Ординатура / Офтальмология / Английские материалы / Strabismus Surgery and Its Complications_Coats, Olitsky_2007

develop intractable diplopia later in life (Chap. 31). There are a few small series that report the successful use of orthoptic therapy in the treatment of vertical deviations [8]. Patients who have been treated successfully generally have small vertical deviations and their long-term prognosis is unknown.

17.4 Prism Correction

Prism correction can be useful in the treatment of strabismus. It may be especially useful when used on a temporary basis or when treating a small angle deviation. Prism may also be useful when a small residual deviation remains following strabismus surgery. Patients who are most likely to respond favorably to prism therapy include patients with comitant, small angle strabismus. Patients who are currently wearing spectacle correction tend to accept prism therapy more readily than those who do not already wear spectacles. It is important to distinguish the difference between needing glasses to improve visual acuity and glasses used in the treatment of diplopia. Patients with a refractive error can often function quite well during many standard activities of daily living. In addition, intermittent squinting to create a pinhole effect allows for improve of visual acuity to a level that allows an excellent degree of function. Thus patients with small refractive errors may prefer not to wear glasses, as they can be quite functional without them. If not already wearing glasses, it is often difficult to convince a patient to succumb to them, even for correction of diplopia.

Prescribed prism can be permanently placed into the spectacle lenses using offsets of the optical center of the glasses or by using ground in prism. A temporary press-on prism (i.e., Fresnel prisms) may be used as a temporary prism trial or as a good long-term option in some patients, especially those requiring large prism correction.

A Fresnel prism is a series of smaller prisms of identical angle and identical power that are placed together on a membrane with no refractive power (>Fig. 17.2). These small prisms cover the entire surface of the lens and therefore all light will be refracted identically as though a simple prism was being used. However, a single prism of equal power would be much thicker and heavier than a Fresnel prism of like power. Fresnel prisms are made of flexible plastic and can be cut to fit either the concave or convex side of a spectacle. They can be placed horizontally, vertically, or obliquely. The advantage of Fresnel prisms is the ability to prescribe large prismatic correction without the extra weight and expense for comparable amounts of permanent spectacle prism. Furthermore, these prisms can be removed easily and they are therefore well suited for the temporary relief of diplopia.

Although these properties of Fresnel prisms make them very useful tools in the treatment diplopia, other factors limit their usefulness in many cases. Fresnel prisms are subject to peeling off spectacles and to discoloration over time. Air bubbles often develop between the surface of the prism and the spectacle lens. Additionally, the use of a Fresnel prism typically

Fig. 17.2. Fresnel prism

results in a mild reduction of visual acuity and contrast sensitivity in the treated eye.

While we most commonly use monocular occlusion to treat diplopia in patients with evolving cranial nerve palsies, we will occasionally prescribe a Fresnel prism for this purpose. They are not optimal for most patients in this setting, because of the marked incomitance that is typical of strabismus due to a cranial nerve palsy. When prescribed, the prism is placed on the spectacle lens of the nondominant eye. This tends to make the small decline in visual function associated with their use less bothersome to the patient.

Patients who present with both a vertical and a horizontal deviation require special consideration. One option would be to prescribe a Fresnel prism for each spectacle lens, one for the horizontal deviation and one for the vertical deviation. This may lead to a bothersome reduction in visual function in both eyes, and is usually not well tolerated. A better option is usually to prescribe prism for the nondominant eye and place the prism obliquely on the spectacle lens to compensate for the bidirectional nature of the strabismus. In order to determine the power and direction of the prism to be used, the two vectors can be placed “head to tail” and the resultant vector (which represents the vector sum) will represent the single prism required to treat the deviation. The magnitude of this vector can be calculated using the Pythagorean theorem. The direction of the vector is determined by the formula:

ArcTangent = (magnitude of vertical prism)/(magnitude of horizontal prism)

The calculated prism can then be cut and placed on the lens surface in the direction needed. Alternatively, the power and vector of the prism can be determined utilizing a graphic approach. An example is demonstrated in Fig. 17.3.

