Incisional Surgery for Natural and Surgically-Induced Astigmatism



Incisional Surgery for Natural and Surgically-Induced Astigmatism


Roger F. Steinert



Regular and irregular astigmatism cause impairment of uncorrected visual acuity and, in moderate to large amounts, loss of best corrected visual acuity. Spectacle correction of higher amounts of astigmatism induces distortion that can be reduced with the use of rigid gas permeable contact lenses or toric soft contact lenses. Surgical intervention to reduce astigmatism provides a permanent alternative solution. In general, the goal of surgical intervention is not the complete elimination of the astigmatism, a goal difficult to achieve because of the variability of the corneal response to astigmatic correction. Rather, the appropriate goal is to reduce the amount of astigmatism to a level that has less functional impact on uncorrected visual acuity and allows a satisfactory correction of the residual smaller astigmatism through the use of spectacles or contact lenses.


HISTORICAL PERSPECTIVE

The first successful surgical intervention for astigmatism occurred in 1885, when Schiotz placed a 3.5-mm limbal penetrating incision in a steep meridian, reducing astigmatism after cataract surgery from 19.5 D to 7 D (1). Lucciola was the first surgeon to employ nonperforating incisions to correct astigmatism (2). Lans (3) first appreciated that the flattening that occurs in a corneal meridian after placing a transverse incision was associated with steepening in the opposite meridian.

He also demonstrated that deeper and longer incisions had more effect. Refinement of astigmatic keratometry has occurred through many investigations, often employing cadaver eye models (4, 5, 6, 7, 8, 9, 10).

Introduction of excimer laser ablation into clinical practice offers the surgeon the ability to directly address astigmatism by reshaping the corneal surface, either with photorefractive keratectomy (PRK) or laser in-situ keratomileusis (LASIK).


CHOOSING A SURGICAL PROCEDURE

Choosing between the surgical alternatives for astigmatism correction begins with the preoperative assessment of the patient. In addition to quantifying the amount of astigmatism and documenting the corresponding corneal topographic change, the surgeon must evaluate the etiology of the astigmatism and the impact of each of the surgical alternatives on the patient’s overall optical balance.

In the case of naturally occurring astigmatism, corneal topography is essential to differentiate between regular astigmatism and astigmatism due to corneal ectasias, principally keratoconus and pellucid marginal degeneration. Astigmatic incisions in the presence of corneal ectasia generally do poorly, with unpredictable and unstable results. Excimer laser ablation will thin an already abnormally weakened cornea, and is generally regarded as contraindicated in the presence of corneal ectasia. In some cases, patients are motivated to seek reduction in astigmatism as an independent procedure. More commonly, surgery for naturally occurring astigmatism becomes a consideration when a patient is undergoing cataract surgery, as surgeons and patients elevate their expectations for high levels of uncorrected visual acuity postoperatively.

Postoperative induced astigmatism is best separated into the separate categories of limbal incision surgically induced astigmatism and postkeratoplasty astigmatism.

Limbal incisions at the time of cataract surgery have been a troublesome cause of astigmatism for the entire history of cataract surgery, until the recent introduction of incisions in the range of 3 mm or less. With phacoemulsification cataract extraction, and the implantation of foldable intraocular lenses, the induction of a functionally troublesome amount of astigmatism is now infrequent. Because some patients still require a larger incision for extracapsular cataract extraction or management of some operative complications, however, cataract surgery remains a major cause of surgically induced astigmatism. In addition, astigmatism is seen not
infrequently in conjunction with glaucoma filtration surgery, particularly if the filtration site is combined with a larger incision for simultaneous cataract surgery.

Postkeratoplasty astigmatism typically includes a combination of regular and irregular astigmatism. Treatment of postkeratoplasty astigmatism is complicated by this combination; surgical intervention for irregular astigmatism is difficult at best. Moreover, the etiology of postkeratoplasty astigmatism resides in the junction of the graft and host, which usually is at a diameter between 7 and 9 mm.

