Comparison of Luneau SA disposable and Goldmann applanation tonometer readings

Abstract

To test the agreement of intraocular pressure (IOP) measurements made with Luneau SA applanators and Goldmann applanator.
A single-blind crossover trial. IOPs were measured in both eyes of subjects with both applanators. Type of applanator was alternated to eliminate systematic bias. Multiple observers were used. Observers were blind to the scale while performing measurements but not to the type of applanator used. The appearance of the meniscus was assessed semiquantitatively. All measurements were combined and presented in a Bland-Altman plot.
A total of 140 eyes of 79 subjects were tested by seven observers. The range of measurements was 6-45 mmHg (mean 17.8 mmHg) for the Goldmann applanator. On average, the Luneau SA applanator (range of measurements 4-36 mmHg) gave a measurement of 2.35 mmHg less than the Goldmann standard. The standard deviation of these differences was 2.13 mmHg, giving an upper 95% confidence limit of 6.53 mmHg and a lower 95% confidence limit of -1.83 mmHg. The measurements agreed in only 24 out of 140 instances. In 28 eyes, the disposable tonometer end point was difficult to assess owing to excessively thick rings. Linear extrapolation suggests an increase in difference with increasing IOP.
The inter-head inaccuracy, tendency to underestimate IOP, and lack of systematic inaccuracy make a corrective algorithm impossible to formulate. The range of variation between the Luneau SA disposable applanator and the Goldmann standard is sufficiently large to influence clinical management decisions. We speculate that one explanation is the interaction of the tonometer with the tear film, making end point determination difficult. Further research is being undertaken.

Full text

Comparison of
Luneau SA
disposable and
Goldmann
applanation
tonometer readings
ACJ Baddon, SF Osborne, SA Quah, M Batterbury
and D Wong
Abstract
Purpose To test the agreement of intraocular
pressure (IOP) measurements made with
Luneau SA applanators and Goldmann
applanator.
Materials and Methods A single-blind
crossover trial. IOPs were measured in both
eyes of subjects with both applanators. Type
of applanator was alternated to eliminate
systematic bias. Multiple observers were used.
Observers were blind to the scale while
performing measurements but not to the type
of applanator used. The appearance of the
meniscus was assessed semiquantitatively. All
measurements were combined and presented
in a Bland–Altman plot.
Results A total of 140 eyes of 79 subjects
were tested by seven observers. The range of
measurements was 6–45 mmHg (mean
17.8 mmHg) for the Goldmann applanator. On
average, the Luneau SA applanator (range of
measurements 4–36 mmHg) gave a
measurement of 2.35 mmHg less than the
Goldmann standard. The standard deviation
of these differences was 2.13 mmHg, giving an
upper 95% confidence limit of 6.53 mmHg and
a lower 95% confidence limit of 1.83 mmHg.
The measurements agreed in only 24 out of 140
instances. In 28 eyes, the disposable tonometer
end point was difficult to assess owing to
excessively thick rings. Linear extrapolation
suggests an increase in difference with
increasing IOP.
Conclusion The inter-head inaccuracy,
tendency to underestimate IOP, and lack of
systematic inaccuracy make a corrective
algorithm impossible to formulate. The range
of variation between the Luneau SA
disposable applanator and the Goldmann
standard is sufficiently large to influence
clinical management decisions. We speculate
that one explanation is the interaction of the
tonometer with the tear film, making end point
determination difficult. Further research is
being undertaken.
Eye (2007) 21, 789–792; doi:10.1038/sj.eye.6702329;
published online 21 April 2006
Keywords: Luneau SA; Goldmann; applanation
tonometer
Introduction
The use of disposable instruments in the
practice of ophthalmology has increased in
response to fears of iatrogenic horizontal
transmission of prion material between patients.
It has been suggested that the very common
practice of measurement of intraocular pressure
(IOP) by Goldmann prism applanation
tonometry may lead to transfer of potentially
infective material.
1–4
Because of this risk, the
Medical Devices Agency has recommended that
‘components of ophthalmic devices that touch
the surface of the eye should be restricted to
single patient use wherever practicable and
where this does not compromise clinical
outcome’.
4
There are a number of disposable
applanation tonometers in use in the NHS.
Silicone and acrylic disposable applanation
tonometers have been the subject of previous
study. Acrylic prisms (Tonosafe
s
) were found
to have good agreement with the Goldmann
applanation tonometer, but silicone
(Tonoshield
s
) prisms were found to
consistently over read by a small margin.
5
The Luneau SA disposable tonometer has
become available. It is placed in the Goldmann
tonometer device in the same way as the
Goldmann prism. The objective of this study
Received: 10 May 2005
Accepted in revised form:
1 February 2006
Published online: 21 April
2006
No proprietary interests
No research funding
Presented as Poster and
Quickfire Presentation,
Royal College of
Ophthalmologists Congress
2005
Department of
Ophthalmology, St Paul’s
Eye Unit, The Royal
Liverpool University
Hospital, Liverpool, UK
Correspondence:
ACJ Baddon,
Department of
Ophthalmology,
Walton Hospital,
Rice Lane, Walton,
Liverpool L9 1AE, UK
Tel: þ 779 860 7703
(Mobile), þ 151 424 1559
(Home);
Fax: þ 779 860 9051.
E-mail: ophthalmologist@
onetel.com
Eye (2007) 21, 789–792
& 2007 Nature Publishing Group All rights reserved 0950-222X/07 $30.00
www.nature.com/eye
CLINICAL STUDY