When small prism correction is needed long-term, we prefer the use of a permanent prism placed in the patient’s spectacle lenses. (Prentice’s rule for induced prism: PD = D · h, where PD

Fig. 17.3. Determination of the power and angle of an oblique prism. The horizontal and vertical prism are depicted using a vector format. The vectors are placed head to tail. In this example, a 6-PD base-in prism is combined with an 8-PD base-down prism. Using the Pytha­ gorean theorem, the magnitude of the oblique prism is calculated to be 10 PD. The angle of the direction of this prism is arc tan (x) = 8/6. Solving for x gives an approximate angle of 37°. Therefore, the new prism should be prescribed as 10 PD with the base down and in

is the power of the induced prism, D is the power of the lens in diopters, and h is the viewing distance from the optical center in cm.) Although occasional patients will tolerate the significant weight and image distortion that can occur with larger degrees of correction, we have found that most patient do not tolerate prism correction greater than 10 prism diopters. We are therefore unlikely to suggest this as a first line of treatment for deviations larger than 10 prism diopters.

Prism can be placed into spectacles in one of two ways. Decentration of the optical center of the lens in accordance with Prentice’s rule can induce prism. To obtain a significant amount of prismatic effect, the patient must have a fairly large refractive error. Prism can also be ground into the spectacle lenses. The decision on how to best create prism in a patient’s spectacles is generally left up to the optician. However, we will specify the use of offset of the optical center of the lens in a small, select group of patients who have relatively small deviations that are incomitant. By offsetting the optical axis of the lens, not only are we able to induce the amount of required prism in the primary position but we are also able to take advantage of the change in prism power in eccentric gaze induced by this method to treat the incomitancy.

Horizontal prism is usually split evenly between the two spectacle lenses, helping to distribute the weight of the spec-

tacles more evenly. However, when a relatively small degree of vertical prism is prescribed, it may be helpful to prescribe this as a base-up correction in only one eye whenever possible. By placing the prism in a base-up orientation, it avoids the problem of the thick prism base resting against the lower eyelid or the cheek, both of which patients find bothersome.

Patients with vertical diplopia only in down gaze may benefit from prism correction in the form of a slab-off prism. This is prescribed in a similar manner to that required for patients who experience diplopia due to anisometropia after cataract surgery. Slab-off, or bicentric grinding, is a technique in which base-up prism is ground on half the lens in either the most minus or least plus lens. Unlike its use for anisometropia following cataract surgery, the purpose of bicentric grinding in patients with diplopia in down gaze is to induce a prismatic effect when looking down, not to reduce or eliminate it as in the patient with anisometropia.

Regardless of which form of prism correction is used, it is important for the amount of prism correction needed to be precise, especially for vertical deviations. Small inaccuracies in the prism prescription can result in asthenopia or persistent diplopia. A Maddox rod may be helpful in measuring vertical deviations. A prism bar or Risley prism can be placed in front of one eye while holding the Maddox rod before the contralateral eye. The Maddox rod is placed in front of one eye in the vertical direction so that the patient sees a horizontal line when looking at a fixation light. The prism bar (or Risley prism) is then adjusted until the patient reports that the line from the Maddox bar runs directly through the fixation light. This method tends to be very accurate and reproducible.

Nonsurgical modalities can and should be available in the treatment arsenal of the strabismus surgeon. Nonsurgical options can be helpful both for patients who do not need surgery and for those who do not want surgery. Nonsurgical measures can also be helpful for patients as a temporizing measure while awaiting surgery and can be invaluable for some patients to treat a small residual deviation after surgery. Some nonsurgical modalities may allow a surgeon to convert a less than ideal surgical outcome to one where the patient is satisfied and has comfortable single vision. While these nonsurgical treatment options are very important, the concept that surgery is a last resort option for the treatment of most instances of strabismus is a myth that should be ignored.

References

1.Iacobucci IL, Archer SM, Giles CL (1993) Children with exotropia responsive to spectacle correction of hyperopia. Am J Ophthalmol 116:79–83

2.Caltrider N, Jampolsky A (1983) Overcorrecting minus lens therapy for treatment of intermittent exotropia. Ophthalmology 90:1160–1165

3.Reynolds J, Wackerhagen M, Olitsky SE (1994) Overminus lens therapy for intermittent exotropia. Am Orthopt J 44:86–91

4.Albert DG, Lederman ME (1973) Abnormal distance – near esotropia. Doc Ophthalmol 34:27–36

5.Pratt-Johnson JA, Tillson G (1985) The management of esotropia with high AC/A ratio (convergence excess). J Pediatr Ophthalmol Strabismus 22:238–242

6.Freeman RS, Isenberg SJ (1989) The use of part-time occlusion for early onset unilateral exotropia. J Pediatr Ophthalmol Strabismus 26:94–96

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7.Hiles DA, Davies GT, Costenbader FD (1968) Long-term observations on unoperated intermittent exotropia. Arch Ophthalmol 80:436–442

8.Cooper J (1988) Orthoptic treatment of vertical deviations. J Am Optom Assoc 59:463–468

While successful implementation of surgical plans is critical in the successful management of a patient with strabismus, the first step to successful surgical care is preoperative management and good surgical decision-making. Errors in preoperative management can be divided into errors involving clinical judgment and errors involving technical or procedural errors. Many such errors can be avoided.

