Tonometry

Tonometry

1. Tonometry
By Dr. Rahul
Moderator Dr.Vijay Shetty

2. Intra ocular pressure can be measured by
1)Manometry
2)Tonometry

3. Manometry
Manometry is only direct measure of IOP
In this method, needle is introduced in AC or in
vitreous
It is then connected to mercury or water manometer

4. 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

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

6. Tonometry
It is an indirect method of measuring the IOP
Three basic types of Tonometers:
o Indentation
oApplanation
oNoncontact

7. History
Malkalov 1885 1
st
Appl tonometer
Schiotz 1905 Indentation tonometry
Friedenwald 1948 &1955 Coefficient of ocular rigidity
Goldmann 1954 prototype Appl. T (constant area)
Grolmann 1972 N. C. T.
Grant Electronic indentation tonometer
Halberg Hand held tonometer

8. Ocular rigidity
Measure of distensibility or resistance to deformation of
ocular coats.
Important in indentation tonometer
Increase in ocular rigidity– increase IOP
Long standing glaucoma
ARMD
Hyperopic eyes
Decrease in ocular rigidity- decrease in IOP
Acutely elevated IOP
Osteogenesis imperfecta
Miotic therapy
Vasodilator therapy
Vitrectomy
Myopic eyes

9. Corneal rigidity
Ability of the corneal tissue to resist deformation
Important in applanation tonometers
Provided by collagen lamellae – 90% of corneal thickness
Increased corneal thickness– increased rigidity– increase
in IOP

10. Classification

12. APPLANATION TONOMETER
1.GOLDMANN
APPLANATION
TONOMETER
PROTOTYPE
I.PERKINS
APPLANATION
TONOMETER
2.MACKAY-MARG
TONOMETER
PROTOTYPE
I.TONOPEN
II.PNUEMATIC
TONOMETER
1.Maklakov –tonometer
VARIABLE FORCEVARIABLE FORCE VARIABLE AREAVARIABLE AREA

13. Classification
Direct
Indirect
Indentation T - Schiotz
Applanation T
Goldmann (prototype)
Goldmann- type- Perkins & Draeger
Mackay- Marg – type T - Tono Pen (hand held)

14. Continued ..
Applanation T
Maklakov tonometer
Maklakov type
Planometer
Tonomat
Halberg
Barraquer
oPneumatic tonometer
Non contact tonometer (NCT)
X –pert T
Grolman airblast T
Keeler pulsair T (hand held)

15. Miscellaneous T
Continuous IOP monitoring devices
Self tonometer
Impact tonometer
Vibrational tonometer
Newer tonometers
Trans –palpebral T
Disposable tonometer
Tonosafe – acrylic biprism
Tonoshield- silicone shield
Dynamic contour tonometer

16. Indentation Tonometry
•It is based on fundamental fact that plunger will
indent a soft eye more than hard eye
•The indentation tonometer in current use is that of
Schiotz
•It was devised in 1905 and continued to refine it
through 1927

17. Basic concept and theory of indentation
As soon as tonometer is placed on cornea different forces
come into play
W - weight of tonometer
A -Area
Vc –volume displaced after indentation
T- tensile force, set up in outer coats of eye at everywhere
tangentially to corneal surface

18. So additional force T to original base line IOP
Resting intraocular pressure (P0) which is artificially
raised to a new value (P1)
Thus the scale reading of tonometer actually measures the
artificially raised IOP

19. Conversion of scale reading to baseline IOP
The conversion of P1 to P0 is obtained from conversion
tables developed by Friedenwald
The calibration was carried by experiments in cadaveric
eyes connected with manometer through cannula
The observation were plotted on semilog scale ,which
serve as Friedenwald nomogram

20. The original conversion tables referred to as 1948 tables,
calculated using average K 0.0245 (coefficient of ocular
rigidity)
The Friedenwald later revised average K to 0.0215 known
as 1955 tables
Subsequent studies indicate 1948
tables agree more closely with
measurement by goldmann AT

21. Parts of schiotz
Handle for holding the instrument in
vertical position on cornea
Footplate which rests on cornea
Plunger which moves freely within a shaft
in footplate
A bent lever whose short arm rests on
upper end of plunger
Long arm which acts as pointer needle

