A detailed review of all the types of tonometer and the technique with the principle is included. Will be very useful for both teachers and students of optometry & ophthalmology

1. TONOMETRY

2. Tonometry
• Tonometry is the procedure performed to determine
the intraocular pressure (IOP)

3. I. DEFINITION
• A. the tissue pressure of the ocular
contents
• B. about 15 mm Hg but does fluctuate
(15.5 +/- 2.57)
• C. normal range of pressures: 10.5 - 20.5

4. III. FACTORS THAT
INFLUENCE IOP
• A. Long Term
1. Genetics - relatives of individuals with open-
angle glaucoma are more likely to have high
IOP
2. Age - IOP increases with increasing age
3. Sex - IOP's equal in the age range 20 to 40,
after menopause women have higher IOP's
4. Race - African-Americans have a higher
incidence of glaucoma than whites

5. III. FACTORS THAT
INFLUENCE IOP
• B. Short Term
1. Diurnal variation - 3 to 6 mm Hg change in 24
hr period; > 10 mm Hg change is pathogenic
• a. Change probably related to aqueous production
and not drainage
2. Sitting - going from a sitting to a lying position
results in an increase in IOP which is even
greater in glaucoma patients
3. Total Body Inversion - causes an increase in
IOP by as much as 15 mm Hg

6. III. FACTORS THAT
INFLUENCE IOP
4. Blinking - raises IOP briefly
5. Exercise - decreases IOP
6. Blepharospasm - increases IOP
7. Coughing - increases IOP
8. Blood pressure - some people believe there
is a link between blood pressure and IOP but
no clear evidence
9. General anesthesia - decrease IOP
10. Alcohol - decreases IOP

7. III. FACTORS THAT
INFLUENCE IOP
11. Cannabis - decreases IOP
12. Tobacco - increases IOP
13. Cholinergic Stimulating Agents (i.e.,
pilocarpine and echothiophate) - decrease
IOP by increasing the aqueous outflow
14. Adrenergic Stimulating Agents (i.e.,
epinephrine, propine, iopidine, alphagan) -
lower IOP by enhancing aqueous outflow
15. Adrenergic Blocking Agents (i.e., timolol and
betaxolol) - decrease IOP by decreasing
aqueous production
16. Carbonic anhydrase inhibitors (i.e., diamox,
trusopt, azopt) - decrease aqueous production

8. Ideal tonometer
• Should give accurate and reasonable IOP
measurement
• Convenient to use
• Simple to calibrate
• Stable from day to day
• Easier to standardise
• Free of maintenance problems

9. IOP VARIATIONS
1. Physiological variations : the IOP normally
fluctuates 2-5mmHg throughout the day :
 with respiration and heart beat
 with the venous pressure
 with the arterial pressure
 with the osmotic pressure of blood.
2. Local mechanical factors :
» dilatation of the pupil
» changes in the solid content of the eye
» pressure from outside

10. 3. Pharmacological factors:
The ciliary muscle is inserted into the
trabeculae , so the contraction of the ciliary
muscle makes the trabecular meshwork
more porous -> increases the facility of
outflow -> reduces IOP
» outflow facility
» reduction of aqueous production
» atropine

11. Direct
Indirect
Manometer
DIGIT
AL
INDENTAT
ION
(Schiotz)
APPLANATI
ON
v
NON CONTACT
TONOMETER
Types of
Tonometer
GAT, Perkins, MMT,
Tonopen,
Keeler pulsair
Grolman airblast
static dynamic
Ballistic T
Vibrationa
l T

12. MANOMETRY
• Manometry is the only direct measure of IOP
• In this method, needle is introduced in
Anterior Chamber or in vitreous humor
• It is then connected to mercury or water
manometer

13. Disadvantages:
• Not practical method for human beings
• Needs general anesthesia
• Introduction of needle produces breakdown of
blood aqueous barrier and release of
prostaglandins which alter IOP.

14. Uses
• It is used for continuous measurements of
IOP
• Used in experiment, research work on
animal eyes

15. Principle:-
-Based on Palpation By the Examiner.
Procedure:-
-Patient looks Down and the examiner palpates the eye.
-Index finger of both hands are used ,one finger push the
eye ball above the tarsal plate and the other finger
senses firmness.
o Soft and indents easily – low IOP
o Firm to touch – normal IOP
o Hard to touch – high IOP
Digital Tonometry

16. • Advantages :-
-Easiest
-No equipment
-No anaesthesia
-No staining
• Disadvantages:-
-Reading is not proper .
-It depends on examiner .
-Minor IOP can’t judged properly.