18.1 Errors in Preoperative Decision-Making

18.1.1 Field of Single Vision

There is usually more than one reasonable surgical treatment plan that can satisfactorily treat most patients with strabismus, and no single technique or philosophy prevails for all patients and all situations. There are some conditions in which one treatment option may have significant advantages over other available plans. Errors in determining which eye and which muscle or muscles should be operated may be more likely in cases where restrictive, paralytic or incomitant strabismus is present. In these situations, alignment of the eyes in the primary position may offer a less than optimal solution. A surgical plan that will provide the largest reasonable field of single vision, in addition to improving primary position alignment, is optimal. Failure to consider the impact of the planned surgery on the field of single vision after surgery may result in a less than fully satisfied patient.

this may occur in a patient with a long-standing strabismus and suppression who develops a cataract which produces monocular diplopia. Strabismus surgery in such a patient will not provide relief from symptoms of diplopia.

Most patients with binocular diplopia will quickly discover closing either eye can eliminate that double vision. Thus a reply that closure of one eye does not result in resolution of diplopia is usually the first clue that the patient has monocular diplopia.

The patient who does not know if monocular eye closure eliminates their diplopia should be asked to close one eye to determine if the diplopia has resolved and then repeat the test with the second eye. This test must be performed for each eye, because the patient who has monocular diplopia in only one eye will report resolution of the diplopia with closure of the problem eye, a situation that can mimic binocular diplopia. The monocular nature of the diplopia, however, can be readily detected upon closure of the contralateral eye, when the patient will report that the diplopia persists. Common causes of monocular diplopia are listed in Table 18.1. In general, monocular

Table 18.1. Causes of monocular diplopia

Optical aberrations of the refractive media of the eye

18.1.2 Monocular Diplopia

Not every patient who presents with a complaint of double vision is found to have strabismus. Monocular diplopia may be the cause of symptoms in patients referred for evaluation of diplopia that is thought to be due to strabismus. Detection of monocular diplopia is important to avoid unnecessary ancillary testing and/or treatments, which will be ineffective in treating the patient’s symptoms. This is especially true when monocular diplopia occurs in a patient who also happens to have strabismus that is not producing symptoms. For example,

Monocular diplopia of cerebral origin (cerebral diplopia) [8]

diplopia that resolves when the patient looks through a pinhole is due to an abnormality anterior to the retina, while failure to resolve when viewing through a pinhole is usually a sign of a retinal etiology. Exceptions to this general rule exist. The cause of monocular diplopia can be determined in most cases with careful history and ophthalmologic examination. Referral to an anterior segment or retinal specialist is required in some cases for treatment, depending on the cause identified.

Incomitant strabismus can sometimes be responsible for all or part of the abnormal head posture. An abnormal head posture that is entirely the result of restrictive or paralytic strabismus is usually obvious. When the incomitant strabismus is responsible for only a fraction of the abnormal head posture, making this determination can be more difficult. Fortunately this situation arises infrequently.

18.1.3Nystagmus and Strabismus in Patients with a Compensatory Head Posture

With small to moderate angle comitant strabismus, the strabismus surgeon may choose to operate on one or both eyes, and the effect of surgery is usually similar regardless of which eye(s) is operated. The choice between procedures that reduce or enhance the effect of muscle contraction on the globe is also, in most cases, up to the discretion of the operating surgeon. In patients with concurrent nystagmus, strabismus, and an abnormal head posture, the choice of procedures is not as simple and procedures are not necessarily interchangeable as in the examples above.

