Download REFRACTION– MYTHS AND FACTS_v07

REFRACTION – MYTHS AND FACTS
Dipl.-Ing. (FH) Olaf Schmidt
Fielmann Akademie Schloss Plön
[email protected]
Abstract.
To effectively perform a refraction a common strategy is to use an autorefractor (AR) and control the
AR values by means of subjective refraction. This implies the belief that a subjective monocular
refraction is more reliable than an autorefractor reading. A number of studies show that subjective
methods are less reliable than autorefractors for cylindrical power and axis. The spherical power
must be controlled subjectively using a binocular balance test because autorefractors do not assess
binocular balance. A subjective control of cylinder power and axis seems to be unneccessary.
On the other hand, in about every 20th refraction the difference between AR values and subjective
refraction exceeds ±0,4 dpt. This difference can deteriorate the patient's satisfaction with his
spectacles. Therefore, a subjective control of AR readings is mandatory to find the best prescription
for a patient.
Keywords:
objective refraction, subjective refraction, autorefractor, binocular balance, reliability
1 Background
Determining the correct refraction of a patient's eyes is part of the standard examination performed
by optometrists. One common strategy to efficiently find the best correction for a customer is to first
perform an objective refraction using an autorefractor (AR) and take the AR values as start values to
test the eyes by means of subjective refraction methods. If subjective and objective measurements
differ from each other then the values taken objectively are rejected. A test for spherical binocular
balancing completes the examination.
This approach implies two beliefs:
1. The determination of the correct sphero-cylindrical prescription is supposed to be highly
reproducible.
2. A subjective monocular refraction is supposed to be more trustworthy than an automated
objective refraction. Therefore monocular AR measurements must be controlled by means
of subjective refraction methods before performing a binocular balance test.
This paper reviews some of the main limitations in the accuracy of finding the refractive error of
the eyes.
2 The repeatability of subjective and objective refraction
Like any other measurement the determination of an eye's refraction underlies systematical and
statistical errors. First of all, the usual increment used in subjective refraction is ±0,25 dpt, while the
true refraction of the eye is, of course, not likely to fit to these steps. Thus, performing only one
subjective refraction will in most cases not lead to the true value of the eye's refraction but give the
best estimation of the refractive error according to the ±0,25 dpt grid. Another critical point in
subjective refraction results out of the fact, that the prescription heavily relies on the answers of the
patient. The refraction of a patient who is a good observer is more likely to lead to the true refractive
error than the refraction of a bad observer. According to Borish and Benjamin a subject needs a
dioptric difference between 0,12 dpt and 1,00 dpt to distinguish between two images an optometrist
asks him to compare [1].
2.1
Repeatability of subjective refraction
Several studies show that a repeated determination of an eye's subjective refraction leads to
different results in any given subject. Horstmann performed objective (N=32) and subjective (N=28)
refractions on four different times of one day. For subjective measurements he found differences of
±0,25 dpt in most of the cases with maximum differences of +0,50 dpt and -0,37 dpt in the spherical
equivalent. Horstmann found the differences in cylindrical power and axis to be "very low" without
giving a number [4]. A study by Krause and Taege supports this finding [6]. Voigt performed a similar
study on 23 subjects. Just like Horstmann he determined the subjects' refraction objectively and
subjectively. For statistical analysis the sphero-cylindrical results were transformed into power vectors
according to the procedure described by Thibos et al [11]. Voigt found that in subjective refraction the
results of the four measurements differ about ±0,25 dpt in the spherical equivalent. The difference was
statistically significant but due to the fact that the absolute magnitude of the difference was only 0,12
dpt no relevance in clinical practice can be stated. Only in one subject the spherical equivalent differed
more than 0,50 dpt over the day. No statistically or clinically relevant differences occured in the
results of the objective or subjective refraction over the day, neither for the cylindrical power nor for
the axis [12].