Residing closer to the optical zone, this keratoplasty wound has a much larger impact on the central corneal optics than a limbal incision. Moreover, because a corneal transplant incision occupies the full 360 degrees, the opportunity for optically significant distortion is greatly magnified compared to a surgical limbal wound. In addition, centration of corneal transplants relative to the visual axis is often suboptimal for a number of reasons, not the least of which is that the visual axis cannot be directly measured. Lessons learned from laser vision correction as well as radial keratotomy emphasize the deleterious impact of decentration of the treatment zone relative to the visual axis.

From these starting points, the surgeon must then determine several other components of the treatment plan. Is the needed correction primarily the reduction of astigmatism, or is it also a major shift in residual hyperopia or myopia? Incisional keratotomy has a modest impact, at most, on the spherical equivalent, particularly with arcuate keratotomy, which is the most commonly employed form of incisional keratotomy. A major shift in the spherical equivalent is better addressed with PRK or LASIK, or, in addition to the astigmatic incision, placement of a secondary intraocular lens or intraocular lens exchange to alter the spherical equivalent.

A second critical component in the treatment plan is to determine whether the principal astigmatic component is excessive flattening in one meridian, or excessive steepening in another. In examining the corneal topography, excessive flattening is readily determined, typically heralded by hemimeridian dioptric values below 40 diopters. A flat hemimeridian suggests significant wound slippage.

It is better to surgically remedy the surgical wound first, before resorting to secondary astigmatic surgery. For example, if a large cataract incision is associated with flattening, the incision should be carefully inspected to see if there is visible wound separation, in which case, the surgeon should evaluate the ability to expose the incision, freshen the wound margins, and resuture the defect. This is particularly prominent when there is an accompanying inadvertent filtering bleb.

In the case of penetrating keratoplasty, excessive flattening is often associated with override of the graft-host junction. The contour of the graft relative to the host can be appreciated not only through the color maps of the typical corneal topography representation but, sometimes more strikingly, in viewing the placido mires of the corneal topography and direct examination of the cornea three-dimensionally at the slit lamp. The placido mires often have a dramatic “teardrop” distortion toward an area of graft-host override. If this type of deformity is a major contributor to the patient’s postoperative astigmatism, the misalignment of the graft and host should be addressed through opening of the defective wound area and repositioning the graft with multiple interrupted 10-0 nylon sutures. Attempting to correct the astigmatism through other measures, while leaving the graft-host junction abnormality, invariably results in a poor outcome.


ASTIGMATIC KERATOTOMY


Patient Selection and Evaluation

Peripheral corneal relaxing incisions (PCRIs), commonly known as limbal relaxing incisions, have the advantage of more reliable wound healing, particularly in elderly individuals where clear corneal incisions heal variably and sometimes poorly, and less potential of inducing irregular astigmatism because of their more peripheral location. PCRIs have less effective power than more centrally located incisions in the clear cornea, however. Decision about the location of the incision, therefore, is driven by an evaluation of the patient’s likely wound healing capability and the amount of astigmatic correction desired. In many cases, adequate improvement in visual function can be obtained with only a partial correction of the full astigmatic error. It is not necessary to target zero residual cylinder, which risks an overcorrection in some patients.

In addition to a comprehensive examination, particular attention should be given to corneal topography and pachymetry. Any areas of previous surgery also need careful inspection to assess the potential that wound slippage is the underlying problem that needs to be addressed rather than superimposing astigmatic surgery.


Surgical Technique

The surgeon must be equipped with surgical markers in order to lay down the pattern of the desired correction, an intraoperative pachymeter to determine the corneal thickness at the time of treatment, a diamond knife designed for astigmatic surgery, and an operating microscope.

With the patient upright, the limbus should be marked with a gentian violet sterile surgical marking pen in order to correct for any cyclotorsion or head malposition that may occur with a patient in the supine position. Some surgeons prefer to make marks at the 3 and 9 o’clock positions, whereas others make marks at the 6 and 12 o’clock positions. If a slit lamp is available that allows rotation of the light beam to a specific setting in degrees, it is possible to directly mark the meridian for the surgery.