The purpose of a compensatory head posture in a patient with nystagmus is generally to move the null zone, the place where the nystagmus intensity is reduced, to the frontal plane of the body to improve visual function. Surgical treatment is often warranted when an abnormal head posture produces symptoms of cervical discomfort, puts the patient at risk for developing facial asymmetry (Chap. 3), or is large and cosmetically unacceptable. Extraocular muscle surgery can reduce or even eliminate a compensatory head posture by rotating the null zone from an eccentric gaze position to the primary position. The basic surgical principle involves surgically moving the eyes in the direction of the abnormal head posture (Chap. 3). If the patient has straight eyes, surgery is performed on both eyes to produce symmetric movement of the eyes in the direction of the head posture. The surgical plan is not so straightforward for patients who have concurrent nystagmus and strabismus. In this setting, if surgery is to result in correction of the compensatory head posture, it must be performed on the eye preferred for fixation, and will have no effect if performed on the nonfixating eye. Thus, two major decisions must be made when planning surgery designed to simultaneously correct a compensatory head posture and strabismus. Surgery to correct the nystagmus is performed on the eye preferred for fixation while surgery to correct the strabismus is performed on the nonfixating eye, if needed. Surgery to correct the nystagmus should be planned first, as it may result in worsening of the patient’s strabismus. The surgical plan for strabismus repair is then devised after considering the impact of the nystagmus surgery on the strabismic deviation, which may improve or worsen depending on the individual circumstances. Both procedures can and should be carried out simultaneously, when indicated. An example of this is demonstrated in Fig. 18.1.

The surgeon should not always assume that an abnormal head posture is compensatory solely in response to nystagmus.

18.1.4 Restrictive Strabismus

If significant restriction of one or more extraocular muscles is present and contributing to the primary position ocular misalignment, failure to identify and address the restriction may result in a less than satisfactory outcome. Some cases may be obvious, such as strabismus due to thyroid orbitopathy. Others cases may not be so obvious. For example, a patient with a longstanding cranial nerve palsy may develop secondary contracture of the antagonist muscle, resulting in strabismus that has both a paralytic and restrictive component. If the restriction is not released, optimal alignment is unlikely, and the field of single vision may not be maximized. Forced duction testing in patients with a preoperative duction deficit is important, and is essential in helping to determine the cause of the duction deficit and in devising a treatment plan. Forced duction testing can be performed either in the examination room preoperatively or at the time of surgery in the operating room.

18.1.5 Paralytic Strabismus

Like duction limitations that are due to restrictive strabismus, a muscle paresis or paralysis should be identified prior to surgery. Diagnosis is usually, but not always, relatively easy. Recognizing that the problem is due to a muscle weakness could not only lead to further systemic investigation, as appropriate, but can also facilitate optimal preoperative surgical planning. In general, such surgery includes operating on the antagonist muscle of the eye with the muscle paralysis. Various treatment schemes have been devised and are considered elsewhere. In cases of complete muscle paralysis involving a rectus muscle, a transposition procedure is often required, as resection of the antagonist muscle in the face of a total muscle paralysis will have little effect on the deviation and may greatly compromise future surgical options. In cases of paralytic strabismus, there exists a primary deviation, the deviation that is present when the nonparalytic eye is fixating, and a secondary deviation, which occurs when the paralytic eye becomes the fixating eye. The secondary deviation is always larger than the primary deviation due to Herring’s law of equal innervation (Chap 2). This issue is important when planning surgery on a patient with paralytic strabismus. If the patient prefers fixation with the paralytic eye, the larger, secondary deviation must be corrected, and this must be addressed in the surgical plan. A significant undercorrection is likely to occur in this setting if the secondary deviation is not addressed.

Fig. 18.1a,b. Surgery for a patient with concurrent strabismus, nystagmus, and a compensatory head posture. Preoperatively the patient had a left face turn and an esotropia with a right eye fixation preference. Surgery to move the null zone toward the primary position requires

18.1.6 Torsional Strabismus

Torsional strabismus commonly occurs in association with vertical misalignment of the eyes, but may also occur in conjunction with horizontal strabismus. It occasionally presents in isolation. Failure to identify a significant torsional component of a strabismic deviation may lead to both an inaccurate diagnosis of the problem and a surgical plan that is inadequate to repair the problem. A classic example that we encounter most frequently is torsional strabismus in patients with bilateral superior oblique paresis. A concurrent V-pattern strabismus may be recognized and a small esotropia that occurs in down gaze, along with complaints of altered vision in the reading position, may lead the ophthalmologist to blame the small horizontal strabismus for the patient’s complaints. Patients may be prescribed reading glasses with prism in an effort to correct the

right lateral rectus recession with or without right medial rectus resection. This will have the secondary result of making the esotropia worse. A horizontal recess-resect operation to address this larger esotropia is required on the left eye, his nonfixating eye

esotropia. Worse, some patients may undergo horizontal strabismus surgery to correct the esotropia, a procedure which will provide no relief from torsion-related diplopia.

