TONOMETRY IN OPTOMETRY & OPHTHALMOLOGY .PPTX

TONOMETRY
ANUJA DHAKAL
OPTOMETRIST

Introduction
• Intra-ocular pressure is the pressure exerted by intra ocular fluids
on the coats of the eye ball.
• The normal level of IOP is maintained by the dynamic equilibrium
between the formation and outflow of the aqueous humour.
• Tonometry is the objective measurement of Intra ocular
pressure(I.O.P)

• A tonometer is an instrument that exploits the
physical properties of the eye to permit measurement
of pressure without the need to cannulate the eye.

USES
• It is performed as a part of a thorough ocular
examination to
– help detect ocular hypertension and glaucoma
– to diagnose ocular hypotony (low IOP)
– in monitoring of antiglaucoma medication

Classification of tonometry
• Applanation Tonometry
• Indentation Tonometry
• Non-applanating type , Contact tonometers
– Dynamic contour tonometer
– Rebound tonometer

Applanation Tonometry
Principle
• It is based on Imbert-Fick Principle:
It states that :For an ideal ,dry ,thin walled sphere , the pressure
inside the sphere is proportional to the force applied to applanate
its surface divided by the area of surface applanated.
P = F/A F=P x A
where , F= external force against the sphere
P= pressure within the sphere
A=area flattened by external force
Assumption: object is perfectly spherical, dry , perfectly flexible and
infinitely thin.

• Since the human eye is not an ideal sphere,it produces
two confounding factors:
-force produced by eye’s scleral rigidity,directed away
from globe
-capillary attraction produced by surface tension of tear
film meniscus tends to pull the tonometer towards the
cornea(towards the globe)

Modified Imbert Ficks principle
• However the corneal rigidity and the capillary attraction
tends to cancel out each other when flattened area has
diameter of 3.06mm
F + S =P x A + B
Where, S= surface tension
B= force produced by relative rigidity of cornea
Therefore,the applanating force required to flatten a
circular area of cornea exactly 3.06mm in diameter is
directly proportional to I.O.P.
-this force (dynes) multiplied by 10 = I.O.P. (mmHg)

Types of applanation tonometers
A) Variable force type (Constant area) applanation
tonometers - Contact Type
–Goldmann tonometer
–Perkin’s tonometer
–Draeger tonometer
–Mackay Marg tonometers
• Tonopen
• Pneumotonometers

B) Variable area (Constant force) type applanation
tonometers – Contact Type
• Maklakow tonometer
C) Variable force type ( Constant area ) applanation
tonometers - Non-Contact Type
• Airpuff tonometer
• Ocular response analyser

GOLDMANN APPLANATION
TONOMETER
• International clinical standard for IOP
measurements
• Most commonly used

APPLANATION TONOMETER
Biprism
(measuring prism)
Feeder arm
Housing
Adjusting knob
Connects to the slit
lamp
Control weight insert

cont…..
The two beam-splitting prism within the
applanating unit optically convert the circular
area of corneal contact in to semicircles

cont….
The instrument is mounted on
a standard slit lamp in such a
way that the examiners view is
directed through the centre of a
plastic Biprism.
 Biprism is attached by a rod
to a housing which contains a
coil spring and series of levers
that are used to adjust the force
of the biprism against the
cornea.
Two beam splitting prisms
within applanating unit optically
convert circular area of corneal
contact in 2 semicircles.

procedure
• The patient is asked not to drink alcoholic beverages as it will lower
IOP and not to take large amounts of fluid (e.g., 500 ml or more) for 2
hours before the test, as it may raise the IOP.
• Councelling regarding purpose of test, reassure and instruct.
• Sterilize tonometer tip ,correctly position the biprism
• Allow the sterilizing solution to dry
• Set tension knob at 1 gram
• Use cobalt blue filter with the slit beam opened maximally

Procedure cont..
• The angle of illumination and the microscope should be 60 deg
• Position the patient
• The palpebral fissure is a little wider if the patient looks up.
However, the gaze should be no more than 15° above the horizontal
to prevent an elevation of IOP.
• After instilling topical anaestheia, Edge of corneal contact is made
apparent by instilling fluorescein while viewing in cobalt blue light.
• The biprism should not touch the lids or lashes because this
stimulates blinking and squeezing.
• The patient should blink the eyes once or twice to spread the
fluorescein-stained tear film over the cornea, and then should keep
the eyes open wide.
Do not to place any pressure on the globe because this raises IOP.

