Quantification of Refractive Error
Amelia G. Bartolone
Daniella Rutner
Quantification of refractive error is one of the key components of a routine pediatric ophthalmic examination. The detection and treatment of uncorrected refractive error can help explain and reverse reduced visual acuity, improve alignment in accommodative esotropia, and rectify avoidance of near work, thus potentially improving academic performance. Hence, proper refraction and spectacle correction reduce the need for vision rehabilitation and increased health care costs. Because of poor visual acuity, children with uncorrected refractive error are also less likely to develop normal fine and gross motor skills necessary in writing and mathematics; reading skills may also be affected, leading to maldevelopment of language skills. Moreover, decreased visual stimulation can have an impact on social skills because the child may not see sufficiently to interact or even recognize peers. The child’s refractive state can also determine if there is an accommodative cause to a strabismus.
Objective refractive techniques are used to determine the refractive status of children’s eyes with no verbal response and minimal cooperation from the patient. These are the primary means to quantify refractive error in infants, preschoolers, and children with special needs. Retinoscopy is the standard method to objectively determine the refractive status of the eye. The technique locates the plane conjugate to the retina with accommodation at a minimum (the far point). Traditional techniques do not work well for preverbal or preschool children because of short attention spans and poor fixation. Therefore, several modifications to standard techniques and new technologies enhance fixation and control accommodation, producing more accurate quantification of refractive error.
This chapter reviews methods used to quantify refractive error and highlights the special needs, modifications, and techniques used in the infant, preschool, and special needs population. The first section discusses the different methods of retinoscopy: dry or static, cycloplegic, and near (Tables 14.1 and 14.2). Subjective refraction is summarized in the second section. The last section focuses on newer technologies such as visually evoked potentials, autorefraction, and photorefraction.
Dry Retinoscopy
Visual deprivation is the diffusion of an image on the retina. It can be caused by a number of processes, including uncorrected refractive error, which can result in amblyopia (1). The time of onset and duration of the deprivation have a profound effect on potential visual function (2,3,4,5,6,7). The earlier the deprivation occurs, the more profound the effect on the visual system (8). Early detection and prompt treatment is important to avoid lifelong visual impairment. Thus, it is imperative to evaluate infants and small children for refractive errors and to
provide them with a clear retinal image, which can prevent or even reverse vision loss resulting from the child’s earlier abnormal visual experiences (8,9,10).
provide them with a clear retinal image, which can prevent or even reverse vision loss resulting from the child’s earlier abnormal visual experiences (8,9,10).
Table 14.1 Common Cycloplegic Agents and Their Ocular and Systemic Side Effects | |||||||||||||||||||||||||
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Table 14.2 Methods of Refraction: Advantages, Disadvantages and Modifications | ||||||||||||||||||||||||||||
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In an adult or older child, refractive error is estimated objectively and verified by subjective refraction. In preverbal infants, young children, or children with special needs, however, limited verification can be obtained subjectively or reliably. As such, clinicians rely heavily on objective findings for the diagnosis and treatment of refractive errors. An accurate measure of refraction is necessary to yield appropriate correction, thereby enabling normal visual development, and clear and comfortable vision (11). The standard method by which objective refractive errors are assessed in adults and children is retinoscopy (12). It is best to perform procedures when the child tends to be more cooperative, such as early in the morning or after a nap to achieve maximal results. Infants tend to be more alert during feedings; it is helpful to suggest that the parents bring a bottle for the child. It is imperative to obtain the maximal amount of information in the minimal amount of time when working with young children because they are easily distracted.
Retinoscopy can be performed in one of two fashions, each eliciting very different information. Practitioners can perform either static retinoscopy with accommodation relaxed for distance refractive error assessment, or dynamic retinoscopy to evaluate near accommodative abilities (13). The focus of this section is distance
refraction evaluation and, as such, only static retinoscopy is discussed.
refraction evaluation and, as such, only static retinoscopy is discussed.
