Bachelor Thesis.
Stability of Distance Refraction in relation to Subjective Plusacceptance at near. 2002

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Stability of Distance Refraction

in relation to

Subjective Plusacceptance at near

Torben Helstrup

(Abstract) (Introduction) (Method) (Results) (Discussion) (References)

Abstract
Most common procedure for distance refraction in Denmark is named " OEP no. 7A ". It is speculated that conditions like Latent Hyperopia and Pseudomyopia will affect the outcome of this procedure and that affected persons will have an abnormal high or low plusacceptance at near distance.

To test this hypothesised relationship a group of 16 prepresbyopic healthy adults with pronounced visual complaints had their refraction evaluated under normal and cycloplegic conditions.

Results strongly indicate, that the OEP no. 7A procedure is very solid and stable and is not influenced by the amount of plusacceptance at near at least from 1.25 Diopter or less. Also meaning that indications for cycloplegic refraction in an adult population are very week.

Key Words: Distance refraction, Cycloplegic refraction, Plus-acceptance at near.

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Introduction
In Danish optometric practice it is common to prescribe the distance prescription according to the refraction procedure named OEP no. 7A (Margach 1979). In short, following monocular refraction both eyes are now blurred with approx. + 1.0 Diopter to a binocular acuity of approx. 6/12 in Snellen notation. Hereafter the excess plussphere is reduced slowly to best visual acuity and the result is named OEP no. 7A. The purpose of this procedure is to avoid unequal activity in the accommodative function in prepresbyopic persons and prevention from prescribing insufficient plussphere to Hyperopes or excess minussphere to Myopes. By definition the OEP no. 7A represents the strongest plussphere or the weakest minussphere to the maximum possible acuity.

If, for some reason, the refractionist is in doubt whether the patient's accommodative function is relaxed under the distance refraction procedure, he can supplement the examination with a measurement of the maximum plusacceptance at near distance. The procedure is named "OEP no. 14B" (Margach 1979). In short, at near distance patients are looking through binocular crosscylinders at a set of vertical and horizontal lines with a very low accommodative demand. Plussphere is reduced from an excess position until the lines appear with equal blackness. The result is the maximum plusacceptance at near distance. For a normal prepresbyopic person without spasm in the accommodative function it is expected to be in the interval of 0,75 to 1,25 Diopter in excess plus above the OEP no. 7A (Margach 1979).

It is therefore a common assumption that when the maximum plusacceptance at near is in the normal range of 0,75 - 1,25 Diopter it is safe to prescribe the OEP no. 7A result for the distance prescription. When this amount is exceeded some accommodative insufficiency should be suspected, and when less than 0,75 Diopter ,or even in the minus range, some spasm in the accommodative function should be suspected. Patients with low plusacceptance at near distance may therefore be placed in a high-risk category of suffering from either Latent Hyperopia or Pseudomyopia, and it is further speculated that the OEP no. 7A result could be affected in the way that the result is too low in plussphere or too high in minussphere.

Another general assumption is that a clinically significant degree of Latent Hyperopia or Pseudomyopia will correspond with the subjective complaint of patients experiencing slow focusing from near to far distance and occasional blurring of distance objects.

The purpose of this study is to test these assumptions by selecting a group of prepresbyopic patients with pronounced visual complaints of slow focusing from near to far distance and regular involuntary blurring of distance objects, and compare the OEP no. 7A refraction result with cycloplegic refraction and further explore any relationship to the OEP no. 14B plusacceptance measurement.

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Method
Subjects: 16 healthy persons participated in the study. 10 women and 6 men, age span 15 - 40 years of age. (Mean 26.62, SD 7.15). Using the definition of Myopia as starting from minussphere refraction 11 subjects were Myopic and the remaining showed a very low amount of Hyperopia.

For the purpose of analyzing for differences in distance refraction after installation of Cyclogyl ¹ the subjects were allocated to one of three groups based on the maximum plusacceptance at near, and for analyzing purposes only the right eye values are calculated.

Group 1: Subjects with OEP no. 14B plusacceptance of <= 0,50 Diopter in addition to OEP no. 7A.

Group 2: Subjects with OEP no. 14B plusacceptance of 0,75-1,25 Diopter in addition OEP no. 7A.

Group 3: Subjects with OEP no. 14B plusacceptance of >= 1,50 Diopter in addition OEP no. 7A.

Number of right eyes and age distribution in each group is therefore:

Group 1: N = 9. Mean age (years): 28,33 (SD 6,58). Span: 15 - 40.

Group 2: N = 5. Mean age (years): 24,00 (SD 7,45). Span: 18 - 37.

Group 3: N = 2. Mean age (years): 25,50 (SD 8,66). Span: 18 - 33.

Inclusion criteria:
a) Persons experiencing occasional blur at distance during daytime on a regularly basis and reports of difficulty in changing focus from near to far distance. (Based on a general and ocular health examination by an ophthalmologist it was considered unlikely that the subjects' symptoms could be ascribed to any illness, and therefore the patients' symptoms in our best opinion represent true visual complaints).

b) Refraction in the - 6,0 Diopter to + 4,0 Diopter interval with maximum cylinders of 2,0 Diopter, and a difference between eyes not exceeding 1,0 Diopter.

c) Willing and able to participate in the study.

