Abstract
purpose. To compare the magnification properties of four different indirect
double aspheric fundus examination lenses for clinical disc biometry.
methods. Experimental study in a model eye. The relationship between the true
size of a fundus object and its image was calculated for each fundus
lens for an ametropic range between −12.5 and +12.6 D using a slit
lamp biomicroscope with adjustable beam length.
results. Equations for determining the correction factor p (degrees per millimeter) were calculated for each fundus lens. The
factor can be used in calculations to determine true optic disc size.
The total change in magnification of the system from myopia to
hyperopia was −21.1% to +24.0% (60-D lens; Volk Opticals, Mentor,
OH), −12.9% to +16.2% (Volk super 66 stereo fundus lens), −13.2%
to +13.9% (Volk 78-D lens), and −13.3% to +14.0% (Volk super-field
NC lens). When the fundus lens position was altered im relation to the
model eye by ±2 mm under myopic conditions, the change in
magnification of the system was −4.3% to +5.7% (60-D lens), −4.6%
to +6.1% (66 stereo fundus lens), −4.9% to +6.3% (78-D lens), and−
5.9% to +7.8% (super-field NC lens). In the hyperopic condition the
change was −2.7% to +3.6%, −3.4% to +4.5%, −3.6% to +4.8%, and−
4.5% to +6.0%.
conclusions. The study has shown that the use of a single magnification correction
value for each fundus lens may not be appropriate. These findings have
important implications for the way in which calculations for
determining the true optic disc size and other structures of the
posterior pole are performed using indirect
biomicroscopy.
In 1953, El Bayadi
1 first examined the fundus with a
plano-convex lens of approximately +60 D using the slit lamp
biomicroscope, but the technique was not widely accepted because of
aberration and difficulty of use. With the introduction of the double
aspheric 60-D lens in 1982 (Volk Opticals, Mentor, OH), the technique
started to gain popularity for routine stereoscopic examination of the
posterior pole.
Since then, many attempts have been made to determine the true size of
the optic disc with several types of high-power positive lenses using
indirect ophthalmoscopy.
2 3 4 5 6 7
The advantages of this technique for determining the true optic disc
size are the immediate availability of the results and the reduced
costs in instruments and personnel compared with sophisticated
techniques such as computer-based analysis of optic disc photographs
(planimetry), scanning laser ophthalmoscopy, video-ophthalmography, and
simultaneous stereo optic disc photography with digital photogrammetry.
In addition, only a few ophthalmologists have access to this expensive
equipment for routine clinical work, and optic disc measurement is
usually performed at the slit lamp biomicroscope.
The purpose of this study was to compare four widely used high-power
positive lenses regarding their magnification over a wide range of
ametropia in the center of the image field, by using a slit lamp
biomicroscope with adjustable beam length.
Four commercially available double aspheric fundus lenses (60-D
lens, 66 stereo fundus lens, 78-D lens, and super-field NC lens)
manufactured by Volk Optical and a calibrated slit lamp biomicroscope
(Haag-Streit 900; Bern–Koeniz, Switzerland) were used for this study.
All lenses provide a stereoscopic view of the fundus and a wide field
of view.
In the slit lamp biomicroscopy the observation system of the slit lamp
is focused at a finite, short distance. The light from the fundus exits
the eye parallel (i.e., from optical infinity); therefore, the slit
lamp cannot be focused on the fundus. With the use of a high-power
positive lens, a real inverted image of the fundus is formed in front
of the slit lamp biomicroscope (toward the observer). For clear
imagery, the slit lamp is focused on this image.
8
The size of this image as measured with the beam length on the slit
lamp is dependent on magnification due to the patient’s eye
(correction factor
q; millimeters per
degree),
9 10 11 12 magnification due to the condensing lens
used to obtain the image (correction factor
p; degrees per
millimeter)
13 and the position of the condensing lens with
respect to the eye
8 ; thus, for calculating the absolute
dimensions of this image the total magnification of the system must be
known.
To measure the change in magnification of the system with refractive
error and variation in condensing lens position, a curved scale in the
form of a quarter of a sphere was fitted with the help of an excimer
laser on the fundus surface of a model eye (based on Gullstrand’s
schematic eye), whereby the center of the spherical fundus, of the
spherical scale and corneal lens are on the same line. This scale has
to be curved because the optics of the slit lamp biomicroscope and
fundus lens are designed for use with a curved field so that a flat
scale only approximates to the retina in practice.
