Anisometropia: what difference does it make?

Optometry in Practice 2013 Volume 14 Issue 1 1 – 10
Anisometropia: what difference does it make?
Paul McCarthy FBDO PG(cert)HE FHEA
Anglia Ruskin University, Cambridge
EV-10774 C-30368
1 CET point for UK optometrists
This article discusses the identification and presence of
anisometropia. The effects on patients with anisometropia,
such as aniseikonia and diplopia, are considered, as are the
issues that the correction of anisometropia presents to both
optometrist and dispensing optician. Lenses that are still
currently available and which help to eliminate or reduce
symptoms of off-axis vision to anisometropes to levels
that are within tolerance are discussed.
Consider for a moment how concerned patients would feel
if, when collecting their first pair of multifocal or progressive
lenses, they find that, on looking down to read, the print
appears to be double and reading is impossible. Maybe they
would feel that the prescription is wrong or the glasses
have not been made correctly. Their sudden fear of having a
serious eye condition should not be discounted.
Fortunately situations like this are unusual as well as
avoidable. Careful analysis of the patient’s prescription would
very likely have identified the presence of anisometropia.
Consideration of the dispensing process should then have
led to an informed decision on how to prevent any
non-tolerance. There is however a growing tendency today
for prescriptions to be transferred directly on to a computer
within the consulting room, which can sometimes lead to an
order being created for a new pair of spectacles with little or
no discussion with the optometrist.
It is imperative therefore that both optometrist and
dispensing optician first recognise an anisometropic
prescription and then if necessary discuss the issues that need
to be considered to ensure patient non-tolerance is avoided.
To do this we need to consider the following points:
• What is anisometropia and how do we recognise it?
• What impact does it have on the patient’s vision?
• How do we avoid the effects of anisometropia?
• What lenses are available?
Anisometropia
Anisometropia is present when a subject’s right and
left prescriptions are unequal: the prefix ‘aniso’ means
‘not the same’.
A significant difference in refractive error between the
two eyes of more than 1.00D in any meridian is often
given as a definition of anisometropia (Harvey and
Gilmartin 2004; Weale 2002). Woodhouse (2012), when
describing the rarity of anisometropia, states that a
difference between the eyes of more than 1.00D should be
considered abnormal.
When the eye looks at a point away from the optical centre
of a lens, or the axis of a plano cylinder, it encounters a
prismatic effect. The amount of this prismatic effect,
determined by the expression P = cF, is the product of the
distance in centimetres from the optical centre and the
power of the lens in the corresponding direction. Looking
through a point 8mm directly below the optical centre of
a lens of power +3.00 will result in a prismatic effect of
2.4Δ (0.8cm × +3.00D). The prism base direction which is
indicated by where the thickest part of the lens is from the
visual point will be up.
If the right and left prescriptions are the same and both
lenses are centred correctly, then each eye will encounter
equal prismatic effects at the corresponding visual points,
which should not present any difficulties to the wearer.
All isometropic progressive lens wearers with equal prism
thinning in each lens encounter this effect with rarely any
difficulties. Both eyes exhibit a version or conjugate movement
where they move together towards the apex of the prism.
Displacement of the image is the same and so each eye
will rotate by the same amount so that binocular vision
is maintained.
Far more problematic is when two eyes have different
prescriptions, especially if the difference is in the vertical
meridian. This will give rise to a differential prismatic effect
Date of acceptance: 11 September 2012. Address for correspondence: P McCarthy, Anglia Ruskin University, Cambridge Campus, East Road, Cambridge CB1 1PT.
[email protected].
© 2013 The College of Optometrists
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P McCarthy
at the same right and left visual points away from the
optical centres. Depending upon the degree of differential
prism it may or may not be possible for one eye to exhibit
a vertical vergence, either supra or infra, to achieve and
maintain binocular vision. If the differential prism is too
great so that fusion of the right and left images cannot be
sustained, it will result in the object being seen double. It is
not always possible to predict the degree of tolerance,
however. Dadeya et al. (2001) have found from the use of
Bagolini lenses that some binocularity can be preserved
even with the presence of 3.00D of anisometropia.