Instrumentation
In static retinoscopy, refractive error is assessed while the patient fixates a distant target in order to relax accommodation. Retinoscopes are electrically or battery powered handheld light sources that direct a beam of slightly divergent light (when the sleeve of the retinoscope is in the down position) into the patient’s eye. The illumination of the retina is reflected back and the examiner can observe movements of the red reflex in the patient’s pupil. The refractive status of the eye is determined by using appropriate correcting lenses to make the far point of the ametropic eye conjugate to the pupil of the examiner’s eye. When this is achieved, the movement of the reflex will be neutralized and no movement will be observed.
Retinoscopes come in several basic styles. The most widely used scopes are the streak retinoscope, which reflects a rectangular beam from a line source, and the spot retinoscope, which reflects a round light from a circular source (13). Although clinically, streak retinoscopes have widely replaced spot retinoscopes because of their ease in viewing the axis of astigmatism, spot retinoscopes are an excellent choice while working with the pediatric population. The spot scope, based on the shape of the reflex, can help detect astigmatism quickly without the need to change the orientation of the light source. Moreover, the round spot enables better observation of pupillary reflex changes indicating fluctuations in accommodation. It is worthwhile for practitioners who service a large pediatric population to invest in pediatric trial frames. The glasses should be stylish to encourage children to keep the glasses on and with flexible hinges to prevent breakage (14). It would be helpful to show the children the trial frame before placing it on the child to acclimate the child to the device being used. For infants or young children who do not allow the placement of the pediatric trial frame, loose lenses or a lens rack may be a useful alternative. If a child is already wearing glasses, Halberg clips, or other trial lens clip holder that can be placed on the glasses so the practitioner can perform an over-refraction by adding standard trial lenses in the grooves (15). Begin by fitting the child in a pediatric trial frame (e.g., the Como Baby Pediatric Trial Frame) and, after a period of acclimation, begin testing for best success. The standard phoropter is cumbersome and impractical for young children for several reasons.
Infants and young children have small interpupillary distance that cannot be accommodated by the phoropter.
Children may be afraid of the phoropter’s imposing size. This can be alleviated in slightly older children by allowing the child to sit on a parent’s lap.
It is difficult for children to maintain proper fixation, which is critical for the accommodative control in static retinoscopy behind the phoropter.
It is difficult for the practitioner to determine the child’s fixation.
Proper fogging of the patient is required before beginning retinoscopy. This can be done by placing fogging lenses in the trial frame, using loose lenses, or by positioning the lens rack horizontally when scoping the patient to ensure relaxation of accommodation.
Fixation Targets
Traditionally with adults, the standard target is a 20/400 projected letter with a bichrome red and green filter to minimize the brightness of the chart’s reflection in the phoropter lenses. With infants and young children, however, creativity is required when devising a distance fixation target. The target must be capable of maintaining the child’s interest for the entire procedure to ensure proper relaxation of accommodation. If no other target is available, it is possible to engage the child’s attention by asking questions about the target: Which letter do you see? What colors do you see? How many lines does the letter have?
Better alternative targets are:
A picture slide projector
Blinking lights
Commercially available toys that can be remote activated with sound, music, or lights
A video recorder playing a child’s favorite video, such as Barney or Sesame Street
An assistant or a parent making faces or noises to attract the child’s attention (14)
Technique
After the fogging lens is in place and an appropriate target is displayed, the practitioner can begin the retinoscopy. First, the practitioner should work at a comfortable working distance. The most common test distances are 1 m (+1.00 D), 67 cm (+1.50 D), or 50 cm (+2.00 D). Turn the retinoscope to the sleeve-down position and begin scoping. Conventionally, examine the patient’s right eye first with your right eye, after ensuring the patient’s left eye is appropriately fogged by rapidly scoping the child’s left eye to see against motion in all meridia.