Exclusion criteria:
Distance monocular acuity of 0,9 or worse, and stereopsis values at near at 100 sec. of arc or higher.

Equipment and materials: All subjective refractive examinations were carried out in a standard test room of 3 meters distance with a Nidek phoropter (acuity letters reflected in a mirror and originating from a 6 meter distance). For the objective refraction an autorefractor was used, model Topcon KR 7500. For the purpose of paralyzing the accommodation Cyclogyl ¹ was used. (¹ Cyclogyl is tradename for Cyclopentolate 1%)

Procedure:
All subjects were referred by their regular optometrist who suspected a moderate degree of accommodative spasm and/or convergence insufficiency, and estimated that accommodative relaxing exercises or convergence training would benefit the person more than new glasses. No attempt to randomize the subjects were carried out. Subjects arriving in the 3 months test period were evaluated as they showed up and were accepted for the study.

All subjects received oral and written information about the procedure and agreed to participate.

All subjects were examined twice. At the first visit they received a complete optometric examination including refraction at distance and near, confirming that new glasses were probably not an option to relieve them of their visual symptoms. One week later the subjective refraction was repeated by the same procedure (measurement of distance refraction OEP no. 7A and maximum plusacceptance at near OEP no.14B). At the second visit the subject furthermore received a complete ophthalmologic examination by an ophthalmologist (T. Haghfelt) followed by objective refraction in cycloplegia using an autorefractor.

Before the cycloplegic refraction patients had two drops of one percent Cyclogyl installed five minutes apart and waited thirty minutes before examination (Davies 1972).

Analysis: For the purpose of analysis, refractive data just before and after the cycloplegic examination are used.

For the purpose of comparing the overall refractive results all values were converted to the spherical equivalent value by the following method: All astigmatism was measured and recorded in minus-cylinder notation and half of the cylinders amount was subtracted from plussphere values or added to minussphere values.

All data analysis was carried out with SPSS version 9, student edition.

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Results
Overview of individual refractive data are displayed in table 1.

Table 1
Refractive status of subjects

Table 1. Actual test results of subjects, conversion to spherical equivalent values and allocation to plusacceptance group.
R. is right eye and L. is left eye.

Overall correlation in refractive spherical equivalents before and after Cyclogyl was installed was calculated
to r = 0,984, p = 0,01 (two-tailed) and displayed with regression statistics in figure 1.

Figure 1
Overall correlation between refractive procedure OEP no. 7A and cycloplegic refraction

Fig. 1. Regression line with 95 % confidence interval of the spherical equivalent values
of the OEP no. 7A refraction and Cycloplegic refraction.

Overall mean values and difference in the spherical equivalents of the two refractive procedures together with regression statistics are shown in table 2.

Table 2.
Overall mean values and regression of the OEP no. 7A and Cycloplegic refraction

Table 2. Means, difference and regression of the two refractive procedures.

Overall regression for the two refractive procedures is expressed by; Y = - 0,327 + 0,897 * X , and calculates the OEP no. 7A result based on the cycloplegic result. Based on the correlation it is obvious that the outcome of these refractive procedures are very closely related and that the OEP no. 7A result can be predicted very closely based on the cycloplegic result.

Subjects were then allocated to one of three groups according to the OEP no. 14B plusacceptance measurement with the purpose of analyzing for group differences. The results are displayed in figure 2 and calculated in table 3.

Figure 2
Mean refractive change in right eyes in the three plusacceptance groups

Figure 2. Mean refractive changes in right eyes with +/- 2 SD interval in the three plusacceptance groups.
N represents number of right eyes.

Actual data for the three groups are shown in table 3.

Table 3
Mean change in the OEP no. 7A refraction after Cyclogyl in the three groups (right eyes)

Table 3. Mean refractive changes in the right eye OEP no. 7A refraction after cyclogyl was
installed in the three plusacceptance groups. N represents number of right eyes.

The results in figure 2 and table 3 show that subjects in group 1 and 2 demonstrated the largest ,and equal, change in refraction after installation of Cyclogyl, and subjects in group 3 half of the change. However all changes are very small and a Simple one-way ANOVA calculates a F (2,13) = 0,057. P 0,945 ( right eye values) and thereby accepts the test null-hypothesis of no difference between the groups. Mean refractive change in the three groups is therefore about equal, or in the same interval, and allocation to one of the three groups has no diagnostic value.

However, there are very few subjects in group 3, and if excluded, it could be stated that an average change in distance refraction about 0,25 Diopter in clinical terms, should be the expected difference between the OEP no. 7A and Cycloplegic procedures regardless of subjective plusacceptance at near from a OEP no. 14B value of 1,25 Diopter and less.