Holladay and Rubin (1988) agree that, whilst horizontal
prism disparities rarely cause problems because of the
large horizontal vergence amplitudes, this is not the case with
the much smaller vertical amplitudes.
A subject with just a 1.00D difference in right and left
refractive errors looking through corresponding points 1cm
away from the optical centres of lenses will encounter 1∆ of
differential prismatic effect.
It is generally accepted that many subjects cannot tolerate
more than about 1∆ of differential prismatic effect for
a prolonged period, especially in the vertical meridian
(Jalie 1984). This is more likely to manifest itself to the
multifocal or progressive lens wearer where, for close-up tasks,
the eyes are required to look down vertically into the near
visual zone. If we consider points at 10mm below the optical
centres of the lenses R +1.00DS, L +3.00DS, a differential
prismatic effect of 2∆ base up in the left (or down in the right)
will result.
Not so obvious at first glance is the effect at the same
visual point with a prescription: R +2.00 / –1.75 × 180,
L +2.75 / –1.00 × 135. Figure 1 shows that for the right eye
the vertical power is just +0.25D. For the left eye the cylinder
power in the vertical direction can be calculated using F sin2θ
where θ is (135 – 90). The cylinder therefore exerts only
–0.50D in the vertical direction, giving a total power of
+2.25D. The differential prismatic effect is again 2∆ base
up in the left (or down in the right) at 10mm below the
optical centres.
R; +2.00D / -1.75 x 180
+2.00D
-1.75D
= +0.25D
+2.00D
Figure 1.
2
L; +2.75D / -1.00 x 135
+2.75D
+2.25D
+2.75D
-1.00D
= +1.75D
The above example shows the importance of considering
the effect of the power and direction of the cylindrical
element on the meridional anisometropia and its impact
on the differential prismatic effect, especially in the more
critical vertical direction.
It is useful to remember that the differential prism at near will
always be base down for the eye requiring the more negative
or less positive lens.
Induced anisometropia
From the author’s experience it was not unusual in the earlier
days of intraocular implants for meridional anisometropia
to have resulted from the astigmatism induced during the
operation. This often led to a notable change from the
patient’s original prescription. Cases do still occur but
thankfully this is now less frequent. The following example
is given where a patient after many years of wearing
bifocals was unable to tolerate the same lens type after
undergoing a cataract removal from the left eye.
Prescription before cataract removal from the left eye:
R: +3.00 / –1.00 × 10 VA 6/9 –1 Add +2.50
L: +2.75 / –1.00 × 45 VA 6/36 Add +2.50
Prescription following left lens implant:
R: +3.25 / –1.00 × 10 VA 6/9 –1 Add +2.75
L: +2.25 / –2.25 × 175 VA 6/6 –1 Add +2.75
Assuming a near visual point (NVP) of 10mm below the
optical centre, with the old lenses the subject would have
encountered a difference of only 0.25∆ base down in
the right, which would not have caused any problems of
diplopia, although, of course, vision in the left eye would
have been poor. With the new prescription the spherical
element of both prescriptions has not greatly altered but
the new cylinder power and axis direction in the operated
eye have resulted in a differential prismatic effect of
about 2¼∆ base down in the left lens at the same NVP. In
contrast to the more common amblyopic anisometrope,
this subject’s good acuities had been restored in both eyes.
It would not have been unreasonable therefore for this
subject to have expected excellent vision through new
bifocals. Unfortunately the 2¼∆ difference at the NVP
gave rise to diplopia, resulting in the patient being unable to
tolerate the new lenses.
Amblyopic anisometropia
Amblyopia is present when the visual acuity (VA) of a
corrected eye which is ophthalmoscopically normal is
reduced compared to the other. Anisometropia is one of
the leading causes of amblyopia, although the mechanism
of anisometropic amblyopia is poorly understood. It is not
clear what levels of anisometropia should be corrected in
children and at what age this correction should take place
to ensure the best chance of visual development (Dadeya
et al. 2001). Weakley (1999) has stated that early detection
Anisometropia: what difference does it make?
and treatment of anisometropia before or early in the
development of amblyopia are likely to produce better
visual outcomes. Weale (2002) states that a disparity
between refractions of the two eyes may cause functional
impairment manifesting in amblyopia. This is mainly the
case with hypermetropic subjects who, if uncorrected
before their early critical period, are only able through
accommodation to focus with the eye requiring the lower
prescription as this requires less accommodative effort.
