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16 Ocular Surface Examination
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199
out anesthesia and with open eyes), Schirmer II
(after nasal stimulation), and Schirmer III (reexstimulated secretion by looking into the sun)
[11]. To avoid confusion, dening the Schirmer
test with or without anesthesia is better than using
I or II [21]. Schirmer test values <10mm indicate
dry eye disease; however, values <5mm are pathological, and between 5 and 10 mm should be
interpreted carefully [22].
For measuring TMH, slit-lamp-based meniscometry and non-invasive optical coherence
tomography (OCT) can be used [20]. Due to its
non-invasive nature and less chances of reex
tearing, OCT-based TMH is more repeatable and
reproducible than the Schirmer test. Time- and
spectral-domain OCT can be used for calculating
TMH, but they are not interchangeable. The
reported normal mean TMH values are 0.29mm
(SD = 0.13 mm, slit lamp), 0.27–0.29 mm
(SD = 0.05–0.12 mm, keratograph), and 0.19–
0.34mm (SD=0.02–0.15mm, SD-OCT) [23].
16.5 Conclusion
The ocular surface examination should be comprehensive and inclusive of all components of the
surface, i.e., cornea, conjunctiva, eyelids, and
eyelashes. Fluorescein stain for ocular surface
evaluation is crucial and should be performed for
every case. Diagnostic devices like ASOCT, ocular surface analyzer, and keratograph for noninvasive tear lm assessment are adjunctive tools
that help in diagnosis and follow-up evaluations.
Funding Hyderabad Eye Research Foundation,
Hyderabad, India.
Disclosure None.
References
1. Kenyon KR.Anatomy and pathology of the ocular
surface. Int Ophthalmol Clin. 1979;19(2):3–35.
2. Cher I. Ocular surface concepts: development and
citation. Ocul Surf. 2014;12:10–3.
3. Srinivas SP, Rao SK. Ocular surface staining: current concepts and techniques. Indian J Ophthalmol.
2023;71(4):1080–9.
4. Han SB, Liu YC, Noriega KM, Mehta JS.Applications
of anterior segment optical coherence tomography in
cornea and ocular surface diseases. J Ophthalmol.
2016;2016:4971572.
5. Kate A, Basu S.A review of the diagnosis and treatment of limbal stem cell deciency. Front Med
(Lausanne). 2022;9:836009.
6. Vazirani J, Donthineni PR, Goel S, etal. Chronic cicatrizing conjunctivitis: a review of the differential diagnosis and an algorithmic approach to management.
Indian J Ophthalmol. 2020;68(11):2349–55.
7. Knop E, Knop N, Zhivov A, etal. The lid wiper and
muco-cutaneous junction anatomy of the human
eyelid margins: an invivo confocal and histological
study. J Anat. 2011;218(4):449–61.
8. Shanbhag SS, Singh S, Koshy PG, etal. A beginner’s
guide to mucous membrane grafting for lid margin
keratinization: review of indications, surgical technique and clinical outcomes. Indian J Ophthalmol.
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9. Singh S. Distichiasis: an update on etiology,
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10. Adewara B, Singh S.Ocular adnexa and antiglaucoma
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and meibomian gland evaluation in dry eye disease: a
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13. Singh S, Shanbhag SS, Basu S. Tear secretion
from the lacrimal gland: variations in normal versus dry eyes. Br J Ophthalmol. 2022;106(6):
772–6.
14. Singh S, Vemuganti GK, Basu S. Morphological
variants of meibomian glands: correlation of meibography features with histopathology ndings. Br
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org/10.1136/bjophthalmol- 2021- 318876.
15. Swiderska K, Read ML, Blackie CA, et al. Latest
developments in meibography: a review. Ocul Surf.
2022;25:119–28.
16. Finis D, Pischel N, Schrader S, Geerling G.Evaluation
of lipid layer thickness measurement of the tear lm
as a diagnostic tool for Meibomian gland dysfunction.
