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Contents :
• IOP Measurement convention & Population Means .
• Types of Tonometers.
• Applanation Tonometers.
• GOLGMANN TONOMETER :
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Parts of the instrument.
Optical principle.
Calibration of tonometer.
Performing Goldman applanation tonometer.
Factors that affect on the accuracy of IOP readings.
Disinfecting the applanation tip.
References.
• Tonometry: is the measurement of
intraocular pressure (IOP), it’s performed
as a part of a thorough ocular examination
to help detect ocular hypertension and
glaucoma and to diagnose ocular
hypotony (Low IOP) in condition such as
iritis, retinal detachement, postoperative
wound leaks & occult perforations of the
glob .
IOP Measurement conventions and
Population Means :
By convention, IOP is measured in millimeters of
mercury (mm Hg).
IOP, like many biologic parameters, varies in the
population as a whole. In large epidemiologic
studies, means IOP is 16 mm Hg, with standerd
deviation of 3 mm Hg. Variables such as the
time of day, age & genetic factors influence IOP.
Although there is no strict cutoff between normal
& abnormal IOP, most people have IOPs
between 10 and 21 mm Hg .
Types of tonometers :
Several types of ophthalmic instruments are used
to performe tonometery. The instrument can be
categorized into two groups based on the way
they determine IOP.
1- Applanation tonometers measure the force
needed to flatten or applanate a small area of
the centeral cornea. The greater the force
needed to flatten a known area of the cornea,
the higher IOP.
2- Indentation tonometers measure the amount of
the indentation of the cornea produced by a
known weight .
Applanation tonometer
Indentation tonometer
Applanation tonometers :
• Some of the most common types of applanation
tonometers and there charactaristics are listed
below :
* The Goldmann Tonometer is the most common
tonometer. Usually mounted on the standard slitlamp biomicroscope. It’s easy to use and
measure the IOP of a seated patient with high
accuracy in most clinical situations.
Measurements are less precise for edematous
and scarred corneas.
The Goldmann Tonometer
* The Perkins tonometer is a handheld, portable
applanation device. The technique for use,
mechanism of action, and relative accuracy are
similar to those of the slit-lamp mounted
Goldmann tonometer, and it can be used with
either a seated or supine patient. It’s portability
makes this device useful at the bedside or in the
opening room. Because it is not mounted to a
stable device.However, the steadiness of both
the patient and the examiner are harder to
control. Nevertheless, with some practice, the
perkins tonometer is a useful instrument.
The Perkins tonometer
* The Peneumatic tonometer is an electronic
pressure- sensing device that consist of a Gasfilled chamber covered with a Silastic
diaphragm. The gas in the chamber escapes
through an exhaust vent. As the diaphragm
touches the cornea, the gas vent is redused in
size and the pressure in the chamber rises. The
instrument supplies a measurement reading
directly in (mm Hg). The Pneumatic tonometer is
portable, can be used with a seated or supine
patient and is specially useful in the presence of
corneal scar or corneal odema.
The Peneumatic tonometer
* The Tonopen like many simlar portable
electronic applaning devices, contains a strain
gauge and produces an electrical signal as the
tip of the instrument applanates the cornea.
These devices use disposable ssterile latex
covers for the applanating tip, can be used with
a seated or supine patient and are useful in the
presence of corneal scars or edema. Some
studies have found that these instruments
underestimate IOP in the higher ranges.
The Tonopen
* The noncontact (Air-puff) tonometer
determines IOP by measuring the time
necessary for a given force of air to flatten a
given area of the cornea . Because the
instrument does not contact the patient’s cornea,
no anesthetic drops are needed. Reading
obtained with these instruments correlate well
with those obtained by Goldmann applanation
tonometery except at high and low extremes of
intraocular pressure.
The noncontact (Air-puff) tonometer
GOLDMANN TONOMETER
Parts of the instrument :
The Goldmann applanation tonometer consist of
four principle operative parts :
I. The Tonometer Tip: The part of the
instrument that contacts the patient’s cornea,
contains biprism (two beam-splitting prisms)
that converts a circular area of conact
between the tonometer tip and the patient’s
cornea into two semicircles. By properly
aligning the semicircles, the examiner can
determine the area of corneal applanation and
measure the intraocular preasure with great
accuracy.