This will commonly lead to poorer VA in the more
hypermetropic eye due to blur – monocular deprivation
(Tunnacliffe 1984). Wu and Hunter (2006) record also
that anisometropia gives rise to amblyopia when one eye,
typically the more hypermetropic, remains blurred.
A subject with a prescription R +0.75DS, L +3.25DS
(Figure 2a) will accommodate by the lower amount in
both eyes, leaving +2.50D uncorrected in the left eye
(Figure 2b), resulting in the likely VA of no better than 6/24.
Early correction would seem essential therefore in these
cases if amblyopia is to be avoided. Duane (1922) measured
accommodation separately for each eye and came across
cases exhibiting different amplitude in the two eyes. This
he attributed to differences in the mechanical properties
of the two lenses.
+0.75
+3.25
Uncorrected. With no accommodation
Figure 2a. Uncorrected, with no accommodation.
Corrected
+2.50
uncorrected
Uncorrected. With +0.75 accommodation
Figure 2b. Uncorrected, with +0.75 accommodation.
Myopic anisometropia rarely causes amblyopia until it is
greater than 2.00D, whereas hypermetropic amblyopia
may arise with as little as 1.00D difference. Astigmatic
anisometropia may cause amblyopia when the difference
becomes greater than 1.50D.
Attempts to improve the VA in the amblyopic eye of
hypermetropes are often carried out by providing a
spectacle correction together with the occlusion of the
fellow eye. Levartovsky et al. (1998) in a study with infants
concluded, however, that those subjects with larger
amounts of hypermetropic anisometropia have a worse
prognosis for successful treatment. Their study found
that those with anisometropia of up to +1.50D were likely
to maintain any improvement in VA after treatment
whereas those with higher values showed a significant
deterioration – from 6/12 to 6/21 over a period of 6.4 years
after treatment. Walsh et al. (2009), in a pilot study in
which they compared two methods of cessation of
amblyopic management, namely abrupt cessation and
therapy tapering, found no difference in the rate of recurrence
of amblyopia for either method.
With myopes having the far points in front of the eyes, both
are able to be stimulated and so amblyopia is less common.
A subject with R –0.25D, L –3.00D could use the right eye
for more distance viewing and the left for distances of
33.3cm or less.
Where VAs are significantly different there is the likelihood
of suppression of the poorer image. If the eye with the
worse VA is accompanied by a significantly higher prescription,
as is usually the case, the expected diplopia with off-centre
viewing may or may not manifest itself to the wearer. It is
possible that little advantage would be gained for the patient
in providing any solution to the anisometropia in these cases
and a balance lens for the worse eye may be advised. An
example is of a contact lens wearer whose prescription was
in the order of R –3.50, L –10.75 with corresponding VAs of
6/6 and 6/36. An infection led to her being advised to change
to wearing her spectacles. Years of dislike regarding the
unsightly nature of the left lens in all her previous glasses
caused much distress for this patient and led to reluctance in
following the advice to cease contact lens wear.
The simple process of neutralising the extra minus power of
the left lens with her glasses being worn indicated that no
difference to her vision had occurred. Lenses of –3.50D for
both eyes were dispensed and the patient happily wore
the glasses. It is likely in this case that even if both VAs had
been good, binocular vision would have been poor due to
unequal image sizes and so a balance lens may still have been
the best spectacle lens option.
We have seen that a differential prismatic effect will only
occur when both eyes look through corresponding points
away from the optical centre of lenses of different powers.
It is usually the multifocal or progressive lens wearer who is
affected by this requirement as the consideration is mainly
focused on the differential prismatic effect at the near visual
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P McCarthy
zone to where the subject lowers the eyes to read. However
it should not be overlooked that both the distance visual
point and fitting cross position on a progressive power lens
will be above the prism reference point. With a fitting cross
height of 6mm which is found on many progressive lenses,
anisometropia of just 1.67D would induce a differential
prismatic effect of 1∆ at the point on the lens that is
likely to be used for prolonged distance vision.