Cornea. 2013;32:1549–53.
17. Singh S, Srivastav S, Modiwala Z, etal. Repeatability,
reproducibility and agreement between three different
diagnostic imaging platforms for tear lm evaluation
of normal and dry eye disease. Eye. 2023;37:2042–7.
18. Cox SM, Nichols KK, Nichols JJ.Agreement between
automated and traditional measures of tear lm break up. Optom Vis Sci. 2015;92(9):257–e263.
19. Singh S, Srivastav S, Mohamed A, Basu S. Noninvasive tear lm assessment in normal population:
effect of age, sex, and interparametric relationship.
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20. Niedernolte B, Trunk L, Wolffsohn JS, et al.
Evaluation of tear meniscus height using different
clinical methods. Clin Exp Optom. 2021;104(5):
583–8.
21. Serin D, Karsloğlu S, Kyan A, Alagöz G.A simple
approach to the repeatability of the Schirmer test
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22. Singh S, Basu S.Effect of topical anesthesia on the
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doi.org/10.4103/IJO.IJO_2808_22.
Intraocular Pressure
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RamyashriS , AparnaRao , andSardarM.Khan
17
17.1 Introduction
Intraocular pressure (IOP) measurement is
essential for diagnosing and managing glaucoma. The Goldmann applanation tonometer
(GAT) is the current gold standard for measuring IOP, although several newer technologies
are currently available for IOP measurement
[1–4]. Some new IOP measurement technologies and devices include dynamic contour
tonometry (DCT), ocular response analyzer
(ORA), Pascal dynamic contour tonometer
(DCT-Pascal), rebound tonometry, Tono-Pen
Avia, and the Bioresonator applanation resonance tonometer (ART). This chapter will
describe the GAT and the newer devices used
for IOP measurement.
R. S
Kallam Anji Reddy Campus, L V Prasad Eye
Institute, Hyderabad, India
Drishti Eye Centre, Hyderabad, India
A. Rao (*)
Kallam Anji Reddy Campus, L V Prasad Eye
Institute, Hyderabad, India
L V Prasad Eye Institute, Bhubaneshwar, India
e-mail: aparna@lvpei.org
S. M. Khan
Gullapalli Pratibha Rao International Centre for
Advancement of Rural Eye Care, L V Prasad Eye
Institute, Siddipet, India
17.2 Goldmann Applanation
Tonometry (GAT)
The technique is named after its inventor, Hans
Goldmann, who discovered it in 1948 [5].
Technology—By applying a known force to
the cornea and measuring the area of applanation
(the attened area), the Goldmann applanation
tonometer (GAT) calculates the IOP based on
Imbert Fick’s law. The device uses a truncated
cone with a surface area of 7.35mm2 and a diameter of 3.06mm mounted on a slit lamp. It is lit
with a blue lter, and the doubling prism embedded in the cone divides the circular tear meniscus
into two equal semicircular halves, which are
aligned at their inner edges (Fig.17.1) to obtain
precise applanation of the cornea. The force
required to atten the corresponding surface of
the cornea is directly proportional to the IOP in
mmHg from the scale of the measuring drum [6].
Technique—The patient is made to sit with
his/her forehead rmly placed against the headband and the chin comfortably placed on the
chinrest of the device. After adequate anesthesia,
the uorescein dye is added to each eye. The
patient’s lateral canthus should align at the mark
on the headrest column, and the patient is
instructed to look straight ahead with eyes wide
open. The clean tip of the tonometer biprism with
the cobalt blue lter is moved toward the eye to
be examined; the lter helps visualize the bright
yellow- green uorescence of the dye. The tip of
the biprism is illuminated at a wide angle, and the
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_17
201
202
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a
c
b
Fig. 17.1 (a) Slit-lamp mounted Goldmann applanation tonometer; (b) the measuring dial; (c) the biprism illuminated
with a blue lter; (d) the mires of the biprism; and (e) the Perkins tonometer
instrument is set at low magnication and highest
illumination. The examiner should look monocularly through the oculus to identify the spot of
uorescein on the cornea that has just been applanated (converted into two semicircular halves).
