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13 Optical Pearls and Pitfalls David L. Guyton, Joseph M. Miller, and Constance E. West O ptics and refraction are often thought of as a dry chapter in ophthalmology, but understanding a few basic principles enables one to avoid errors and complications when treating both pediatric and adult strabismic patients. REFRACTION AND REFRACTIVE ERROR IN CHILDREN Retinoscopy need not be limited to preverbal children following cycloplegia. Dry retinoscopy is useful both in evaluating the ability to accommodate and in serving as a quick assessment of the present pair of glasses. To check the present correction, two free lenses, a 1.50 D and a 2.00 D, are grasped between the thumb and forefinger of one hand and held in front of the two eyes. The patient is instructed to look at the distance fixation target through the 2.00 D lens, thus relaxing accommodation. The eye being evaluated is then checked with the 1.50 D lens with the retinoscope on axis for neutrality. Dynamic retinoscopy, performed to evaluate the effectiveness of accommodation, is performed without free lenses. One eye of the subject is occluded. A fixation target is held just below the peephole of the retinoscope, and the subject is instructed to look first at a distance target, then at a near one. If the subject is able to focus on the near target, the observer will see neutralization of the retinoscopy reflex. This test is most useful in assessing the need for bifocal correction in an amblyopic eye. If the child cannot readily accommodate and neutralize the reflex at near, even if there is no element of accommodative esotropia, a reading add should be considered. Performing dynamic 520 chapter 13: optical pearls and pitfalls 521 retinoscopy with both the patient’s eyes open provides a good screen for anisometropia or sphere imbalance in the glasses. Cycloplegic refraction is an essential part of the examination of strabismic children and may be effected by several drugs with different cycloplegic and mydriatic characteristics. The agents most commonly used by strabismologists are atropine, cyclopentolate, and tropicamide. Atropine blocks parasympathetic activity by competing with acetylcholine and therefore prevents contraction of the ciliary muscle and iris sphincter. Mydriasis is fully developed at 35 to 45 min, while cycloplegia is not completed until 1 h after instillation of eyedrops. Atropine has the longest duration of cycloplegia (up to 48 h) and mydriasis (up to several days) of the parasympatholytic drugs. Tropicamide 1% is a short-acting (3–6 h duration) mydriatic with a rapid onset of cycloplegia (20–30 min). Cyclopentolate, like tropicamide, is a synthetic parasympatholytic but seems to be a more effective cycloplegic with peak accommodative paresis between 25 and 35 min. Its mydriatic action may last for 24 h. One cannot measure accommodative amplitude, reading adds cannot be determined, and strabismic deviations are affected after the administration of cycloplegic agents. The authors’ preferred practice with children is to anesthetize the conjunctiva with a topical anesthetic, followed by instillation of 1% cyclopentolate. The anesthetic seems to lessen the discomfort caused by the cyclopentolate and has the advantage of increasing its penetration into the anterior chamber. Cyclomydril (cyclopentolate 0.2% and phenylephrine hydrochloride 0.5%) or 0.5% cyclopentolate should be used in neonates and infants. In adults who require a cycloplegic refraction, we use 1% tropicamide because of its shorter duration of cycloplegia. When adequate cycloplegia cannot be effected in the office (usually in children with darkly pigmented irises), prescribe atropine sulfate 1%, one drop in each eye, morning and evening for 2 days before the next visit. On the day of the visit, a drop should be instilled in each eye 1 h before the appointment. Local allergic (hypersensitivity) reactions manifested by conjunctivitis, swollen lids, and periocular dermatitis are occasionally seen with atropine administration but rarely, if ever, with tropicamide or cyclopentolate. All cycloplegic medicines have potential systemic side effects: flushing, fever, dry skin and mucous membranes, tachycardia, restlessness, hallucinations, seizures, and even death, especially in the smallest and most 522 handbook of pediatric strabismus and amblyopia lightly pigmented children. Severe reactions are rare but may require administration of intravenous physostigmine (0.5– 1.0 mg in children, 1–4 mg in adults, administered as a 0.2 mg/ml solution over at least 2 min). Systemic side effects may be lessened by occluding the canaliculi and preventing absorption by the nasal mucosa. Special care must be taken when atropine is given for administration at home, where the dose given is less controlled than in the office. One 50-l drop of 1% atropine sulfate contains 0.5 mg of atropine, whereas the dose of atropine in resuscitation of the infant and child is 0.01 to 0.03 mg/kg! Be particularly careful in small babies and children with heart disease. Most neonates (approximately 75%) are hyperopic.2 The hyperopia is usually symmetrical and less than 4 D.8 It is also known that the degree of hyperopia usually increases until about the age of 7 