was to compare the IOP readings of the two devices and
to assess the end point features of each.
Materials and methods
The IOPs of a consecutive series of patients were
measured with both the Luneau SA disposable
applanator tonometer (Figure 1) and Goldmann
applanator tonometer. Each patient’s pressures were
measured by only one of the seven observers. All
observers were of consultant or specialist registrar
grades. The type of applanator and the side measured
first were alternated to eliminate systematic bias. The
tonometers’ calibration was checked before and after the
study by use of the calibration bar. The measurements
were made by setting the tonometer dial to zero and then
applanating the cornea and aligning the inner aspect of
the two half menisci. Measurements were taken with the
eye in the primary position and apart from the
applanation, there was no external pressure exerted on
the eyeball. The observers were blinded to the scale on
the tonometer dial. Observers were not blinded to the
type of applanator being used. The appearance of the
menisci was assessed semiquantitavely, without
reference to a standard photograph, by all observers as
too thin, too thick, or acceptable. To illustrate agreement,
all measurements were combined and presented in a
Bland–Altman plot
6
with calculation of 95% confidence
limits of agreement. A paired t-test was used to analyse
the difference between measurements.
Results
A total of 140 eyes of 79 subjects were tested by seven
observers. The range of measurements was 6–45 mmHg
(mean 17.7 mmHg) for the Goldmann applanator. The
range of measurements for the Luneau SA applanator
was 4–36 mmHg (mean of 16.3 mmHg). On average, the
Luneau SA applanator gave a measurement of
2.35 mmHg less than the Goldmann standard. The range
of difference in these paired readings was from 6to
þ 9 mmHg. The standard deviation of these differences
was 2.13 mmHg, giving an upper 95% confidence limit of
6.53 mmHg and a lower 95% confidence limit of
1.83 mmHg for measurements made with the
disposable prism in comparison to the Goldmann
standard. The measurements agreed in only 24 out of 140
instances (Figure 2). A paired, two-tailed t-test showed
that the difference between the means of the two data
sets was very highly significant (Po0.001) (Table 1). In 28
eyes, the disposable tonometer end point was difficult to
assess owing to excessively thick rings. In three eyes, thin
poor quality rings were seen with the disposable
tonometer. With these paired data removed, a paired
Figure 1 Luneau tonometer prism.
+9
-6
y = 0.0816x
-6.5
-4.5
-2.5
-0.5
5 101520253035404550
1.5
3.5
5.5
7.5
9.5
Goldman IOP (mmHg) ->
Difference between Goldman and Disposable
(ie Positive numbers = Goldman reading higher)
Figure 2 Bland–Altman plot.
Comparison of Luneau SA and Goldmann applanators
ACJ Baddon
790
Eye