II.
A metal rod : it connects the tonometer tip
to the instrument housing.
III. The tonometer housing : contains a
mechanism that can deliver a measured force
controlled by the force adjustment knob on
the housing to the tonometer tip.
IV. The force adjustment knob: on the
housing is used to vary the amount of force
needed to applanate the cornea. The scale
reading on the knob is multiplied by 10 to
express IOP in mm Hg.
Parts of the instrument :
Optical principle :
• The Goldmann Applanation tonometer
relies on an interesting physical principle :
“For an ideal, dry, thin-walled sphere, the
pressure inside a sphere is proportional to
the force applied to it’s surface.” Unlike an
ideal sphere, however, the human eye is not
thin-walled and it is not dry. This produces two
confounding forces : (1) a force produced by the
eye’s scleral rigidity (because the eye is not a
thin-walled),wich is directed away from the glob,
and (2) a force produced by the surface tension
of the tear film (because the eye is not dry), wich
is directed toward the glob.
Goldmann determined that when a flat surface is
applied to the cornea with enough force to
produce a circular area of flattening 3.06 mm in
diameter, the force caused by scleral rigidity
exactly cancels out the force caused by surface
tension. Therfore, the applanating force required
to flatten a circular area of cornea exactly 3.06
mm in diameter is directly proportional to the
intraocular pressure.
Calibration of Goldmann tonometer
• It is possible to check the calibration of the
tonometer; this should be done every six
months. Calibration is done at dial positions 0, 2,
and 6 (equivalent to 0, 20, and 60 mmHg).
• Insert the prism in the holder and place the
tonometer on the slit lamp.
• At dial position 0, the feeler arm should be in
free movement. If the dial is turned backwards
a small way (to the equivalent of position -0.05),
the arm should fall towards the examiner.
• If the dial is turned forwards a small way (to the
equivalent of position +0.05) the arm should fall
towards the patient.
• If the arm doesn’t respond in the above way, the
tonometer is inaccurate at dial position 1.
• To check dial positions 2 and 6, the check weight
is used (this is normally found in the case with
the tonometer prisms or in the drawer of the slit
lamp). There are five markings engraved on the
bar. These represent 0 centrally,
then 2 on either side, and 6 towards the edges.
• Line up the adjustable holder with index mark
2 on the weight. With the longer end of the bar
facing you, put it into the slot on the side of the
tonometer and push it all the way in.
• Repeat the above steps (for dial position 0), with
the dial now at position 2. This time, turn the dial
backwards to the equivalent of 1.95 and
forwards to the equivalent of 2.05.
• To check dial position 6, move the weight bar to
the end position. Repeat the steps at dial
position 6, turning the dial backwards to the
equivalent of 5.9 and forwards to the equivalent
of 6.1 .
• If the tonometer is inaccurate at any of these dial
positions, it should be returned to the
manufacturer for recalibration.
Performing Goldman applanation
tonometer.
1) Insert clean tonometer tip in the biprism holder.
The 180 degree marking on the tonometer tip
should be aligned with the white line on the
biprism holder.
2) Instill a topical anesthetic drop and fluorescein
dye into each of the patient’s eyes. Many
clinics use a single solution containing both the
anesthesia and dye fluorescein for this test.
3) Seat the patient at the slit lamp with the
patient’s forehead firmly against the headrest
and chin comfortably on the chin rest.
Wetting the fluorescein with
anesthesia
The patient’s eye should be aligned with the black
band on the headrest column. Instruct the
patient to look straight ahead and to open the
eyelids wide. The examiner should be seated
facing the patient, behind the slit- lamp
oculars.
4) Position the cobalt filter in front of the slit-lamp
illumination device. The cobalt-blue light
causes the fluorascien dye on the patient’s eye
to fluoresce a bright yellow-green.
5) Set the magnification of the slit lamp at low
power, with the light beam at high intensity and
shinig on the tonometer up at a wide angle
(about 60 degree)
6) Looking from the side, use the slit-lamp control
handle to align the tonometer tip with the
patient’s right cornea. Adjust the numbers on
the tonometer force adjustment knob to read
anywhere between 1 and 2 (10 and 20 mm
Hg).
