 |
Slide 1 |
Barry Milder, MD
Anisometropia is an unequal refractive error in both eyes of an individual. Because truly identical refractive errors in eyes is not the norm, clinical usage of the term anisometropia is reserved for patients with significantly asymmetric refractive errors, usually a difference of 2 D or more between the eyes. However, symptoms may occur with less asymmetry.
When humans see the world, each of the two eyes presents a different sensory input to the brain, where a single perception is created. If the two messages are too dissimilar, the brain has difficulty fusing the messages into a single, interpretable image. The result may be blurred or degraded acuity, headache, reading intolerance, asthenopia, diplopia, or amblyopia (if the anisometropia is present in early childhood). On the other hand, the result may be no problem at all
The degree of anisometropia, age of onset, rate of refractive change, refractive type of anisometropia (e.g., myopic, hyperopic, antimetropic, astigmatic), the individual's level of tolerance, previous fusion ability and motility status, and previous use of spectacles are factors in the production of symptoms in an anisometropic patient. Some patients are uncomfortable with 1 D of difference between their two eyes, whereas other patients accept differences of 5 D, 6 D, or 8 D without apparent adverse effects. In children, the most important factor is usually the refractive type of anisometropia. In adults, the rate of change and the difference between a patient's new glasses and old glasses are significant concerns.
The problems related to anisometropia fall into three categories: unequal acuity (anisoacuity), unequal motility (anisophoria), and unequal image size (aniseikonia).
|
Unequal visual acuity is inevitable in all types of uncorrected anisometropia. No accommodative adjustment can bring both eyes into focus at the same time. Subnormal acuity in one eye or both eyes may become a serious problem. However, myopic anisometropia is common in children and is well tolerated by a majority of them (Slide 1, left). The "natural monovision" of moderate unilateral myopia in a preteenager or teenager rarely requires correction, as one eye sees distance and the other near. When the distance eye develops myopia, full correction of both eyes is almost always tolerated without symptoms.
Other forms of anisoacuity are more likely to require therapy. Unilateral high myopia, almost any degree of hyperopic anisometropia, and astigmatic anisometropia (to a lesser extent) are potential causes of amblyopia in children. A child with any of these types of anisometropia should be treated as soon as the refractive asymmetry is discovered.
A child with unilateral high myopia may do well with full spectacle correction of the refractive error, though a contact lens can be equally as beneficial if a patient or parent can manage its use. Even refractive surgery may be appropriate for such a child. Because anisometropic myopia is usually due to asymmetric axial length, aniseikonia is minimal and tolerable for most children. Amblyopia from such anisometropia sometimes improves with refractive correction alone, though patching therapy is required to achieve maximum visual acuity in many children.
Amblyopia from such anisometropia sometimes improves with refractive correction alone, though patching therapy is required to achieve maximum visual acuity in many children.
Hyperopic anisometropic children are especially likely to develop amblyopia (Slide 1, right). Even when the less hyperopic eye is focused with accommodation, the other eye remains blurred, unlike the moderately myopic anisometrope. Small children are tolerant of this defect, and the problem may not be found until school age, by which time amblyopia is present. Although detection efforts have improved through screening programs, hyperopic anisometropia remains the most common cause of amblyopia. In esotropic children, full correction of hyperopia is necessary. However, in other children, it is appropriate to correct the difference between the eyes, but to leave 1 D to 2 D of uncorrected hyperopia in each eye to encourage accommodation, which may be a factor in reducing the total hyperopia.
The presence of asymmetric acuity makes fusion difficult, and children who have an underlying phoria or intermittent tropia may benefit from optical correction of their anisometropic error with spectacles or contact lenses. In many instances, correction of anisometropia reduces or eliminates the need for prism or surgery to control a patient's strabismus.
|
Anisophoria is an iatrogenic problem caused by spectacle correction of anisometropia. Unequal prism is induced in front of both eyes as the eyes move the direction of gaze away from the optical centers of unequal power lenses.
|
Prentice's rule (Slide 2) identifies the amount of prism induced by off-axis viewing through a lens as equal to the power of the lens times the distance of the line of sight from the optical center of the lens (measured in centimeters). In a patient with OD 2.00 D and OS -3.00 D (Slide 3), left gaze induces base out prism in front of both eyes, leaving the eyes exophoric; that is, the eyes must converge to fixate bifoveally on a target. In right gaze, the base in induced prism leaves the eyes esophoric, requiring divergence for fusion. Understandably, horizontal anisophoria may produce symptoms, including asthenopia, nausea, panoramic headaches, and spectacle intolerance. If fusional amplitudes are exceeded, diplopia results. This is especially common when vertical anisophoria is present because vertical fusion amplitudes are minimal in most people. Anisometropic presbyopes often experience diplopia when reading because the use of bifocals forces them to look away from their spectacles' optical centers, inducing vertical anisophoria.