Anesthesia is achieved typically with topical 0.5% proparacaine or tetracaine with three or four applications every 5 minutes. More profound anesthesia can be obtained with lidocaine jelly, but the residual gel on the ocular surface is undesirable at the time of performing the surgery, and the extra anesthesia is generally unnecessary. Many surgeons apply one to two drops of 0.5% povidone-iodine solution and employ a sterile small lid speculum such as a Barraquer wire lid speculum. Draping the skin and lashes is preferred by some surgeons; others feel that the drape becomes an encumbrance as well as causing patient anxiety.

The surgical incisions are usually centered relative to the entrance pupil. The apical light reflex is not necessarily the visual axis, and the center of the pupil is a more reliable guide for centration in the absence of an abnormal pupil.

With a patient fixating on the coaxial light of the operating room microscope, the surgeon places the appropriate marks depending on the desired treatment pattern (see below). If the cornea is relatively dry, the impression of the marker on the corneal epithelium may be sufficient. Some surgeons prefer to place gentian violet ink from a sterile surgical marker on the corneal marking device in order to leave a readily visible guide for the incisions. Pachymetry is then performed immediately prior to the incisions. In clear corneal incisions, a calibrated diamond knife is usually set at 90% to 100% of the pachymetry value, depending on the surgeon’s experience. The goal is to achieve an incision as deep as possible without perforating through Descemet’s membrane. In the case of PCRIs, most surgeons use a standard setting of either 600 or 650 μm for the periphery.

While the patient is fixating on the operating microscope light, the surgeon fixates the limbus with a forceps and the knife is inserted at the beginning of the incision. The surgeon should pause at this point to allow the blade to penetrate to the full depth, and then slowly and steadily advance the knife along the incision.

Most surgeons prefer a front-cutting knife in order to obtain good visibility.

At the completion of the procedure, a drop of a broad-spectrum antibiotic and a nonsteroidal antiinflammatory drug are typically applied. Routine patching, application of a bandage contact lens, and cycloplegia are usually not necessary. If a perforation has occurred, the cornea should be observed for sealing. If a slow, spontaneous leak persists, an interrupted 10-0 nylon suture should be placed.


Choice of Astigmatic Keratotomy Pattern

Clear corneal incisions are typically either straight (transverse keratotomy) or curved parallel to the limbus (arcuate keratotomy). Peripheral corneal relaxing incisions are always in an arc parallel to the limbus, just inside the vascular arcade. In the past, transverse incisions were sometimes applied with the addition of radial keratotomy incisions, but now this is rarely employed. These different patterns of incisions are illustrated in Figure 71-1. Transverse incisions have increasingly fallen out of favor relative to arcuate incisions. The effect of a transverse incision is maximal at its center, where the incision is closest to the optical center of the cornea, with less effect in the periphery, where the straight incision becomes more distant from the optical zone. In contrast, an arcuate keratotomy incision maintains the same distance from the optical zone throughout its length. Surgical experience has led many surgeons to favor arcuate keratotomy for all clear corneal incisions, with a feeling that a longer arcuate keratotomy and a larger optical zone are preferable to a shorter transverse keratotomy closer to the optical zone.






FIGURE 71-1. Patterns of astigmatic keratotomy.


Coupling refers to the phenomenon that placement of the incision in the steep meridian leads to flattening of the steep meridian, but accompanied by some degree of steepening of the untreated flat meridian 90 degrees away from the incision.

If there is an equal amount of steepening of the flat meridian compared to the flattening of the steep meridian, the coupling ratio is 1:1, and there is no change in the spherical equivalent. In most applications of astigmatic keratotomy, the coupling ratio is close to 1:1. At most, there is a mild hyperopic shift amounting to between 0.5 and 1 D. In the past, transverse keratotomies were sometimes combined with adjacent radial incisions to either side, which was reported to achieve a coupling ratio as high as 5:1. This degree of flattening of the steep meridian with markedly less steepening of the flat meridian 90 degrees away would achieve a more dramatic hyperopic shift, and could be used to manipulate a patient’s spherical equivalent to reduce myopia. Lack of predictability of this procedure, however, has largely caused it to be abandoned. In most cases where a large shift in the spherical equivalent is desired, consideration should be given to PRK or LASIK.