Procedure cont..
• In some patients, it is necessary for the examiner to hold the eyelids
open with
the thumb and forefinger
of one hand against the
orbital rim.
• By manually rotating a dial calibrated in grams, the force is adjusted
by changing the length of a spring within the device.
• The prisms are calibrated in such a fashion that inner margin of
semicircles touch when 3.06 mm of the cornea is applanated.
• The Intra ocular pressure is then read directly from a scale on the
tonometry housing.

cont….
The fluorescent semicircles are
viewed through the biprism and the
force against the cornea is adjusted
until the inner edges overlap.
The fluorescein rings should be
approximately 0.25–0.3 mm in
thickness – or about one-tenth
the diameter of the flattened area.

Potential Sources of Error – During
Measurement

Effect of astigmatism and its correction

Potential Sources of Error – During Measurement

TONOMETRY IN OPTOMETRY & OPHTHALMOLOGY .PPTX

sterilization
• Applanation tip should be soaked for 5-15 min in diluted
sodium hypochlorite, 3% H2O2 or 70% isopropyl alcohol or
by wiping with alcohol, H2O2, povidone iodine or 1: 1000
merthiolate.
• Other methods of sterilization include: 10 min of rinsing in
running tap water, wash with soap and water, cover the tip
with a disposable film, and exposure to UV light.
• Disposable tonometer tips may also be used

• When using disposable tips, they have a smooth
applanating surface.
• The acrylic disposable tips seem to be somewhat more
accurate than the silicone ones.
• While disposable shields or tips may be safer than
disinfection solutions, they are not 100% protective against
prion disease.

• Possible infections transmitted
:epidemickeratoconjunctivitis, hepatitis B, Jacob-
Kreutzfeld and, theoretically, acquired
immunodeficiency syndrome.
• Sterilizing solution should be completely rinsed off the
tonometer tip( toxic to corneal epithelium)

SAFETY REGULATIONS
• No examination should be undertaken in case of eye infections (or)
injured corneas.
• Only clean and disinfected measuring prism should be used.
• No damaged prisms should be used.
– If the measuring prism come in to contact with the cornea without
the drum having previously been correctly set, vibration can
occur in the feeler arm, which will produce unpleasant feeling for
the patient.
– The tonometer tips should be examined periodically under
magnification as the antiseptic solutions and mechanical wiping
may cause irregularities in the surface of the tip that can, in turn,
injure the cornea.

Perkins tonometer
• It uses same prisms as Goldmann
• It is counterbalanced so that tonometry is performed
in any position
• The prism is illuminated by battery powered bulbs.
• Being portable it is practical when measuring IOP in
infants / children, bed ridden patients and for use in
operating rooms.
• It underestimates IOP,and underestimation increases
when the true IOP increases.

Draeger Tonometer
• Draeger tonometer is similar to Perkins
• It has a different set of prisms
• It operates with a motor.

Mackay-Marg Tonometer
• Contains a movable plunger of 1.5mm
diameter
• Transducer
• Paperstrip
• When the instrument touches the
cornea, the plunger and its supporting
spring are apposed by IOP and the
corneal bending pressure
• When the instrument is advanced to a
point of applanation,the corneal
bending pressure is transferred to the
footplate and a notch is seen in the
pressure tracing

Advantage of Mackay-Marg tonometer
• Useful for measuring IOP in eyes with scarred
,irregular or edematous cornea because the endpoint
does not depend on the evaluation of a light reflex
sensitive to optical irregularity

Tono pen
• Portable
• Battery operated .
• Same principle as that of
Mackay-Marg tonometer.