22. Weights - a 5.5 gm weight is permanently fixed to
plunger, can be increased to 7.5 and 10 gm

23. Techinque of schiotz tonometry
A metal sphere used as dummy cornea. radius of curvature
- 15mm
Use - for testing the tonometer & calibration
When the tonometer is placed on the metal sphere, there
is no indenting movement of the plunger
The pointer logically should be at 0 marked on the scale
because there is no downward movement of the plunger

24. Techinque of schiotz tonometry
It is customary to start with 5.5 gm
Greatest accuracy is attained if deflection of lever is
between 3 to 4
if the scale reading is < 3, additional weight is added
to plunger to make it 7.5 gm or 10 gm
Sterilisation - by dipping in ether, absolute alcohol or
acetone

25. Advantages
-easy to use, simplicity, low price
Disadvantage
Gives false reading when used in eyes with abnormal
scleral rigidity
False low levels of lOP with low scleral rigidity seen in
high myopes n following ocular surgery

26. Errors of indentation tonometry
1)Errors inherent in the instrument
These may be due to difference in weight, size ,shape
and curvature of footplate
2)Errors due to contraction of extra ocular muscles
- tend to increase IOP

27. 3) errors due to accommdation
patient look at the tonometer and thus
accommodation comes into play
Contraction of ciliary muscle increases the facility of
aqueous outflow by pulling on trabecule
Thus causes some lowering of IOP
4)Errors due to ocular rigidity

28. 5) Errors due to variation in corneal curvature
-Steeper or thicker cornea will cause greater
displacement of fluid
-Causes falsely high IOP readings
Errors may arise in cases of –
-Microphthalmos
-Buphthalmos
-High myopia
-Corneal scars

29. 6)Moses effect
- At low scale reading the cornea may mould into
space between Plunger and hole
- Pushing the plunger up and leading to falsely high
pressure reading

30. 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

31. 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

32. 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

33. These two forces cancel each other
when flattened area has diameter of 3.06 mm

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

35. GOLDMANN TONOMETER
Most popular and accurate tonometer
It consists of double prism mounted on slit lamp
The prism applanates the cornea in an area of 3.06
mm diameter

36. 36
Goldmann tonometer
Measures the force required to applanate
the cornea over a circular area of diameter
3.06mm..
Applanates an area of diameter 3.06 mm
for 3 reasons.
Amount of fluid displacement is negligible (approx.
0.5ml).
Surface tension force and the force required to
counteract the corneal rigidity act opposite to each
other.
Tonometer force becomes equal to the force in
mmHg.
Area applanated on the cornea is 7.35mm.

37. 37
CALIBRATION OF
APPLANATRION
TONOMETER
Appl, tonometer is supplied with calibration
bar.
 it should be done once in a week
The appl. Pressure spring is calibrated with
calibration bar .
In ZEISS model rod is placed at the junction of
balance arm.
The rod is moved towards the patients .
The center is at ring (o) and is set for tension (o) + or – 0.50.
The next mark is at 2 gm this represent tension of 19.50 and the
when the rod is moved to position 3 that is at 6 gm the tension is
between 59 to 61.

38. Technique
Topical anesthesia
Staining tear film with fluorescein
The cornea and biprisms are illuminated with cobalt
blue light
Biprism is the advancd until it just touches the apex
of cornea
At this point two fluorescent semicircles are viewed
through prism

39. Applanation force against cornea is adjusted until
inner edges of two semicircles just touch
Intraocular pressure is determined by multiplying
dial reading with ten

40. Potential errors
Patient related
Thin cornea
Thick cornea
Astigmatism
Irregular cornea

41. Technical
Tonometer out of calibration
Repeated tonometry
Pressing on the eyelids or globe
Squeezing of the eyelids
Observer bias (expectations and even numbers)
Potential errors

42. Perkins Tonometer
It uses the same biprism as the Goldmann applanator.
The light source is powered by battery.
The readings are consistent and compare quite well with
the Goldmann applanator.