years.1 The increase in hyperopia during early childhood also seems to apply to neonates born myopic and results in loss of myopia in those neonates born with a small amount of myopia.5 Thus, the majority of children examined have some degree of refractive error. PRESCRIBING GUIDELINES AND LENS TYPES Once the refractive error has been determined, a decision must be made about whether to give the correction. In the absence of strabismus, the decision as to when to prescribe the correction must be made based on the magnitude of the error, the patient’s ability to accommodate, the visual needs of the individual, and the risk of refractive and/or anisometropic amblyopia. There are few data regarding who should receive glasses, but some common sense and general guidelines are helpful. Myopic children should receive correction when their uncorrected binocular visual acuity is 20/30 or worse. This level of acuity frequently occurs at 1.50 D in both eyes and is the threshold to follow for simple, symmetrical myopia. Hyperopia has no such simple guideline, as there is a tremendous variation in how children respond to an accommodative demand. Many children will not accommodate consistently at a level above 5.00 D and will require at least partial correction to allow for normal visual development. For high hyperopia, which is usually accompanied by subnormal accommodation, prescribe chapter 13: optical pearls and pitfalls 523 the full hyperopic correction (perhaps cut by 0.50 D), especially when there is abnormal visual function. It is a little easier to determine when to prescribe glasses in the presence of anisometropia. If both eyes are developing normal visual acuity and normal binocular function is present, no glasses are given. However, if anisometropic amblyopia is present (usually in the more ametropic eye), glasses must be prescribed. In anisometropic hyperopic amblyopia, the full correction need not be prescribed so long as the correction is reduced equally in each eye. In anisometropic myopic amblyopia, the full correction should be given. Pay careful attention to accommodative abilities when children are forced to fix with an amblyopic eye. A reading add may hasten treatment of the amblyopia during occlusion or atropine penalization therapy, although this has not been proven conclusively. If glasses are to be prescribed for a significant spherical error, any astigmatic error should be corrected as well. Astigmatic correction is given by itself when the child is not developing normal visual acuity; this usually occurs with 1.50 D or more of astigmatism. Children readily accept the full cylindrical correction at the proper axis, and it should be prescribed as such (not always the case with adults). Strabismus surgery can affect the refractive error, particularly the astigmatic component, and refraction should be rechecked after strabismus surgery. In the presence of high refractive errors, it is best to overrefract the individual and then read the resultant correction by placing both the free lenses and the glasses in a lensometer. Errors induced by changes in pantoscopic tilt or vertex distance will be eliminated. When strabismus coexists with a refractive error or an abnormal accommodative convergence/accommodation ratio, the full cycloplegic refraction should be given, adding bifocals if an esodeviation is still present at near. If alignment is not attained or maintained with spectacle correction, surgery may be considered. Bifocals, when used for the treatment of accommodative esotropia with a high accommodative convergence/ accommodation ratio, should be fit high, usually with the top of the segment bisecting the pupil. Executive-style bifocals are commonly prescribed, but large, “D”-shaped (flat-top) segments are frequently less expensive, lighter in weight, and provide adequate field in pediatric patient frames. Progressive style bifocals have been advocated by some authors,3 but one should remember that the transitional zone is usually 12 mm in vertical extent 524 handbook of pediatric strabismus and amblyopia and may render the most powerful part of the segment useless to pediatric patients. It is often prudent to specify a lens with high impact resistance (polycarbonate) for monocular and amblyopic children; special recreation spectacles are particularly appropriate for this population. Lens coating and filters are sometimes included in children’s corrective lenses. Ultraviolet protection should be considered for children with aphakia, lens implants, or maculopathy and for those children undergoing atropine penalization. Tinted and photochromic lenses, both of which are now available in glass or plastic, often provide comfort for patients with aniridia, ocular albinism, or oculocutaneous albinism. THE CORNEAL LIGHT REFLEX AND STRABISMUS The corneal light reflex (the first Purkinje–Sanson image) is a virtual image located 4 mm behind the cornea and may be thought of as located on an imaginary string connecting the center of curvature of the cornea with the fixation light. To avoid errors from parallax in the Hirschberg or Krimsky4 test, the examiner’s eye must be directly behind the fixation light. To produce Hirschberg test photographs of strabismic patients, the electronic flash should be held directly