Introduction to Refraction
Epidemiology of Ametropia
The prevalence and distribution of refractive
errors varies greatly with age. At birth refractive errors are
normally distributed. Early in infancy the majority of children
become emmetropic or slightly hyperopic. During the school years
children become myopic in increasing numbers, and by age 18 25%
of the population is myopic. Relatively little change occurs
during the early adult years. By age 45 presbyopia has occurred
in most adults and latent hyperopia begins to manifest itself.
After age 55 there is a widening of the distribution curve for
refractive errors with some people increasing in hyperopia, some
people remaining the same, and other people becoming more myopic
due to nuclear changes in the crystalline lens.
Astigmatism changes little with age. The majority
of children and young adults have small amounts of with the rule
(axis 180) astigmatism. During the later adult years there is
a tendency toward a reduction in with the rule astigmatism and
an increase in against the rule astigmatism.
Variability of the Eye's Refractive Components
Over the years many researchers have attempted
to determine how variations in the refractive components of the
eye correlate with refractive errors. There is a wide range in
the refractive components of the emmetropic eye:
axial length: 21 to 26 mm
corneal power: 38.00 to 48.00 D
lens power: 17.00 to 26.00D
For refractive errors less then 4.00D, the values
for refractive components of the eye are usually within the normal
ranges for emmetropic eyes. Ametropia is due to faulty correlation
of the individual refractive components. This is not as common
as chance might make it due to a process known as emmetropization.
For refractive errors greater than 4.00 D axial length is usually
outside the normal range for emmetropia.
Anomalies of Refraction
Ametropia is the general term for any refractive
condition other than emmetropia, or a condition in which there
is a refractive error or refractive anomaly.
Myopia
Myopia is a condition in which, with accommodation
relaxed, parallel rays of light converge to a focus in front of
the retina. This can be due to an eye of normal axial length
with a shorter than normal focal length or an eye of normal focal
length with a longer than normal axial length. Research has shown
that small amounts of myopia are due to a combination of axial
lengths and focal lengths within the ranges that are normal for
emmetropic eyes, however, moderate and larger amounts of myopia
(>4.00D) are due to axial length outside the normal range.
Correction of Myopia
Myopia can be "corrected" with a negative,
or diverging, lens. The minus lens must be of a power such that
the secondary focal point of the lens coincides with the far point
of the eye. Clinically this is achieved by determining the weakest
minus lens that provides sharp visual acuity.
Hyperopia
Hyperopia is a condition in which, with accommodation
relaxed, parallel rays of light converge to a focus behind the
retina. This can occur because axial length of the eye is normal
but focal length is longer than normal or because the focal length
is normal but the axial length is shorter than normal. As with
myopia, low amounts of hyperopia occur with axial lengths and
focal powers within the normal ranges for emmetropic eyes. Moderate
to high amounts of hyperopia are usually due to axial lengths
shorter than the emmetropic eye.
Correction of Hyperopia
Hyperopia is corrected by using positive, or
converging, lenses. The correcting lens must be of a power such
that the secondary focal point of the lens coincides with the
far point of the eye.
Introduction to Refraction
Far Point
The far point (punctum remotum or PR) is the point conjugate to the sharply focused retinal image. Knowing the distance of the far point from the retina doesn't help us to determine the amount of refractive error unless we know parameters such as axial length and powers of the ocular lenses.
To be clinically useful, we need to know the incident vergence which is required to produce a sharp retinal image. The reciprocal of this vergence is the far point. With the emmetropic correction in place, the far point shifts to infinity, and the optical image is focused on the retina. A -10.00D lens held in front of a 10D myopic eye corrects it optically (anatomically it remains myopic). Emmetropia is created by optically shifting the far point from 10cm to infinity. We measure this shift in retinoscopy, not by actually locating the far point (this would be impossible when it is virtual) but rather by moving it with lenses to the plane of the retinoscope.
The far point of any myopic eye can be measured
with a sliding target. The farthest point at which the target
is seen clearly represents the far point. This is the basis of
an instrument known as the optometer. This would be a relatively
simple way to measure myopia, but, due to depth of focus this
is not necessarily an accurate way to measure ammetropia.
Spectacle and Ocular Refraction
In geometric optics, object distances are measured to the optical center of the lens. In visual optics the far point is measured to the equivalent plane of the eye. The equivalent plane of the eye is close enough to the cornea for it to serve as a reference point for clinical refraction. The power at the spectacle plane differs from ocular refraction due to effectivity.
The spectacle prescription depends on vertex
distance. It is best to measure the vertex distance when refracting
a patient. In refractive errors greater than 4.00 diopters, the
power of a contact lens must be adjusted for vertex distance due
to effectivity. It takes a stronger convex lens (plus) or a weaker
concave lens (minus) to correct the eye at the plane of the cornea.
Astigmatic Correction
Astigmatic eyes have two far points, one for
each principle meridian. The location of the far points leads
to the classification of the astigmatism
Simple Astigmatism: one focal line is on the retina
Compound Myopic Astigmatism: both focal lines are in front of the retina
Compound Hyperopic Astigmatism: both focal lines are behind the retina
Mixed Astigmatism: one
focal line is in front of the retina and one is behind the retina
If the more myopic meridian is vertical or near
vertical, the astigmatism is with the rule (correcting
cylinder axis 180). If the more myopic meridian is horizontal
or near horizontal, the astigmatism is against the rule
(correcting axis 090). If the more myopic meridian is more than
30 degrees from vertical or horizontal, the astigmatism is called
oblique. If the principle meridians are 90 degrees apart
the astigmatism is regular, if not, it is irregular.
Astigmatic Correction with the Clock Dial (sunburst)
Chart
The principle of correcting ocular astigmatism
is to collapse the interval of Sturm with cylinders. Since the
eye has been converted to compound myopic astigmatism by fogging,
we can use either plus cylinders to advance the focal line nearest
the retina or minus cylinders to displace the more blurred line
closer to the retina. In the first case the patient must judge
when all dial lines appear equally blurred; in the second, the
end point is when they appear equally clear. The second method
is easier for the patient to judge and therefore more reliable.
Using the clock dial chart the patient is asked
to identify the lines on the chart which appear clearest under
20/40 fogging. The lines which appear clearest are in the meridian
of the most minus (correcting lens) power. The axis of the correcting
cylinder required is 30 times the lower number corresponding to
the clock hours on the line which is clearest. For example, if
the line from 12 to 6 is clearest, the axis of the correcting
minus cylinder lens is 180 degrees.
If two lines appear equally clear then the meridian
of most minus power lies between the two. For example, if the
lines from 1 to 7 and from 2 to 8 are equally clear then the axis
of the correcting cylinder is at 45 degrees (midway between 30
and 60 degrees). If the line from 2 to 8 is clearer than the
line from 1 to 7, but both are clearer than all the other lines,
then the axis of the astigmatism is between 45 degrees and 60
degrees.
Clinically, astigmatic correction can be measured using the clock dial chart. The axis is determined as above and then cylinder lenses are added until the lines 90 degrees away are equally clear. At this point if all the astigmatism has been corrected, all the lines on the chart should appear equal.