Tonopen Cont….
Advantages
• Useful in health fairs , on
ward rounds (portable)
• Useful in children as
readings are taken quickly
• More accurate in irregular
corneas
• Small applanating area
allows finding the smoothest
part of cornea to applanate.
Disadvantages
• Not as accurate as
GAT(underestimates)
• Overestimates the IOP in
infants so less useful in
cases of infantile glaucoma
• Not useful in Band
Keratopathy where
pathology is harder than
normal cornea

Pneumatonometer or pneumatic tonometer
• It is like Mackay-Marg tonometer.
• The sensor is a air pressure like electronically controlled
plunger in Mackay-Marg tonometer.
• It can also be used for continuous monitoring of IOP.

• Silicon diaphragm
• Central probe
• Transducer
• Amplifier and recorder
• Air supply

Maklakow tonometer
• A known force is applied to the eye, and the area of
applanation is measured
• Consists of a wire holder into which a flat-bottom weight
,ranging from 5-15 gms is inserted
• The surface of the weight is painted with a dye(silver protein
mixed with glycerine)
• 1 sec contact

• IOP = W / π (d/2) 2
• weight (W)
• Diameter of the area of applanation (d)
• Intraocular pressure is measured in grams per square centimeter and
is converted to millimeters of mercury by dividing by 1.36.
• widely in Russia and China
• This instrument displaces a greater volume of aqueous humor and
thus IOP readings are more influenced by ocular rigidity.
• Many instruments similar to the Maklakow device have been
described,like the Applanometer, Tonomat, Halberg tonometer, and
GlaucoTest.

The Ocuton tonometer
• The Ocuton™ tonometer
• hand-held tonometer
• works on the applanation principle
• probe is so light that it is barely felt
• needs no anesthetic in most patients.
• It has been marketed in Europe for home tonometry
• useful to get some idea of the relative diurnal variation in IOP if
the patient or spouse (etc.) can learn to use it.

Non contact tonometer
• Noncontact tonometer (NCT) was introduced by Grolman.
• Original NCT has 3 subsystems:
1. Alignment system: It aligns patient’s eye in 3 dimensions.
2. Optoelectronic applanation monitoring system:
It comprises transmitter, receiver and detector, and timer.
a. Transmitter directs a collimated beam of light at corneal apex.
b. Receiver and detector accept only parallel coaxial rays of light
reflected from cornea.
c. Timer measures from an internal reference to the point of peak
light intensity.
3. Pneumatic system: It generates a
puff of room air directed against cornea

PRINCIPLE
• A puff of room air creates a force that momentarily flattens the
cornea. The corneal apex is deformed by a jet of air
• The force of air jet which is generated by a solenoid activated
piston increases linearly over time.
• When the reflected light is at peak intensity, the cornea is
presumed to be flattened.
• The time elapsed is directly related to the force of jet necessary to
flatten the cornea and correspondingly to IOP.
• The time from an internal reference point to the moment of
flattening is measured and converted to IOP.

• A puff of air of known area is generated against cornea (B).
• At the moment of corneal applanation,a light (T), which is usually
reflected from the normal cornea into space, suddenly is reflected (R)
into an optical sensor (A).
• When the sensor is activated by the reflected light, the air generator is
switched off. The level of force at which the generator stops is
recorded, and a computer calculates and displays the intraocular
pressure.

• NCT is accurate if IOP is nearly normal, accuracy decreases with
increase in IOP and in eyes with abnormal cornea or poor fixation.
• It is useful for screening programs because it can be operated by non-
medical personnel
• It does not absolutely require topical anesthesia .
• There is no direct contact between instrument and the eye.
• The non-contact tonometer measures IOP over very short intervals, so
it is important to average a series of readings.
• New NCT, Pulsair is a portable hand held tonometer.

Ocular Response Analyzer
• It is an adaptation of the non-contact tonometer.
• It directs the air jet against the cornea and measures not one
but two pressures at which applanation occurs
1) when the air jet flattens the cornea as the cornea is bent inward and
2) as the air jet lessens in force and the cornea recovers.

