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