In principle, a reversed coupling ratio, where there is a net myopic shift, can be achieved with arcuate incisions greater than 90 degrees in length. The instability of such a large incision makes it inadvisable under most circumstances, however.


Variables Effecting Outcome

An increased effect occurs with incisions closer to the visual axis, longer incisions, arcuate rather than straight incisions, increased number of incisions, increased depth of incision, increasing patient age, and male gender.

Typical nomograms for arcuate, transverse, and peripheral corneal relaxing incisions are given in Tables 71-1, 71-2, 71-3 and 71-4.


Complications


Irregular Astigmatism

Irregular astigmatism occurs more commonly with incisions closer to the visual axis, decentered incisions, and poor wound healing. For example, an asymmetrical overresponse, where one hemimeridian flattens more than the other due to excessive wound separation, should be recognized and treated immediately with placement of 10-0 nylon sutures to reappose the excessively separated incision. When there is a choice, a conservative surgeon will choose the surgical technique that places the incision as peripheral as possible, but not exceeding a maximum of 75 to 90 degrees in arc length.


Overcorrection

Symmetrical overcorrection should be treated by cleaning out the incision and resuturing with interrupted 10-0 nylon sutures. If at all possible, the surgeon should avoid attempting to treat overcorrection by placing further astigmatic incisions in the meridian 90 degrees opposite the original meridian. The combination of astigmatic incisions in multiple meridians commonly leads to irregular astigmatism.








TABLE 71-1. NOMOGRAM FOR PERIPHERAL CORNEAL RELAXING INCISIONS TO CORRECT KERATOMETRIC ASTIGMATISM DURING CATARACT SURGERY (TEMPORAL 3.2- TO 3.5-mm CLEAR-CORNEAL INCISION)






































































Preoperative Astigmatism (D)

Age (year)

Number

Length (Degrees)


WTR

0.75-1

<65

2 (or 1 × 60 degrees)

45


≥65

1

45

1.01-1.50

<65

2

60


≥65

2 (or 1 × 60 degrees)

45

>1.50

<65

2

80


≥65

2

60


ATR

1-1.25a

_

1

35

1.26-2

_

1

45

≥2

_

2

45


a Especially if cataract incision is not directly centered on steep meridian.


From Koch D, Lindstrom RL, Wang L, Osher RH. Control of astigmatism in the cataract patient. In: Steinert RF, ed. Cataract surgery, 2nd ed. New York: Elsevier, 2004, with permission.



Undercorrection

Undercorrection can be treated by redeepening the original incisions (utilizing a blade with sharp edges only at the depth of the blade in order to stay within the original incision track), lengthening the original incision, or placing a new set of incisions closer to the optical zone.


Infectious Keratitis

An opportunistic infection may occur in the corneal incisions. For that reason, use of povidone-iodine to prepare the globe immediately preoperatively and use of a broad-spectrum antibiotic postoperatively until wound healing occurs is advisable. Even then, breakdown in the epithelial barrier may allow a late-onset opportunistic infection. The patient should be advised to report the onset of redness, pain, or a shift in vision immediately.


Corneal Perforation

A small perforation may spontaneously seal, or respond well to simple pressure patching or a bandage soft contact lens. If in doubt, however, one or more interrupted 10-0 nylon sutures should be placed immediately. Rare cases of endophthalmitis or epithelial ingrowth into the anterior chamber have been reported after perforations following radial keratotomy and/or astigmatic keratotomy.