43. Perkins Tonometer
Perkins –
Handheld
Horizontal as well as vertical
Infants, children, O T, recumbent patients

44. Mackay Marg Tonometer
Plunger plate has diameter of 1.5mm
Surrounding Sleeve has 3 mm
Force required to keep the plate flush with the
sleeve is electronically monitored – recorded on a
paper strip

46. Source of error-
 >3 mm flattening – high IOP
Multiple readings to compensate ocular pulsation
Specific utility- irregular and edematous cornea

47. Pneumatic tonometer

48. Pneumatic tonometer
Cornea is applanated by touching apex by silastic
diaphragm covering sensing nozzle
It is connected to central chamber containing
pressurized air
There is pneumatic to electronic transducer
It converts the air pressure to recording on paper
strip and IOP is read

49. Principle
The principle is similar to the MacKay-Marg
tonometer.
Corneal contact of the pencil-like tip records both the
IOP and the force required to bend the cornea.
 advancement of the tip transfers the latter force to
the surrounding “collar.”
The “plunger” is replaced by a column of air and the
contact surface is a Silastic membrane

50. Principle
The air column is continually vented via a port.
 Changes in pressure in the column records via a
transducer on a moving strip of paper.
 instrument is useful with edematous and irregular
corneas

52. Noncontact tonometer
It is an applanation tonometer and works on the principle
of a time interval.
Measuring the time from initial generation of the puff of
air to cornea gets flattened (in milliseconds) to the point
where the timing device stops.
It takes less time for the puff of air to flatten a soft eye
than it does a hard eye.

53. •Three subsystems:
•Alignment system
•Optoelectric applanation monitoring system
•Transmitter
•Receiver and detector
•Pneumatic system
•Time for max light detection= time to applanate
the cornea = corelated with IOP
•Limitations
•Ocular pulse
•Glaucomatous eyes
Average of 3 readings

55. 57
CALIBRATION NON-CONTACT
TONOMETER
•The use of logic circuit in the instruments,
which are necessary to measure and record
IOP , enables the operator to check the
calibration of pneumatic electronic network as
under
•Turn instrument to on [red dot ]
•Remove the objective cap for 30 sec for
warm up.
•Depress the trigger switch –display at –68.

56. Noncontact tonometer

57. Tono pen
This is handheld Mackay Marg type tonometer
It is a computerised pocket tonometer
It converts IOP into electric waves

58. Tono pen
The wave form is internally analyzed by a
microprocessor.
Three to six estimations of the pressure are then
averaged.
The instrument is 18 cm in length and weighs 60 g.

59. For pressures from 6 to 24 mmHg, it measured an
average of 1.7 mm higher than the Goldmann
tonometer.
Above 24 mmHg, the readings were similar.

60. Dynamic Contour Tonometer
The PASCAL (DCT) is a slit lamp–mounted device
It measures IOP independent of corneal rigidity or
thickness.
 It was commercially launched in
August 2004.

61. Principle
DCT uses the principle of contour matching instead of
applanation.
The tip contains a hollow miniature pressure sensor in its
centre.

62.  when the contours of the cornea and tonometer match,
then the pressure measured at the surface of the eye
equals the pressure inside the eye (B).

63. Principle
The probe is placed on the pre-corneal tear film
on the central cornea
 The integrated piezoelectrical ( electricity
resulting from pressure) pressure sensor records
data, measuring IOP 100 times per second.
The tonometer tip rests on the cornea with a
constant appositional force of one gram.

64.  When the sensor is subjected to a change in
pressure, the electrical resistance is altered
 The PASCAL's computer calculates a change in
pressure according to the change in resistance.
A complete measurement cycle requires about 8
seconds of contact time.

65. It is less influenced by corneal thickness than
other methods
 As the tip shape is designed for the shape of a
normal cornea, it is more influenced by corneal
curvature.

66. Ocular Response Analyzer
It is similar to Reichert’s current generation NCT
and provides a Goldmann-equivalent IOP reading.
 It analyzes the signal obtained from the corneal
response to measure the biomechanical properties
of the corneal tissue.

67. Principle
It utilizes a dynamic bi-directional applanation
process to measure pressure of the eye.
 During measurement, a precisely metered
collimated-air-pulse applies force to the cornea.

68. Principle
Under the force of the air pulse, the cornea moves
inwards, past applanation, and into a slight
concavity
As the air pulse pressure decreases, the cornea
return to its normal configuration.
In the process, it once again passes through an
applanation state.

69. Principle
An advanced electro- optical system monitors the
changes in curvature of the cornea
Two independent pressure values are derived the
inward and outward applanation events.

70. Due viscous damping in the cornea causes
delays, resulting in the different pressure values.
The average of these two pressure values
provides Goldman-Correlated IOP value (IOPG).
The difference between these two pressure values
is Corneal Hystersis.