Distilled water, which has a refractive index similar to that of the
media in the eye (1.336) was introduced in the anterior and vitreous
chambers. In this situation the equivalent power of the model eye is+
59.4 D. By means of a screw ruled in micrometers, the vitreous
chamber depth can be precisely varied to produce axial ametropia.
The fundus object was viewed with each fundus lens as in a
routine examination of the optic nerve head. The instruments were
aligned perpendicularly to the model eye’s cornea and the fundus
object was brought into focus by moving the biomicroscope away from the
condensing lens until a sharp image of the fundus object was provided
into the center of view.
Figure 1 shows the optical diagram of the experimental setup.
A narrow slit beam, with width maintained at 0.2 mm, was progressively
reduced in size from 8 mm until it coincided with the diameter of the
fundus object. The beam length was then recorded from the millimeter
scale at the top of the instrument. Because the slit lamp beam length
is calibrated in 0.1 mm, the reading was judged to the nearest 0.05 mm.
After each reading, the millimeter scale was reset to 8 mm.
Measurements were taken with the vitreous depth of the model eye
set at values corresponding to a range of ocular refraction from −12.5
to +12.6 D. The pupil width was 8.0 mm in diameter. For each fundus
lens, three measurements were obtained at each value of the ametropia
setting. This process was repeated at another session for each fundus
lens, and the 95% confidence intervals for repeatability were
calculated.
14
The fundus lens correction factor
p was then determined
according to the formula:
\[p{=}(K/17.453){\cdot}(t/s)^{13}\]
where
K is the ametropia of the eye + equivalent
power of the eye,
t is the fundus object size (the
diameter), and
s is the measured size of the fundus
object’s diameter on the slit lamp biomicroscope.
To investigate the change in magnification of the system with variation
in condensing lens position, measurements of the fundus object size
were obtained as described when the fundus lens position was altered by±
2 mm to the model eye’s cornea under myopic and hyperopic
conditions.
The interindividual variability in optic disc size is
morphogenetically and pathogenetically important.
15 Therefore, for clinical purposes, it is usually sufficient to know
whether the optic disc is abnormally large, medium, or abnormally
small.
With the help of the fundus lens correction factor
p (see
Table 1 ), ophthalmologists can determine the true size of the optic
disc (
t) according to Littmann’s formula:
where
s is the optic disc diameter measured at the slit
lamp biomicroscope, which must be calibrated before the measurement,
and factor
q is the relationship of the real diameter of the
optic disc measured in millimeters to the angular diameter, with which
the optic disc is reflected through the optical system of the eye in
its exterior space. It is a variable dependent on the optical
dimensions of the patient’s eye and not the fundus imaging system.
Several methods are available for determining the ocular factor
(
q in millimeters per degree) for a human eye based on
ametropia and keratometry,
9 10 11 ametropia and axial
length,
9 10 11 and axial length only.
12
Another accurate method to determine the true optic disc size is
according to the formula:
in which
m is the total linear magnification of the
system. It can be determined according to the formula:
\[m{=}\mathrm{-}{\{}\ \frac{F_{\mathrm{e}}{+}A}{F_{\mathrm{c}}-A{\cdot}{[}1-(F_{\mathrm{c}}{\cdot}e){+}(F_{\mathrm{c}}{\cdot}d){]}}{\}}\]
where
F e is the equivalent power
of the eye,
A the ametropia of the eye,
F c the power of the condensing lens,
e the position of the first principal plane of the eye, and
d the working distance of the condensing lens. The minus
sign indicates an inverted image. It is not possible to collect this
amount of data in clinical practice, and thus the true optic disc size
has to be estimated on the basis of fewer, easily obtained variables,
as has been described.
It is equally important that the slit lamp and fundus lens be aligned
correctly in front of the patient’s cornea and the optic disc be
centered in the image field to maximize the repeatability of the
experimental setup.
Figures 2 and 4 show that the measurements are
highly dependent on the distance of the condensing lens from the eye,
particularly in presence of a high refractive error. In clinical disc
biometry, the degree of error can be minimized if the operator is
careful when positioning the slit lamp biomicroscope and condensing
lens and if the total magnification of the system is taken into
consideration as described.
If the true minimal and maximal diameters of the optic disc and cup are
known, ophthalmologists can determine the optic disc, cup, and
neuroretinal rim areas according to the formula of an
ellipse.
17 This is especially important in the
identification of optic discs with glaucomatous optic
neuropathy.