Three possible movements are observed during retinoscopy: with, against, or neutral. With motion appears in hyperopic eyes or in eyes with a lesser degree of myopia than that of the practitioner’s working distance. As the practitioner directs the streak or spot source of light into the eye and moves the light from side to side, the reflected image that appears in the patient’s pupil will move in the same direction as the movement of the retinoscope. This is an indication to add plus or convex lenses. Against motion appears in myopic eyes that exceed the practitioner’s working distance. As the practitioner directs and moves the light source from side to side in the eye, the reflex moves in the opposite direction to that of the retinoscope. This is an indication to add minus lenses. Neutrality is achieved when the patient’s far point is conjugate with the practitioner’s retinoscope and no movement is observed as the retinoscope is moved side to side. Astigmatic eyes have different powers in different meridia. When performing retinoscopy on an astigamatic eye, therefore, it is necessary to determine the refractive power of each principal meridian separately. Astigmatism should always be corrected with minus cylinders to facilitate accommodative control.
Neutralization should adhere to the following sequence:
Locate the two principal meridia in the right eye with the retinoscope in front of your right eye.
Determine the meridian that can be neutralized with the most plus or least minus lens.
Neutralize that meridian.
Confirm neutrality by either
Moving the sleeve all the way up, creating a concave mirror; the reflex should appear neutralized.
Moving in closer to the patient should cause the with motion to return; moving away should cause the against motion to appear.
Place an extra +0.25 D sphere; an against motion should appear.
Repeat the neutralization in the meridian 90° away. This is accomplished by rotating the sleeve on the streak retinoscope. During spot retinoscopy, no need is seen to rotate the sleeve, just scope along the meridian.
Move across to the other side of the patient, being careful not to obscure the distant target. Neutralize the patients left eye with the retinoscope in front of your left eye. The patient’s right eye will be fogged by an amount equivalent to the dioptric value of your working distances and no further fog is required.
Recheck both eyes again and record your results, taking the working distance into account.
Always record retinoscopy findings in terms of correcting lenses, not neutralizing lenses. Therefore, to make the subject emmetropic for optical infinity, the refractive correction needs to be reduced by an amount equal to the dioptric value of the working distance.
Undoubtedly, some children will not allow the placement of phoropter, loose lenses, lens racks, or trial frames during retinoscopy. In such cases, the practitioner can move closer to or further from the patient until a neutral reflex is achieved and then measure the distance at which neutrality was observed. Convert this distance into diopters.
Several sources of error in retinoscopy can yield inaccurate results and, thus, should be avoided:
Incorrect working distance: It may be helpful to attach a string the length of the desired working distance to the retinoscope that can be routinely used to verify working distance.
Scoping off the patient’s visual axis: It is important that retinoscopy is performed as close as possible to the visual axis. Accordingly, use your right eye to examine the patient’s right eye, and the left eye to examine the patient’s left eye.
Patients failing to fixate the distance target: To mitigate this, routinely ask the patient questions about the target to ensure proper fixation. In addition, fluctuation observed in the patient’s pupil size may be an indication of accommodative fluctuations. Measurements should be assessed when pupil size is at a maximum.
Failure to obtain reversal: This can result in over- or undercorrection.
Failure to locate the principal meridia: The axis of astigmatism can be determined by evaluating the break and width of the streak. When the streak is aligned with the principal meridian, the break effect disappears and the width of the reflex appears the narrowest and brightest. With spot retinoscopy, you should observe that both the patch of light and the reflex move along the same meridian. If the reflex does not move in the same meridian as the patch of light, this indicates that you are not working along one of the principal meridia.