The height of the slit lamp is adjusted such that
the two halves of the applanated cornea are equal.
The force adjustment knob or measuring dial is
moved in the appropriate direction such that the
inner edges of the two semicircles touch each
other but do not overlap. The number on the measuring dial multiplied by 10 gives the IOP in
mmHg (Fig.17.1).
Clinical Application—Goldmann applanation tonometry provides accurate and reproducible IOP measurements; nevertheless, this
depends on the physician’s (user’s) experience.
Interobserver variability of ±2 mmHg may be
observed [7]. The measurement of IOP using
malities such as scarring or other irregularities
can result in inaccurate IOP measurements. The
technique may also underestimate IOP in thin
corneas and overestimate IOP in thick corneas
[7, 8]. Other drawbacks of using GAT include
interference in IOP measurement due to tear
lms with too little or too much uorescein dye,
quenching of the dye, and other calibration
errors [9].
Recent advancements in technology and
design have improved the accuracy and reproducibility of GAT.Newer models use a digital display to provide more precise IOP measurements,
and some integrate with electronic medical
records for easier tracking and analysis of collected data. A portable version of GAT is the
Perkins tonometer, which works on the same
principle as the Goldmann applanation tonometer
but is useful for people lying supine.
GAT, however, is not without limitations. Since
the technique requires the device to contact the
cornea, it is a potential source of corneal abra-
17.3 Dynamic Contour Tonometry
sion and infection. The device is slit-lamp
mounted and requires that the patient is in an
upright position. Additionally, the IOP value
obtained by GAT can be affected by corneal
parameters like corneal thickness, curvature, and
Dynamic contour tonometry (DCT) is a lowcontact method of measuring IOP.The technique
uses a small probe gently placed near the eye to
measure the eye pressure.
other biomechanical properties [8]. Any abnor-
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Technology—DCT uses a small, sensorequipped probe that lightly touches the surface of
the eye and records the changes in the pressure as
the eye is deformed. The sensor of the probe has
a exible and curved surface that conforms to the
shape of the cornea, allowing it to measure the
pressure accurately [10, 11].
Clinical Application—DCT is more accurate
than applanation tonometry, especially in patients
with corneal abnormalities, such as scarring or
irregular curvature. In addition, DCT is easy to use
and requires no special skills or training. However,
DCT is affected by eye movements and can result in
inaccurate readings, and the devices used are associated with high costs. The comparison of DCT and
GAT, and other tonometers is shown in Table17.1.
New developments and versions of DCT technology [12] include:
• Corvis ST: This is a new generation DCT
device developed by Oculus (Switzerland).
The Corvis ST measures corneal deformation
responses and IOP using a high-speed
Scheimpug camera that captures images of
the cornea when a puff of air deforms it. It also
provides biomechanical information on the
cornea, such as corneal hysteresis.
• DCT Pro: This is an upgraded version of the
Pascal DCT tonometer developed by Zeimer
(Switzerland). The DCT Pro uses a smaller
and more ergonomic probe, which improves
patient comfort and ease of use for the operator. It also features enhanced measurement
stability, thus improving the accuracy and
reproducibility of IOP measurements.
• Bidirectional DCT: This is a new DCT technology developed by Nidek (Japan). The bidirectional DCT uses a two-way air jet to
measure IOP, improving accuracy and reducing corneal biomechanics' effect on IOP
measurements.
• DCT with Ocular Pulse Amplitude (OPA):
This is a new feature added to some DCT
devices, such as the Reichert ORA.The OPA
measures the pulsatile component of IOP,
which reects changes in blood ow and ocular perfusion [13, 14]. Adding OPA to DCT
measurements provides additional information about ocular health and can help diagnose
and manage glaucoma and hypertension.
