| No matter how sophisticated the contact lens practitioner becomes in
designing a contact lens or using new lens materials, the type of astigmatism must be determined prior to selecting the proper lens modality for a
given patient. First some background and basics:
CAUSE OF ASTIGMATISM
As background, astigmatism may be defined as an “optical defect” in
which refractive power is not uniform in all meridians. Light rays entering
the eye are bent unequally by different meridians, preventing formation of
a sharp focus on the retina, according to Cassin, Review of Eye Terminology, second edition.
Causes of astigmatism may
arise from any or all of the
following: 1. Astigmatism due
to the cornea, 2. Astigmatism
due to the crystalline lens, 3.
The fovea of the eye’s occupying an eccentric position in
relation to the visual axis, 4.
Changes in vitreous gel,
resulting in a change of
index, irregularity in the surface of the retina, according
to Hartstein J., Review of
Refraction.
INCIDENCE OF
ASTIGMATISM Up to about 83 percent of all
patients wearing eyeglasses
have an astigmatism correction. 60 percent of the population show astigmatism
between .50 and 1.50
diopters. That is made up of
almost 20 percent with less
that .50 diopter. About 45 percent show astigmatism between .50 and
1.00 diopter and another 15 percent show astigmatism between 1.00 and
1.50 diopters. Another 10 percent demonstrate astigmatism between 1.50
and 2.00 diopters, and finally another 7 percent show astigmatism
between 2.00 and 3.00 diopters. It is interesting to note that only about 3
percent of the population shows astigmatism in excess of 3.00 diopters.
(From Hartstein, J., Review of Eye Terminology.)
TYPES OF ASTIGMATISM
Total astigmatism of the eye consists of both corneal and lenticular astigmatism. To determine the type and amount of astigmatism, the following
relationship may be utilized: Total astigmatism consists of corneal and
lenticular astigmatism (internal).
Total astigmatism — Corneal Astigmatism + Lenticular astigmatism (TA
= CA + LA) (From Contact Lenses and Corneal Disease, A. Gasset.)
Corneal Astigmatism — Astigmatism found on the cornea and measured
by keratometry. The difference between the two principal meridians is
the amount of corneal astigmatism.
Lenticular Astigmatism — Found within the eye and cannot be determined by keratometry.
Total astigmatism is reflected in the patient’s refraction or spectacle correction; for example in the prescription: -3.00 -.50 x 180, the total astigmatism = -.50 x 180.
Corneal astigmatism can be calculated by keratometer (K) readings, for
example K = 42.50 @ 180/43.00 @ 90. The amount of corneal astigmatism is equal to a +0.50 x 90 or -0.50 x 180.
Lenticular astigmatism cannot be measured by keratometry, but can be
calculated by using the following formula: Lenticular Astigmatism = Total
Astigmatism - Corneal Astigmatism
(LA = TA – CA) (Gasset A., Contact Lenses and Corneal Disease)
Example:
Rx -2.50-1.50x180
“K” 44.00@180/45.00@90
LA = TA – CA
LA = -1.50 x180 – (- 1.00 x 180)
LA = -1.50 x180 +1.00 x 180
LA = -0.50 x 180 or +0.50 x 90
Most corneas have at least 0.50 diopter difference in curvature between the horizontal and vertical meridians. Astigmatism may be classified as
with-the-rule astigmatism (WTR), against-the-rule (ATR) or as oblique
astigmatism.
WTR astigmatism is identified as the curvature of greatest power in the
vertical meridian and the weakest power in the horizontal meridian.
Therefore, WTR astigmatism will have the flattest curve along the horizontal meridian. An example of a WTR astigmatic cornea is:
45.00@180/46.00@90. WTR astigmatism is corrected with a - cyl x 180
or + cyl x 90. WTR astigmatism is more frequently found on the cornea
than ATR astigmatism.
ATR astigmatism can be identified as the curvature of greatest
power in the horizontal meridian and the weakest power in the vertical meridian. ATR astigmatism will have its flattest curve along the
vertical meridian. An example of ATR astigmatism may be written as
46.00@180/45.00@90. ATR astigmatism is corrected with a - cyl x 90
or + cyl x 180.
Lenticular astigmatism or
internal astigmatism will
usually manifest as ATR
astigmatism and therefore
neutralize WTR astigmatism. As mentioned earlier,
this astigmatism cannot be
measured by keratometry,
but can be calculated by
using the LA = TA - CA
formula.
Oblique astigmatism is a
condition where the two
major meridians are at some
intermediate axis other than
the horizontal or vertical
meridians. The greatest
powers in oblique astigmatism lie between 30th and
60th meridian and 120th and 150th meridian. An example of oblique
astigmatism may be written as 42.00@135/43.00@45.
With or Against or Oblique astigmatism can be corrected with eyeglasses, rigid or soft contact lenses. Sometimes, the front surface of the
cornea is irregular i.e., the meridians are not uniform in curvature and/or
the principal meridians are not crossed at 90 degrees. Irregularities of the
front surface of the cornea can be seen with a keratometer. This condition is called irregular astigmatism. Glasses will not accurately correct
irregular corneal astigmatism, however rigid contact lenses are usually
the modality of choice to enhance a patient’s vision.