The cited studies have in common that all refractions were performed by the same examiner. Other
studies took the inter-examiner variations into account. Leinonen et al. compared two refractions
performed by different examiners under clinical conditions. In subjects with a visual acuity of ≥0,7
they found a difference of 0,05 ±0,51 dpt (Mean ± 95% Confidence Interval) between the two
measurements while in subjects with poorer visual acuity a larger 95% Confidence Interval (±1,14
dpt) was found [7]. MacKenzie let 40 registered optometrists perform a refraction on one
asymptomatic male subject. He found an average inter-ocular difference of -0,04 ±0,20 dpt (Mean
±standard deviation) with maximum differences of +0,50 dpt and -0,50 dpt for the spherical
equivalent. Mean differences for 0°/90° and 45°/135° astigmatism were -0,23 ±0,084 dpt and -0,14
±0,086 dpt, respectively [8]. Gramenz performed a similar study with one subject and 20 optometrists
in Northern Germany. She found an inter-examiner range of 0,75 dpt in both the spherical equivalent
and the cylindrical power of the prescriptions [3].
2.2
Repeatability of autofractor measurements
In contrast to subjective refraction where only one measurement is taken modern
autorefractometers take 3 to 5 measurements in a fraction of one second. The user of the autorefractor
gets the average result out of these measurements. Even if each single measurement taken by an AR
has a relatively high variation, the fact that a number of measurements are averaged increases the
probability to find the correct refractive error. Indeed, studies (for instance [2], [13]) show that an AR
has 95% limit of agreement of about ±0,3 D which is better than the reproducibility of a subjective
refraction.
Pseudovs examined inter-examiner repeatability of autorefractor measurements and subjective
refraction and concluded that objective refraction has a better repeatability than subjective refraction
throughout all examiners [9]. Zadnik et al. examined the repeatability of the measurement of several
ocular components. For the refractive error of the eye they found automated refraction under
cycloplegia to be the most reliable technique with 95% limits of agreement of ±0,3 dpt [15].
Numerous other authors come to similar findings (for instance [14], [10]).
3 On the necessity of a monocular subjective control of autorefractor data
The common habit of rejecting monocular autorefractor values in favour of subjectively
determined monocular refraction values implies the belief that autorefractor values are less reliable.
Hence, optometrists control the AR values by means of subjective monocular refraction. A number of
studies have shown that autorefraction delivers reliable values for the sphero-cylindrical refraction of
the eye. Of course, monocular refraction can not deliver a good estimation for binocular balance. The
high reliability of automated refraction gives reason to ask if a monocular subjective control of AR
values is appropriate.
Könitz compared the sphero-cylindrical results of a refraction using two different settings. In the
first setting she followed the common routine: An AR measurement was taken and the spherocylindrical values were controlled subjectively using the cross-cylinder method. After that a binocular
balance test was conducted. In the second setting the subjective monocular control was left out. The
AR values were put into the trial frame and a binocular balance test was conducted. Könitz found a
significant difference in the spherical equivalent between the AR values and the values after the
binocular balance test. There was, however, no significant difference between the results in the
spherical equivalents of the two different settings (Figure 1). The mean difference between the
spherical equivalents with and without performing a subjective control of the sperical component after
the AR readings is 0,01 dpt, the 95% limit of agreement is ±0,34 dpt, which is in good agreement with
the repeatability of a refraction. Obviously the binocular balance test makes an important difference
[5].
Figure 1 - Differences vs. mean for the Spherical Equivalent for refractions with and without
monocular subjective control of the AR readings. The MBA values result out of the AR readings
plus binocular balance test, Msubj includes a monocular subjective control of the AR readings
before the binocular balance test. There is no statistically significant difference between the
results (p<0,05), but 5% of the differences exceed ±0,35 dpt which might be critical for the
satisfaction of the customer.
In cylindrical power and axis there was no statistically significant difference between AR values
and the values after subjective control (Figure 2). The mean difference is -0,0004 dpt, the 95% limits
of agreement between the cylindrical AR values and the values after a subjective control are ±0,36 dpt
[5].
Figure 2 - Differences vs. mean for the Cylindrical Power for refractions with and without
monocular subjective control of the AR readings. The CAR values are the AR readings, Csubj
displays the AR readings plus a subjective monocular control. There is no statistically
significant difference between the results (p<0,05), but 5% of the differences exceed ±0,36 dpt
which might be critical for the satisfaction of the customer.