7) Instruct the patient to focus on your right ear,
blink once (to spread the fluorescein dye), and
then try to avoid blinking. If it is necessary to
hold the patient’s lids open, secure them
against the bony orbit ; do not apply pressure
to the globe.
Cobalt filter & gently touch of the
cornea
8) If the patient wears soft lenses you should
flush the eyes out very well, cleaning the
patient up to remove all the fluorescein, then
advise them not to wear their lenses for at
least one (1) hour. Fluorescein will be
absorbed into the soft lens turning it yellow.
Patients who wear hard lenses should be
advised not to wear their lenses as long as
they would normally. Topical anesthetics soften
the cornea’s epithelium making an abrasion
more likely. All patients should have has much
of the fluorescein dye cleaned off as possible.
Soft contact lenses
Hard contact lenses
Soft & Hard lenses
 You do not have to pull the probe off the
patients eye to make fine adjustments for
alignment; this can be done with small
movements while on the cornea. Removing the
probe from the patients eye then putting it back
on until you have proper alignment will result in
drying and staining of the cornea.
9) Using the slit-lamp control handle, gently move
the biprism forward until it just touches the
cornea. Looking through the slit-lamp oculars,
confirm that the biprism has just touched the
cornea: the spot of fluorescein will break into
two semicircles, one above and one below a
horizontal line. Raise and lower the slit-lamp
biomicroscope with the control handle until the
semicircles are equal in size. The semicircles
can be viewed monocularly through only one
of the slit-lamp oculars; in most slit lamps the
semicircles are viewed through the left ocular.
A. If the patient has a large amount of corneal
astigmatism, the semicircles seen by the
examiner through the instrument ocular will
look elliptical rather than circular. An error will
be introduced into the pressure determination.
In this situation, rotate the tonometer tip so that
the dividing line between the semicircles is 45
degree to the major axis of the ellipse.
10) Slowly and gently turn the force adjustment
knob in the direction required to move the
semicircles until their inner edges just touch
and do not overlap.
A. If the semicircles are separated, the pressure
reading will be too low if the semicircles
overlap, the pressure reading will be too high.
B. If there is too fluorescein or if the examiner is
applying pressure to the glob while holding the
patient’s eye open, the semicircles will appear
thick and an inaccurate pressure reading will
result. A small pulsatile motion of the
semicircles may be apparent, synchronous
with the patient’s pulse.
Normal pressure need some
correction
Too high pressure
Too low pressure (image “a”)
Too Fluorescein or the Probe is too
far forward
11) With the slit-lamp control handle, pull the
tonometer biprism away from the
patient’s eye. Note the reading on the
numbered dial of the force adjustment
knob. Multiply the number by 10 to obtain
the intraocular pressure in mm Hg and
record the pressure in the patient’s
medical record.
12)Repeat the procedure for the left eye.
Factors that affect on the accuracy
of IOP readings:
• The accuracy of applanation tonometry is
reduced in certain situations. Corneal edema
predisposes to inaccurately low readings,
whereas pressure measurements taken over a
corneal scar will be falsely high. Tonometry
performed over a soft contact lens gives falsely
low values. Alterations in scleral rigidity may
compromise the accuracy of measurements; for
example, applanation readings that follow scleral
buckling procedures may be inaccurately low.
Disinfecting the Applanation Tip:
1. Remove the tonometer up from the biprism
after each use.
2. Disinfect by one of the following methods:
 Swab the tip with a cotton-tipped applicator
that has been soaked in isopropyl alcohol.
 Soak the tonometer tip in a 10% solution of
sodium hypochlorite (household bleach) or 3%
hydrogen peroxide or isopropyl alcohol for 5
minutes.
3. Rinse the tonometer tip with water and dry with
tissue or gauze to remove residual disinfecting
solution, which could damage the corneal
epithelium.
Isopropyl alcohol
Books:
References:
• Optics, Refraction & Contact lenses (American
Academy of ophthalmology).
• Practical ophthalmology
(American Academy of ophthalmology).
• Clinical ophthalmoptics ….By Mohamed El-Rifi
(Cairo – Egypt).
Websites:
• American Academy of optometry.
• www.webmd.com/eye-health/tonometry .
• medical-dictionary.thefreedictionary.com/
applanation+tonometer