Whilst anisometropic single-vision lens wearers are able to
move their head to ensure the eyes view objects through
the optical centres, there may be circumstances, such as
prolonged or constant use for close-up work, where visual
difficulties may be encountered. To overcome this issue,
lowering the optical centres to coincide with the pupils when
looking down to read will ensure the anisometropic subject
encounters no differential prismatic effect at this point.
In order to comply with manufacturers’ fitting criteria it
is recommended that the optical centres of single-vision
lenses for normal use should be lowered from the pupil
centre position in relation to the pantoscopic tilt of the front
of the frame. With the eyes likely to be positioned at least
5mm above the optical centres in the primary position of
gaze, a 2D difference in right and left lens powers will result
in a prolonged differential prismatic effect of 1∆. It may be
worth considering a reduced pantoscopic tilt with optical
centres on or close to the pupil centres in these cases.
Assessing patient tolerance
From the earlier discussions it could be supposed that a
subject with a significant difference in right and left
prescriptions would require a compensating prism at the
off-centre visual point of one or both of the lenses, which is
usually at the NVP. Some anisometropic subjects however
are able to adapt to a degree of differential prism and exhibit
no symptoms. Some will just suppress, especially at higher
levels. Others, at lower levels, may have good fusional
reserves and tolerate the differential prism. Subjects with
marked anisometropic amblyopia will often benefit from
prism compensation even though, when corrected, the VA in
one eye is poor.
Pouliquen de Liniere et al. (1998) studied 34 progressive
lens-wearing anisometropes. Near vision fusion as well as
phorias, VA and binocular vision were some of the checks
carried out to support the evaluation of tolerance to
progressive lenses over a 6-month period. The majority
were satisfied with their lenses (62%), whilst the rest
either abandoned them or preferred single-vision lenses for
prolonged reading.
It should be reasonably straightforward for the dispensing
optician to determine whether a subject will be tolerant
of the differential prismatic effect resulting from an
anisometropic prescription.
4
Subjects must be instructed to look through the near
visual zone of their single-vision reading or distance lenses,
depending upon whether they are first-time presbyopes or
not, whilst observing print close to their resolution limit,
making any problems more obvious. This would normally
require subjects to lower the eyes about 10mm below the
optical centre. The required compensating prism is placed
before the corresponding eye and, if a ‘better’ response is given,
prism compensation would be beneficial (Tunnacliffe 1998).
It is not uncommon for anisometropic subjects who appear
to suffer no symptoms with existing lenses that have not
been prism-compensated to be dispensed with the same
lens type. It is simple to employ the method above to
determine whether a solution to the anisometropia offers any
added benefit to the wearer.
Aniseikonia
When a solution using spectacle lenses is provided in order
to eliminate the differential prism induced by an
anisometropic prescription, we may still be left with the
problem of the lenses producing different image sizes as
detected by the brain.
Most anisometropias of greater than 2.00D result from
differences in the axial length of the eye rather than
the refractive power (Bradley et al. 1983). It should
theoretically, according to Knapp’s law, be possible to
equate the retinal image sizes of axial anisometropes by
placing the spectacle correction at the anterior focal plane
of the eyes. Bradley et al. (1983) found, however, that
with confirmed axial anisometropia and accurate lens
positioning, aniseikonia was reported by subjects with equal
retinal image sizes. This raised the question of either some
intervention between the retinal and ocular images or changes
within the eye itself.
It is worth briefly reminding ourselves of the difference
between retinal image size and aniseikonia. Aniseikonia
is a binocular vision anomaly in which two eyes perceive
images of different size and/or shape (De Witt 2007). Both
aniseikonia and different retinal image sizes may occur for
both corrected and uncorrected eyes. It could be possible
for a subject to have different retinal image sizes but report
no indication of aniseikonia.
The two axial myopic eyes in Figure 3 show that the
uncorrected blurred retinal image in the less myopic
shorter eye is smaller than that in the more myopic
eye. De Witt (2007) describes that, with a greater axial
length, retinal stretching around the fovea may increase by
a greater proportion; therefore in an uncorrected myopic
eye with a relatively large retinal image, the number of
photoreceptors that are stimulated may be smaller, giving
rise to a smaller perceived image. The difference in the
perceived images indicates aniseikonia. Changes in the retina
may not be uniform and therefore variations in aniseikonia
may occur in other parts of the visual field.