18 19
The slit lamp biomicroscopic measurement of the optic disc diameter,
using a high-power positive lens, shows an acceptable intraobserver and
interobserver variability in routine clinical work compared with other
sophisticated and time-consuming methods for clinical disc
biometry.
2 3 20 21
Previous studies have investigated biomicroscopic measurement of
the optic disc with different indirect double aspheric fundus
examination lenses,
4 5 6 by comparing the optic disc
diameter measured at the slit lamp biomicroscope and its true size
obtained by photogrammetry or Heidelberg Retina Tomograph, (Heidelberg
Engineering, Heidelberg, Germany) to assess a conversion factor or
equation for each fundus lens. A single correction factor or equation
can provide only a rough estimation of the optic disc size (see
Fig. 3 ).
16 Spencer and Vernon
2 showed that
according to a single conversion factor, the Volk 78-D lens gives
larger measurements of the optic disc than photogrammetry (using
ametropia and keratometry)
22 by 0.41 mm.
A potential problem of these studies is that the obtained measurements
were analyzed by using correlation coefficients.
4 5 6 However, in the analysis of measurement method comparison data, neither
the correlation coefficient nor techniques such as regression analysis
are appropriate.
14 23 The correlation coefficient measures
the strength of a relation between two variables, not the agreement
between them. Perfect agreement is attained only if the points of
measurements lie along the line of equality, but perfect correlation is
attained if the points of measurements lie along any straight
line.
23
The fundus lens correction factor
p (see
Table 1 ) allows
slit lamp biomicroscopic measurement of the real optic disc size for
any degree of ametropia and makes biomicroscopic measurements with
different fundus lenses comparable. Furthermore, the factor
p can be used in the measurement of other structures of the
posterior pole, such as retinal or choroidal tumors, and in the
assessment of macular degeneration, by measuring the extent of the
disease and its distance from the center of the foveal avascular zone.
This would be of particular importance when comparing the morphometric
characteristics of a fundus landmark of interest between individuals
with regard to diagnosis and therapy. Therefore fundus landmarks should
be measured in absolute size units (millimeters), instead of using the
interindividually variable disc diameter as a measurement unit. It
should be considered, however, that the ocular correction factor
q can be used with sufficient accuracy within ±20° of the
optical axis.
9 12
The present study had the advantage of using the ideal optical
condition of a model eye. The purpose of our next study will be to
investigate the potential of this technique for clinical disc biometry,
in comparison with magnification-corrected photogrammetry of the optic
disc.
Submitted for publication May 17, 2000; revised August 16, 2000; accepted September 12, 2000.
Commercial relationships policy: N.
Corresponding author: Siamak Ansari–Shahrezaei, Department of Ophthalmology and Optometry, Vienna University School of Medicine, Anilingasse 2/25, A-1060 Wien, Austria.
[email protected]
Table 1. Values and Equations of Regression Lines of Correction Factor
p for Indirect High-Power Positive Lenses Using Slit
Lamp Biomicroscopy
Table 1. Values and Equations of Regression Lines of Correction Factor
p for Indirect High-Power Positive Lenses Using Slit
Lamp Biomicroscopy
Fundus Lens | Factor p (deg/mm) | Range of Factor p (deg/mm) | 95% Confidence Interval for Repeated Measurements of s (%) | Range of Ocular Refraction Investigated (D) |
Volk 60-D lens | 3.04± 0.04 (mean± SD) | 2.96–3.08 | | −12.5 to +12.6 |
| 0.001 A+ 3.03 | 2.96–3.08 | +3.24 to−3.67 | −12.5 to+12.6 |
Volk 66 stereo fundus lens | 3.56± 0.06 (mean± SD) | 3.46–3.63 | | −5 to+5 |
| 0.020 A + 3.54 | 3.26–3.75 | +3.89 to−4.25 | −12.5 to+12.6 |
Volk 78-D lens | 3.69± 0.06 (mean± SD) | 3.61–3.78 | | −5 to+5 |
| 0.017 A+ 3.65 | 3.36–3.93 | +3.97 to −4.38 | −12.5 to+12.6 |
Volk super-field NC lens | 4.61± 0.03 (mean± SD) | 4.57–4.66 | | −5 to+5 |
| 0.025 A+ 4.58 | 4.22–4.92 | +4.50 to −4.93 | −12.5 to+12.6 |
The authors thank Bernhard Rassow of the Medical Optics
Laboratory, University of Hamburg, Germany, for providing the model
eye.
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