Cycloplegic Retinoscopy
Medications
Medications are used to facilitate the determination of refractive status of the eye. Among them are atropine, tropicamide, and cyclopentolate (Table 14.1). They each function by decreasing ciliary muscle activity on the crystalline lens, thereby diminishing or eliminating fluctuations of accommodation. Thus, these cycloplegic agents improve the accuracy with which the refraction can be performed (17). Cycloplegic retinoscopy is a useful procedure when it is imperative to control the accommodation of the child. Most would argue that cycloplegic refraction is the standard of care in children who have high amounts of hyperopia, greater than 1.00 D of anisometropia, and strabismus, especially esotropia (16). It is not necessary, however, for most children being examined. In addition to cycloplegia, these medications are mydriatics and result in pupillary dilation, which facilitates the dilated fundus examination.
Atropine is an antimuscarinic drug that inhibits the action of acetylcholine. When applied topically to the eye in 1% concentration it produces mydriasis after 10 to 15 minutes, reaching optimal dilation and cycloplegia levels in 30 to 40 minutes (18) and can last up to 14 days. In children, atropine ointment (0.5% concentration in lightly pigmented eyes and 1% concentration in darkly pigmented eyes) (14) is instilled in both eyes twice daily for 3 days before the examination. Atropine provides the most complete cycloplegia of all the available agents. Ointment is the ideal preparation for use in children and can be easily administered while they sleep.
Atropine, however, has some significant ocular and systemic side effects. Ocular side effects include allergic reaction, increased intraocular pressure (IOP), photophobia, and decreased lacrimation. Systemic side effects include dryness of skin, mouth, and throat; restlessness, irritability, or delirium; tachycardia, flushed skin, ataxia, convulsions, high fever, coma, and death from general central depression, which causes decrease in blood pressure, circulator collapse, and respiratory failure. The estimated fatal dose in children is 10 mg; one drop of 1% atropine solution contains 0.5 mg. A fatal dose is 20 drops. In almost all the cases in the literature that resulted in death, the children were mentally or physically disabled. As such, caution should be used when dealing with the specials needs population (18).
Tropicamide is another antimuscarinic (parasympatholytic) drug (14). The onset of action of the drug begins in 15 to 30 minutes after instillation. Its duration of action is 4 to 6 hours, having the shortest half-life of all the
drugs. Because it is eliminated quickly from the body, it is ideal for children with special needs and other instances of hypersensitivity to cholinergic agents (19). Tropicamide has fewer side effects then the other cycloplegics. Side effects include increased IOP, psychotic reactions, behavioral disturbances, and cardiorespiratory collapse in children and some adults More common side effects are transient stinging, dryness of the mouth, blurred vision, photophobia with or without corneal staining, tachycardia, headache, parasympathetic stimulation, or allergic reaction. Tropicamide has the most residual accommodation after maximal effect from all the cycloplegic agents; 40% to 60% of accommodation remaining in brown eyes and 20% to 40% residual accommodation in blue eyes (15,17). A 1% tropicamide solution is only 60% to 70% as effective as atropine in producing cycloplegia (20).
drugs. Because it is eliminated quickly from the body, it is ideal for children with special needs and other instances of hypersensitivity to cholinergic agents (19). Tropicamide has fewer side effects then the other cycloplegics. Side effects include increased IOP, psychotic reactions, behavioral disturbances, and cardiorespiratory collapse in children and some adults More common side effects are transient stinging, dryness of the mouth, blurred vision, photophobia with or without corneal staining, tachycardia, headache, parasympathetic stimulation, or allergic reaction. Tropicamide has the most residual accommodation after maximal effect from all the cycloplegic agents; 40% to 60% of accommodation remaining in brown eyes and 20% to 40% residual accommodation in blue eyes (15,17). A 1% tropicamide solution is only 60% to 70% as effective as atropine in producing cycloplegia (20).