71. How it works
The ORA produces a rapid air impulse and uses an
electro-optical system to monitor the deformation.
The device records two applanation events: inward
movement ; the other as it returns.
The difference between the “in” and “out” pressure
values is known as corneal hysteresis

73. Ocular Response Analyzer
The Ocular Response Analyzer (A) utilizes a collimate
air pulse to applanate the cornea, along with an
infrared electro-optical detection system (B).

74. Hysteresis
•The phenomenon was identified, and the term
coined, by Sir James Alfred Ewing in 1890.
•Hysteresis is a property of physical systems that
do not instantly follow the forces applied to them,
but react slowly, or do not return completely to
their original state.

75. Corneal Hysteresis
 It is the "energy absorption capability" of the cornea
 This because of the speed at which the cornea is
deformed during the dynamic bi-directional
applanation process in ORA

76.  average value of (CH) in normal subjects is
approximately 11 mmHg.
 However, it is very likely that CH values will vary
depending on age and race.

77. CRF
CRF is a measurement of the cumulative effects of
both the viscous and elastic resistance
encountered by the air jet while deforming the
corneal surface.
 CRF exhibits the expected property of increasing
at significantly elevated pressures.

78. Rebound tonometry
It determines IOP by bouncing a small plastic
tipped metal probe against the cornea.
 The device uses an induction coil to magnetize
the probe and fire it against the cornea.

79. As the probe bounces against the cornea and back
in to the device, it creates an induction current
from which the intraocular pressure is calculated.

It is portable
no eye drops
 suitable for children and non-cooperative
patients

80. OCT Tonometry
Non-contact tonometry using optical coherence
tomography (OCT) is currently under
development.
It works as a force being applied to the cornea
and simultaneous measurement of the corneal
reaction.

81. In the case of OCT tonometry, the force applied
to the cornea can be
-air pressure in the form of a high pressure jet
-a shock or acoustic wave
- low pressure air using air pumped into a sealed
chamber around the eye (like scuba mask).

82. Transpalpebral tonometer
No contact directly with the eyeball
The test is done through the upper eyelid
No risk of infection during the test
No anesthesia drops and staining agents
Comfortable for the patient
IOP measuring in immobilized patients and in
children

83. Special conditions
Corneal astigmatism
Corneal Edema
Keratoconus
Flat AC
Penetrating Keratoplasty
Contact Lenses

84. Corneal astigmatism
Mires unequal
Underestimating IOP –with the rule
Overestimating IOP – against the rule
 1 mm Hg for every 4 D
Irregular astigmatism– unpredictable
Recommendations
irregular corneas – Mackay Marg principle-Tono Pen

85. Corneal Edema
Cornea with epithelial or stromal edema – easier to
indent
Hence underestimation by 10 – 30 mm Hg
Recommendation- Mackay Marg T

86. Corneal Scars
Increased corneal rigidity in the area of the scar-
increased IOP
Recommendation- Mackay Marg T,
Pneumotonometer

87. keratoconus
Corneal thinning- Low IOP measured
Increased curvature- Low IOP measured
Decreased corneal rigidity – reduces overall ocular
rigidity – K value differs hence Schiotz is also
inaccurate
Recommendation- Mackay Marg T, Tono Pen away
from the cone

88. Flat AC
Unreliable with applanation- errors upto 51 mm Hg
Flat A.C. post Trab. –
Overfiltration- has low IOP
Aqueous misdirection- high IOP
Diagnosis difficult with tonometer due to unreliability
Recommendation- digital pressure

89. Laser Refractive Surgery
LASIK – reshape – CCT decreases – falsely Low IOP by
applanation
Peripheral Tono Pen & Goldmann readings unchanged
Central & peripheral Pneumotonometer readings
unchanged
Post op steroid induced increase IOP may mask the
underestimated goldmann readings

90. Recommendations-
Tono Pen or Pneumotonometer
Correction factor(C) : P1-P2
P1 = pre op IOP
P2 = post op IOP after 6 months and at least off
steroids for 1 month
True IOP= appl IOP + C

91. Penetrating Keratoplasty
Irregular
Edema
Scarring
Astigmatism
Recommendation- Mackay Marg T considered the
best for irregular and scarred cornea, Tono Pen

92. Contact Lenses
Applanation – unreliable
Recommendation- Pneumotonometer, Tono Pen

93. THANK YOU