t-test shows that the statatistical significance remains
high (Po0.001). Linear extrapolation suggests an
increase in measurement difference with increasing IOP.
Discussion
Our results show that, under conditions of normal
clinical use, the Luneau SA disposable tonometer
underestimates IOP compared with the reference
standard Goldmann nondisposable tonometer in a
nonsystematic way that, in many eyes, is clinically
significant. While only 13.6% of our readings were above
21 mmHg, accurately recording a lower pressure may, in
many cases, have clinical significance.
In this study, there was no measure made to assess
inter-observer agreement. Neither were the observers
blinded to the prism type, owing to the differing
appearance of the Luneau SA disposable prism as
compared to the Goldmann prism. Judgements on the
quality of menisci were not standardised against
photographs of a poor or satisfactory meniscus, although
all observers were ophthalmologists who were very
experienced in applanation tonometry.
Our findings are contrary to those of Maino et al,
7
who
found good agreement between disposable and
nondisposable tonometer prisms by practitioners trained
in applanation tonometry; however, each eye received
one applanation from each of the three types of prism in
every case in random order. Our readings, although
lesser in total number, were mostly within the normal
range, from a population of a catchment area adjacent to
that of Maino et al.
Our impression before setting up this study was that
for the Luneau SA prism the end point of measurement,
the alignment of the semicircular menisci, was difficult to
determine because of the occurrence of thick rings. This
impression is supported by observation in our results.
The disposable prism menisci appearance was judged to
be of acceptable quality in only 80% of cases, whereas
there was no case of the Goldmann menisci reported as
being of poor quality.
Goldmann applanation tonometry relies on the Imbert-
Fick principle. Imbert-Fick states that the force of
flattening (F) equals the pressure (P) multiplied by the
surface area flattened (A), or F ¼ P A. However, this
principle assumes that the surface is dry, perfectly
flexible, infinitely thin, and spherical. In the design of the
Goldmann applanator prism, two additional factors are
included: the surface tension of the tear film, which tends
to draw the tonometer head toward the cornea and
corneal rigidity, and the tendency of the cornea to resist
being flattened during applanation. Goldmann assumes
the corneal thickness to be exactly 520 mm and that all
corneas are equally rigid with parallel anterior and
posterior surfaces. Given these assumptions, at a
diameter of applanation of 3.06 mm, scleral rigidity and
surface tension cancel each other out.
8
We speculate that one aspect of the poor agreement
between the Luneau SA and the Goldmann is the
interaction between the prism and the tear film such that
the surface tension is altered. Not only might this change
the measurement of IOP
8–11
but also may result in thick
menisci that make end point determination inaccurate.
We also speculate that different combinations of
fluorescein and local anaesthetic agent might lead to a
range of tear film surface tensions, which might affect the
accuracy of the IOP measurement. Further studies are
being undertaken to test these hypotheses.
Design of disposable applanation prisms may
necessitate the use of different materials or the surface
modification of current materials and this in turn might
require the modification of the body of the tonometer or
its dial. This modification would of course require that
there be a systematic error. In the case of a nonconstant
error in tonometry measurement, it would seem logical
to balance the risk of horizontal transmission of
infections between patients against the risk of the over-
or underestimation of their ocular pressure and the
impact of this on treatment. The materials we received
from Luneau did not include any specification as to the
level of accuracy expected with their prism.
The inter-head inaccuracy, tendency to underestimate
IOP, and lack of systematic inaccuracy make a corrective
algorithm impossible to formulate. The range of variation
between the Luneau SA disposable applanator and the
Goldmann standard is sufficiently large to influence
clinical management decisions. This lack of agreement
means that the Luneau SA prism in its present form
cannot replace the nondisposable prism.
References
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Ironside JW. Retention of corneal epithelial cells following
Goldmann tonometry: implications for CJD risk. Br J
Ophthalmol 2003; 87(5): 583–586.
Table 1 Summary of results
N ¼ 140 eyes Goldmann
(G)
Disposable
(D)
Difference
(GD)
Mean 17.7 16.34 2.35
Standard deviation 2.13
95% CI 74.17
Range 6 to 45 4 to 36 6to þ 9
For paired t-test, t ¼ 13.69, Po0.001.
All values are in mmHg.
Comparison of Luneau SA and Goldmann applanators
ACJ Baddon
791
Eye

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