Binocular vision can be disturbed by large differences in the refractive error between the two eyes: anisometropia . When this is left uncorrected, central suppression areas can develop in the eye with the more blurred vision. Anisometropia over 1.50D results in a significant increase in the risk of amblyopia and decrease of binocular function ( Weakley, 2001 ) and higher anisometropia is likely to be associated with worse amblyopia and stereoacuity ( Rutstein & Corliss, 1999 ). Poorer stereoacuity is associated with reduced performance at motor tasks ( Hrisos, Clarke, Kelly, Henderson, & Wright, 2006 ) and at driving ( Bauer, Kolling, Dietz, Zrenner, & Schiefer, 2000 ; Gresset & Meyer, 1994 ). A surprising finding is that approximately a quarter of hypermetropic anisometropic patients exhibit, in their amblyopic eye, less accommodation at near than at distance ( Toor, Horwood, & Riddell, 2018 ). This was resolved with spectacle correction, emphasising the importance of optometric care for these patients. These authors noted that aniso-accommodation is more common than previously suggested, even in a normal control group.
Anisometropia over 1.00D occurs in 0.7% of preschool children ( Donahue, 2006 ). Anisometropia in children is an indication for cycloplegic refraction ( Table 2.11 ). If anisometropia occurs in young patients, and particularly before the age of 6 years when the visual system is still not firmly established, amblyopia may also be present. Often in these cases, the vision is very good in one eye, so that the anisometropia and reduced vision in the other eye is not discovered. The older the child, the more difficult it is to treat the amblyopia and restore full acuity ( Chapter 13 ). The importance of early eye examination is obvious, and the procedures for the examination of young children are dealt with in Chapter 3 . There is no doubt that many cases of anisometropic amblyopia are preventable by early examination and correction by spectacles. The treatment of anisometropic amblyopia is covered in Chapter 13 , together with other types of functional amblyopia.
With patients of any age, prescribing glasses to correct anisometropia may present two additional problems:
Prismatic effects: when the patient is not looking through the optical centres of the lenses, a difference in prismatic effect between the two lenses can make binocular vision difficult or impossible. These prismatic effects present more difficulties when the patient looks above or below the centres, as the vertical tolerance to prisms is very much less than the horizontal. For some patients, a vertical prismatic effect of 0.5Δ can impair stereopsis (Jimenez, J.R., Rubino, M., Diaz, J.A., Hita, E., & Jimenez-del Barco, L., 2000).
Aniseikonia: when the lenses are of different powers, there will be a larger retinal image in one eye than the other owing to the difference in spectacle magnification.
These two problems are discussed in more detail later. In both cases, these difficulties will cause more problems in older patients, with previously uncorrected patients, or where a large change in prescription is given.
The main factor in recognising a difficulty due to prismatic effects is the presence of the anisometropia itself. It may also be found that older children and teenage patients with anisometropia have spasm of accommodation, and cycloplegic refraction is especially important in these cases. The symptoms of anisometropia are typically those due to the type of refractive error in the better eye: asthenopia for near vision in hypermetropia and blurred distance vision in myopia. Some patients may be hypermetropic in one eye and myopic in the other. In these cases, they may use one eye for distance vision and the other for close work. If there is no significant refractive error in one eye, the patient may have no symptoms. This may also be true in cases where no glasses have been worn and suppression has developed.
Many patients will experience no problems when spectacles are prescribed; the younger the patient when glasses are first worn, the more likely that trouble can be avoided. This is probably because patients with stable binocular vision or compensated heterophoria can usually adapt to prismatic effects in a very short time ( Carter, 1963 ). The symptoms that occur when the patient does not adapt to the correction for anisometropia consist of difficulties in adjusting to the new glasses: typically, headache or intermittent diplopia. Troubles seldom occur when the anisometropia is less than 2D. If spectacles that fully correct the anisometropia can be tolerated in childhood, the prognosis for successful spectacle wear in adult (pre-presbyopic) life is good, as anisometropia usually gradually reduces over the years ( Ohlsson, Baumann, Sjostrand, & Abrahamsson, 2002 ).
