In addition, this young population typically does not have coexisting conditions that may make local or topical anesthesia unsafe, such as hearing difficulties, impaired mental status, and involuntary movements. However, it should also be remembered that younger patients undergoing elective surgery may be more anxious than older patients and may therefore require more sedation.

The choice of anesthesia for phakic IOL correction will depend somewhat on the lens and incision site. Foldable phakic IOLs, such as the Implantable Contact Lens (ICL) (STAAR Surgical, Monrovia, Calif) and the Phakic Refractive Lens (PRL) (Medennium, Inc/CIBA Vision, Atlanta, Ga), can be inserted through a clear corneal incision because of their smaller size. This lends itself to the use of topical anesthesia. However, these lenses have been inserted using a variety of techniques, including topical lidocaine, lidocaine gel, regional blocks, and general anes- thesia.3-5

Angle-supported phakic IOLs, such as the Baïkoff NuVita MA20 (Bausch & Lomb Surgical, Rochester, NY), may be more effectively inserted using retrobulbar and peribulbar techniques rather than topical anesthesia. A larger incision is necessary to insert the nonfoldable varieties, and iris manipulation is common during the insertion. Regional blocks were used in most of the published clinical studies of angle-supported lenses.6-8

The Artisan phakic IOL (Ophtec USA Inc/Allergan, Boca Raton, Fla) requires a large (6.5 mm) incision. Several

48 Chapter 6

incision types have been used, including clear cornea, corneoscleral, limbal, or scleral tunnel. In addition, the Artisan lens requires iris manipulation when enclavating the lens. Therefore, topical anesthesia alone may not be adequate. However, topical anesthesia combined with intracameral lidocaine has been used successfully,9 even though the safety of this technique in phakic patients has not been studied. Depending on patient needs and surgeon preference, other anesthesia techniques used in the series of Artisan patients published to date have included general, retrobulbar, and peribulbar blocks.10,11

TOPICAL/INTRACAMERAL

History

The first topical anesthetic used for ocular surgery was cocaine, extracted from Erythroxylon coca. Koller described the use of topical cocaine for ocular surgery in 1884.12 In that same year, Knapp reported a technique for cataract removal under topical anesthesia using frequent instillation of cocaine.13 His technique was largely abandoned secondary to local toxicity (exposure keratopathy and corneal ulceration) and complications associated with the retrobulbar use of cocaine. Since that time, less toxic anesthetics have become available. Common topical agents used today include benoxinate 0.4%, tetracaine 0.5%, and proparacaine 0.5%.

Fichman was the first to report the successful use of topical anesthesia in modern day cataract surgery.14 In 1995, Gills reported the use of intracameral lidocaine as an adjunct to topical anesthesia.15 Many studies in the past decade have found the intracameral administration of lidocaine to be a safe and effective method of providing additional pain control during cataract surgery.15-17

Description/Technique

Topical anesthesia aims to block the superficial branches of the nasociliary and lacrimal nerves to the cornea and conjunctiva. There is minimal intraocular penetration of the anesthesia; therefore, intraocular sensations, such as increased intraocular pressure, anterior chamber irrigation, and iris manipulation, may be felt by the patient. These sensations can be minimized by the use of intracameral lidocaine. The addition of lidocaine to the anterior chamber allows the anesthetic to diffuse into the iris stroma and be absorbed by the unmyelinated small sensory nerve fibers of the long and short posterior ciliary nerves. Lidocaine is taken up quickly by the iris/ciliary body and cornea and is rapidly removed upon washout with balanced salt solution.18

The technique of topical anesthesia requires the placement of several drops of an anesthetic agent on the eye

shortly before surgery. The number and timing of administration varies according to surgeon preference. In general, however, there are two reasons to administer the anesthetic shortly before surgery. First, topical agents have a short half-life and the anesthetic effect may wear off before the conclusion of surgery. Second, unnecessary application of drops given long before the procedure commences may increase the risk of epithelial toxicity. Some surgeons prefer to use topical lidocaine, noticing reduced epithelial toxicity and increased duration of action as compared to tetracaine, benoxinate, or proparacaine.19,20 The drops can be placed directly on the surface of the eye or soaked in the anesthetic agent and placed in the conjunctival cul-de-sac in the preoperative period. Alternatively, 2% lidocaine gel can be used and has the benefit of acting as a lubricating agent.16,21

If intracameral anesthesia is used, typically 0.2 to 0.5 mL of unpreserved (methylparaben-free) 1% lidocaine hydrochloride is injected into the anterior chamber immediately after the paracentesis incision and before viscoelastic injection.15

Advantages/Disadvantages

The benefits of topical anesthesia include immediate onset of action, short duration, early return of visual acuity, and preservation of full ocular motility with good postoperative cosmesis. A patch is not required (most beneficial in monocular patients) and periocular bruising from an injection does not occur. In addition, there are no needleassociated risks, such as globe perforation or hemorrhage. Therefore, it is a preferable method of anesthesia in patients on systemic anticoagulation medication.