Anisometropia: what difference does it make?
-1.00
Small uncorrected
retinal image size
-3.00
Larger uncorrected
retinal image size
Figure 3. Axial myopia.
A further issue that is likely to be present in unilateral
aphakics is the breakdown of binocular vision through
diminution of the amplitude of sensory fusion and
disturbance of motor fusion (Crone and Leuridan 1973).
When dispensing anisometropes with first-time spectacles
the issue of aniseikonia may need to be investigated and
addressed, especially if both eyes have good acuities. There
would be little point in correcting for differential prism only
to find that fusion of the two images is prevented because
of the degree of aniseikonia.
It is worth remembering that the curvature, thickness
and refractive index of a lens will affect the shape factor
1
element, given by 1 – (t/n)F
, of the magnification of a lens.
1
Refractive ametropes with equal axial lengths are unlikely
to suffer retinal stretching and so the difference in the
uncorrected retinal image sizes should theoretically be the
same as the perceived image sizes.
Reed (2011) discusses Knapp’s law at length, with mention
of it falling short in clinical practice. Part of the law states
that the refractive power of the eye must be equal to that of
the standard emmetropic eye, implying that different retinal
image sizes resulting from refractive ametropia cannot be
equalised with spectacle lenses.
It is not surprising that in view of the above issues contact
lenses are often the favoured method of correction of
both anisometropia and aniseikonia by most optometrists.
Kowal et al. (2005) suggest a case for supplying contact
lenses to all spectacle-wearing myopic anisometropes for at
least a few hours to assess the aniseikonic response.
An extreme example of refractive anisometropia is with
aphakia of one eye where a difference of the right and
left prescription could be over 10.00D. If a spectacle lens
correction were given, this could result in at least 30% of
aniseikonia, which together with the anisometropia will
mean that, especially away from the optical centre, any
fusion of the images will be prevented.
If an anisometropic subject is currently wearing
spectacles and has no symptoms, he or she will either be
suppressing or will have adapted to different right and
left cortical image sizes. Adaptation is unusual in subjects
with more than 5% aniseikonia and symptoms may even be
suffered by patients with aniseikonic values of 3% or less
(De Witt 2007). Crone and Leuridan (1973) reported mean
tolerances of 7%, which is the average residual aniseikonia
when the example above of unilateral aphakia is corrected
with contact lenses. Intraocular lens implants, which are
now the preferred method of correcting aphakia, greatly
reduce the problem of differing perceived image sizes.
Changing the vertex distance and combining spectacles
with contact lenses are also ways of altering the
magnification factor of the correcting system.
A further, more complex issue to assess and control is
dynamic aniseikonia, which is defined as a heterophoria
which varies in magnitude with the prismatic effect as the
eyes deviate from the optical centre of the spectacle lens
(De Witt 2007). The tolerance of this will depend upon
the fusional reserves of the patient.
It should be said that, for current spectacle lens wearers,
provided that any changes in the above parameters are
minimal, there is unlikely to be any additional problems
with regard to aniseikonia.
It should be apparent from the discussions so far that
many factors need to be considered before deciding upon
the most suitable solution for a patient with an
anisometropic prescription. These factors include:
• the degree of anisometropia
• the patient’s tolerance of any differential prism
• the likely improvement in the vision with
prism compensation
• the degree of aniseikonia and retinal image size differences
• the ability to fuse the different image sizes
• the visual acuities
• any suppression of one eye
• whether the use of a balance lens would be preferred
The above points, where possible, should be routinely used
by the dispensing optician in collaboration with the
optometrist as a systematic checklist to ensure that the most
satisfactory outcome for anisometropic patients is achieved.
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P McCarthy
Solutions for anisometropia
If it is established that the dispensing of lenses with no
prism compensation for an anisometropic subject is likely to
lead to diplopia, we need to consider the available options
for eliminating or reducing the induced differential prism
responsible for this problem. In nearly all cases apart from
contact lenses, which will be discussed later, the critical
point will be the near visual zone of the spectacle lens.