Cyclopentolate HCl is the drug of choice for cycloplegic refraction because of its rapid onset of action, minimal side effects, and minute residual accommodation (17). Cyclopentolate HCl is available in 0.5%, 1%, and 2% concentrations; however, 2% cyclopentolate HCl is rarely used because of its increased risk for significant side effects (11). One percent cyclopentolate does not yield significantly greater cycloplegia than 0.5%. Moreover, 0.5% cyclopentolate produces greater control of accommodation than 1% tropicamide and yields just less than 0.50 D hyperopia as compared with atropine (17,21). Cyclopentolate HCl is used in 0.5% concentration in infants under the age of 3 months and 1% concentration for children over the age of 3 months. Cycloplegia is maximal within 30 minutes and returns to normal in 24 hours with 80% of accommodative amplitude recovered after 7 hours (11,17). A reduced effect of cycloplegia was seen when comparing brown irises than blue irises; however, recovery from cycloplegia with cyclopentolate was slower for brown irises than blue irises (17,20).
The ocular side effects of cyclopentolate include transient stinging, increased IOP, the precipitation of angle closure, and hyperemia. The systemic side effects usually occur in 2% concentration or multiple doses of 1% and include central nervous system disturbances such as cerebellar dysfunction and visual and tactile hallucinations; milder allergic drug reactions can also occur and are usually manifested in a localized rash. Patients may also complain of weakness, dizziness, difficulty breathing, or a loss of consciousness (22,23).
Methods
Cycloplegic refraction is performed at the end of the examination just before fundus evaluation, but after completion of all binocular testing. Several methods of drop instillation are used with children: These include
One drop in each eye as quickly as possible. Wait 5 minutes and repeat. It is best with the child lying supine in a parent’s lap with the head toward the parent’s knees. If the parent needs to hold the child in the crook of an arm, instill the drop in the eye closer to the parent because once the drop is administered, most children will turn their heads toward the parent and then you can instill the drop in the eye closer to you.
Some advocate the use of proparacaine hydrochloride 0.5%. Proparacaine causes a deepening of cycloplegia in brown versus blue irises 20 minutes after the instillation of cyclopentolate. It does not have an impact on the recovery from cycloplegia in either cyclopentolate or tropicamide (17). Although proparacaine will reduce the intensity of the stinging sensation, the instillation of more drops than absolutely necessarily in small children is difficult and may prevent the instillation of the cycloplegic agent. If you can only get one application of solution in the eye, you would want it to be the cycloplegic drop.
An alternate method is to instill drops onto the closed lids of the child. Ask the child to tip the head back and close the eyes. Apply the cycloplegic solution to the nasal portion of the upper lid. When the child opens the eyes, the drop will drip into the eye. Do not give the child tissues, as they will wipe away the medication. No statistical difference was seen with this method when compared with that of instillation of drops to the open eye (23).
Spray is another option in the instillation of cylcoplegia in small children. Spray represents an alternative, less irritating and intimidating method of cycloplegia. The spray solution is a combined preparation of 3.75 mL of cyclopentolate 2%, 3.75 mL of phenylephrine 10%, and 7.5 mL tropicamide 1%, yielding a final concentration consisting of cyclopentolate 0.5%, phenylephrine 2.5%, and tropicamide 0.5%. The combination of agents produces adequate mydriasis for fundus evaluation and allows effective cycloplegia in 20 to 30 minutes. Studies have found that spray administered to the closed eyelid is as effective as an ophthalmic solution drop instilled to the open eye with no major side effect reported (23).
After the instillation of a cycloplegic agent, a second dose is usually advisable after 5 minutes. Punctal occlusion is recommended. Also, having the child close the eye will collapse the nasal lacrimal canal, reducing systemic absorption (15). No more than three doses of cyclopentolate should be used to avoid the risk of toxic reactions. Cycloplegic retinoscopy can be performed 30 minutes after instillation of cycloplentolate and should be completed approximately 40 minutes after installation of the cycloplegic agent. A nonvariable reflex observed during retinoscopy is a good indicator of cycloplegia. If variability is present, you can consider another drop of cyclopentolate, waiting a few more minutes or, if necessary, atropinization (14).
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