Investigation and Evaluation
Often, the difficulties described above can be avoided by anticipation. A partial correction is given in the more hypermetropic eye in those cases where there has been no previous correction or where there is a large difference between the previous correction and the new one ( Howell-Duffy, Umar, Ruparelia, & Elliott, 2010 ). The extent of this modification to the prescription can be determined by the Mallett fixation disparity test ( Chapter 9 ). The patient looks at the fixation disparity vertical target through the full correction and is asked to move the head vertically up and down in a nodding movement, so the eyes look through the lenses above and then below the optical centres. If a vertical fixation disparity is induced, the prescription is modified until this does not occur.
As a rough guide, the prescription for the more hypermetropic eye is reduced by one-third of the change in the anisometropia (the difference between the two eyes) compared with the previous prescription. This will mean that it is reduced by one-third of the anisometropia in the case of a patient who has worn no previous glasses. However, it must not be assumed that all patients with anisometropia will experience difficulties with their new glasses. Some patients with marked anisometropia will settle very readily to a new prescription, whereas others with low degrees will experience symptoms.
Patients often learn very quickly to turn the head rather than the eyes, so that they always look through the optical centres of the lenses. It sometimes helps to encourage patients to do this. If the patient needs bifocals, round top segments of different sizes in each lens can be used to control the vertical prismatic power in the reading portion. A more detailed coverage of this subject can be found in Rabbetts (2007) . Optically, the best approach is to fit contact lenses, which move with the eyes, so no prismatic effect is induced, and which also reduce aniseikonia ( Evans, 2006a ). Refractive surgery has been advocated for similar reasons ( Paysse, Coats, Hussein, Hamill, & Koch, 2006 ).
Aniseikonia Due to Spectacle Magnification Differences
Most aniseikonia arises from the difference in spectacle magnification that accompanies anisometropic corrections, acquired optical aniseikonia . Other types will be considered separately later in this chapter. Interestingly, everyone experiences aniseikonia in asymmetrical convergence of the eyes, for example, when converging to an object in our peripheral vision which will be closer to one eye than the other, and this may become of the order of 5%–10% or more. This physiological aniseikonia appears to be automatically compensated and gives rise to no symptoms ( Romano, 1999 ).
The possibility of an aniseikonic problem occurring can be foreseen largely from the presence of anisometropia, and particularly when there is a difference in spectacle magnification of more than 2% (some authors say 5%). This means that anisometropia of as little as 1.25D may cause clinically significant aniseikonia, although the precise value will depend on the prescription, back vertex distance, and relative ocular dimensions ( Rabbetts, 2007 ). Typically, 1D of anisometropia causes between 1% and 1.5% of aniseikonia ( Borish, 1975 ).
There is a large variation between people in the amount of aniseikonia they can tolerate ( Romano, 1999 ). Again, symptoms can lead to nontolerance of new glasses, and sometimes headache and intermittent diplopia. A symptom more characteristic of aniseikonia is a disturbance in spatial perception: the floor appears to slope, or other horizontal objects appear tilted when looking through the new glasses. Induced aniseikonia (using size lenses) of 3%–5% causes a reduction in stereoacuity ( Jimenez, Ponce, Jimenez-del Barco, Diaz, & Perez-Ocon, 2002 ) and of 5% or more significantly reduces binocular contrast sensitivity and binocular summation ( Jimenez, Ponce, & Anera, 2004 ).
Aniseikonia can be investigated with an eikonometer ( Morrison, 1993 ), although this apparatus is uncommon. There are two types of eikonometer:
Ames eikonometer , which presents a separate image to each eye, so the patient can make a direct comparison of the image sizes. Polarising filters can be used ( Romano, 1999 ).
Space eikonometer , which allows the patient to recognise distortions of space perception, such as a tilting of the frontoparallel plane.