below, or above, the camera lens, with a fixation object placed between the flash and the lens. Reflection of the camera flash in the patient’s glasses can be detected by a handlight before taking the photograph and avoided by raising the temples, thus increasing the pantoscopic tilt of the glasses. MEASUREMENT AND CORRECTION OF STRABISMIC DEVIATIONS WITH PRISMS Misalignment of the visual axes may be measured in degrees or prism diopters (PD). While strabismic deviations are measured in degrees in Europe, it is more common to quantify them in PD in the United States. Glass and plastic prisms are made with nonparallel surfaces that deviate light rays passing through them. The power of a prism (glass or plastic) in PD () is equal to the displacement, in centimeters, of a light ray passing through the prism, measured 100 cm from the prism (Fig. 13-1). chapter 13: optical pearls and pitfalls 525 FIGURE 13-1. A 15 prism displaces a light ray 15 cm when measured 100 cm from the plane. Remember, when converting PD to degrees, that each degree is not exactly equal to 2 ; the relationship is a trigonometric one (degrees tan1(/100). For amounts less than 45° (100 ), the relationship of 2 per degree is roughly correct but, beyond 45° (100 ), the number of PD per degree increases rapidly without bounds, rising to an infinite number of PD at 90°. Variability in strabismus surgery may result, in part, from incorrect use of prisms when measuring strabismic deviations preoperatively. Knowledge of these potential errors helps the ophthalmologist minimize their effects. These errors occur when prisms are incorrectly positioned or stacked in the same direction and when measuring deviations through high minus and high plus lenses. Ophthalmic prisms are made of either glass or plastic, and the amount of strabismic deviation neutralized (or produced) by the prism varies with the position in which it is held. There are three commonly used positions for holding ophthalmic prisms: Prentice position, minimum deviation position, and frontal plane position (Fig. 13-2). Glass prisms are calibrated for use in the Prentice position, which requires the patient’s line of sight to strike the rear (or front) surface of the prism at right angles. Small errors in holding glass prisms may produce large errors in the amount of deviation neutralized. For example, if the rear surface of a 40 glass prism is held in the frontal plane rather than in the Prentice position, the effect is only 32 .9 Plastic prisms and prisms bars are calibrated for use in the position of minimum deviation and, in this position, the line of 526 handbook of pediatric strabismus and amblyopia FIGURE 13-2. The three common positions for use of ophthalmic prisms with fixation at a distance: left, Prentice position; center, minimum deviation position; right, frontal plane position at distance (solid lines) and near (dashed lines). Plastic prisms should be held in the frontal plane position, and glass prisms are calibrated to be held in the Prentice position. sight makes an equal angle with each of the faces of the prism. In clinical practice, however, the position of minimum deviation may be difficult to judge. Holding the rear surface of the prism in the frontal plane of the patient very nearly produces the minimum deviation for that prism. Note, however, that if the rear surface of a 40 plastic prism is held in the Prentice position (a large error in holding a plastic prism) rather than in the frontal plane, the effect is 72 rather than 40 . Small errors in holding plastic prisms (in the frontal plane position instead of the minimum deviation position) produce only small errors in the amount of deviation neutralized. Thus, plastic prisms are less prone to position error than glass prisms and are preferable for this reason. The common practice of “stacking” two prisms together in the same direction to measure large deviations (greater than 50 ) may induce large errors. Glass prisms are available to a maximum of 40 and plastic prisms are available to a maximum of 50 . Prisms do not add linearly when stacked together in the same direction and should never be stacked together in that manner. Even though the rear surface of one of the prisms may be held in the correct position, the other prism is far from its calibrated position, and a much greater effect is produced than anticipated. For instance, a 3 plastic prism added to a 50 plastic prism gives a 58 effect.9 When measuring large deviations, prisms are best held before both eyes, although there is chapter 13: optical pearls and pitfalls 527 still some additivity error in doing this. The additivity errors induced have been tabulated by Thompson and Guyton9 or may be calculated with a formula. Fortunately, additivity error is not significant when adding a vertical prism to a horizontal one. Thus, a vertical and a horizontal prism may be stacked together with no significant interaction between the two when measuring a combined horizontal and vertical deviation. When measuring strabismic deviations with a fixation target at near, the distance from the eye to the prism must be acknowledged. The amount of prism necessary to neutralize a deviation at near fixation increases as the prism is held farther away from the eye; this effect