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A Prospective Trial Comparing Scleral Pneumotonometry to Goldmann Applanation Tonometry
Sara Duke, MD, Usiwoma Abugo, BS, Shuchi Patel MD
Loyola University Medical Center, Department of Ophthalmology, Maywood IL
Abstract
Introduction
Purpose
To determine if scleral pneumotonometry (PT) will
provide an accurate and reliable assessment of
intraocular pressure (IOP) either directly or via a
predictable relationship such that IOP can be
extrapolated from scleral measurement.
Methods
A prospective non-randomized trial of individuals 18
years of age and older. Each had his/her IOP measured
with the Goldmann applanation tonometer (GAT) first,
then corneal PT and finally scleral PT. The patient’s age,
gender, ocular history, refractive error, current
medications and pachymetry were recorded. Pearson’s
correlation was used for statistical analysis.
Results
107 eyes from 54 patients (32-87 years old) have been
examined to date. Mean GAT IOP was 15.03 mmHg
while mean corneal PT was 18.47 mmHg. Mean
difference between corneal PT and GAT was 3.39 mmHg
(SD 2.29). Mean scleral PT was 29.37 mmHg with an
average difference between scleral PT and GAT of 14.27
mmHg (SD 9.74). Mean spherical equivalent refraction
(SE) was -0.98 D, mean CCT was 561.56 µm, mean AL
was 24.06 mm. A correlation plot between scleral PT and
corneal PT reveals a statistically significant positive
correlation (r 0.196, p 0.047), but the plot between
scleral PT and GAT shows no statistically significant
correlation (r 0.084, p 0.359). The difference between
scleral PT and GAT did not correlate with SE (r 0.08, p
0.46), CCT (r -0.11, p 0.34) or AL (r 0.01, p 0.98).
Conclusions
Scleral PT has shown consistently higher IOP
measurements than the corneal IOP measurements.
While scleral PT has a statistically significant positive
correlation to corneal PT, it unfortunately does not
correlate well to the gold standard of IOP measurement,
GAT. Initial analysis has been unable to account for this
discrepancy by factoring in SE, CCT or AL separately.
With further data collection and analysis we hope to
determine if an equation could be formulated that would
provide the true IOP as a function of scleral PT
measurements by taking SE, AL and CCT into
consideration concomitantly.
Current devices used to estimate intraocular
pressure (IOP) perform the measurements on the
cornea. This is not possible in some patients such
as those with a keratoprosthesis or other corneal
pathology. Glaucoma is known to occur in about
75% of patients following a keratoprosthesis, but
accurate pressure readings to monitor for
progression are not possible. Thus, we sought to
determine if a predictable relationship exists
between Goldmann applanation tonometry (GAT)
and scleral pneumotonometry (PT) to provide an
accurate and reliable assessment of intraocular
pressure (IOP) via scleral measurements.
Results
Data
Methods
•IRB approved, prospective case control study
•54 patients who presented to the Loyola University
Hospital outpatient ophthalmology clinic for routine
examination
Average
Standard
Deviation
Range
GAT
(mmHg)
15.03
3.34
9 – 24
Corneal PT
(mmHg)
18.47
3.89
11 – 28
Scleral PT
(mmHg)
29.37
9.35
6.5 – 52.5
Spherical
Equivalent
(D)
CCT
(µm)
-0.98
2.42
-9.38 – +2.50
561.56
55.16
450 – 727
Axial Length
(mm)
24.06
0.98
22.58 – 26.20
Conclusions
•Patients were provided appropriate informed consent
and were aware of the risks and benefits of participation
•The first IOP measurement was taken on the central
cornea with a Goldmann applanation tonometer AT900®
(Haag-Streit International)
• Scleral PT shows consistently higher
measurements
than
the
corneal
measurements from the same eye
•The second IOP measurement was taken on the central
cornea
with a Model 30 Classic Pneumatonometer
(Reichert Ophthalmic Instruments)
• Scleral PT has a statistically significant positive
correlation to corneal PT, however it does not
correlate to GAT
•The third and final IOP measurement was taken on the
inferotemporal scleral approximately 2 mm from the
limbus
with
the
same
Model
30
Classic
Pneumatonometer (Reichert Ophthalmic Instruments)
•Corneal pachymetry was measured with the DGH Model
555 PACHETTE 3 (DGH Technology Inc)
•The patient’s age, gender, ocular history, refractive error,
and current medications were recorded
•Pearson’s correlation was used for statistical analysis
Scleral PT vs Corneal PT
Pearson r
0.196
P value (two-tailed)
0.047
Scleral PT vs GAT
Pearson r
P value (two-tailed)
0.084
0.359
References
1. Chew HF, Ayers BD, Hammersmith KM et al. Boston keratoprosthesis outcomes and complications. Cornea 2009 Oct;28(9):989-96.
2. Barraza RA, Sit AJ. Investigation to determine a relationship between scleral and corneal tonometry. ARVO poster presentation; May
02, 2010.
3. Eisenberg DL, Sherman BG, McKeown CA, Schuman JS. Tonometery in adults and children: a manometric evaluation of
pneumatonometry, applanation and TonoPen in vitro and in vivo. Ophthalmology: vol 105; 1173-1181.
4. Erlich JR, Haseltine S, Shimmyo M, Radcliffe NM. Evaluation of agreements between intraocular pressure measurements using GAT
and Goldmann correlated intraocular pressure by Reichert’s ocular response analyzer. Eye: 2010; 1555-1560.
5. Shen CC, Downs J, Mansberger SL. Assessment of intraocular pressure along the limbus and sclera using contact and indentation
tonometry. ARVO poster presentation; May 02, 2010.
IOP
IOP
• The difference between scleral PT and GAT did not
correlate with SE, CCT or AL individually
• Scleral PT may be a useful method of IOP
measurement when SE, AL and CCT are taken into
consideration concomitantly
• Further studies are needed with a larger number of
participants in order to create an equation which
may be able to provide IOP as a function of scleral
PT measurements
Acknowledgement: The Richard A. Perritt Charitable Foundation.