The presence of symptoms from anisophoria is dependent on multiple factors. Lens power and degree of anisometropia are major ones, as are fusion amplitudes and preexisting phorias. Position of the spectacle's optical centers before the eyes is also significant, as an anisometrope may be diplopic even when looking straight ahead if the line of sight is above the centers of the glasses. Vertex distance is a factor because the line of sight to an off-axis target is farther from the optical center if the vertex distance increases (Slide 4). Lens size plays a role, because larger lenses allow greater off-axis viewing and, thus, greater potential for symptoms.
|
Although treating anisophoria may have a financial impact on the patient and spectacle frame choices may be limited, undertreating anisophoria results in unhappy patients. The following are some basic rules for when treating anisophoria is appropriate:
Compensating for Anisophoria
When reducing anisophoria symptoms, a clinician must calculate the prism induced by the spectacle prescription using Prentice's rule and the following basic assumptions:
|
When oblique astigmatism is present, the effective power must be known in the vertical meridian to determine the risk of diplopia when reading and in the horizontal meridian to evaluate phorias in lateral gaze. Table 1 shows the effective power in the vertical and horizontal meridians of a 1.00 D cylinder oriented at various axes.
Symptoms of horizontal anisophoria may be minimized by careful frame selection. Avoiding large frames reduces the degree that ocular movements may carry the line of sight away from the optical center. Also, keeping vertex distance at a minimum further limits induced prism. Patients will have fewer problems when they turn their head to look at peripheral targets through the center of the glasses, rather than turning their eyes to the side to view the same target. Of course, contact lenses would eliminate anisophoria, but contact lenses cannot be used by all anisometropes.
Anisophoria in the vertical meridians is likely to produce diplopia, but can usually be controlled successfully. Distance vision diplopia occurs only rarely and usually because the glasses were fit poorly. For reading difficulties, the primary technique of compensating is slab grinding of one lens (Slide 5). Slab grinding is a method of adding prismatic effect in the reading area of a lens without producing prism in the distance vision area.
Slab off, or base up slab, grinding is achieved by changing the center of curvature of the grinding tool that is cutting the front lens surface, without changing the curvature that is being cut. Because the front curve remains stable, the lens power (or difference between front and back curves) is the same across the lens. Essentially, a base down prism wedge is removed inferiorly, leaving a base up effect in that area of the lens. The slab on grinding, or base down slab, is the reverse of the slab off method, in which a base down wedge is added to (or left on) the inferior area of the lens, adding base down prism. Slab off grinding is always performed on the more minus lens. Slab on grinding is always performed on the more plus lens. One can slab off 1 to 3 prism diopters, and slab on 1.5 to 5 prism diopters. Slab calculation is summarized in the following steps:
One alternative to slab grinding would be single vision reading glasses with the optical center set 8 mm below the primary gaze position, providing relief for patients who cannot tolerate bifocals. Dissimilar bifocals may be accepted by a few patients. These could be used to induce prism counteraction of the anisophoria of the distance correction. A round seg induces base down prism because the reading position is above its optical center, whereas a flattop seg may induce a slight base up prism if the reading position is below its optical center or none if the reading is at its optical center. A contact lens in one or both eyes may also be offered to patients .
|
Aniseikonia is a difference in size or shape of the focused images presented to the brain by the two eyes (Slide 6). It may be an asymmetrical size from spherical anisometropia (one larger than the other) or an asymmetrical shape from meridional distortion from anisometropic astigmatism.
Aniseikonia is considered to be inherent (or primary) aniseikonia if it is a property of the eyes themselves, and may be described as either anatomical or optical. Anatomical aniseikonia results when there are differences in the spacing of retinal receptors in the two eyes, as may occur in unilateral central serous retinopathy or unilateral epiretinal membrane with foveal distortion.