Prism compensation within this area may be provided
through the following methods:
• slab-off
• different round bifocal segment sizes
• Franklin split
• glass prism-controlled bifocals
• cemented or bonded bifocal segments
Slab-off
This is arguably the most common as well as cosmetically
attractive method employed on both single-vision and
multifocal lenses to remove or neutralise unwanted
vertical prismatic effect in the near vision zone only.
With progressive power lenses this may also be extended
to removing the differential prism at the distance vision
zone. More commonly the technique is performed on bifocal
lenses, where the horizontal line, showing the apex of
the slabbed-off prism, is made to coincide with the segment
top (Figure 4d).
a
b
Base Down
removed
from F2.
c
Equal Base
Up removed
from F1 above
segment.
Finished lens
with required
prism below
segment top.
When performed on an executive bifocal, the line becomes
indistinguishable from the segment top. However it should
be remembered that, with the optical centre of the
segment now not on the dividing line, there will be a
displacement of the image as the eye passes into the
reading segment of the lens, so removing the no-jump
property of this type of bifocal.
Figure 4 shows the procedure carried out on the less plus
(or more minus) bifocal, this being the lens exhibiting the
more base down prismatic effect at the NVP. In Figure 4a
the whole of F2 is surfaced to remove the required amount
of base-down prism. The same amount of base-up prism
is removed from the portion of the lens above the segment
top in Figure 4b, neutralising the prism introduced in
Figure 4a in this part of the lens. This leaves the required
amount of prism just in the reading zone of the lens.
The bifocals in Figure 4d show the right lens, which is the
less positive, having been slabbed off with the line of the
apex of the prism at the segment top. We can see that,
although the left images are more magnified, their vertical
positioning within the bifocal segment is almost the same
as that of the right lens.
The process shown on Figure 4 is that carried out on a
glass-fused bifocal where the front of the segment has the
same curvature as the main lens and so there is no ridge at
the top of the bifocal. The process is more challenging with
the more common plastics lenses where the protruding
segment on the moulded front surface requires the entire
slab-off process to be performed on the concave rear
surface only (Norville Prescription Companion 2006).
The slab-off technique for progressive power lenses involves
removing the differential prism at the distance reference
point by working prism over the whole prescription surface
and then removing the required base-down prism again from
the lower part of the weaker plus (or higher minus) lens.
Vision will however be restricted at the height of the prism
measuring point due to the slab-off edge (Carl Zeiss ND).
As stated previously, most single-vision lens wearers can
lower the head to maintain vision through the optical
centres of their lenses when needing to look down to read.
A lowering of the optical centres for those primarily involved
in close-up tasks may be beneficial, provided the
distance vision is not compromised. Compensation for
anisometropia for the single-vision lens wearer should not
be overlooked, however, if appropriate to the patient’s
needs and likely to provide better optical comfort.
Figure 4a-d. Slab-off.
6
Slab-off may be performed on a single-vision lens creating
a bicentric lens which will have a separate optical centre
in the near portion that is positioned to eliminate or
reduce any differential prism within the near visual zone.
A bicentric lens is made by cementing a cover on to the
lens (Figure 5b) and then the required amount of prism
is removed by grinding the lens at the correct angle
Anisometropia: what difference does it make?
(Figure 5c). This is continued until the lower edge of the
lens is at a precalculated thickness, placing the dividing line
at the required distance below the optical centre of the
distance zone of the lens (Figure 6).
a
b
c
d
Left eye: OS
NVP = 13mm. (1.3 x 1.00D = 1.3∆ base down)
Right eye: OS
NVP = 6mm. (0.6 x 1.00D = 0.6∆ base down)
Differential prism per dioptre of add at the NVP = 0.7 base
down (Alternatively; the product of the difference in segment
radii and 1.00D gives us 0.7∆.)
R; +1.00D
Add +3.00
10mm
•
4mm
L; +3.00D
Add +3.00
OD
10mm
NVP
OS
•
•
4mm
12mm
•
NVP
OD
•
•
OS
Figure 7. Different segment sizes.
