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

50 Chapter 6

Figure 6-1. Retrobulbar block. Atkinson's original description with the globe position up and in before the needle is advanced parallel to the floor of the orbit. In this position, the needle may penetrate the optic nerve. The currently recommended technique is with the globe in primary position prior to advancement of the needle. The optic nerve is further from the course of the needle (reprinted from Lindquist TD, Lindstrom RL. Ophthalmic Surgery: Looseleaf and Update Series: Anesthesia in Ocular Surgery. Orlando, Fla: Elsevier Science; 1990:IA7, with permission from Elsevier Science).

Hyaluronidase enhances the ability of a local anesthetic to spread through the orbital tissues. Therefore, the addition of 1.0 mL of hyaluronidase per 10 mL of anesthesia may reduce the time to onset of akinesia and anesthesia. There is very poor evidence, however, that hyaluronidase enhances the degree of akinesia produced.23 Some surgeons have tried warming or buffering the anesthetic agents with sodium bicarbonate to reduce pain on injection and to improve the effectiveness of anesthesia. However, these techniques have not in general been found to be efficacious.23

The use of epinephrine in retrobulbar blocks is controversial. On one hand, epinephrine retards the systemic absorption of the anesthetic and, therefore, prolongs the anesthetic effect. Epinephrine, however, is a vasoconstrictor and may affect the central retinal artery or ophthalmic artery. Retrobulbar injection of lidocaine with epinephrine has been shown to reduce ophthalmic artery pressure by 50%.40 Therefore, it may be contraindicated in patients with poor circulation or poor optic nerve function.

Advantages/Disadvantages

Advantages of retrobulbar blocks include akinesia, reliability of anesthetic effect, and improved pain control over topical anesthesia.23 Compared to peribulbar blocks, retrobulbar blocks are more reliable, have a quicker onset, and are associated with less chemosis. Retrobulbar anesthesia, however, does not block the frontal and lacrimal nerve branches that enter the orbit above the annulus of

Zinn. As a result, patients may still have sensation in structures innervated by these branches, such as the superior and lateral conjunctiva and skin of the eyelid. Therefore, patients may need a supplemental facial or eyelid block.

Complications

There are a variety of needle-associated complications of retrobulbar blocks. These complications are important considerations in patients undergoing phakic IOL implantation because many of them are high myopes with long eyes. The risk of globe perforation with retrobulbar and peribulbar blocks is higher in eyes with longer axial lengths.41,42 In a series of 20 eyes with globe perforation following retrobulbar and peribulbar blocks, 45% had an axial length greater than 26 mm.42

There are a host of complications of retrobulbar blocks including, but not limited to, globe perforation, retrobulbar hemorrhage, retinal vascular occlusion, extraocular muscle injury, ptosis, optic nerve damage, brainstem anesthesia from subarachnoid or intradural injection, and cardiac and respiratory arrest.43-50

Peribulbar

History

Kelman introduced the technique of peribulbar anesthesia for cataract surgery and it was later popularized in the mid-1980s by Davis and Mandel.51 Their original description is of multiple injections at multiple sites in both the anterior and posterior orbit. There have been many variations of the technique since their original description.

Description/Technique

Peribulbar blocks are administered in a similar fashion as retrobulbar blocks. The site of injection is either the standard juncture of the lateral third and medial two-thirds of the infraorbital rim or midline, just superior to the rim (Figure 6-2). The needle penetrates the orbital septum and it is directed posteriorly along the floor of the orbit (Figure 6-3). The needle is not advanced superiorly and medially as in a retrobulbar block; therefore, the muscle cone is not entered and the agent is delivered to the extraconal peribulbar space. A shorter 25or 27-gauge needle (18 to 24 mm) can then be used. Higher volumes of anesthesia (5 to 10 mL) are needed as compared to retrobulbar blocks because this technique relies on the diffusion of the anesthetic through the orbital tissues. Following administration of the block, pressure is applied to the eye with a Honan balloon. If additional akinesia is needed (specifically to block the superior oblique muscle), a medial periconal injection can be administered in the supratrochlear region, 2 mm medial and inferior to the supraorbital notch. Alternatively, the injection may be given between the medial canthus and caruncle.