However, careful questioning of the patient may elicit the true nature of the complaint, helping the surgeon to recognize that torsional strabismus is the problem. Tests for torsional strabismus, such as the double Maddox rod test, should be routinely used to detect torsional strabismus in patients with vertical strabismus and in patients with small angle strabismus who are unable to fuse with prism in place. Testing for torsion may also be of value in any patient who presents with diplopia in the presence of strabismus that does not adequately explain the patient’s symptoms. A fundus examination for evidence of objective torsion of the globe can also be of value, especially in children where subjective tests are difficult or impossible to perform.

18.1.7Misdiagnosis of Apparent Duction Abnormalities

18.1.7.1 Apparent Duction Deficits

While some abnormalities of ocular rotations are easy to overlook unless they are specifically considered, such as the aforementioned torsional strabismus, others may be easily seen but the findings misinterpreted. Patients with large angle esotropia may display a pseudo abduction deficit. This is classically seen in young children with large angle infantile esotropia. The presence of cross fixation may lead the observer to believe that an abduction deficit exists when it does not. If the patient has good vision in both eyes, an erroneous diagnosis of bilateral sixth nerve paresis may be made. When one eye is amblyopic, only the preferred eye may cross fixate and a unilateral palsy may be diagnosed. This erroneous diagnosis can lead to unnecessary neuroimaging and a mistaken belief that a transposition procedure is necessary to realign the eyes. A similar pattern may be seen in patients with a large angle exotropia in which an adduction deficit may appear to be present. Occluding one eye will often allow demonstration of normal ductions. Occlusion may be needed in some cases for a prolonged period of time to make this distinction.

18.1.7.2 Pseudo Oblique Overaction

Pseudo inferior oblique overaction has been described by Kushner [1]. Patients with this relatively unusual motility disturbance appear to have over elevation of the adducting eye in the field of action of the inferior oblique muscles, often also demonstrating a Y-pattern with a large exotropia in up gaze. This has been suggested to represent a problem of co-innerva- tion. Weakening of the inferior oblique muscles does not appear to offer any benefit. Differentiation from true inferior oblique overaction can often be made by noting the rapid development of exotropia in up gaze in these cases. This is in contrast to the

Chapter 18

gradual development and increase of exotropia in cases of true inferior oblique overaction associated with a V-pattern.

Pseudo inferior oblique overaction can also be seen following anterior transposition of the inferior oblique muscles [2]. This phenomenon has been labeled the anti-elevation syn­ drome and tends to occur when the lateral aspect of the transposed inferior oblique muscle is placed too anteriorly and/or when the new inferior oblique insertion is spread too widely (Chap 25). In such cases, there is limitation of elevation of the abducting eye due to restriction produced by the transposed inferior oblique muscle. This results in fixation duress of the restricted eye with “over elevation” of the adducting eye due to Hering’s law of equal innervation. Treatment involves revising the transposed inferior oblique insertion, moving it more posteriorly and/or narrowing the width of the transposed inferior oblique insertion.

Capó and coworkers [3] described pseudo overaction of the superior and inferior oblique muscles in both eyes of patients with large angle exotropia (>Fig. 18.2). Upon correction of the exotropia alone, the oblique function returns to normal. They explained this phenomenon as occurring because of the shape of the orbits and demonstrate that, in a patient with exotropia, elevation of the abducting eye is checked sooner than elevation of the adducting eye by the shape and structure of the orbit (>Fig. 18.3). We have seen patients who have previously undergone weakening procedures of all four oblique muscles at the time of surgery for exotropia, when in reality only exotropia surgery was probably warranted.

18.2 Errors in Measurements of Strabismus

18.2.1 Primary Position Measurement Errors

Errors in the measurement of strabismus may occur due to mistakes made when positioning the patient for evaluation or in measuring the angle of deviation. The deviation angle should be measured in the primary position for all patients. Some patients will adopt a compensatory head posture in order to fuse.

The ophthalmologist must be mindful of this strong tendency and deliberately straighten the patient’s head when performing preoperative measurements. Even a very small abnormal head posture can mask a large deviation. Unless the ophthalmologist is alert to this possibility, it can be easily overlooked and the patient’s primary position deviation underestimated or overlooked altogether. Measurements in other fields of gaze are often helpful and are often necessary when incomitant strabismus is present. Ideally, strabismus measurements in the nine positions of gaze should always be performed in a consistent gaze position. In routine practice, this is impractical, and in general, measurements are made in the most extreme positions of gaze possible. Patients with vertical strabismus should also be evaluated with their head tilted to either side in order to assist in the diagnosis of an isolated cyclovertical muscle palsy, if present.