Despite the advantages of other IOP measure-
ment systems, GAT is more widely used and
remains the standard of care.
Table 17.1 Comparisons of different tonometers
Schiotz tonometer Working principle Indentation
Goldmann
Applanation
tonometer
Perkins
tonometer
Relation with cornea Contact
Corneal biomechanics Not considered, but affects the measurements
Clinical application Useful in low-resource countries
IOP in post-Kpro eyes with scleral indentation
Availability/cost Available easily
Inexpensive
Working principle Applanation
Relation with cornea Contact
Corneal biomechanics Affected by corneal biomechanics and corneal thickness
Clinical application In all cases, but not accurate when corneal scarring is present
Availability/cost Easily available and routinely used by all
Current gold standard
Working principle Indentation
Relation with cornea Contact
Corneal biomechanics Same as GAT
Clinical application Can be used in supine position
Availability/cost Can be used as a replacement for GAT
(continued)
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Table 17.1 (continued)
Tono-pen Working principle Indentation/applanation
Relation with cornea Contact
Corneal biomechanics Same as GAT
Clinical application Underestimates IOP value
Availability/cost Moderately available
DCT Working principle Contour matching
Relation with cornea Contact
Corneal biomechanics Independent of corneal properties
Clinical application Precise but difcult to use
Availability/cost Not available easily
ORA Working principle Applanation
Relation with cornea Non-contact
Corneal biomechanics Provides information on corneal biomechanics and IOP
Clinical application Useful in post-refractive surgery; self-calibration possible
Corvis ST Availability/cost Expensive
Working principle Indentation/applanation
Relation with cornea Non-contact
Corneal biomechanics Provides information on corneal biomechanics and IOP
Clinical application Useful in post-refractive surgery cases and glaucoma cases
Availability/cost Expensive
iCare Rebound
tonometer
Bioresonator ART Working principle Applanation, based on the acoustic impedance
Sensimed
Triggersh
Working principle Rebound principle using a ballistic probe
Relation with cornea Contact
Corneal biomechanics Inuenced by corneal properties
Clinical application Can be used for screening purposes and in GA cases
Availability/cost Expensive but feasible to use
Relation with cornea Contact
Corneal biomechanics Affected by corneal properties
Clinical application No need for uorescein, self-calibrated, provides a quality
Availability/cost Expensive
Working principle Scleral stretch monitoring
Relation with cornea Contact
Corneal biomechanics Affected by corneal biomechanics
Clinical application Used for continuous 24-h IOP monitoring
Availability/cost Expensive
Highly variable and poor repeatability
Expensive
Needs high patient cooperation
Expensive
corrected for corneal biomechanical property
Provides a quality index, but there is limited evidence for
application
corrected for corneal biomechanical property
Tends to underestimate the GAT values
index
Has a learning curve to measuring IOP
Does not provide direct IOP (only an estimate)
R. S et al.
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17.4 Ocular Response Analyzer
(ORA)
Technology—ORA measures the biomechanical
properties of the cornea. It rapidly delivers a puff
of air to the cornea that deforms/indents the corneal surface [15–18]. The ORA measures two
key parameters of the cornea’s response to this
puff: (a) corneal hysteresis (CH) and (b) corneal
resistance factor (CRF).
The CH measures the cornea's ability to
absorb and dissipate energy during deformation.
It reects the cornea's elasticity or “bounceback,” and a low CH may indicate a stiffer cornea
associated with a higher risk of developing glaucoma [16]. The CRF is a measure of the overall
resistance of the cornea to deformation. A higher
CRF value suggests the cornea’s ability to resist
deformation, which may be protective against
glaucoma.
Clinical Application—ORA measurements
help identify individuals at increased risk of
developing glaucoma, even if their IOP is within
the normal range; this can aid in early and more
individualized interventions.