17. Schiotz tonometry
• Schiotz (1905, Modified 1924/1926)

18. Parts of schiotz tonometer
scale
needle
Weight 5.5g
plunger
holder
Foot plate
lever
3mm diameter
ROC 15mm
Tonometer weight = 11g
Additional weights
7.5,10,15g

19. Schiotz tonometer
It consists of :
• Handle for holding the instrument in vertical position on the
cornea .
• Foot plate which rests on the cornea.
• Plunger which moves freely within a shaft in the foot plate.
• Bent lever: whose short arm rests on the upper end of the
plunger and a long arm which acts as a pointer needle .The
degree to which the plunger indents the cornea is indicated by
the movement of this needle on a scale
• Weights: a 5.5g weight is permanently fixed to the plunger
,which can be increased to 7.5 and 10gm.

20. 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.

21. Plunger of
tonometer
Indenting the
cornea.
Base plate of
tonometer
resting on
cornea.
In this the cornea is
indented

22. 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.

23. 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

24. 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

25. 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.

26. 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

27. Factors Affecting Scleral Rigidity
• High Scleral Rigidity
• hyperopia
• long standing glaucoma
• ARMD
• vasoconstrictors

28. Factors Affecting Scleral Rigidity
•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.

29. 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

30. Advantages of schiotz tonometer
• Simple technique
• Elegant design
• Portable
• No need for SlitLamp or power supply
• Reasonably priced
• Anodized scale mount which is highly resistant to
sterilizing water.
• Schiotz tonometer is still most widely tonometer.

31. 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.

32. 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.

33. Differential tonometry
• It is done to get rid from ocular rigidity.
• A reading is taken with one weight on the Plunger and then a second
reading' in taken with a different weight.
• Making a diagnosis of glaucoma in a pt. with myopia presents
unusual difficulties. The low ocular rigidity in these eyes result in
Schiotz readings within normal limits.
5.5g 10g Ocular
rigidity
IOP
18 mm Hg 15 mm Hg lower >18
18 mm Hg 21 mm Hg higher <18
18 mm Hg 18 mm Hg equal 18

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

35. Applanation tonometry
• The concept was introduced by goldmann is 1954
• It is based on IMBERT FICK LAW
• It states that the pressure inside an ideal sphere
(P) is equal to force (W) reqired to flatten(A)
P=W/A

36. • P can be determined if
Force F is fixed or
Area A is fixed
• The ideal sphere is dry, thin-walled and
flexible.
• The cornea is not ideal sphere

37. • Two extra forces acting on cornea -
– Capillary attraction of tear meniscus (T), tends to
pull tonometer towards cornea
– Corneal rigidity (C) resists flattening
• Thus,
F = PA , becomes
F + T = PA + C , or
P =( F + T - C) / A

38. • These two forces cancel each other
when flattened area has diameter of 3.06 mm

39. Applanation tonometers
1) Goldman tonometer
2)Perkins AT
3)Pneumatic tonometer
4)Pulse air tonometer
5)Tono pen

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

41. 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.

42. 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.
• The angle between the illumination and the microscope
should be approximately 60°.
• The room illumination is reduced.
• A fixation light may be placed in front of the fellow eye.
• The tension knob is set at 1 g. If the knob is set at 0, the prism
head may vibrate when it touches the eye and damage the
corneal epithelium.
• The 1 g position is used before each measurement.

43. Procedure cont..
• 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.

44. 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.

45. 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.

46. Potential Sources of Error – During Measurement

47. Effect of central corneal thickness (CCT):
• A thinner cornea may require less force to applanate it, leading to
underestimation of true IOP while a thicker cornea would need
more force to applanate it, giving an artificially higher IOP.
• The Goldmann applanation tonometer was designed to give
accurate readings when the CCT was 520 μm.
• The deviation of CCT from 520 μm yields a change in applanation
readings of 0.7 mm Hg per 10 μm.
• IOP measurements are
also modified after PRK and
LASIK.
• Thinning of the central
cornea is gives lower readings
on applanation.