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Discussion
This study had two objectives; The first was to examine the validity of the OEP No. 7A distance refraction procedure in an adult group of subjects with pronounced visual complaints, and the second is to examine if the OEP no. 14B measurement of maximum plusacceptance with the binocular crosscylinder method can be used as indicator of either Latent Hyperopia or Pseudomyopia.

Regarding the first objective, the results of this study point to the fact that the No. 7A refraction procedure is a very stable and valid measurement of distance refraction.. The correlation coefficient between the OEP no. 7A procedure and cycloplegic refraction states that 96,8 % of variation in one of the procedures can be explained by the other.

Often it is expected that subjective refraction has a very sharp endpoint, but in a study of repeatability of subjective refraction in normal adults by Rosenfield et.al (1995), the 95 % limit of agreement in spherical equivalents could be assessed to +/- 0,29 Diopter and this value is suggested as a minimum threshold of a significant change in refractive error.

In another study by Grosvenor et.al (1984) comparing subjective non-cycloplegic and subjective cycloplegic refraction (using Cyclogyl) in a young adult population, 41 out of 60 subjects had refractive findings between +/- 0,25 Diopter, and the remaining distributed between 0,50 - 1,25 Diopter more plus in the cycloplegic refraction. Similar results were obtained by Nayak et.al. (1987) comparing subjective refraction with cycloplegic autorefractor results. The percentage within + 0,25 Diopter was 86 % and the percentage within + 0,50 Diopter was 96 %.

Finally, some variation in Autorefractor measurements should be expected. In a Study by McCaghrey et.al. (1993) of eight different autorefractors, the instrument that performed most consistent produced following example: A person with a subjective refraction of + 2,25 DS/-0,50 DC x 140 degrees was measured 50 times with a mean and standard variation of
+ 2,07 DS (+/- 0,31) / - 0,35 DC (+/- 0,20) x 148 degrees (+/-26).

Considering this normal reported variation in available test methods, the refractive results of this study appear to be in good agreement. It means, on average, it should be expected that the No. 7A procedure consistently produce a slightly higher minus value (or less plus value) at the amount of 0,25 Diopter in clinical terms.

Regarding the validity of the plusacceptance measurement at near there are no reports of reliability studies in the literature, but the general clinical impression is that the measurement at least can be measured within 0,50 Diopter of precision in individual prepresbyopic subjects. Normal performance, on the other hand, has received much attention. Morgan (1944) published a table of expected test values originating from comparison and recalculation of different investigators in the field. Based on many studies, including several thousand subjects, the normal binocular crosscylinder value (OEP no. 14B) was determined to be + 0,50 Diopter (SD 0,25) above the distance subjective.

In light of these baseline data the second objective was evaluated, and at least for the OEP no. 14B plusacceptance values in the range of 1,25 Diopter or less, there are no systematic or significant influence on the distance refraction whether measured with the OEP no. 7A procedure or cycloplegic refraction, and in this clearly symptomatic sample the measurement was not indicative of any significant degree of Latent Hyperopia or Pseudomyopia.

In summary. The standard procedure named OEP no. 7A appears to be a very solid and stable measurement of distance refraction even for adults with pronounced visual complaints. It also means, that there is no indication for cycloplegic refraction in adult prepresbyopic persons. Furthermore, the OEP no. 14B plusacceptance value was not indicative of any hidden refractive errors, and cannot be used to predict this. For this sample it demonstrates that, besides refractive errors, other sources of discomfort should be investigated.

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References:
Davies, P.H.O. (1972). The actions and uses of ophthalmic drugs. In: Grosvenor T.P (eds.) Primary care optometry.
Professional Press 1989, USA. Second edition, pp. 274.

Grosvenor, T., Perrigin, D.M., Perrigin, J., Moorehead, A., Lamp, M. (1984). A comparison of cycloplegic refractive findings on a group of young adults. In: Grosvenor T.P (eds.) Primary care optometry.
Professional Press 1989, USA. Second edition, pp. 277.

Margach, C.B. (1979): Introduction to Functional Optometry.
Optometric Extension Program Foundation, Inc.Duncan, Oklahoma 73533. USA.

McCaghrey, G.E., Matthews F.E. (1993). Clinical evaluation of a range of autorefractors. In: Bennett & Rabbetts' (eds.) Clinical visual optics.
Butterword-Heinemann Ltd. 1998. Oxford, United Kingdom. Third edition, pp. 361.

Morgan, M.V. (1944). Analysis of clinical data. In: Borish, I.M. (eds.) Clinical refraction.
Professional Press 1970, USA. Third edition, pp. 914.

Nayak, B.K., Ghose, S., Singh, J.P. (1987). A comparison of cycloplegic and manifest refraction on the NR-1000F (An objective Auto Refractometer). In: Bennett & Rabbetts' (eds.) Clinical visual optics.
Butterword-Heinemann Ltd. 1998. Oxford, United Kingdom. Third edition, pp. 361.

Rosenfield, M et.al (1995): Repeatability of subjective and objective refraction.
Optometry and Vision Science vol. 72, no. 8, pp. 577 - 579.

(Abstract) (Introduction) (Method) (Results) (Discussion) (References)

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