Original Article

Intraocular Pressure Values using Icare® Rebound
Tonometer and Correlation with Postconceptional Age

in Premature Infants
Catarina Monteiro, MD; Maria Vivas, MD; Júlio Almeida, MD; Mário Ramalho, MD, FEBO; Mafalda Mota, MD

Susana Teixeira, MD; Isabel Prieto, MD

Hospital Professor Doutor Fernando da Fonseca, Amadora, Portugal

ORCID:
Catarina Mimoso Monteiro: https://orcid.org/0000-0001-6057-3546

Abstract

Purpose: This study aimed to determine a normative range of intraocular pressure (IOP) values
measured with Icare rebound tonometer in premature infants and evaluate IOP variation over
time and its correlation with the progression of postconceptional age (PCA). By doing so, we also
evaluated advantages of this IOP-measuring method in this population when compared to more
traditional methods.
Methods: We conducted a single-center prospective study that included premature infants
(gestational age ≤32 weeks) who were admitted to the neonatal intensive care unit (NICU)
in Hospital Professor Doutor Fernando Fonseca. The study took place between January and
December 2021. IOP was measured using Icare tonometer on the occasion of the first retinopathy
of prematurity (ROP) screening requested by the NICU and again after a two-week interval if PCA
was still ≤37 weeks. IOP measurements were stopped at 37 weeks or if the infant was discharged.
The evaluated outcomes were mean IOP values and their correlation with PCA.
Results: Thirty-four eyes of 17 preterm infants with a mean gestational age of 29.4 ± 2.3 weeks
and a mean birth weight of 1222.9 ± 361.9 gr were evaluated. The mean IOP registered was 16.1
± 6.4 mmHg, with a median value of 15.3 mmHg. The top 90𝑡ℎ percentile was 22.1 mmHg and
the bottom 10𝑡ℎ percentile was 9.0 mmHg. The average IOP reduction was 4.8 ± 6.7 mmHg (P =
0.0019) within the two-week interval of PCA.
Conclusion: The mean IOP in premature infants was 16.1 ± 6.4 mmHg and this value significantly
decreased by 4.8 ± 6.7 mmHg every two weeks of PCA.

Keywords: Intraocular Pressure; Postconceptional Age; Premature Infants; Rebound Tonometry

J Ophthalmic Vis Res 2023; 18 (3): 267–271

Correspondence to:

Catarina Mimoso Monteiro, MD. IC19 276, 2720-276
Amadora, Portugal.
Email: catarinammonteiro@campus.ul.pt
Received: 09-07-2022 Accepted: 04-01-2023

Access this article online

Website: https://knepublishing.com/index.php/JOVR

DOI: 10.18502/jovr.v18i3.13774

This is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.

How to cite this article: Monteiro C, Vivas M, Almeida J, Ramalho
M, Mota M, Teixeira S, Prieto I. Intraocular Pressure Values using
Icare® Rebound Tonometer and Correlation with Postconceptional
Age in Premature Infants. J Ophthalmic Vis Res 2023;18:267–271.

© 2023 Monteiro et al. THIS IS AN OPEN ACCESS ARTICLE DISTRIBUTED UNDER THE CREATIVE COMMONS ATTRIBUTION LICENSE | PUBLISHED BY KNOWLEDGE E 267

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IOP Values in Premature Infants; Monteiro et al

INTRODUCTION

In order to detect developmental abnormalities
and congenital diseases in the eye, it is crucial to
have a good understanding of normal intraocular
pressure (IOP) values and their changes over time
in premature infants.[1] The characteristics of the
anterior chamber angle in the premature infant
differ from that in adults, and parameters such as
anterior chamber depth, trabecular-iris angle and
iris thickness seem to show a linear relation with
postconceptional age (PCA).[2] To this date and
to the best of our knowledge, very few studies
have reported IOP in preterm infants, and with
the advent of new handheld tonometers such
as the iCare, measuring IOP could become part
of the routine evaluation of these children. The
purpose of this prospective longitudinal study is
to determine average IOP values for premature
infants and to evaluate their progression over time.