Figure 6-2. Peribulbar block. The standard inferior approach places the needle just above the inferior orbital rim through the skin. A wooden applicator or finger is used to move the skin inferior and adjacent to the globe (reprinted from Lindquist TD, Lindstrom RL. Ophthalmic Surgery: Looseleaf and Update Series: Anesthesia in Ocular Surgery. Orlando, Fla: Elsevier Science; 1990:IA8, with permission from Elsevier Science).

Figure 6-3. Peribulbar block. The needle is directed posteriorly along the floor of the orbit as the anesthesia is injected (reprinted from Lindquist TD, Lindstrom RL. Ophthalmic Surgery: Looseleaf and Update Series: Anesthesia in Ocular Surgery.

Orlando, Fla: Elsevier Science; 1990:IA8, with permission from Elsevier Science).

Advantages/Disadvantages

An advantage of a peribulbar block is the improved reliability of anesthesia and akinesia as compared to topical anesthesia. Peribulbar blocks act on the long sensory root of the ciliary ganglion; therefore, all of the trigeminal nerve branches going to the globe are blocked. Subsequently, there is often no need for a facial nerve block. There may also be less needle-associated risks with peribulbar anesthesia as compared to retrobulbar anesthesia. In the peribulbar technique, the muscle cone is not entered, so the sequelae of retrobulbar hemorrhage and optic nerve compression are less likely and less severe. There is also less intraoperative posterior pressure and less intraoperative and postoperative amaurosis.52

Peribulbar blocks, however, take longer to achieve efficacy, may require more than one injection, and cause significant chemosis and ecchymosis.52 There is good evidence that retrobulbar and peribulbar blocks provide equal pain control and akinesia.23

Complications

Complications of peribulbar blocks are similar to those of retrobulbar blocks and include globe perforation, hemorrhage, retinal vascular occlusion, extraocular muscle injury, and brainstem anesthesia.46,53-56 However, the incidence of direct injury to the optic nerve, globe perforation, and retrobulbar hemorrhage should be less since the needle does not directly invade the muscle cone.

Parabulbar/Sub-Tenon’s

History

Turnbull described injecting 4% cocaine into a cut made through the conjunctiva and Tenon’s capsule before enucleation in 1884.57 It wasn’t until the early 1990s, as reports of complications of retrobulbar and peribulbar blocks increased, that this technique underwent a resurgence.58,59

Technique

The conjunctiva is anesthetized using topical anesthesia. Greenbaum’s classic technique consists of an incision made 2 to 3 mm posterior to the limbus through the conjunctiva and Tenon’s capsule.60 The administration site chosen should be away from the rectus muscles to avoid toxicity; the inferior quadrants are most commonly used. The conjunctiva can be cauterized prior to the incision to reduce subconjunctival hemorrhage. A polyethylene cannula (Greenbaum cannula [Alcon Labs Inc, Fort Worth, Tex]) or a curved blunt cannula is then introduced into the incision. The Greenbaum cannula is designed to encourage posterior dissection of the anesthetic along the globe and minimize anterior fluid release. Approximately 1.0 to 1.5 mL of a lidocaine/bupivacaine mix is injected. Alternatively, either agent alone could be used, depending on the degree and length of anesthetic effect desired. To be effective, the agent must diffuse to the posterior globe near the optic nerve and anesthetize the posterior ciliary

nerves as they pass through Tenon’s capsule to penetrate the sclera. Alternatively, an incision can be made more posterior, through the conjunctiva and Tenon’s capsule, and a curved, blunt, flattened tip cannula can be inserted posteriorly behind the equator (Figures 6-4 and 6-5). If only superficial anesthesia is needed, subconjunctival injections have been used safely and efficaciously.61-63 More extensive anesthetic effect without akinesia is obtained by circumferentially spreading subconjunctival anesthetic around the globe.64,65