In both cases, measurements of image size differences are made by an afocal optical system of variable magnification which is adjusted until a normal appearance is reported. Neither of these instruments gives very consistent results. Several readings are taken, and if the spread of readings is less than the mean value, this mean value may be taken as the size difference. Its use may be more necessary in types of aniseikonia other than acquired optical.
Most eyecare practitioners do not have an eikonometer, and a software package called the Aniseikonia Inspector was developed for the investigation and management of aniseikonia ( de Wit, 2003 ). This produces more accurate results in the vertical direction than the horizontal, but underestimates aniseikonia ( Rutstein, Corliss, & Fullard, 2006 ; Antona, Barra, Barrio, Gonzalez, & Sanchez, 2007 ). A later version of the test may overestimate aniseikonia induced by size lenses ( Fullard, Rutstein, & Corliss, 2007 ).
It is possible to obtain a diagnosis and qualitative estimate of aniseikonia using commonly available refracting equipment which dissociate the right eye from the left eye images. For example, many projector charts have a muscle balance test comprising a pair of ‘square brackets’, one of which is seen by the right and the other by the left eye. Patients can directly compare the size of these to give an estimate of aniseikonia. A similar technique can be used with letter charts having cross-polarised letters or targets. A more accurate measure can be obtained if a tangent screen is available. The two eyes are dissociated with a vertical prism that is too great to be fused (e.g., 8Δ) and the position of numbers on the smaller image of the tangent scale is compared to the position of the same numbers on the larger to calculate the magnification difference. This approach can be improvised using a line of Snellen letters for distance vision or a centimetre rule for near vision.
Anticipation of the difficulties is again very important. The following should be considered:
Warn the patient that difficulties in space perception may occur during the first few days of wearing the new glasses. It is usually adequate to say that the patient’s prescription is of the type that may require a few days to settle to the new glasses. In most cases, these problems will disappear after a short time, particularly if some of the factors mentioned next have been considered. Warn the patient not to drive or operate machinery until they have adapted.
Some strabismic patients may be less able to tolerate optically induced aniseikonia than patients with normal binocular vision ( Bucci, Kapoula, Bernotas, & Zamfirescu, 1999 ), so they may be less able to tolerate large refractive changes.
Reduce the difference in spectacle magnification by considering the factors that contribute to it ( Fig. 11.1 ):
Lens power: the higher the power, the higher the spectacle magnification. A partial correction for one eye can be considered, again reducing by about one-third of the change in the anisometropia. In some cases, a partial correction in both eyes may be appropriate, as this will leave the patient to exert the same accommodative effort in both eyes. With young patients, such a binocular reduction will give sufficient correction to relieve any symptoms of hypermetropia, but because both lenses are less powerful, the difference in spectacle magnification will be less.
Lens form: the deeper the meniscus (i.e., the higher the base curve), the greater the spectacle magnification. The lenses should be dispensed with the more positive lens in a ‘flatter’ form than the other (e.g., aspheric front surface). This will reduce the spectacle magnification a little, and it will also result in the front surfaces of the lenses being more similar in appearance. The lenses can be made to an aspheric design, which is thinner, flatter, and lighter.
Lens thickness: the thicker the lens, the greater the spectacle magnification. The least powerful positive lens can be thicker than normal, so that its spectacle magnification is increased slightly. This will also have the effect of helping to balance the weight of the two lenses. Clearly, the more powerful lens needs to be kept as thin as is consistent with the type of frame or mount used. This will maintain the spectacle magnification and the weight at a minimum. One possibility is to use a higher refractive index material for the thicker lens, although care should be taken to ensure the antireflection coatings do not appear different.
Back vertex distance: the closer the lens is to the eyes, the less will be the spectacle magnification. It is not possible to mount one lens closer to the eyes than the other, but if the back vertex distance is kept to a minimum, the spectacle magnification for both eyes will be at a minimum and therefore the difference between them less.