METHODS

This single-center prospective study included
preterm infants with either gestational ages
between 24 and 32 weeks or a birth weight
of <1500 gr, who were evaluated by the
ophthalmology department as part of the
routine retinopathy of prematurity (ROP) screening
program. Children with chromosomal abnormalities
and dysmorphic syndromes, congenital ocular
abnormalities and children under invasive
ventilation were excluded from the study. We
included some eyes with early stages of ROP
(stages I and II) but excluded all eyes with ROP in
more advanced stages and those eyes submitted
to laser therapy or anti-VEGF treatment. The
study was conducted in the neonatal intensive
care unit (NICU) of our hospital and all infants
were examined while still hospitalized. PCA was
defined as the gestational age plus the number of
weeks since birth. Gestational age of the infants
was determined based on obstetric history and
first-trimester obstetric ultrasound.

Two experienced ophthalmologists specialized
in pediatric ophthalmology performed the ocular
examination in all studied infants. If the infant
was irritable or crying, IOP measurement was
postponed until the child was calm to avoid a
Valsalva-like effect. No sedative drugs or muscle
relaxants were given to the infants immediately
before or during the examination, as these may

have artificially lowered the IOP values. The
evaluation was only performed if the child was
clinically stable. The infant was laid on his/her back
in the incubator and IOP was measured with an
iCare® IC200 rebound tonometer, without the use
of topical anesthesia. The right eye was the first
to be examined in all cases and measurements
were obtained between 9 and 12 am. Three
IOP measurements were taken for each eye and
average IOP was calculated. IOP measurements
were repeated every 2 weeks and stopped after 37
weeks of PCA or if the infant was discharged.

The study protocol was reviewed and approved
by the Ethics Committee of the Hospital Professor
Doutor Fernando da Fonseca, Amadora, Portugal
under approval number 39/2022. The protocol
of the study complied with the guidelines for
human studies and the World Medical Association
Declaration of Helsinki.

RESULTS

This study included 34 eyes from 17 premature
infants, 26 males and 8 females, with a mean
gestational age of 29.4 ± 2.3 weeks (24.9–32.7
weeks) and a mean birth weight of 1222.9 ±
361.9 gr. As there was no statistically significant
difference between right and left eyes (P = 0.8), the
results were analyzed together. The mean PCA was
33.9 ± 1.6 weeks with a range of 32 to 41 weeks.
Twelve patients underwent two evaluations and ten
were only submitted to one observation. Of the 34
included eyes, 6 had ROP stage I or stage II; none
of these eyes were submitted to laser or anti-VEGF
therapy.

At the end of the experiment, it was
determined that the mean IOP in both eyes
for all measurements was 16.1 ± 6.4 mmHg. The
mean IOP in the right eye was 15.9 ± 5.7 mmHg
(7.7 to 34.0 mmHg) and the mean IOP in the left eye
was 16.4 ± 7.1 mmHg (7 to 37.7 mmHg). The median
was 15.3 mmHg. The top 90𝑡ℎ percentile (P90) for
both eyes was 22.1 mmHg and the bottom 10𝑡ℎ
percentile (P10) was 9.0 mmHg. As represented in
the box and whisker plot [Figure 1], the interquartile
range (P25–P75) ranged between 12.0 and 19.6
mmHg.

Out of the 34 examined eyes, 24 had at least
two IOP measurements, separated by two weeks.
In this subset, the mean IOP showed an average

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IOP Values in Premature Infants; Monteiro et al

Figure 1. Box and whisker plot of IOP values. The box extends from the lower to the upper quartile values of the data (P25–P75)
with a line at the median. The whiskers extend from the box to show the range of the data. Beyond the whiskers, data is considered
as outliers and is plotted as individual points.

reduction of 4.8 ± 6.7 mmHg (P = 0.0019) every two
weeks of increased PCA, as represented in Figure
2.

DISCUSSION

In 1999, Ricci et al[3] evaluated the IOP of 20
preterm infants and concluded that the mean IOP
following birth decreased progressively. As many
other studies have shown since then, our study also
reports that the IOP of preterm infants decreases
significantly with the increase of PCA. Normative
values are not established as most studies use
different tonometers and inclusion criteria. Ng et
al[4] reports that of all recent studies regarding IOP
in premature babies, the mean/median IOP varies
between 10.1 and 18.6 mmHg. In our sample, the
average IOP ranged between 12 and 19.6 mmHg
(P25–P75) with a median value of 15.3 mmHg.
These values are in agreement with McKibbin
et al[5] and Axer-Siegel et al[6] whose analyses
obtained mean IOP values ranging between 15.5
and 16.3 mmHg.

