Tonometry

1. T
onometry
By:
Dr. Rashida Riaz

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

3. Manometry
Manometry is only direct measure ofIOP
In this method, needle is introduced in AC or in
vitreous
It is thenconnected 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. T
onometry
It is an indirect method of measuring the IOP
Three basic types ofTonometers:
o Indentation
o Applanation
o Noncontact

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

8. Ocular rigidity
Measure of distensibility or resistance to deformation of
ocular coats.
Important in indentation tonometer
Increase in ocular rigidity– increaseIOP
Long standing glaucoma
ARMD
Hyperopic eyes
Decrease in ocular rigidity- decrease inIOP
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. APPLANATIONTONOMETER
1. GOLDMANN
APPLANATION
TONOMETER
PROTOTYPE
I. PERKINS
APPLANATION
TONOMETER
2. MACKAY
-MARG
TONOMETER
PROTOTYPE
I. TONOPEN
II. PNUEMATIC
1. Maklakov –tonometer
VARIABLE FORCE VARIABLE 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
o Pneumatic 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. Basicconcept and theory of indentation
As soon as tonometer is placed on cornea differentforces
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 tooriginal base line IOP
Resting intraocular pressure (P0) which isartificially
raised to a new value(P1)
Thus the scale reading of tonometer actually measures the
artificially raised IOP

19. Conversion of scalereading to baselineIOP
The conversion of P1to P0 is obtained fromconversion
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 toas 1948 tables,
calculated using average K0.0245 (coefficient of ocular
rigidity)
The Friedenwald later revised average Kto 0.0215known
as 1955 tables
Subsequent studies indicate 1948
tables agree more closely with
measurement by goldmann AT

21. Parts of schiotz
Handle for holding theinstrument in
vertical position on cornea
Footplate which rests on cornea
Plunger which moves freely within ashaft
in footplate
A bent lever whose short arm rests on
upper end ofplunger
Long arm which acts as pointer needle

22. Weights - a 5.5 gm weight is permanently fixedto
plunger, can be increased to 7.5 and10gm

23. Techinque of schiotztonometry
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 theplunger
The pointer logically should be at 0 marked on the scale
because there is no downward movement of the plunger

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

25. Advantages
-easy to use, simplicity, lowprice
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 indentationtonometry
1)Errors inherent in the instrument
These may be due to difference in weight, size ,shape
and curvature offootplate
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 ofIOP
4)Errors due to ocularrigidity

28. 5) Errors due to variation in corneal curvature
-Steeper or thicker corneawill 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 FICKLAW
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 Ais fixed
The ideal sphere is dry, thin-walled andflexible.
The cornea is not idealsphere

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 eachother
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. GOLDMANNTONOMETER
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.
0.5l).
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
CALIBRATIONOF
APPLANATRI
ON
TONOMETER
Appl, tonometer is supplied with calibration
bar.
it should be done once in a week
The appl. Pressure spring is calibratedwith
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.

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 adjusteduntil
inner edges of two semicircles justtouch
Intraocular pressure is determined bymultiplying
dial reading with ten

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

41. Technical
Tonometer out ofcalibration
Repeated tonometry
Pressing on theeyelids or globe
Squeezing of the eyelids
Observer bias (expectations and evennumbers)
Potential errors

42. PerkinsTonometer
It uses the same biprism as the Goldmann applanator.
The light source is powered bybattery.
The readings are consistent and compare quite well with
the Goldmann applanator.

43. PerkinsTonometer
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 3mm
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 touchingapex by silastic
diaphragm covering sensing nozzle
It is connected to central chambercontaining
pressurized air
There is pneumatic to electronictransducer
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 Silasticmembrane

50. Principle
The air column is continually vented via a port.
Changes in pressure in the column records via a
transducer on a moving stripof 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 devicestops.
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 3readings

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. Tonopen
This is handheld Mackay Marg typetonometer
It is a computerised pocket tonometer
It converts IOP into electricwaves

58. Tonopen
The wave form is internally analyzed bya
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 1.7mm higher than the Goldmann
tonometer.
Above 24 mmHg, the readings weresimilar.

60. Dynamic ContourTonometer
The PASCAL (DCT) is a slit lamp–mounted device
It measures IOP independent of corneal rigidity or
thickness.
It was commercially launchedin
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 isaltered
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 ResponseAnalyzer
It is similar to Reichert’s current generation NCT
andprovides 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 monitorsthe
changes in curvature of the cornea
Two independentpressure values are derived the
inward and outward applanationevents.

70. Due viscous damping in the cornea causes
delays, resulting in thedifferent 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 itworks
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 otheras it returns.
The difference between the “in” and “out” pressure
values is known as corneal hysteresis

73. Ocular ResponseAnalyzer
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. CornealHysteresis
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 1
1mmHg.
However, it is very likely that CH values will vary
depending on age and race.

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

78. Reboundtonometry
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 theintraocular pressure is calculated.
It is portable
no eye drops
suitable for children andnon-cooperative
patients

80. OCTTonometry
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 thecornea 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 theeyeball
The test is done through the upper eyelid
No risk of infection during the test
No anesthesia drops and stainingagents
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
1mm Hg for every 4 D
Irregular astigmatism– unpredictable
Recommendations
irregular corneas – Mackay Marg principle-TonoPen

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

86. CornealScars
• 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 overallocular
rigidity – Kvalue differs hence Schiotz is also
inaccurate
Recommendation- Mackay Marg T, Tono Pen away
from the cone

88. Flat AC
Unreliable with applanation- errors upto 51mm 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. LaserRefractive 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
P1-P2
Correction factor(C) :
P1= pre op IOP
P2 = post op IOP after 6 months and at least off
steroids for 1month
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