However, there are shortcomings to the topical administration of anesthesia. For example, the patient is awake with no akinesia. This may not be an ideal surgical circumstance for a nervous or anxious patient. In addition, there may be inadequate blockade of sensory and motor nerves in the iris/ciliary body from incomplete absorption or dilution by tears. Therefore, patients may experience intolerance to the operating light microscope, discomfort from iris manipulation, and changes in pressure dynamics inside the eye.22 A recent review of literature showed convincing evidence that both retrobulbar and peribulbar anesthesia provide better pain control during cataract surgery than topical anesthesia alone.23

The introduction of intracameral lidocaine, however, has minimized pain associated with topical anesthesia.23,24 Two studies revealed that no pain was experienced by 77% and 90% of patients undergoing cataract surgery with intracameral lidocaine vs 47% and 74% in the topical group alone.15,25 Even fewer patients may experience pain with the addition of an intravenous sedative or narcotic. However, patients should not be overly sedated, as their cooperation is required throughout the surgical procedure.

In addition, the use of intravenous agents has been shown in one study to increase the risk of medical events (eg, myocardial infarction, ischemia, heart failure, arrhythmias, hypertension, and hypotension).26 This study, however, was conducted in patients undergoing cataract surgery with an average age in the mid-70s.

Topical anesthesia with or without intracameral lidocaine requires constant communication between the surgeon and the patient. Surgeons or patients who are not good communicators are not ideal for topical anesthesia. Additionally, language barriers, movement disorders, or deafness may preclude the use of topical anesthesia. Patients with nystagmus and large angle strabismus also are not good candidates for topical anesthesia because of fixation difficulties.

Complications

Complications of topical anesthetics for ocular surgery are mainly superficial, including epithelial keratopathy and alteration of the tear film.20 The addition of intracameral lidocaine has been shown in several studies to be well tolerated by the cornea.17 Corneal toxicity in rabbits has been demonstrated with the use of intracameral bupivacaine probably because of a higher lipid solubility than lidocaine27,28; however, corneal toxicity was not found with the clinical use of intracameral bupivacaine.27 Intracameral lidocaine has been associated with transient corneal edema29 and transient dose dependent retinal changes.17 There is one report of no light perception visual acuity after intracameral injection of lidocaine with full visual recovery.30

ORBITAL BLOCKS

Retrobulbar

History

Retrobulbar blocks using 4% retrobulbar cocaine for enucleation were first described by Knapp in 1884.13 Cocaine was largely abandoned due to episodes of syncope, excessive stimulation, hallucinations, and even death. Procaine hydrochloride was introduced in 1905 when it was discovered that it could be used for nerve blocks without the effects of cocaine. It wasn’t until the 1930s, however, that retrobulbar injection of procaine became popular.31 Subsequently, other amide anesthetics, principally lidocaine, were introduced and are now used more frequently due to better diffusion and a longer duration of action than procaine. The addition of hyaluronidase was found to further increase anesthetic diffusion and improve the rapidity of onset.32

Although the classic technique of retrobulbar injections was described by Atkinson,33 there have been many variations reported since his original description. These variations include multi-injection techniques; changes in gaze position; and altering needle design, sharpness, and length.34,35

Description/Technique

In general, a sharp 25or 27-gauge retrobulbar needle is placed just above the infraorbital rim at the junction of the lateral third and medial two-thirds of the orbital rim. The needle can either be placed through the skin or the skin can be pulled down and the needle injected subconjunctivally in the lower fornix. An index finger is used to elevate the globe out of the path of the needle. The needle is passed parallel to the floor of the orbit until the tip of the needle is past the equator of the globe (1.5 cm) or until the midshaft of the needle has reached the plane of the iris.36 The needle is then directed superiorly and medially toward the intraconal space, aiming for an imaginary point behind the macula. This technique also aims to minimize the risk of inadvertently damaging the inferior oblique muscle. Three “pops” should be heard as the needle penetrates the skin, orbital septum, and intraconal space. After aspiration, 3 to 5 mL of the anesthetic agent is slowly injected. This may be followed by lid ptosis, pupil dilation, akinesia, and amaurosis. A Honan balloon (Katena Products Inc, Denville, NJ), or other external pressure, is typically applied to the eye for at least 15 minutes.

In Atkinson’s original description of the retrobulbar technique, patient gaze was superior and medial.33 However, it was later discovered that this position rotates the optic nerve, ophthalmic artery, superior orbital vein, and posterior pole of the globe into the path of the needle (Figure 6-1A).37,38 The optic nerve is also stretched in this position, which increases the chance of perforating the nerve. Therefore, primary gaze is the currently recommended eye position during administration of a retrobulbar block so that the optic nerve is directed away from the path of the needle (Figure 6-1B).37 The type of needle is also important in minimizing the risk of globe perforation, with shorter (31 mm) needles being safer than longer (38 to 40 mm) needles.39

Common anesthetic agents for retrobulbar and peribulbar blocks are lidocaine 1% to 2%, bupivacaine 0.25% to 0.75%, etidocaine 0.5% to 1%, and mepivacaine 1% to 2%. A combination of these agents may be used to maximize the rapidity of onset and duration of anesthesia. For example, lidocaine has a rapid onset of action and relatively short duration. Bupivacaine has a longer duration of action, but slower onset than lidocaine. Therefore, depending on the anticipated length of surgery, any combination of agents may be used.