TABLE 71-2. ARCUATE KERATOTOMY 6-mm OPTICAL ZONE NOMOGRAM

















































































































































































































































































































































































































































































Surgical Option


Age (y)

1 × 30 Degrees

2 × 30 Degrees or 1 × 45 Degrees

1 × 60 Degrees

2 × 45 Degrees or 1 × 90 Degrees

2 × 60 Degrees

2 × 90 Degrees


20

0.60

1.20

1.80

2.40

3.60

4.80


21

0.62

1.23

1.85

2.46

3.69

4.92


22

0.63

1.26

1.89

2.52

3.78

5.04


23

0.65

1.29

1.94

2.58

3.87

5.16


24

0.66

1.32

1.98

2.64

3.96

5.28


25

0.68

1.35

2.03

2.70

4.05

5.40


26

0.69

1.38

2.07

2.76

4.14

5.52


27

0.71

1.41

2.12

2.82

4.23

5.64


28

0.72

1.44

2.16

2.88

4.32

5.76


29

0.74

1.47

2.21

2.94

4.41

5.88


30

0.75

1.50

2.25

3.00

4.50

6.00


31

0.77

1.53

2.30

3.06

4.59

6.12


32

0.78

1.56

2.34

3.12

4.68

6.24


33

0.80

1.59

2.39

3.18

4.77

6.36


34

0.81

1.62

2.43

3.24

4.86

6.48


35

0.83

1.65

2.48

3.30

4.95

6.60


36

0.84

1.68

2.52

3.36

5.04

6.72


37

0.86

1.71

2.57

3.42

5.13

6.84


38

0.87

1.74

2.61

3.48

5.22

6.96


39

0.89

1.77

2.66

3.54

5.31

7.08


40

0.90

1.80

2.70

3.60

5.40

7.20


41

0.92

1.83

2.75

3.66

5.49

7.32


42

0.93

1.86

2.79

3.72

5.58

7.44


43

0.95

1.89

2.84

3.78

5.67

7.56


44

0.96

1.92

2.88

3.84

5.76

7.68


45

0.98

1.95

2.93

3.90

5.85

7.80


46

0.99

1.98

2.97

3.96

5.94

7.92


47

1.01

2.01

3.02

4.02

6.03

8.04


48

1.02

2.04

3.06

4.08

6.12

8.16


49

1.04

2.07

3.11

4.14

6.21

8.28


50

1.05

2.10

3.15

4.20

6.30

8.40


51

1.07

2.13

3.20

4.26

6.39

8.52


52

1.08

2.16

3.24

4.32

6.48

8.64


53

1.10

2.19

3.29

4.38

6.57

8.76


54

1.11

2.22

3.33

4.44

6.66

8.88


55

1.13

2.25

3.38

4.50

6.75

9.00


56

1.14

2.28

3.42

4.56

6.84

9.12


57

1.16

2.31

3.47

4.62

6.93

9.24


58

1.17

2.34

3.51

4.68

7.02

9.36


59

1.19

2.37

3.56

4.74

7.11

9.48


60

1.20

2.40

3.60

4.80

7.20

9.60


61

1.22

2.43

3.65

4.86

7.29

9.72


62

1.23

2.46

3.69

4.92

7.38

9.84


63

1.25

2.49

3.74

4.98

7.47

9.96


64

1.26

2.52

3.78

5.04

7.56

10.08


65

1.28

2.55

3.83

5.10

7.65

10.20


66

1.29

2.58

3.87

5.16

7.74

10.32


67

1.31

2.61

3.92

5.22

7.83

10.44


68

1.32

2.64

3.96

5.28

7.92

10.56


69

1.34

2.67

4.01

5.34

8.01

10.68


70

1.35

2.70

4.05

5.40

8.10

10.80


71

1.37

2.73

4.10

5.46

8.19

10.92


72

1.38

2.76

4.14

5.52

8.28

11.04


73

1.40

2.79

4.19

5.58

8.37

11.16


74

1.41

2.82

4.23

5.64

8.46

11.28


75

1.43

2.85

4.28

5.70

8.55

11.40


Find patient age, then move right to find result closest to refractive cylinder without going over.


(From Koch D, Lindstrom RL, Wang L, Osher RH. Control of astigmatism in the cataract patient. In: Steinert RF, ed. Cataract surgery, 2nd ed. New York: Elsevier, 2004, with permission.)

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Sep 18, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Incisional Surgery for Natural and Surgically-Induced Astigmatism

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