Inherent optical aniseikonia is presumed to be present in anisometropia, which may be the result of either refractive or axial asymmetry. Refractive anisometropia occurs when the difference in refractive error of the two eyes is the result of asymmetric corneal and/or lens power (e.g., a patient with keratometry OD = 43.00 sphere and OS = 48.00 sphere). The effect on image size is actually mild. If an emmetropic eye were made ametropic by changing only the refractive elements, the blurred retinal image would be about 0.25% per diopter different in size from the emmetropic eye's image. Myopic eyes would have smaller images; hyperopic eyes larger.
Axial anisometropia produces considerably more significant aniseikonia than refractive anisometropia. Compared to emmetropia, an eye made ametropic by lengthening (myopia) or shortening (hyperopia) would have a retinal image 1.5% per diopter different from the emmetropic eye's image. The myope's image would be larger; the hyperope's smaller.
|
All lenses used to correct for refractive errors have magnification effects and alter the image size on the retina. If the corrective lens is placed at the anterior focal plane of the eye, it alters the image by approximately 1.5% per diopter. The effect is less if an appropriate power corrective lens is closer to the eye. It is least when a contact lens is used. Hyperopic corrective lenses enlarge, whereas myopic lenses minify. In refractive anisometropia, the asymmetric magnification would result in significant aniseikonia (1.5% per diopter), because the magnification of the lenses is additive to the inherent magnification effect. In axial anisometropia, however, the corrective lens at the anterior focal plane counteracts the inherent effect, canceling any magnification change compared to the emmetropic eye (Knapp's rule). In the clinical environment, anisometropia is usually a combination of asymmetric refractive and axial components. Glasses are not often at the anterior focal plane of both eyes, so aniseikonia is generally estimated to be 1% per diopter of anisometropia.
|
The symptoms of aniseikonia are the result of the brain's attempt to fuse the dissimilar images it receives from the eyes. When this function is stressed to its limit, patients experience headaches, asthenopia, pseudovertigo, a "swimming" of vision, poor depth perception, and spectacle intolerance. Clinicians vary in their opinion regarding tolerance of aniseikonia. It appears that most patients can tolerate 1% to 1.5%, but greater aniseikonia is likely to produce symptoms. When aniseikonia exceeds 5%, symptoms may actually decrease, as most people stop attempting to fuse images of such large size disparity. However, there is clear evidence in clinical practice that some individuals tolerate a broad range of aniseikonia without loss of function and some individuals are intolerant of minimal aniseikonia.
|
Lens Magnification
Unlike the textbook explanation of thin lens optics, all spectacle lenses have magnification effects. These effects are a result of both the
shape and power of the lenses. The total magnification is the shape factor (Ms) times the power factor (Mp).
|
The shape factor formula (Slide 7) shows that increasing either lens thickness (t) or front curvature (D1) will decrease the denominator and increase the lens magnification. Also, increasing the index of refraction of the lens (n) will decrease the lens magnification.
The power factor formula (Slide 8) shows that increasing vertex distance (h) decreases the denominator and, thus, increases the effect of the lens, which is magnification if a plus lens (Dv = plus) and minification if a minus lens (Dv = minus). Increasing plus power decreases denominator, resulting in increased magnification of more than 1.00. Increasing minus power increases denominator, resulting in decreased magnification of less than 1.00.
|
Using these formulae, nomograms have been created for the shape factor (Slide 9) and the power factors for minus (Slide 10) and plus lenses (Slide 11). Measuring the thickness with calipers (Slide 12, bottom), the front curve with a lens clock (Slide 12, top), and the vertex distance with a Distometer (Slide 13) provides all the necessary information to determine the magnification of a pair of glasses. A simple straight edge connecting the known parameters will indicate the shape and power factors and multiplying the shape and power factors results in the total magnification of the lens. These nomograms are based on standard plastic lenses. The effect of newer lenses that have a high index of refraction is discussed below.
|
Routinely, spectacles have a steeper front curve for more plus powers and a flatter front curve for more minus. Increasing plus powers increases the thickness of a lens, and all minus power lenses are made with the minimum allowed center thickness, formerly 2 mm to 2.2 mm, although newer lenses are often created with centers as thin as 1.3 mm. The shape magnification formula shows that these standard techniques enhance aniseikonia rather than minimize it, unfortunately.
Reduction of Aniseikonia
The magnification formulae involve variables that can be adjusted to reduce aniseikonia - power, front curve, thickness, vertex distance,
and index of refraction. Changing power is rarely the proper approach, because it means prescribing inaccurate spectacles resulting in
suboptimal acuity.