Over the years, the design and technology of
ORAs have evolved. The most recent version, the
ORA G3 (2016), features a waveform analysis
that provides more detailed information on the
corneal response to the puff of air. The ORA G3
also includes a more user-friendly interface and a
redesigned, easy-to-use probe that is less prone to
operator error. Other improvements in the ORA
G3 include individualized measurement protocols and the ability to integrate with optical
coherence tomography (OCT). The proposed
developments in ORA technology that may
improve its accuracy and reliability and expand
its diagnostic capabilities include corneal biomechanical imaging [16, 18], articial intelligence
integration [19–22], and improved waveform
analysis.
The ORA and Goldmann applanation tonometer differ in their approaches to measuring IOP
and the information they provide. In certain conditions, ORA may be more accurate and reliable,
but GAT remains the gold standard for measuring
IOP in most patients.
17.5 Telemetric 24-h IOP
Monitoring
Technology—Telemetric 24-h screening of IOP
is a non-invasive method for measuring IOP over
an extended period, typically 24h [23–27]. This
method uses a tiny wireless sensor implanted
inside the eye to transmit data to an external
receiver/recorder. The data is then analyzed to
obtain a comprehensive prole of the uctuations
in the patient's IOP over 24h.
Currently, there are several modes/types of
telemetric 24-h screening of IOP for monitoring
glaucoma. These include (a) continuous recording mode: the sensor records the IOP continuously over 24h, (b) pulsatile mode: the sensor
records the IOP at intervals during the cardiac
cycle, allowing for the detection of IOP uctuations related to changes in blood pressure, (c)
triggered mode: the sensor is triggered by a specic event or activity, such as physical activity or
changes in posture, to record the IOP during that
specic period, (d) hybrid mode: a combination
of two or more of the above modes to provide a
more comprehensive prole of the patient’s IOP
uctuations over the 24h. The choice of a specic
telemetric mode depends on the specic research
question or clinical need.
Telemetric 24-h screening of IOP in glaucoma
is a growing area of research and clinical practice, with potential applications in the early
detection and management of glaucoma and
other eye diseases. Several investigators are currently working on the development and commercialization of such devices. Here are some
examples:
1. Eyemate-SC (Implantable Ophthalmic
Products, Germany): The system consists of
a tiny pressure sensor implanted inside the
eye and a wireless handheld device reading
the sensor's data. The pressure sensor is
implanted in a minimally invasive surgical
procedure under local anesthesia. The device
is powered by a small battery that can last up
to 5years, after which the device can be easily replaced. It is used with the EyeMate
Reader, a handheld device that wirelessly
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reads the data from the pressure sensor. The
data from the Eyemate- SC system is transmitted to a secure cloud- based platform
where physicians and other healthcare professionals can analyze it. The Eyemate-SC
system has received regulatory approval in
Europe and is currently undergoing clinical
trials in the USA [28].
2. Triggersh (Sensimed AG, Switzerland)
[26]: This device uses “continuous monitoring of ocular dimensional changes” (CMOD)
technology, which measures changes in the
diameter of the eye over 24h. Changes in the
diameter of the eye are directly related to
changes in IOP. During the monitoring
period, the patient wears a contact lens with
the sensor, and the data is recorded by a portable recorder that can be worn on a belt or
carried in a pocket. The data is then transferred to a computer for analysis. The
Triggersh device has been approved by the
US Food and Drug Administration (FDA)
and is available in the USA and several other
countries.
3. EyeSeal (Bausch + Lomb, USA): The
EyeSEAL is a small, disposable patch worn
on the skin around the eye. The patch contains
a tiny wireless sensor capable of measuring
changes in the shape of the eye due to changes
in IOP. The sensor wirelessly transmits the
data to a small portable device worn by the
patient and can store up to 24h of data. The
device is currently undergoing clinical trials
and is not yet commercially available.
4. IOP Watcher (NovaSight, Israel): The system consists of a small wireless sensor
placed on the skin near the eye and a mobile
app that wirelessly reads the data from the
sensor. It provides a comprehensive view of
a patient's IOP over 24h. It can also help
identify the peak and trough times in IOP
values, which can be used to determine the
most effective time for administering glaucoma medications. The IOP Watcher system
has been approved for use in Europe and is
currently undergoing clinical trials in the
USA.