Advantages/Disadvantages

There are many advantages to parabulbar anesthesia as compared to traditional orbital blocks. One advantage is that the onset of anesthesia is immediate. Akinesia, however, may take approximately 5 minutes to achieve because the motor axons are large caliber myelinated nerves. The level of sensory blockade is better with a parabulbar block than a retrobulbar block because all three branches of the ophthalmic division of the trigeminal nerve are addressed. Therefore, facial blocks are not needed because patients do not perceive the sensory stimuli. In addition, there is no significant increase in intraocular pressure, which alleviates the need for ocular compression after the injection. Furthermore, as compared to retrobulbar or peribulbar blocks, pain during administration and during surgery is lessened.23 Finally, there are no needlerelated complications, such as globe perforation, because a blunt probe is used for injection.

There are few disadvantages to a sub-Tenon’s approach. It should be done under sterile conditions; therefore, it is not well suited for the preoperative holding area. Additionally, compared to topical anesthesia, an incision into the conjunctiva must be made. This may cause patient discomfort and contribute to subconjunctival hemorrhage and chemosis. Discomfort during administration of the anesthesia can be minimized by prior topical anesthetic application or the addition of intravenous sedation. Postoperative cosmesis and return of visual acuity is inferior as compared to topical anesthesia.

Figure 6-5. Sub-Tenon's block. A blunt-tip cannula is introduced in the sub-Tenon's space. The cannula is directed posteriorly and the anesthetic agent is injected (reprinted from Lindquist TD, Lindstrom RL. Ophthalmic Surgery: Looseleaf and Update Series: Anesthesia in Ocular Surgery. Orlando, Fla: Elsevier Science; 1990:IA9, with permission from Elsevier Science).

Complications

Complications of parabulbar anesthesia are rare but could include injury to the sclera and ciliary nerves and periorbital hemorrhage. If a needle is used instead of a blunt cannula, needle-related injuries are possible.

GENERAL ANESTHESIA

History

Since the introduction of local anesthesia, general anesthesia for ocular surgery has traditionally been reserved for young children and during extensive orbital surgery. For elective ocular surgery, it is mainly reserved for situations in which patients are not suitable for local anesthesia, including patients who are anxious, uncooperative, or uncommunicative; have involuntary movements (ie, head tremor, nystagmus); or are unable to lie still. In the past, bleeding disorders were considered a relative indication for general anesthesia. However, with the development of topical and parabulbar anesthesia, the risks of retrobulbar hemorrhage are largely avoided.

Description/Technique

A thorough physical exam should be performed before deciding if a patient is a good candidate for general anesthesia. A review of medications should be employed, as medications can interact with the anesthetic agents (ie, monoamine oxidase inhibitors).66 A history of prostate enlargement should be sought, as this may contribute to urinary retention in patients undergoing general anesthe-

sia.66 Previous reactions to general anesthesia in the patient or family members should be addressed. Additionally, patients that may be difficult to intubate, such as those with cervical spondylosis, should be identified and evaluated prior to surgery.

The traditional method of providing general anesthesia involves endotracheal intubation, paralysis, and ventilation. Induction of anesthesia is done with thiopental, methohexital, or propofol. Maintenance of anesthesia can be maintained with inhalational agents such as nitrous oxide, halothane, enthurane, and isoflurane. Neuromuscular blockade is achieved by using depolarizing agents (ie, succinylcholine) or nondepolarizing agents (ie, pancuronium). Alternatively, a laryngeal mask can be inserted and anesthesia maintained with a propofol infusion or volatile agent. The use of the laryngeal mask enables faster turnaround times and reduces the cough associated with extubation.

Advantages/Disadvantages

The advantages of general anesthesia include patient comfort, ideal operating conditions, no needle-associated risks of local anesthetic blocks, no akinesia or amaurosis at the conclusion of surgery, and better conditions for prolonged cases or teaching. As the patient population for phakic IOLs is skewed toward higher myopes with long eyes, general anesthesia may be a safer alternative to regional blocks in some patients.