Regarding IOP decrease with PCA, Ng et al[4]
reported an IOP reduction of 0.11 mmHg (P < 0.001)
for each week increase in the PCA, however, it
included evaluations of children with more than
40 weeks of age. Lindenmeyer et al[7] reported a
reduction of 0.29 mmHg (P = 0.047) per PCA week.
In the current study, IOP reduction was significantly

larger, 4.8 ± 6.7 mmHg (P = 0.0019) with every two
weeks increase in PCA. First of all, this may be
explained by the fact that we included extremely
preterm infants with very low gestational ages who
were not included in other studies. Secondly, the
IOP measuring method used was also different as
Tonopen II was used. Thirdly, artificial elevation
of IOP may occur in infants who are awake and
react with vigorous resistance to the ophthalmic
examination.[4]

Although the theory of IOP decrease with
increasing PCA seems to have a relative
consensus as shown in recent studies, the exact
physiopathological mechanism behind this fact
is still not well established. Physiopathological
mechanisms that may explain this consistent IOP
decrease with increasing PCA include increasing
size of ocular structures, anterior chamber angle
maturation and progressive improvement of the
aqueous drainage system and neuroendocrine
changes. The exact mechanism is yet to be
discovered.

It is well known that infants with ROP may have
increased IOP values due to several treatment
options such as laser and anti-VEGF therapy
and changes in advanced ROP stages that may
lead to angle closure. There are other proposed
physiopathological mechanisms that may cause
increases in IOP measurements which are not
unique to ROP but are related to prematurity in

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IOP Values in Premature Infants; Monteiro et al

 

 

 

A 

 

 

 

 

A

Figure 2. (A) The figure shows a graphic representation of IOP values variation with PCA (in days) for each of the 24 preterm
infants evaluated. (B) The graph shows an average IOP decrease of 0.47 mmHg per day

itself such as incomplete trabecular meshwork
development, decreased anterior chamber depth,
anteriorly displaced iris planes, and increased lens
thickness.[9] In our study, we only included a small
number of children with early stages of ROP (stage I
or II). As none of them were submitted to laser or to
anti-VEGF therapy that could alter IOP values, it is
highly unlikely that early ROP changes would lead
to significant IOP changes in these eyes and we
chose to include them in the study.

Most studies regarding IOP values in premature
infants included the use of an eyelid speculum and
anesthetic drops for the IOP measurements which
were obtained with the use of TonoPen XL𝑇𝑀.

We chose to use iCare IC200 for measuring IOP
in preterm infants. This method has the advantage
of not needing anesthetic drops or eyelid speculum
placement. There have been studies that reported
that the use of an eyelid speculum may raise IOP
by an average of 4 mmHg in children.[8]

Haus et al[10] compared the iCare IC200
rebound tonometer (ICT) with the TonoPen XL
(TP) measurements in premature infants and
concluded that IOP values evaluated by the
ICT were significantly lower than that of the
TP. Tonopen use requires anesthetic eye drop
instillation and a large applanation area which
implicates an increased width of the eyelid
opening. This may cause discomfort to the child
leading to falsely elevated IOP values. Therefore,
the iCare rebound tonometer might be a better
option to measure IOP in premature children. Its
main advantages include the extensive flexibility in
positioning with the possibility of the patient being
placed in a supine position, no need to anesthetic
drops and little or no discomfort for the child.

As far as we are aware, this is the first
prospective longitudinal study using iCare IC200

tonometer in preterm infants. This IOP-measuring
method is fast, accurate, and reliably repeatable,
and thus it should be considered a first-line option
for the IOP evaluation in premature children.

Our study had certain inherent limitations. On
one hand, eyelid edema after birth made some
of the readings hard and probably less reliable
as the infant had difficulty opening one or both
eyes, and manual opening of the eyelid might have
resulted in inadvertent pressure on the scleral-
corneal interface that might have influenced IOP
measurements. On the other hand, the sample
size of this study was relatively small, which
may limit the statistical strength of the findings.
We are also of the view that the study would
have been strengthened if the measurement of
corneal thickness and its correlation with IOP
values were considered. Therefore, further studies
with prospective, controlled design, and a larger
number of included patients are needed to confirm
our findings.

Financial Support and Sponsorship

This research did not receive any specific grant
from funding agencies in the public, commercial, or
not-for-profit sectors. The authors also declare they
have no relevant financial or non-financial interests
to disclose.

Conflicts of Interest

There are no competing interests.

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8. Epley KD, Tychsen L, Lueder GT. The effect of an
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