RESIDUAL ASTIGMATISM
When a spherical rigid lens is placed on a cornea, the tear lens corrects
the astigmatism on the cornea but not the astigmatism due to the other
components of the eye. When evaluating where the astigmatism on the
eye is present, residual astigmatism may be calculated depending on the
type of contact lens you may wish to fit. Calculating residual astigmatism
is determined by subtracting keratometer astigmatism (KA) from the
total astigmatism (TA): CRA = TA - KA.
When a patient requests to be fitted with contact lenses, the contact
lens practitioner should consider that an astigmatic error might manifest
itself when a contact lens is placed on the patient’s cornea.
The term “residual astigmatism,” as used in the contact lens field, can
be defined as the “remaining astigmatic refractive error that is present
when a contact lens is placed upon the cornea to correct the existing
ametropia.” (From R. Mandell, Contact Lens Practice.)
Residual astigmatism may be subdivided into physiological residual
astigmatism or induced residual astigmatism.
Physiological residual astigmatism may be
caused by any or all of the following
conditions. (after Mandel)
- That portion of the anterior corneal surface that is not
neutralized by the tear lens.
- The difference in curvature of the principal meridians of the
posterior corneal surface.
- The difference in curvature of the principal meridians of the
crystalline lens.
- Tilt of the crystalline lens.
- Variability of the refractive index of the cornea, crystalline
lens or vitreous.
- Oblique incidence of light upon the cornea.
- An eccentric position of the fovea in relation to the visual axis.
- Some irregularity in the shape of the retina.
Induced residual astigmatism is the
astigmatism that is introduced into the
contact lens system by the contact lens
itself. Possible causes of induced residual
astigmatism may result from:
- Tilt or decentration of the contact lens;
- Lens warpage; or
- Lens flexure
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Residual astigmatism caused by either physiological or induced astigmatism can reduce acuity; decrease visual performance and cause
asthenopia (fatigue). When the amount of residual astigmatism is great
enough to cause any of these problems for a particular patient, residual
astigmatism must be corrected or a different lens type must be chosen
to reduce the residual error. When residual astigmatism is 0.50 diopter
or less, it seldom reduces acuity to cause visual distress or asthenopia.
(Mandel) If over 0.75 diopter of residual astigmatism is present, correction may become essential.
It has been established that a very large percentage of patients fitted
with spherical contact lenses can manifest a certain amount of residual
astigmatism. In one study, for example, comprising 105 patients fitted
with spherical contact lenses, 84 percent showed .50 diopter or more of
residual astigmatism in one or both eyes. Of the 210 eyes in the given
study, 56.4 percent had .50 diopter or more of residual astigmatism with
37 percent having .75 diopter or more, according to N. Bier N. and G.E.
Lowther, Contact Lens Correction. It would appear that the incidence
of residual astigmatism is relatively high and the contact lens practitioner
must be prepared to offer alternatives to the prospective patient.
Anticipation of residual astigmatism is the single largest factor in determining what type of contact lens is recommended to a patient.
Residual astigmatism may be calculated if the keratometer readings and
spectacle refraction are known. Calculated residual astigmatism (CRA)
is the difference between the keratometer readings (KA) and the refractive astigmatism (TA): CRA = TA - KA.
Calculating residual astigmatism can be used to predict whether or not
a significant amount of residual astigmatism may be present with a certain type of contact lens. However, from a clinical standpoint, another
method of verifying residual astigmatism is to perform a subjective spherocylinder over-refraction over a spherical diagnostic contact lens to
determine the measured or actual residual astigmatism. The difference
between calculated residual astigmatism and measured residual astigmatism is that calculated is predicted through a formula and actual residual
is determined subjectively with a contact lens on the patient’s eye. Specific studies have shown there can be significant differences between calculated and measured residual astigmatism. Although residual astigmatism is common, clinically measured residual astigmatism is often less
than calculated residual astigmatism.
A number of reasons can be given to account for the difference
between the calculated and measured residual astigmatism. They are:
1. Errors of measurement in refraction or “K” readings probably reduce
a random difference.
2. Keratometer readings may not give a valid description of the corneal
toricity for the segment of the cornea through which the line of sight
passes.
3. The tilt or decentration of the contact lens system relative to the line of
sight produces a small but significant amount of oblique astigmatism.
4. Flexure of thin corneal lenses will produce an induced residual astigmatism that may increase or decrease the total residual astigmatism.
(From Clinical Refraction, Vol. 2, I.M. Borish).
ASTIGMATISM AND CONTACT LENSES BY EXAMPLE
The following examples illustrate a method for anticipating residual astigmatism and the type of lens to recommend based on the amount of
residual astigmatism calculated.
Example #1
Spectacle Rx: -3.50 -.50 x 180
K reading 43.50@180/44.00@90
In this case, there is -0.50 x 180 WTR astigmatism in the spectacle prescription.