A disadvantage of general anesthesia is that if paralytics are used, they may wear off before the surgery is complete. Alternatively, the patient may remain paralyzed at the completion of the surgery, delaying spontaneous respirations. Because of the time associated with inducing general anesthesia and waiting for spontaneous respiration to ensue, turn around times between cases may be prolonged. Increased turn around time and more extensive equipment and monitoring make general anesthesia a more expensive alternative. This is often not desirable when the surgery is an out-of-pocket expense. In one study, there was an increased risk of intraoperative myocardial ischemic events in elderly patients undergoing cataract surgery with general anesthesia as compared to a peribulbar block.67 However, the patients in that study had risk factors for myocardial ischemia, which are uncommon in the patient population undergoing phakic IOL implantation.

Complications

One of the most feared complications of general anesthesia is malignant hyperthermia. Malignant hyperthermia is a rare consequence of general anesthesia with an incidence in adults of approximately 1 in 50,000.68 It is an autosomal dominant defect in muscle metabolism that is triggered by anesthesia agents and succinylcholine. A his-

tory of reactions to anesthesia in family members should be sought. Physicians should also be familiar with the signs and symptoms of malignant hyperthermia, which include tachycardia, tachypnea, hypercarbia, muscle spasms, acidosis, hyperkalemia, hypovolemia, and hyperthermia. Treatment consists of cooling the patient with ice baths, iced saline, and cold water lavage. Oxygen, bicarbonate, and dantrolene sodium should also be on hand. Local anesthesia is preferred in patients who may be susceptible to malignant hyperthermia.

Ocular complications of general anesthesia are rare. Postoperative extubation difficulties, coughing, or vomiting can lead to valsalva retinopathy, retinal detachment, bleeding, wound dehiscence, or loss of vitreous.69 Additionally, if the unoperative eye is not taped during surgery, it may lead to exposure keratopathy or a corneal abrasion.

CONCLUSIONS

There are multiple techniques to achieve adequate anesthesia for phakic IOL placement. Although supportive literature of anesthetic techniques is derived from cataract surgery patients, it can be extrapolated to phakic IOL placement. The decision as to which technique to use should be individualized and based on the patient’s characteristics and desires, the type of surgery performed, surgeon preference, and risk factors associated with the various techniques.

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3.Gimbel HV, Ziemba SL. Management of myopic astigmatism with phakic intraocular lens implantation. J Cataract Refract Surg. 2002;28:883-886.

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9.El Danasoury MA, El Maghraby A, Gamali TO. Comparison of iris-fixed Artisan lens implantation with excimer laser in situ keratomileusis in correcting myopia between -9.0 and -19.5 diopters. A randomized study. Ophthalmology. 2002;109:955-964.

10.Budo C, Hessloehl JC, Izak M, et al. Multicenter study of the Artisan phakic intraocular lens. J Cataract Refract Surg. 2000;26:1163-1171.

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13.Knapp H. On cocaine and its use in ophthalmic surgery. Arch Ophthalmol. 1884;13:402-448.

14.Fichman RA. Use of topical anesthesia alone in cataract surgery. J Cataract Refract Surg. 1996;22:612-614.

15.Gills JP, Cherchio M, Raanan M. Unpreserved lidocaine to control discomfort during cataract surgery using topical anesthesia. J Cataract Refract Surg. 1996;22:612-614.

16.Hasan SA, Edelhauser HF, Kim T. Topical/intracameral anesthesia for cataract surgery. Surv Ophthalmol. 2001;46:178-180.

17.Karp CL, Cox TA, Wagoner MD, et al. Intracameral anesthesia: a report by the American Academy of Ophthalmology. Ophthalmology. 2001;108:1704-1710.

18.Anderson NJ, Woods WD, Kim T, Rudnick DE, Edelhauser HE. Intracameral anesthesia: in vitro iris and corneal uptake and washout of 1% lidocaine hydrochloride. Arch Ophthalmol. 1999;117:225-232.

19.Marr WG, Wood R, Senterfit L, Siegelman S. The effect of topical anesthesia on regeneration of corneal epithelium. Am J Ophthalmol. 1957;43:606-610.

20.Rosenwasser GOD. Complications of topical ocular anesthetics. Int Ophthalmol Clin. 1989;29:153-158.