Ocular response analyser
• The first is the resting intraocular pressure.
• The difference between the first and the second applanation
pressure is called corneal hysteresis
• corneal hysteresis is a measure of the viscous dampening
and, hence, the biomechanical properties of the cornea.
• The biomechanical properties of the cornea are related to
corneal thickness and include elastic and viscous dampening
attributes.

• IOP correlate well with Goldmann tonometry but, on
average, measure a few millimeters higher.
• Further , while IOP varies over the 24-hour day, hysteresis
seems to be stable.
• Congdon et al found that a ‘low’ hysteresis reading with
the ORA correlates with progression of glaucoma,
whereas thin central corneal thickness correlates with
glaucoma damage.
• It has practical value in the management of glaucoma.

Indentation tonometry
• Schiotz tonometer
• Electronic schiotz tonometer
• Impact rebound tonometer
• Transpalpebral tonometer

Schiotz tonometry
Schiotz (1905, Modified 1924/1926)

Parts of schiotz tonometer
scale
needle
Weight 5.5g
plunger
holder
Foot plate
lever
3mm diameter
Additional weights
7.5,10,15g

Schiotz tonometry - characteristics
• The extent to which cornea is indented by plunger is
measured as the distance from the foot plate curve to
the plunger base and a lever system moves a needle
on calibrated scale.
• The indicated scale reading and the plunger weight
are converted to an IOP measurement.
• More the plunger indents the cornea, higher the
scale reading and lower the IOP
• Each scale unit represents 0.05 mm protrusion of
the plunger.

PRINCIPLE
• The weight of tonometer on the eye increases the actual IOP (Po)
to a higher level (Pt).
• The change in pressure from Po to Pt is an expression of the
resistance of the eye (scleral rigidity) to the displacement of
fluid.
• P(t) = P(o) + E
• IOP with Tonometer in position Pt =
Actual IOP Po + Scleral Rigidity E
• Determination of Po from a scale reading Pt requires conversion
which is done according to Friedenwald conversion tables.

Friedenwald formula
• Friedenwald generated formula for linear relationship between the log
function of IOP and the ocular distension.
Pt = log Po + C ΔV
• This formula has ‘C’ a numerical constant, the coefficient of ocular
rigidity which is an expression of distensibility of eye. Its average
value is 0.025
• ΔV is the change in volume

Friedenwald conversion table
•
Plunger Load
Scale Reading 5.5 g 7.5 g 10 g 15 g
3.0 24.4 35.8 50.6 81.8
3.5 22.4 33.0 46.9 76.2
4.0 20.6 30.4 43.4 71.0
4.5 18.9 28.0 40.2 66.2
5.0 17.3 25.8 37.2 61.8
5.5 15.9 23.8 34.4 57.6
6.0 14.6 21.9 31.8 53.6
6.5 13.4 20.1 29.4 49.9
7.0 12.2 18.5 27.2 46.5
7.5 11.2 17.0 25.1 43.2
8.0 10.2 15.6 23.1 40.2
8.5 9.4 14.3 21.3 38.1
9.0 8.5 13.1 19.6 34.6
9.5 7.8 12.0 18.0 32.0
10.0 7.1 10.9 16.5 29.6

TECHNIQUE
• Patient should be anasthetised with 4% lignocaine or 0.5%
proparacaine
• With the patient in supine position, looking up at a fixation target
while examiner separates the lids and lowers the tonometer plate to
rest on the anesthetized cornea so that plunger is free to move
vertically .
• Scale reading is measured.
• The 5.5 gm weight is initially used.
• If scale reading is 4 or less, additional weight is added to plunger.
• Conversion table is used to derive IOP in mm Hg from scale reading
and plunger weight.

SOURCES OF ERROR
• Accuracy is limited as ocular rigidity varies from eye to eye.
• As conversion tables are based on an average coefficient of ocular
rigidity; eye that varies significantly from this value gives erroneous
IOP.
• Repeated measurements lower IOP.
• steeper or a thicker cornea causes greater displacement of fluid
during tonometry and gives a falsely high IOP measurement.
• Schiøtz reads lower than GAT

Factors Affecting Scleral Rigidity
• High Scleral Rigidity
• hyperopia
• long standing glaucoma
• ARMD
• Vasoconstrictors
• high ocular rigidity -----
falsely low scale reading -----
falsely high IOP.