If a rigid lens is fit to this cornea, the tear lens will neutralize
the .50 diopter WTR astigmatism on the cornea.
Therefore, there is no residual astigmatism in this case.
CRA = TA - KA
= -.50 x 180 - - .50 x 180
= -.50 x 180 + .50 x 180 = 0
Lens recommendation: Rigid or gas permeable lens
If a soft lens were being considered, the contact lens practitioner must
realize that soft lenses will not correct corneal astigmatism. Residual astigmatism for soft spherical lenses is the patient’s refractive astigmatism.
CRA = TA - KA
= -.50 x 180 - 0
= -.50 x 180
The calculated residual astigmatism is -0.50 x 180. Because the residual astigmatism is 0.50 diopters, the cylinder power can be ignored
because most patients will tolerate up to 0.50 diopter of uncorrected
astigmatism. In this case, the patient’s vision will not be compromised if
the patient is fit with a soft lens. However, there are occasions when the
spherical equivalent of the spectacle prescription may be incorporated.
Spherical equivalent = one half the spectacle cylinder + sphere, e.g., -3.50
-.50 x 180, power in contact lens = -3.75.
Example #2
Spectacle Rx: -4.00
K reading 42.25@180/42.25@90
Since there is no astigmatism correction present in the spectacle prescription or on the cornea, a rigid or soft spherical lens would correct
this patient’s ametropia.
Lens recommendation: Rigid or soft spherical lens
Example #3
Spectacle Rx: -4.00
K reading 42.00@180/43.00@90
In this case study, there is no astigmatism in
the spectacle prescription. However, there is
1.00 diopter WTR astigmatism on the cornea. If a
rigid lens were fitted on this patient, the tear lens would neutralize the
1.00 diopter WTR astigmatism on the cornea. This would then leave
1.00 ATR astigmatism uncorrected. If the contact lens practitioner were
to fit this patient, an induced residual astigmatism of -1.00 x 90 would
manifest itself and would diminish the person’s visual acuity. In this particular case, it would be more advisable to fit the patient with a soft lens.
This would not induce any residual astigmatism because the soft would
not compromise the astigmatism on the cornea.
Lens recommendation: A spherical soft lens would be recommended
for this patient
Example #4
Spectacle Rx: -3.75 - 1.50 x 90
K reading 43.50@180/43.00@90
In this case, there is -1.50 x 90 ATR astigmatism in the spectacle prescription. However, on the cornea, there is 0.50 diopter of ATR astigmatism. If a rigid lens is considered for this patient, the tear lens will neutralize only the .50 diopter ATR of the total 1.50 of the spectacle
prescription. Because the total and corneal astigmatism are both ATR,
the residual astigmatism with a spherical rigid lens would be -1.00 x 90.
CRA = TA - KA
= -1.00 x 90 - - .50 x 90
= -1.00 x 90 + .50 x90
= -1.00 x 90 residual
If a soft lens were recommended, the calculated residual astigmatism
would be -1.50 x 90.
CRA = TA - KA
= -1.50 x 90 - 0
= -1.50 x 90
In both cases, there is a significant amount of residual astigmatism that
the prospective contact lens wearer may not adapt to. In either case, the
practitioner may wish to design a front rigid toric or use a soft toric lens
to correct the ametropia.
Lens Recommendation: A front rigid toric or use a soft toric lens
Example #5
Spectacle Rx -2.75 - 1.00 x 90
K reading 42.50 x 180/43.50 x 90
In this case study, there is -1.00 x 90 ATR in the spectacle prescription
and the cornea has 1.00 WTR astigmatism. If a rigid lens were fitted on
this patient, the tear lens would neutralize 1.00 diopter of WTR astigmatism on the cornea. However, an additional -1.00 X 90 ATR astigmatism would manifest itself over the -1.00 x 90 already present in the spectacle prescription. By fitting a rigid spherical lens, an induced residual
astigmatism of -2.00 x 90 would be present.
CRA = TA - KA
= -1.00 x 90 - - 1.00 x 180
= -1.00 x 90 + 1.00 x 180
= -1.00 x 90 -1.00 x 90*
= -2.00 x 90
*To combine two cylinders of a different axis, transpose one to a
- cylinder to get the same axis.
If a soft lens were recommended, the residual astigmatism would only
be -1.00 x 90.
By not neutralizing the cornea, the spectacle cylinder would only have
to be considered.
CRA = TA - KA
= -1.00 x 90 - 0
= -1.00 x 90 residual
In a practical application where both cases manifest residual astigmatism, the soft lens option would induce less residual astigmatism, making it an easier option for both the fitter and the patient.
Lens recommendation: A toric soft lens would be recommended for this
patient
CONCLUSION
In conclusion, anticipation and calculation of residual astigmatism is possible, if the contact lens practitioner takes the time to evaluate the
options and consequences of different lens modalities before fitting the
prospective contact lens patient. In doing so, chair time can be reduced
and the possibility of poor visual acuity and frustration by the patient
can be avoided. ■
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