21.Barequet IS, Soriano ES, Green WR, O’Brien TP. Provision of anesthesia with single application of lidocaine 2% gel.

J Cataract Refract Surg. 1999;25:626-631.

22.Fichman RA. Topical anesthesia. In: Gills JP, Hustead RF, Sanders DR, eds. Ophthalmic Anesthesia. Thorofare, NJ: SLACK Incorporated; 1993:166-171.

23.Friedman DS, Bass EB, Lubomski LH, et al. Synthesis of the literature on the effectiveness of regional anesthesia for cataract surgery. Ophthalmology. 2001;108:519-529.

24.Crandall AS, Zabriskie NA, Patel BC, et al. A comparison of patient comfort during cataract surgery with topical anesthesia versus topical anesthesia and intracameral lidocaine. Ophthalmology. 1999;106:60-66.

25.Carino NS, Slomovic AR, Chung F, Marcovich AL. Topical tetracaine versus topical tetracaine plus intracameral lidocaine for cataract surgery. J Cataract Refract Surg. 1998;24:1602-1608.

26.Katz J, Felman MA, Bass EB, et al. Adverse intraoperative medical events and their association with anesthesia management strategies in cataract surgery. Ophthalmology. 2001;108:1721-1726.

27.Anderson NJ, Nath R, Anderson CJ, Edelhauser HF. Comparison of preservative-free bupivacaine vs lidocaine for intracameral anesthesia: a randomized clinical trial and in vitro analysis. Am J Ophthalmol. 1999;127:393-402.

28.Judge AJ, Katayoun N, Lee D, Miller K. Corneal endothelial toxicity of topical anesthetics. Ophthalmology. 1997;104:1373-1379.

29.Kim T, Holley GP, Jong HL, et al. The effects of intraocular lidocaine on the corneal endothelium. Ophthalmology. 1998;105:125-130.

30.Hoffman RS, Fine IH. Transient no light perception visual acuity after intracameral lidocaine injection. J Cataract Refract Surg. 1997;23:957-958.

31.Russell DA, Guyton JS. Retrobulbar injection of lidocaine (Xylocaine) for anesthesia and akinesia. Am J Ophthalmol. 1954;38:78-84.

32.Atkinson WS. Use of hyaluronidase with local anesthesia in ophthalmology: a preliminary report. Arch Ophthalmol. 1949;42:628-633.

33.Atkinson WS. The development of ophthalmic anesthesia. Am J Ophthalmol. 1961;51:1-14.

34.Gills JP, Hustead RF, Sanders DR, eds. Ophthalmic Anesthesia. Thorofare, NJ: SLACK Incorporated; 1993.

35.Hamilton RC. Technique of orbital regional anesthesia. Br J Anaesth. 1995;75:88-92.

36. Hamilton RC. Retrobulbar block revisited and revised. J Cataract Refract Surg. 1996;22(9):1147-1150.

37.Unsold R, Stanley J, DeGroot J. The CT-topography of retrobulbar anesthesia. Graefes Arch Clin Exp Ophthalmol. 1981;217:125-136.

38.Liu C, Youl B, Moseley I. Magnetic resonance imaging of the optic nerve in extremes of gaze: implications for the positioning of the globe for retrobulbar anesthesia. Br J Ophthalmol. 1992;76:728-733.

39.Katsev DA, Drews RC, Rose BT. An anatomic study of retrobulbar needle path length. Ophthalmology. 1989;96:1221-1224.

40.Hørven I. Ophthalmic artery pressure during retrobulbar anesthesia. Acta Ophthalmol. 1978;56:574-586.

41.Churchill AJ, James TE, Lacey V. Should myopes have routine axial length measurements before retrobulbar or peribulbar injections? Br J Ophthalmol. 1996;80:498.

42.Duker JS, Belmont JB, Benson WE. Inadvertent globe perforation during retrobulbar and peribulbar anesthesia: risk fac-

tors and outcome in 50,000 consecutive injections.

J Cataract Refract Surg. 1991;98:519-526.

43.Rainin EA, Carlson BM. Postoperative diplopia and ptosis: a clinical hypothesis based on the myotoxicity of local anesthetics. Arch Ophthalmol. 1985;103:1337-1339.