Figure 5a-d. Slab-off technique: single-vision lens.
In this example the distance powers will induce a
differential prismatic effect of 2∆ (base up in left) at the
NVP. With a +3.00D reading add, the different segment
sizes will produce 2.1∆ (0.7∆ × 3) base down in the left at
the NVP, resulting in just 0.1∆ of differential prism, which
will be easily tolerated by the wearer.
If the difference in segment diameters is required, the
× δF
can be used, where c is the distance from
formula 2cAdd
the optical centre to the NVP (in mm) and δF is the
difference in lens powers.
Franklin split
Figure 6. Bicentric lens showing distance and near
optical centres (OC).
Different bifocal round segment sizes
Although not cosmetically attractive, the correct choice
of round segment size for each eye can reduce, if not
always eliminate, the differential prismatic effect at the
NVP to a level the wearer may tolerate.
Round segments exert base-down prism at the NVP
with the amount of prism dependent upon the reading add
and the distance between the NVP and the optical centre
of the segment (OS).
From the geometry of the lenses in Figure 7, we can calculate
that the round 38mm segment in the left eye will exert
0.7∆ base down more per 1.00D of reading add at the NVP
than the round 24mm segment in the right eye.
There cannot be many 250-year-old inventions that may
still be offered as a solution to a current-day problem. With
correct positioning of the optical centres of the reading
lens portion, the Franklin split can be used to eliminate
differential prism arising from anisometropia. The bifocal,
which is simply two lenses cemented together, may be used
to overcome other issues apart from differential prism. These
include prism in either the near or distance portions only
and different cylinder axes in the near prescription. Although
it is indeed a very versatile option, the lens is somewhat
cosmetically unattractive (Figure 8). Interestingly, it is often
the first solution given by many dispensing examination
candidates to these more challenging problems but whether
the number of recommendations in practice reflects this
choice as an examination answer is doubtful.
Despite its versatility, it is unlikely that Franklin in the
1760s would have considered the potential of the lens
where the optical centres of each separate lens could be
positioned to create a desired prismatic effect. The lenses
would most certainly have been just for distance and
near vision, although with his close association with many
famous artists of the day, eg Reynolds and West, it is not
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P McCarthy
beyond doubt that, to help them with their work, an
intermediate portion could have been used, thus creating
the world’s first occupational lens!
Bonded segments
As with the Franklin split, when one lens is bonded on
to another, both optical centres can be positioned
independently to provide a desired prismatic effect at a
given point. When removing differential prism the reading
segment, with its compensating prism, is bonded on to the
main lens at the near-vision zone. It is preferential to use
this prism compensation on the main lens that requires
base down in order to prevent a thick ridge at its upper edge.
Other options
A Fresnel prism is not a permanent option because of the
poor cosmetics and durability and the reduction in VA.
It is mainly used to assess the wearer’s acceptance of the
compensating prism before a more permanent solution is
decided upon.
Figure 8. Franklin split.
Prism-controlled solid bifocal
Although only produced in glass material, another versatile
lens is the 30mm round segment prism-controlled bifocal,
which has been available for many years. It can produce
prisms in any direction within the segment of up to
6∆, although for the control of anisometropia-induced
differential prism, we are primarily concerned with vertical
compensation. For cosmetic reasons the normal procedure
would be to split the differential prism compensation
between the two lenses. Where an unwanted prism of 3∆
base down in the left needs to be compensated, for example,
the right segment would incorporate 1½Δ base down with
the left having 1½Δ base up. If one lens only were to be used
for prism compensation, then all of the 3∆ base up would
be incorporated in the left lens to avoid the ridge at the
dividing line (Figure 9). The right eye would be matched with
a standard solid R30 bifocal lens.
Single-vision distance and reading spectacles remove the
need for the eyes to look away from the optical
centres. However, the benefits of avoiding the issues of
anisometropia are offset by the inconvenience of needing to
change spectacles for different viewing distances.