4 Conclusion
These findings seem to suggest that a subjective monocular control of AR values is obsolete, as
long as a binocular balance test is conducted. A closer look at the data shows that this is not true. If
95% of all data lie within a tolerable variance then 5% lie outside these borders. In other words: Every
20th patient's sphero-cylindrical prescription might be wrong and lead to complaints and dissatisfied
patients. Facing the relatively small amount of time a subjective control of AR values takes it seems
reasonable to minimize this risk and conduct a subjective refraction with every patient.
5 References
[1] I. M. Borish and W. J. Benjamin: Monocular and binocular subjective refraction. WB Saunders,
1998.
[2] M A Bullimore, R E Fusaro and C W Adams: The repeatability of automated and clinician
refraction, Optometry and Vision Science, Vol 75, No 8, p. 617-622
[3] Alexandra Gramenz: Reproduzierbarkeit der Refraktion eines Menschen in unterschiedlichen
augenoptischen Fachgeschäften, BSc-Thesis Lübeck University of Applied Sciences, Studiengang
Augenoptik/Optometrie, Mai 2011
[4] Dieter Horstmann: Tageszeitliche Schwankungen in der Refraktion, In Die Fachvorträge des
WVAO-Jahreskongresses 1974 in Baden-Baden, pages 33–36, WVAO, 1974.
[5] Aiga Könitz: Ist eine subjektive monokulare Refraktionsbestimmung noch zeitgemäß?, BSc-Thesis
Lübeck University of Applied Sciences, Studiengang Augenoptik/Optometrie, 2012
[6] Krause, K. und Taege, A.: Diurnal Fluctuations in Human Refraction. Klinische Monatsblätter fur
Augenheilkunde (1988) 192(2), 53-57
[7] Jaako Leinonen, Eero Leinonen and Leila Laatikainen: Repeatability (test-retest variability) of
refractive error measurement in clinical settings, Acta Ophthalmologica Scandinavica 2006:84:532536
[8] Graeme E MacKenzie: Reproducibility of sphero-cylindrical prescriptions, Ophthalmic Physiol
Opt, 28(2):143–50, Mar 2008
[9] Konrad Pesudovs, Katrina E Parker, Han Cheng, and Raymond A Applegate: The precision of
wavefront refraction compared to subjective refraction and autorefraction, Optom Vis Sci, 84(5):387–
92, 2007
[10] M. Rosenfield and N. N. Chiu: Repeatability of subjective and objective refraction, Optom Vis
Sci, 72(8):577–579, 1995
[11] Larry N. Thibos, PhD FAAO, William Wheeler, PhD, and Douglas Horner, OD, PhD, FAAO:
Power Vectors: An Application of Fourier Analysis to the Description and Statistical Analysis of
Refractive Error, Optometry and Vision Science, Vol 74, No. 6, 1997, 367-375
[12] Christian Voigt: Tageszeitliche Schwankungen in der Refraktion, BSc-Thesis Lübeck University
of applied Sciences, Studiengang Augenoptik/Optometrie, 2010.
[13] J. J. Walline, K. A. Kinney, K. Zadnik, and D. O. Mutti. Repeatability and validity of astigmatism
measurements. J Refract Surg, 15(1):23–31, 1999
[14] PD Dr. W. Wesemann: Funktionsprinzipien ud Messgenauigkeit moderner Autorefraktometer,
Deutsche Optiker Zeitung, 10/2004, p. 38-44
[15] K Zadnik, D O Mutti, and A J Adams. The repeatability of measurement of the ocular
components. Invest Ophthalmol Vis Sci, 33(7):2325–33, Jun 1992
List of figures
Figure 1 - Differences vs. mean for the Spherical Equivalent for refractions with and without
monocular subjective control of the AR readings. The MBA values result out of the AR
readings plus binocular balance test, Msubj includes a monocular subjective control of the
AR readings before the binocular balance test. There is no statistically significant
difference between the results (p<0,05), but 5% of the differences exceed ±0,35 dpt
which might be critical for the satisfaction of the customer. .............................................3
Figure 2 - Differences vs. mean for the Cylindrical Power for refractions with and without
monocular subjective control of the AR readings. The CAR values are the AR readings,
Csubj displays the AR readings plus a subjective monocular control. There is no
statistically significant difference between the results (p<0,05), but 5% of the differences
exceed ±0,36 dpt which might be critical for the satisfaction of the customer. .................4