44.Duker JS, Belmont JB, Benson WE, et al. Inadvertent globe perforation during retrobulbar and peribulbar anesthesia: patient characteristics, surgical management, and visual outcome. Ophthalmology. 1991;98:519-526.

45.Sullivan KL, Brown GC, Forman AR, et al. Retrobulbar anesthesia and retinal vascular obstruction. Ophthalmology. 1983;90:373-377.

46.Hay A, Flynn HW, Hoffman JI, Rivera AH. Needle penetration of the globe during retrobulbar and peribulbar injections. Ophthalmology. 1991;98:1017-1024.

47.Morgan CM, Schatz H, Vine AK, et al. Ocular complications associated with retrobulbar injections. Ophthalmology. 1988;95:660-665.

48.Javitt JC, Addiego R, Friedberg HL, et al. Brainstem anesthesia after retrobulbar block. Ophthalmology. 1987;94: 718-724.

49.Nicoll JM, Acharya RA, Ahlen K, et al. Central nervous system complications after 6000 retrobulbar blocks. Anesth Analg. 1987;66:1298-1302.

50.Feitl ME, Krupin T. Retrobulbar anesthesia. Ophthalmol Clin North Am. 1990;3:83-91.

51.Davis DB, Mandel MR. Posterior peribulbar anesthesia: an alternative to retrobulbar anesthesia. J Cataract Refract Surg. 1986;12:182-184.

52.Davis DB, Mandel MR. Peribulbar anesthesia. A review of technique and complications. Ophthalmol Clin North Am. 1990;3:101-110.

53.Edge KR, Davis A. Brainstem anesthesia following a peribulbar block for eye surgery. Anaesth Intensive Care. 1995;23:219-221.

54.Puustjarvi T, Purhonen S. Permanent blindness following retrobulbar hemorrhage after peribulbar anesthesia for cataract surgery. Ophthalmic Surg. 1992;23:450-452.

55.Weiss JL, Deichmann CB. A comparison of retrobulbar and periocular anesthesia for cataract surgery. Arch Ophthalmol. 1989;107:96-98.

56.Hamilton RC, Grizzard WS. Complications. In: Gills JP, Hustead RF, Sanders DR, eds. Ophthalmic Anesthesia. Thorofare, NJ: SLACK Incorporated; 1993:187-202.

57.Turnbull CS. Editorial. Med Surg Rep. 1884;628.

58.Greenbaum S. Anesthesia for Eye Surgery. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology.

Philadelphia, Pa: Lippincott, Williams and Wilkins; 2000:1- 32.

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60.Greenbaum S. Anesthesia in cataract surgery. In: Greenbaum S, ed. Ocular Anesthesia. Philadelphia, Pa: WB Saunders; 1997:1-55.

61.Anderson CJ. Subconjunctival anesthesia in cataract surgery. J Cataract Refract Surg. 1995;21:103-105.

62.Petersen WC, Yanoff M. Subconjunctival anesthesia: an alternative to retrobulbar and peribulbar techniques. Ophthalmic Surg. 1991;22:199-201.

63.Anderson CJ. Combined topical and conjunctival anesthesia in cataract surgery. Ophthalmic Surg Lasers. 1995;26:205208.

64. Anderson CJ. Circumferential perilimbal anesthesia.

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INTRODUCTION

Astigmatism management represents an important and challenging facet of any refractive surgical procedure, and this certainly pertains to the use of phakic intraocular lenses (IOLs). Estimates of the incidence of significant, naturally occurring astigmatism vary from 7.5% to 75%.1 It is thought that 3% to 15% of eyes may have two or more diopters (D) of astigmatism.2 Although some degree of pre-existing astigmatism may be reduced when using phakic IOLs, a substantial number of patients will clearly require treatment for this condition. Patient selection, quantification of astigmatism, treatment options, and techniques are all important considerations and will be discussed below.