48. • Wider meniscus or improper vertical alignment gives higher
IOP readings
• If the two semicircles are not equal in size, IOP is
overestimated.
• For every 3D increase in corneal curvature, IOP raises about 1
mm Hg as more fluid is displaced under steeper corneas
causing increase in ocular rigidity
• More than 6 D astigmatism produces an elliptical area on
applanation that gives erroneous IOP. 4D with-the-rule
astigmatism underestimate IOP and 4D against-the-rule
astigmatism overestimate IOP.
• Mires may be distorted on applanating on irregular corneas .

49. • Elevating the eyes more than 15° above the
horizontal causes an overestimation of IOP.
• Widening the lid fissure excessively causes an
overestimation of IOP
• Repeated tonometry reduces IOP, causing an
underestimation of the true level. This effect is
greatest between the first and second
readings, but the trend continues through a
number of repetitions.
• A natural bias for even numbers may cause
slight errors in readings.

50. Applanation - Possible Errors
• Falsely low IOP
– too little flouroscein
– thin cornea
– corneal edema
– with the rule astigmatism
• 1mm Hg per 4 D
– prolonged contact
– Repeated tonometry
• Falsely high IOP
– too much flouroscein
– thick cornea
– steep cornea
– against the rule
astigmatism
1mm Hg per 3D
– wider meniscus
– Widening the lid fissure
excessively
– Elevating the eyes more
than 15°

51. Potential Sources of Error – During Measurement
If the fluorescein rings are too wide, the patient’s eyelids should be blotted
carefully with a tissue, and the front surface of the prism should be dried with
lint-free material.
An excessively wide fluorescein ring can cause IOP to be overestimated

52. Potential Sources of Error – During Measurement
If the rings are too narrow, the patient should blink two or three times to
replenish the fluorescein; additional fluorescein may be added if necessary.
If the fluorescein rings are too narrow,IOP is underestimated.

53. Potential Sources of Error – During Measurement

54. Potential Sources of Error – During Measurement

55. Potential Sources of Error – During Measurement

56. Potential Sources of Error – During Measurement

57. Potential Sources of Error – During Measurement

58. Potential Sources of Error – During Measurement

59. Potential Sources of Error – During Measurement

60. Potential Sources of Error – During Measurement

61. CALIBRATION
• GAT should be calibrated periodically, at least monthly. If
the GAT is not within 0.1 g (1 mmHg) of the correct
calibration, the instrument should be repaired; however,
calibration errors of up to 2.5 mmHg may still be tolerated
clinically.

62. • Following checks are necessary:
• Check position 0: Turn the zero calibration on the
measuring drum downwards by the width of one
calibration marking, against the index marker.
• When the feeler arm is in the free movement zone, it
should then move itself against the stop piece in the
direction of the examiner.
• Check position 0.05: Turn the zero calibration on the
measuring drum upwards by the width of one
calibration marking, against the index marker.
• When the feeler arm is in the free movement zone, it
should then move itself against the stop piece in the
direction of the patient.

63. • Check position at drum setting 2: For checking this
position, check weight is used.
• Five circles are engraved on the weight bar.
• The middle one corresponds to drum position 0, the
two immediately to the left and right to position 2
and the outer ones to position 6.
• One of the marks on the weight corresponding to
drum position 2 is set precisely on the index mark of
the weight holder.
• Holder and weight are then fitted over the axis of the
tonometer so that the longer part of the weight
points towards the examiner.

64. • Check position 1.95: The feeler arm should
move towards the examiner.
• Check position 2.05.The feeler arm should
move in the direction of the patient.
• Check at measuring drum setting 6: Turn the
weight bar to scale calibration 6, the longer
part shows in the direction of the examiner.
• Check position 5.9/6.1 as performed for drum
setting 2.

65. 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

66. 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.

67. 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.

68. 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.

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

71. Mackay marg tonometer

72. Mackay-Marg Tonometer
• 1.5 mm diameter plunger
• rigid spring
• rubber sleeve.
• Movement of plunger is electronically monitored by a
transducer and recorded on a moving paper strip.
• This instrument is useful for measuring IOP in eyes with
scarred, irregular, or edematous corneas because the end
point does not depend on the evaluation of a light reflex
sensitive to optical irregularity, as does the Goldmann
tonometer.
• It is accurate when used over therapeutic soft contact lenses.