17.5.1 Clinical Application
ofTelemetric IOP Monitoring
While 24-h monitoring of IOP can provide valuable information for diagnosing and managing
glaucoma, this technology has safety concerns
and pitfalls. These include an increased risk of
infection, damage to the cornea, false positive or
negative results [22], high cost, and the possibility of poor patient compliance. Currently, there is
also limited evidence of the utility of this form of
monitoring. The clinical application of 24-h
monitoring in glaucoma practice may take time
to evolve and will need radical ways of dealing
with the enormous amount of data from such
measurements. Novel AI-based algorithms may
help incorporate the data into clinical treatment
decisions in glaucoma practice in the future.
17.6 Rebound Tonometer
Technology—A rebound tonometer is a portable
handheld tonometer that uses a disposable probe
propelled from the tonometer toward the cornea
by an electrical-pulse generator to create a magnetic eld via a solenoid. The speed with which
the probe returns to the device changes the
magnetic eld, and the tonometer uses this value
to calculate the IOP [29, 30]. In patients with
higher IOPs, the probe touches the cornea for a
shorter time and therefore rebounds into the
tonometer faster.
Clinical Application—This tonometer does
not require topical anesthesia, making it useful
for IOP measurements in less cooperative patients
and children [31]. It is also used for IOP monitoring at home for patients who cannot follow-up
regularly but require frequent IOP assessment
[2]. The other advantages of this tonometer are
lightweight and portable, easy to use, there is no
need for staining with uorescein, and there is a
lower risk of eye infections since it uses a disposable probe [31]. In addition, this tonometer can
also measure IOP in people with corneal abnormalities [32]. However, the disadvantages of this
device are its high cost and that the IOP measure-
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ments made with the device may vary with variations in eyelid position.
17.7 Corvis ST
Technology—The Corvis ST combines tonometry with corneal imaging to provide a more comprehensive assessment of the eye. It uses a
high-speed Scheimpug camera to capture
images of the cornea as it responds to a puff of
air. The camera captures over 4300 images per
second, allowing for a detailed analysis of the
corneal responses. In this device, an all-blue
UV-free light (of wavelength 455nm) covers an
8.5mm horizontal slit that evaluates the dynamic
corneal deformation resulting in 140 images captured during the 30 ms after a puff of air is
directed at the cornea. The bidirectional corneal
deformation by the air puff is recorded during the
entire process [33]. When a puff of air is directed
at the cornea, it causes the cornea to applanate
(A1 or rst applanation) and then deform (to the
point of highest concavity or HC) and then
recover with second applanation (A2 or second
applanation) before it returns to the primary position. With the help of algorithms, this device
identies the anterior and posterior limits of the
deformation, and the IOP is measured at A1.
Clinical Application—The Corvis ST provides
parameters related to the shape of the cornea, IOP,
and depth, like deformation amplitude (DA), timing, and velocity [34, 35]. The advantages of this
device are its ease of use (specially trained personnel not required), it is non-contact (reduces the risk
of eye infection and contamination), has diverse
applicability (can be used for ectatic corneas like
those in patients with keratoconus or glaucoma),
and is accurate (not affected by the central corneal
thickness (CCT)). The disadvantages of the device
include its high cost and currently limited evidence
of precise IOP measurement.
17.8 Tono-Pen
Technology—The Tono-pen is a handheld device
that works on the principles of applanation and
indentation. The Tono-pen XL and the more
recent Tono-pen Avia (Reichert Ophthalmic
Instruments, Depew, NY, USA) are portable
battery- operated devices. The measured IOP is
displayed on the digital screen with a standard
deviation of an average of 10 consecutive
readings.