•Low Scleral Rigidity
•increasing age
• high myopia
•miotics
•vasodilators
•Postoperative after RD surgery (vitrectomy, cryopexy, scleral
band)
•intravitreal injection of compressible gas.
•keratoconus
•Low ocular rigidity ----- falsely high scale reading -----
falsely low IOP.

LIMITATIONS
Instrumental errors
• Standardisation - testing labs for certification
• Mechanical obstruction to plunger etc.
Muscular contractions
• Of extra ocular muscles increase IOP
• Accomodation decreases IOP
Variations in volume of globe
• Microphthalmos
• High Myopia
• Buphthalmos
It can be recorded in supine position only

Advantages of schiotz tonometer
• Simple technique
• Elegant design
• Portable
• No need for SlitLamp or power supply
• Reasonably priced

calibration
• The instrument should be calibrated before each use by
placing it on a polished metal sphere and checking to be
sure that the scale reading is zero.
• If the reading is not zero, the instrument must be repaired.

sterilization
• The tonometer is disassembled between each use and the
barrel is cleaned with 2 pipe cleaners, the first soaked in
isopropyl alcohol 70 % or methylated spirit and the
second dry.
• The foot plate is cleaned with alcohol swab.
• All surfaces must be dried before reassembling.
• The instrument can be sterilized with ultraviolet radiation,
steam, ethylene oxide.
• As with other tonometer tips, the Schiotz can be damaged
by some disinfecting solutions such as hydrogen
peroxide and bleach.

Electronic schiotz tonometer
• Has continuous recording of IOP that is used for
tonography
• Scale is also magnified ,which makes it easier to
detect small changes in IOP

Rebound tonometer(I care)
• It is a new and updated version of an indentation
tonometer
• Portable

• can be used without anesthetizing the eye.
• A very light, disposable, sterile probe is propelled forward into the
cornea .
• The time taken for the probe to return to its resting position and the
characteristics of the rebound motion are indicative of the IOP.
• The time taken for the probe to return to its resting position is longer
in eyes with lower IOP and faster in eyes with higher IOP.

• It is comparable to the GAT.
• It correlates with central corneal thickness like the Goldmann
• used in screening situations, when patients are unable to be seated or
measured at the slit lamp, or when topical anesthetics are not feasible
or usable.
• Not useful in scarred corneas (as does the Goldmann).

Trans palpebral tonometry
• used in situations where other, more accurate, devices are not practical,
such as in young children, demented patients and severely
developmentally-challenged patients.

• In addition to all the problems facing indentation tonometry, such as
scleral rigidity, transpalpebral tonometry adds variables such as the
thickness of the eyelids, orbicularis muscle tone and potential
Intra palpebral scarring.
• Portable. patients can measure their own IOP at home
• pressure on the eyelid in most eyes produces retinal phosphenes.
• The pressure on the eyelid required to induce these phosphenes is
proportional to the intraocular pressure.
• It is not accurate always. inter observer and intra observer variability
was large.subsequent studies failed to confirm the accuracy of this
device.

Dynamic contour tonometer(pascal)

• Introduced by Kanngiesser
• It is based on a totally different concept other than
indentation or applanation tonometry.
• Principle : By surrounding and matching the
contour of a sphere (or a portion thereof ), the
pressure on the outside equals the pressure on the
inside.
• The tip of the probe matches the contour of the
cornea.
• A pressure transducer built into the center of the
probe measures the outside pressure, which should
equal the inside pressure, and the IOP is recorded
digitally on the liquid crystal display (LCD).

• The concept developed from a previous contact lens tonometer called
the ‘Smart Lens”.
• It superior in accuracy to Goldmann tonometry and pneumotonometry .
• IOP is not affected by corneal thickness.
• IOP is not altered by corneal refractive surgery that thins the cornea.

TONOMETRY IN OPTOMETRY & OPHTHALMOLOGY .PPTX

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