PATIENT SELECTION

It is generally agreed that uncorrected astigmatism of greater than 0.5 D will lead to symptoms of ghosting and shadows. When using myopic phakic IOLs, it has been our experience that some degree of pre-existing cylinder is dampened, presumably due to image magnification. This is consistent with the frequent finding of an increase in bestcorrected visual acuity postoperatively. With this in mind, one may be slightly less aggressive in his or her approach to the treatment of astigmatism, but levels of greater than 1.00 D are likely to leave the patient symptomatic and, therefore, should be addressed.

Important also is the location of the astigmatism, age of the patient, and status of the fellow eye. Because most individuals will drift against-the-rule over their lifetime, many surgeons advocate a less aggressive approach to the reduction of with-the-rule cylinder. The widely held tenet that residual (myopic) with-the-rule astigmatism favors better distance vision and against-the-rule benefits near vision pertains more to the pseudophakic state and, as such, maximizing the position of the conoid of Sturm becomes less relevant in the context of phakic IOLs. It is well known, however, that oblique cylinder is poorly tolerated, and one must opt to avoid a disparity in the axis of cylinder between each eye in order to minimize meridional aniseikonia.

The surgeon must also factor into account the astigmatic effect of the incision needed to implant the phakic lens. Most foldable lens styles and their small incisions will have negligible effect, particularly when they are inserted temporally.3 These small incisions have also been shown to stabilize rapidly and show little drift.4 This cannot, however, be said of incisions in the range of 6.0 mm, which are typically needed to insert rigid lens styles, and it is generally agreed that superiorly placed incisions will have greater postoperative drift and wound flattening.5 This phenomenon may in fact be used to reduce pre-exist- ing astigmatism when the incision is placed upon the steep meridian as described later in the chapter. If a larger incision is needed to accommodate a particular implant style, the astigmatic effect may be minimized through incision design.6

58 Chapter 7

TREATMENT OPTIONS

The first decision faced by the surgeon is whether to address pre-existing astigmatism at the time of implantation or to defer and treat the cylinder separately. One could argue that for the highest level of accuracy, sufficient wound healing should take place and a stable refraction ought to be documented prior to astigmatic correction. This consideration is more important with rigid lenses and larger incisions. In addition, residual spherical error may be corrected along with astigmatism at a second sitting by utilizing a bioptics approach, as described later in the chapter. Many surgeons, however, feel that once they have determined the astigmatic effect of the implant incision and it is factored into their strategy, concomitant treatment of pre-existing astigmatism is a more efficient approach. It is more favored because it is likely to save the patient from having to undergo a second procedure.

The next major decision is whether to treat the astigmatism utilizing a lenticular approach (ie, to employ a toric IOL or a keratorefractive technique). From a theoretical standpoint, it is hard to argue against the use of a toric phakic IOL. However, not unlike toric pseudophakic implants, limited designs currently exist and similar intraoperative challenges arise as to proper axis alignment. Additionally, long-term stability will need to be confirmed.7,8 Early studies are nonetheless quite promising, and the use of such lenses will likely increase in the future.9

As noted, one can positively affect pre-existing cylinder by manipulating the implant incision’s location, size, and design.10 Specifically, one can center the incision upon the steep meridian and then increase or decrease the amount of flattening that will occur by increasing or decreasing the length of the incision. Similarly, one can increase wound flattening by moving closer to the visual axis or by creating a more circumparallel incision to the limbus. This approach, however, presents logistical challenges by requiring movement about the operating table and may potentially require awkward hand positions. This is important when considering phakic intraocular surgery in which very delicate maneuvers may be required. For these reasons, as with pseudophakic lens surgery, a more common incisional strategy is to utilize a consistent and reproducible implant incision, typically located temporally and as astigmatically neutral as possible, and then correct significant pre-existing astigmatism separately. Currently, this latter element most commonly takes the form of corneal or limbal relaxing incisions (LRIs).