Contact lenses offer the most natural and obvious solution
to any anisometropic prescription in that they remove the
issue of any unwanted prismatic effect that spectacle
lenses create when the eyes look away from the optical
centres. Contact lenses also give more natural vision in that
the differences in retinal image sizes compared to those
of spectacle lenses are far less and so images produced
by eyes needing different ocular corrections can be more
easily integrated. There are a number of variables to
consider but, as an example, a +4.00D contact lens will
give a magnification of about 1.7% compared to about
5% with a spectacle lens fitted at, say, 12mm from the eye.
A subject with anisometropia of 4.00D is therefore unlikely
to tolerate uncompensated spectacle lenses but should
suffer no symptoms when fitted with contact lenses.
It would be expected therefore that, where suitable, either
single-vision or multifocal contact lenses should always
be strongly recommended to patients with these types
of prescriptions.
Different reading additions
A point that is rarely considered when providing a bifocal
solution to anisometropia, but one that may affect the
wearer’s comfort, is that accommodation will vary depending
on the given prescription. A difference in prescription of
about 4.00D will lead to the eyes needing to accommodate
by a difference of about 0.25D. We can determine from
Figure 10 that, to see at 0.3m away, the left eye with a
prescription of –1.00 will need to accommodate by nearly
0.25D more than the right eye and therefore may benefit
from an additional 0.25D in the reading prescription.
Figure 9. Prism-controlled solid bifocal.
8
Anisometropia: what difference does it make?
-5.00DS
Summary
R
33.33cms
12mm
K = 1000/-212mm = -4.717D
L1 = -3.00D, L1' = -8.00D
L 2 = 1000/ -137mm = -7.30D
A = -4.717 – (-7.30) = 2.583D
-1.00DS
Anisometropia frequently raises the question in the
mind of the optometrist or dispensing optician:
‘To correct or not to correct’. When the eye with the
higher prescription frequently has noticeably worse
visual acuity, especially with hypermetropes, a balance
lens may often be the most suitable solution.
The decision to provide the correct prescription may well
lead to difficulties for the wearer:
• Retinal and/or cortical image sizes may differ
• Differential prism at off-axis points is likely to lead
to diplopia
• Monocular vision due to suppression may still occur
L
• The correcting lenses are likely to be cosmetically
unacceptable
33.33cms
12mm
K = 1000/-1012mm = -0.988D
L1 = -3.00D, L1' = -4.00D
L 2 = 1000/ -262mm = -3.817D
A = -0.988 – (-3.817) = 2.828D
Figure 10.
Conclusion
Differences in right and left prescriptions in corresponding
meridians signify the presence of anisometropia. We have
considered the fundamental aspect of identifying these
differences and the likely effects the resultant differential
prismatic effects can have on our patients if they are not
compensated for.
The anisometropic prescription is, however, just one of a
number of associated factors that need to be considered
when deciding whether any value will gained for the patient
in providing a correcting lens with prism compensation.
The key to the final decision will often lie in the discussion
between the dispensing optician and the optometrist
concerning the patient’s likely acceptance of the prescription
in either its prescribed or modified form.
The solutions to the issues of anisometropia in terms of
correcting lenses have been discussed in detail. These lenses
are still available and so offer dispensing opticians the
means of providing their anisometropic patients with the best
possible vision.
Contact lenses remove many of the issues faced by
the anisometropic subject but for those unwilling
or unable to wear them a number of spectacle lens
solutions are still available.
References
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• yourself (improved knowledge, performance, confidence)?
• your colleagues?
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CET Multiple Choice Questions
This article has been approved for one non-interactive point
under the GOC’s Enhanced CET Scheme. The reference
and relevant competencies are stated at the head of
the article. To gain your point visit the College’s website
www.college-optometrists.org/oip and complete the
multiple choice questions online.
2. How might you assess/measure this impact?
CPD Exercise
After reading this article can you identify areas in which
your knowledge of anisometropia has been enhanced?
How do you feel you can use this knowledge to offer
better patient advice?
Are there any areas you still feel you need to study and
how might you do this?
Which areas outlined in this article would you benefit
from reading in more depth, and why?
Reflection
1. What impact has your learning had, or might it have, on:
• your patients or other service users (eg those who refer
patients to you, members of staff whom you supervise)?
10
To access CPD Information please click on the following link:
college-optometrists.org/cpd