73. At 1.5 mm of corneal area applanation, tracing reaches a
peak and the force applied = IOP + force required to deform
the cornea.
At 3 mm flattening, force required to deform cornea is
transferred from plunger to surrounding sleeve, creating a
dip in tracing corresponding to IOP.
Flattening of >3 mm of area gives artificial elevation of
IOP.

74. Tonopen

75. Tono pen
• Portable
• battery operated .
• same principle as that of Mackay-Marg tonometer.
• It is particularly useful in community health fairs, on ward rounds
,children, irregular surfaces, measuring through an amniotic
membrance patch graft, to read from the sclera .
• Tono-Pen tends to overestimate the IOP in infants so its usefulness
in congenital glaucoma screening and monitoring is somewhat
limited.
• In band keratopathy where the surface of the pathology is harder
than normal cornea, the Tono-Pen tends to overestimate the IOP
• A disposable latex cover which is discarded after each use
provides infection control.

76. 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.

77. • It gives significantly higher IOP estimates.
• It has a sensing device that consists of a gas chamber
covered by a polymeric silicone diaphragm.
• A transducer converts the gas pressure in the
chamber into an electrical signal that is recorded on
a paper strip.
• The gas in the chamber escapes through an exhaust
vent between the diaphragm and the tip of the
support nozzle.
• As the diaphragm touches the cornea, the gas vent
is reduced in size, and the pressure in the chamber
rises.

78. Maklakov tonometer
•Indentation
•Pt supine
•wire holder
 Dumb-bell-shaped metal
cylinders with flat end plates
of polished glass
 Diameter of 10 mm
The surface of the weight is
painted with a dye, such as
mild silver protein (Argyrol)
mixed with glycerin.
1 sec contact
imprint on end plate

79. • 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.
• It does not correct for corneal bending, capillary
attraction, or tear encroachment on the layer of dye.
• Many instruments similar to the Maklakow device have
been described,like the Applanometer, Tonomat,
Halberg tonometer, and GlaucoTest.

80. 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.

81. Rebound tonometer

82. • 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.

83. • 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).

84. 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.

85. • Portable. patients can measure their own IOP at
home, DVT
• 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.

86. 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

87. PRINCIPLE
• A puff of room air creates a constant 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.

88. • 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.

89. • 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 patient should be warned that the air puff can
be startling.
• 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.

91. 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.

92. 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.

93. • 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.

94. Dynamic contour tonometer

95. • 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).

96. • 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.

97. • Because the DCT measures IOP in real time,
the actual measurement, like the IOP, is
pulsed.
• The internal electronics ‘call’ the IOP as the
bottom of the pulsed curve and indicate it
digitally on the LCD ..
• IOP readings with the DCT are generally lower
than GAT because, when properly done,
indicates the average difference between the
maximum and minimum pressures whereas
the DCT reads the minimum.

98. Tonometry for Special Clinical Circumstances
• Tonometry on Irregular Corneas
• The accuracy of Goldmann and Tono-Pen tonometers and the
noncontact tonometers is limited in eyes with irregular
corneas.
• The pneumatic tonometer has been shown to be useful in
eyes with diseased or irregular corneas .
• Tonometry over Soft Contact Lenses
• Pneumo tonometry and the Tono-Pen can measure with
reasonable accuracy the IOP through bandage contact lenses .
• pneumotonometer correlates well with manometrically
determined IOP, whereas the Tono-Pen consistently
underestimates the pressure.

99. • Tonometry with Gas-Filled Eyes
• Intraocular gas affects scleral rigidity,
rendering indentation tonometry
unsatisfactory.
• pneumatic tonometer and Tono-Pen used.
• A pneumatic tonometer underestimates
Goldmann IOP measurements in eyes with
intravitreal gas
• Tono-Pen compares favorably with Goldmann
readings.
• Both instruments significantly underestimated
the IOP at pressures greater than 30 mm Hg .

100. • Tonometry with Flat Anterior Chamber
• IOP readings from the Goldmann applanation
tonometer, pneumotonometer, and Tono-Pen do not
correlate well with manometrically determined
pressures.
• Tonometry in Eyes with Keratoprostheses
• In patients at high risk for corneal transplant
rejection, implantation of a keratoprosthesis is now a
viable option for vision rehabilitation .
• Most keratoprostheses have a rigid, clear surface, it
is impossible to measure IOP by using applanation or
indentation instruments.
• In such eyes, tactile assessment appears to be the
most widely used method to estimate IOP.