Limbal Relaxing Incisions

Our experience with the use of LRIs originated from the work of Dr. Stephen Hollis. Through refinement of his nomogram, along with the addition of Dr. Spencer

Thornton’s age modifiers, the author now utilizes a system for astigmatism reduction that is more forgiving and less demanding than that using smaller optical zones and true “corneal” astigmatic relaxing incisions. Other surgeons have experienced similar results with comparable techniques,11,12 and published reports have subsequently documented the safety and efficacy of LRIs.13,14

An advantage of LRIs includes less tendency to cause axis shift. This presumably occurs because the need to center the incisions precisely upon the steep meridian decreases. Perhaps more importantly, these more peripheral incisions are less likely to induce irregular corneal flattening and, hence, irregular astigmatism. Technically, they are easier to perform than shorter and more central corneal astigmatic incisions, and patients generally report less discomfort.

Yet another advantage gained by moving out to the limbus relates to the “coupling ratio,” which describes the amount of flattening that occurs in the incised meridian relative to the amount of steepening that is induced 90 degrees away. LRIs exhibit a very consistent 1:1 coupling ratio; therefore, little if any change occurs in the spheroequivalent, thus obviating the need to alter the power of the phakic IOL. Admittedly, these incisions are less powerful than corneal incisions, but one can correct up to 3 D of astigmatism depending upon the age of the patient. One must keep in mind that the goal is to reduce the patient’s cylinder without overcorrecting or shifting the axis.

Measuring Astigmatism

Perhaps the most challenging and often frustrating aspect of astigmatism surgery deals with the determination of the quantity and exact location of the preoperative cylinder that needs to be corrected. Unfortunately, preoperative measurements—keratometry, refraction, and topography—do not always agree. Lenticular astigmatism may account for some of this disparity; however, the author’s experience supports the notion that traditional measurements of astigmatism (eg, those obtained with standard keratometry [only 2 points measured in each meridian]) do not always adequately describe the astigmatic state of a patient.

When confounding measurements are obtained, one can compromise and average the disparate readings. This is frequently done when using LRIs at the time of cataract and implant surgery.15 Unlike pseudophakic surgery, lenticular astigmatism is not eliminated when using phakic IOLs and, as such, more emphasis is placed upon the patient’s manifest refraction rather than keratometry. Corneal topography is often helpful when the refraction and keratometry differ and may act as a “tie-breaker.” Topography, of course, is also helpful in detecting subtle corneal pathology, such as keratoconus fruste, which would likely negate the use of LRIs.

Figure 7-1. Nomogram design. Note relative disparity in incision length between a large and small corneal diameter if measured in millimeters. Degrees of arc lend consistency irrespective of corneal size.

Once the amount and location of the astigmatism has been determined, a number of different nomograms may be consulted.16 As mentioned, the author’s preferred nomogram is based upon that of Hollis and incorporates the surgical principles described by Thornton (Table 7-1).17 This nomogram utilizes paired incisions rather than longer single incisions to optimize symmetric corneal flattening and is expressed in degrees of arc rather than millimeters. This is done to avoid over and under corrections that may occur in unusually small or large corneas because corneal diame-

ter may significantly influence the relative arc length of the incision and its resultant effect (Figure 7-1).

Surgical Technique

Increasing evidence indicates that significant cyclotorsion may occur when assuming a supine position.18 For this reason, most surgeons advocate placing an orientation mark at the 12:00 or 6:00 limbus while the patient is in an upright position. According to Euler’s theorem, an axis deviation of 5, 10, or 15 degrees will result in 17%, 33%, and 50% reduction, respectively, in surgical effect.19 This is particularly important when employing injection anesthesia wherein unpredictable eye rotation may occur. An additional measure to help accurately center the relaxing incisions is to identify the steep meridian (plus cylinder axis) intraoperatively by using some form of keratoscopy. The steep meridian over which the incisions are centered corresponds to the shorter axis of the reflected corneal mire. A simple hand-held device such as the Maloney keratoscope (produced by a number of manufacturers) works nicely, or a more elaborate microscope-mounted instrument may be used, such as the Mastel Ring of Light (Mastel Precision, Rapid City, SC). The steep meridian may also be identified by aligning a Mendez ring (Rhein Medical, Tampa, Fla) or similar degree gauge with the previously placed 12:00 or 6:00 limbal orientation mark, and then locating the cylinder axis on the 360-degree marker.