Case Study
When crown glass and standard plastic were the only lens materials available, aniseikonia was managed by varying curvature, thickness,
and vertex distance from the "normal" spectacle parameters. The following case study demonstrates the technique.
|
|
Sam was unhappy with his new glasses (Table 2), which had 3.25 D of anisometropia. The glasses had been manufactured with standard lenses, the more minus having a flatter curve. In the problem glasses, the right lens shape factor equals 0.8% or 1.008. The left lens shape factor equals 1.003 (0.3%). The right lens power factor equals 0.981 (-1.9%), whereas the left equals 0.942 (-5.8%). Multiplying the shape and power factors shows that the right lens magnification equals 0.989 (-1.1%) and the left 0.945 (-5.5%). Thus, the right lens produces an image 4.4% larger than the left. These factors are enough to produce his symptoms.
|
|
Management.
The left lens image must be enlarged and/or the right diminished. It is more effective to work on the more powerful lens. If the left lens is
made thicker than necessary at 3 mm and the front curve steepened (increased) to 4 D, both changes will enlarge the image because
thicker and steeper always magnify. Increasing vertex distance increases the effect of lens power, which means minifying because the
lens is minus. The opposite is true if the vertex distance decreases; that is, it will magnify, compared to the 13-mm vertex. It is possible to
have asymmetric vertex distances in minus lenses by placing the bevel that holds the lens in the frame forward or back on the lens edge
(Slide 14). All of these variations from a standard lens reduce the image size of the left spectacle lens. Opposite changes
in the right lens, including flattening the front curve and thinning the center thickness, also help. Because it is a much weaker lens,
the effect is minor.
The result of creating glasses with these nonstandard lenses (Table 3) slightly minifies the right image to 0.986 (-1.4%) and magnifies the left to 0.960 (-4%), resulting in a reduction in aniseikonia to right larger than left by 2.6%, which it is hoped that Sam will tolerate. The glasses look different from average glasses, but they work better for him.
|
Although standard plastic lenses have an index of refraction of 1.49, higher density lenses are now available with an index of refraction up to 1.70. The advent of high index of refraction spectacle lenses has increased the opportunities for aniseikonic compensation. The formula for the shape factor showed that increasing the index of refraction decreases the lens magnification (Slide 7). The higher index also allows a certain lens power to be produced by a thinner lens with a flatter base curve (Slide 15). Because virtually all spectacle lenses have a meniscus profile, flattening the front curve will also flatten the back curve and bring the lens closer to the eye, that is, the vertex distance decreases. In plus powers, these lenses are created with aspheric front surfaces which allow further flattening. The effect on aniseikonia can be enhanced by using two lenses of different indices of refraction, rather than both with the highest index available. Slide 16 is a photograph of 4 D in each lens. However, the right is a standard CR-39 plastic lens with a 1.49 index of refraction; the left is an aspheric lens with a 1.6 index of refraction.
|
If a patient has myopic anisometropia, using a higher index lens for the more myopic eye will result in the lens being flatter and, therefore, its vertex distance is shorter than usual, which magnifies. Increasing the lens' thickness to higher than the standard 1.5 mm (e.g., to 2.5 mm or more) further magnifies. If a lower index lens is used for the other eye, a patient's glasses adequately compensate for the aniseikonia. In myopic prescriptions, one may even increase the vertex distance of the less myopic eye and/or decrease that of the more myopic eye to achieve additional reduction in aniseikonia.
|
In a hyperopic anisometrope, the higher index lens is still used with the more ametropic eye. That eye will have a flatter and thinner lens than a lens made of standard plastic. Both of these changes reduce the magnification of the more hyperopic lens. If the vertex distance is minimized, the power factor is less important, so these lenses should be fit close to the lashes. For either type of anisometropia, the higher index aspheric lens is used on the higher power eye.
These efforts may be considered laborious but patients like Sam will benefit especially because he had already experienced the results of glasses without aniseikonic compensation. When there is a significant anisometropic change in new prescription glasses, compared to the old glasses, modifying standard spectacle lenses should be considered. Modify if a patient has had problems adjusting to new glasses in the past, or has never been comfortable with any glasses. Compensate if the last glasses were compensated for aniseikonia, and consider compensating aniseikonia in anxiety prone, type A personality patients.
Anisometropia is common in daily practice. Therefore, it is important for ophthalmologists to understand the principles of anisometropia management. Following is an outline of the information discussed in this tutorial.