may lead to overcorrections when the surgery is calculated on the basis of the near deviation.10 An additional error may result when measuring strabismic deviations through glasses, even when prisms are held in the proper position.7 This error is also present when measuring the deviation by the Krimsky prism reflex test or subjective methods. Both lines of sight of a strabismic patient cannot pass through the optical centers of the respective spectacle lenses; thus, glasses produce a prismatic change of the deviation as measured in front of the glasses. This peripheral prismatic effect begins to become clinically significant with spectacle lenses of more than 5 D (minus or plus). Minus lenses increase the measured angle of deviation, and plus lenses decrease the measured angle, whether the deviation is esotropia, exotropia, or hypertropia. The distance deviation is changed by approximately (2.5) (D)%, where D is the spectacle power. For example, a 10 D bilateral high myope with 40 of exotropia will measure (2.5) (10)% more than 40 , or 50 , through the glasses. A helpful mnemonic is “minus measures more.” When calculating and prescribing oblique prisms, remember that prisms add as ordinary vectors, so a horizontal prism may be combined with a vertical prism and prescribed as a single prism at an oblique angle. The power and orientation of the prism may be determined by using a prism nomogram (Fig. 13-3), or by marking off proportional distances from the corner of a piece of paper, forming two sides of a right triangle. The third side of the triangle is proportional to the amount of oblique prism needed, and the orientation can be determined by folding the paper and measuring the appropriate angle with the protractor on a trial frame. The orientation of the prism base should be specified in the appropriate meridian, but note that over the left eye, for example, “base in the 135° meridian” is ambiguous. 528 handbook of pediatric strabismus and amblyopia PR IS M RO T IO AT N G AN LE FIGURE 13-3. Prism nomograph. To use the nomograph, determine the amount of vertical and horizontal prism needed to neutralize the deviation and then locate their intersection on the nomograph. The quarter circle nearest their intersection is the power of the prism to be used. Locate the intersection of this quarter circle and the amount of vertical prism determined on prism and cover test. A line drawn through this point and the origin intersects the prism rotation angle scale and determines the proper orientation of the oblique prism. The base must be specified either as “base up and in at the 135° meridian” or as “base down and out in the 135° meridian.” Horizontal, vertical, or oblique prisms may be ground into spectacle correction, or Fresnel Press-On prisms may be applied to existing lenses. When one measures incomitant deviations with the prism and cover test,6 the deviation should always be neutralized with the prisms placed before each eye in turn. Only the eye not looking through the prism is truly pointing in the desired direction of gaze during testing. In this case, therefore, the “fixing eye” must be defined as the eye not looking through the prism; chapter 13: optical pearls and pitfalls 529 the position of the cover makes no difference. Once an incomitant deviation is neutralized with prism(s), no movement of either eye should be seen on movement of the cover from one eye to the other, unless a dissociated horizontal or vertical deviation is also present. When determining the amount of prism ground into, applied to, or caused by decentration (intentional or unintentional) of spectacles, it is important to measure the effective prism in the part of the lens through which the patient is looking. While the patient is looking through the spectacles, mark that point with the edge of a piece of paper tape. Then, measure the amount and orientation of the prism by placing this mark in the center of the nosecone of the lensometer. References 1. Brown EVL. Net average yearly change in refraction of atropinized eyes from birth to beyond middle age. Arch Ophthalmol 1938;19: 719–734. 2. Cook RC, Glasscock RE. Refractive and ocular findings in the newborn. Am J Ophthalmol 1951;34:1407–1412. 3. Jacob J-L, Beaulieu Y, Brunet E. Progressive addition lenses in the management of esotropia with a high accommodation/convergence ratio. Can J Ophthalmol 1980;15:166–169. 4. Krimsky E. Fixational corneal light reflexes as an aid in binocular investigation. Arch Ophthalmol 1943;30:505–521. 5. Mohindra I, Held R. Refractions in humans from birth to 5 years. In: Fledelius HC, Alsbirk PH, Goldschmidt E (eds) Documenta Ophthalmologica Proceeding Series, vol 28. The Hague: Junk, 1981. 6. Repka MX, Kelman S, Guyton DL. Prism measurement of incomitant strabismus. Binoc Vis 1985;1:45–49. 7. Scattergood KD, Brown MH, Guyton DL. Artifacts introduced by spectacle lenses in the measurement of strabismic deviations. Am J Ophthalmol 1983;96:439–448. 8. Slataper FJ. Age norms of refraction and vision. Arch Ophthalmol 1950;43:466–481. 9. Thompson JT, Guyton DL. Ophthalmic prisms: measurement errors and how to minimize them. Ophthalmology 1983;90:204–210. 10. Thompson JT, Guyton DL. Ophthalmic prisms: deviant behavior at near. Ophthalmology 1985;92:684–690.