Clinical application—The Tono-pen is commonly used in clinical practice because it is a
portable and easy-to-use device that can quickly
and accurately measure IOP.However, the variability in intra-session measurements is quite
high (±4.3mmHg) [36]. It is particularly useful
when traditional GAT is difcult or impossible
to use, such as in patients with corneal abnormalities or those who cannot sit upright.
However, CCT is known to impact the IOP measurements by the Tono-pen [37]. The advantages
of the Tono-pen include its portability, ease of
use, quick and accurate measurements, and
reduced risk of corneal infection since the
device uses a disposable latex cap. The disadvantages of the device include its high cost, the
need for calibration before every use, and regular maintenance requirements. Studies have also
shown that the Tono-pen consistently underestimates IOP in high ranges (at IOP values
>30mmHg) [38]. Recent advances in Tono-pen
technology have focused on improving its accuracy, reliability, and ease of use. For example,
newer models may include automatic calibration, data storage, and improved algorithms for
calculating IOP. Additionally, some devices
may incorporate additional diagnostic tools,
such as pachymetry and/or imaging.
17.9 Schiotz Tonometry in Today’s
Practice
Introduced by the Norwegian ophthalmologist
Professor Magne Schiotz in 1905 [39], Schoitz
tonometry has since been widely used in ophthalmic practice. Technology—The footplate of the
tonometer is placed on the anaesthetized cornea
with known standard weights via a plunger that
indents the cornea. The indentation distorts the
globe with aqueous displacement inside the anterior chamber. The movement of the plunger from
its “0” position is correlated to the deformation of
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Scale
Needle
Weights
Lever arm
Plunger
Holder
Barrel
Foot plate
Fig. 17.2 Schiotz tonometer. Left panel: the device; middle panel: the nomogram to translate the scale reading to
intraocular pressure in mmHg; right panel: the tonometer
in use. (Source for left and middle panels: Understanding
the cornea and can be translated via a calibrated
scale read with the help of the lever that indicates
it. Friedewald’s formula relates to it and needs a
constant “K,” which is the ocular rigidity coefcient [39]. The IOP can be obtained from the
table, which calculates this value using the weight
and the scale reading (Fig.17.2).
Technique—After the cornea is adequately
anesthetized, the mid-temporal sclera (3 mm
from the temporal corneoscleral limbus) is
tested by asking the patient to look at the medial
side for good exposure; in other cases, the
extreme temporal gaze is used with a point midway between the medial canthus and medial
corneoscleral limbus. The readings are taken
with 5.5g weights. If the reading is less than 4,
the weight is increased to 7.5g, and the IOP is
measured again. The measured reading is converted to IOP using the nomogram. Figure17.2
(right panel) shows Schiotz tonometry performed on the Kpro eye at an extreme temporal
gaze while the nasal sclera is widely exposed
for tonometry.
Clinical applications—Senthil etal. (2019)
showed that the IOP values recorded using
and caring for a Schiotz tonometer. Comm Eye Health,
2014; 27(87): 57. Ismael Cordero. Copyright 2014, used
under the Creative Common Attribution 2.0 license)
scleral Schiotz tonometry had better agreement
with GAT [40]. The main advantages of this
method are that it is simple to use, affordable,
can be used in people in supine position, and
does not require other equipment like a slit
lamp. The disadvantages of this method are that
it is operator dependent, needs frequent calibration, and may cause corneal abrasion and infection. Due to these limitations, Schiotz tonometry
has been replaced by the current gold standard,
GAT, and the other newer tonometers due to
their greater accuracy and reproducibility.
However, there are certain situations where
GAT/newer tonometers are unreliable or cannot
be used, such as post- keratoprosthesis, postkeratoplasty, or in scarred irregular corneas
where the central cornea cannot be used for the
measurement of the IOP.
Modications and advances in Schiotz tonometry include (a) a digital scale to measure the
force required to indent the cornea, thus eliminating the need for the operator to read the scale
manually, and (b) the use of a disposable plunger,
which reduces the risk of transmitting infections
between patients.
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