18.2.2 Krimsky and Hirschberg Tests

Alternate cover testing with prisms is the most accurate method of determining the angle of deviation in patients with strabismus. Some uncooperative patients, or patients with fixation too poor to allow alternate cover testing, may be measured with a modified Krimsky technique. However, it should be remembered that measurements made using the Krimsky method may vary significantly, even when made by experienced stra-

Fig. 18.3a,b. Mechanical explanation for pseudo oblique overaction. a Note symmetric upgaze when the eyes are aligned and b asymmetric checking of up gaze in exotropia due to orbital anatomy resulting in pseudo oblique overaction. {With permission from Capó H, Mallette RA, Guyton DL (1988) Overacting oblique muscles in exotropia: a mechanical explanation. J Pediatr Ophthalmol Strabismus 25:281–285 [3]}

bismus surgeons [4]. The examiner should be positioned directly in front of the prism to avoid parallax errors that can result in over or under estimation of the deviation. It is our opinion that the Hirschberg method of measuring strabismus is too inaccurate to be useful in the surgical management of most strabismus.

18.2.3 Prism Measurement Errors

18.2.3.1 Improper Prism Position

Whether managing a child or an adult, diagnostic accuracy errors can and do occur. We prefer the term estimation of a patient’s deviation rather than measurement of a patient’s deviation for describing the process of strabismus quantification. This is because ocular alignment is a dynamic process and subject to change depending upon a variety factors including level of alertness, level of participation, gaze position, accommodative factors, and others. The frequency and degree of preoperative diagnostic accuracy errors can be reduced by recognition of several common problems. For example, the total deviation of a ray of light produced by a prism is the sum of the deviation produced at each surface of the prism. The total angle of deviation produced changes depending upon the orientation of the prism. The minimum angle of deviation is produced when a ray of light undergoes equal refraction at the two faces of a prism. Plastic prisms, the type generally used in ophthalmologic practice, are best held in the frontal plane position (>Fig. 18.4), a position that is easy to obtain and repeat and that will have an effect very close to the calibrated power of the prism. The total deviation induced by a prism changes as the angle of incidence of light striking a prism changes (>Fig. 18.5). Figure 18.6 presents a graph of the large degree of variation that can occur for various angles of incidence. Thus failure to properly orient a prism during quantification of strabismus can result in significant errors. Errors may be more likely to occur during measurement of deviations in eccentric positions of gaze, and even small errors in prism orientation can have important clinical effects when large prisms are used [5].

The apparent power of a prism also changes as the distance of the prism from the eye changes, with the apparent power of a prism becoming reduced as the distance of the prism from the eye is increased [6]. Thus the position of the prism relative to the corneal plane should be controlled and minimized.

18.2.3.2 Addition of Stacked Prisms

Caution should be used when attempting to add prism powers together because the calibrated prism powers may not be arithmetically added by stacking two prisms together [6, 7]. A formula is available to calculate the prism power created by stacked prisms [6], but is cumbersome to use. A table that was developed by Thompson and Guyton [6] simplifies this process (>Table 18.2).

18.2.3.3Addition of Prisms Held in Front of Both Eyes

Strabismic angles measured in prism diopters are not additive. For small deviations, the error is clinically insignificant. Fortunately, it is only necessary to hold a prism in front of both eyes when a deviation is extremely large. When two large prisms are used to measure a deviation, a significant under estimation of the deviation will occur if the powers of the two prisms are simply added together. The measurements must be converted to degrees and then back into prism diopters. A table developed by Thompson and Guyton [6] simplifies this process (>Table 18.3).

18.2.4Spectacle-Induced Measurement Errors

18.2.4.1 Large Refractive Errors

The preoperative measurements on patients with large refractive errors who wear glasses during the measurements may also require adjustment [7]. The use of spectacles to correct high myopia introduces base-in prism for patients with esotropia and base-out prism for those with exotropia, increasing the apparent deviation for both conditions. Likewise, the use of spectacles to correct high hyperopia will reduce the measured deviation compared to the actual deviation present. These effects may become clinically important with spectacle correction of more than 5 diopters. In order to accurately determine the magnitude of the deviation, the patient may be measured with contact lens refractive correction, or the surgeon may calculate the true deviation after standard measurements while the patient is wearing spectacles. The formula to express the true deviation as a percentage of the measured deviation is as follows: