A presentation on various techniques and principle of tonometry.

1. TONOMETRY
Dr Saurabh Kushwaha
Resident Ophthalmology

2. SCOPE
 Introduction
 Tonometry techniques
 Manometry
 Digital tonometry
 Schiotz tonometry
 Goldmann applanation tonometry
 Non Contact tonometry
 Pascal’s DCT

3. INTRODUCTION
 Intra Ocular Pressure : IOP is the pressure within the
eyeball
 Intra Ocular Tension : IOT is the pressure exerted by the
intraocular contents on the outer coats of eye
 Normal IOP : It is that pressure that does not lead to
glaucomatous damage of the optic nerve head.
 Normal mean IOP = 15.5 +/- 2.57 mmHg & 2 SD above
mean is 20.5 mmHg

4. TECHNIQUES FOR MEASURING IOP
Techniques
Manometry
Indentation
1. Von Graefe
2. Schiotz
Instrumental Digital
Direct Indirect (Tonometry)
Applanation
Non Contact
Contact
1. Grolman NCT
2. Ocular response
analyzer
Variable force Variable area
Contour
Pascal’s DCT
Maklakov
1. Goldmann
2. Perkins

5. FACTORS AFFECTING TONOMETRY

6. MANOMETRY
 Catheter is inserted into AC of eye
 Other end is connected to a manometric device
 MOST accurate method
 NOT feasible in humans (invasive procedure)

7. DIGITAL TONOMETRY
 Response of eyeball to pressure applied by pulp of finger
 METHOD: rest both hands on patient’s forehead &
alternately apply just enough pressure on the globe (above
the superior tarsal plate) to indent it slightly with 1 index
finger while feeling the compliance with the other
 Indents easily/ firm to touch
 NOT accurate (subjective)

8. DIGITAL TONOMETRY
 Advantages
• Easiest to perform
• No equipments
• No anesthesia
• No stain
• Estimation of IOP with irregular corneas, where
applanation tonometry is not possible.
 Disadvantages
• Reading inaccurate
• Subjective
• Over-estimation or under-estimation of IOP

9. AN IDEAL TONOMETER
 Accurate & reasonable IOP measurement
 Convenient to use
 Simple to calibrate
 Stable from day to day
 Easier to standardize
 Free of maintenance problems

10. INDENTATION TONOMETRY
 Shape of deformation -
TRUNCATED CONE
 Precise shape - variable &
Unpredictable
 Displace large intraocular
Volume
 Conversion tables based
on empirical data used
 Prototype - Schiøtz
tonometer
APPLANATION TONOMETRY
 Shape of deformation -
FLATTENING
 Precise shape – constant
 Displace small intraocular
Volume
 Mathematical calculations
for IOP
 Differentiated on the basis
of variable measured

11. SCHIØTZ TONOMETER
 Handle : to hold the instrument
 Foot plate : rests on the cornea
 Plunger : moves freely within a
shaft in the foot plate
 Bent lever : short arm and long
arm acting 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 : 5.5 g weight is
permanently fixed to the plunger,
which can be increased to 7.5 and
10gm.

12. 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
 Because the tonometer actually measures Pt, it is
necessary to estimate Po for each scale reading & weight.

14.  On the basis of,
Friedenwald formula, a set of
conversion tables for IOP was
made
 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

15. GOLDMANN APPLANATION
TONOMETER (1954)
 Goldmann based his concept of tonometry on the
Modified Imbert- Fick Law
W + S = PA1 + B
 When A1 = 7.35 mm2 S balances B & W equals P
 This internal area of applanation is obtained when diam.
of external area of applanation is 3.06 mm
 Volume of displacement produced by applanating this
area is approx. 0.50 mm3
 So P is very close to actual IOP & ocular rigidity does
not significantly influence measurements

16. GOLDMANN APPLANATION
TONOMETER (1954)
 Applanation tonometry is based on the Imbert-Fick
principle, which states that the pressure (p) inside an ideal
dry, thin walled sphere equals the force (F) necessary to
flatten its surface divided by the area of the flattening (A).
 Cornea being aspherical, wet, and slightly inflexible fails
to follow the law.
 Moisture creates surface tension (S) or capillary attraction
of tear film for tonometry head.
 Lack of flexibility requires force to bend the cornea (B)
which is independent of internal pressure.
 The central thickness of cornea is about 0.55 mm and the
outer area of corneal flattening differs from the inner area of
flattening (A1). It is this inner area which is of importance.

17.  The pressure (P) of a body of fluid encapsulated within
a sphere is directly proportional to the force (W) required
to applanate an area (A) of the sphere:
W = PA
IMBERT – FICK PRINCIPLE (1885)

18. W + S = PA1 + B
W = tonometer force
S = surface tension of pre-corneal tear film
P = intra-ocular pressure
A1 = inner corneal area of applanation
B = corneal rigidity
MODIFIED IMBERT-FICK’S LAW

19. PARTS OF GOLDMANN TONOMETER

20.  The two beam-splitting prism within the
applanating unit optically convert the circular area of
corneal contact into 2 semicircles
BIPRISM PROBE

21.  The fluorescent semicircles
are viewed through the biprism
and the force against the cornea
is adjusted until the inner edges
overlap.

22. FALSELY LOW IOP
 Inadequate flourescein
 thin cornea
 corneal edema
 with the rule astigmatism
(1mm Hg per 4 D)
 prolonged contact
 Repeated tonometry
FALSELY HIGH IOP
 Excess flourescein
 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°

23. POTENTIAL ERRORS
 Thin cornea
 Thick cornea
 Astigmatism > 3D
 Irregular cornea
 Inadequate fluorescein
 Too much fluorescein
 Tonometer out of calibration
 Repeated tonometry
 Elevating eyes > 15°
 Pressing on the eyelids or globe
 Squeezing of the eyelids
 Observer bias (expectations and even Numbers)

24. PERKINS TONOMETER (1965)
 Developed by ES Perkins
 Hand-held applanation tonometer
 Uses same prism tips as GAT
 The prism is illuminated by battery powered bulbs
 Advantages Over GAT:
• Portable & can be used in any position of pt.
• Infants/ children
• in the O.T.
• at the bedside for non-ambulatory pt.

25. GROLMAN NCT (1972)
 Introduced by Grolman in 1972
 NCT has 3 subsystems:
1. Alignment system: It aligns patient’s eye in 3
dimensions (axial/ vertical/ lateral)
2. Optoelectronic applanation monitoring system:
a. Transmitter directs a collimated beam of light at
corneal apex
b. Receiver & 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

26. GROLMAN NCT (1972)

27. GROLMAN NCT (1972)
 Advantages
• Screening procedure
• Can be operated by non-medical personnel
• No anesthesia required
• No contamination
• No chance of corneal abrasion
 Limitations
• IOP is near normal, accuracy decreases with increase
in IOP & in eyes with abnormal cornea or poor fixation
• Sub-epithelial air bubbles after repeated use of NCT
(rare)

28. PULSAIR TONOMETER (1986)
 New Non Contact Tonometer
 Keeler Pulsair Intellipuff is a portable hand-held tonometer
 Launched in European market in April 2007
 Based on the same principle as the Grolman NCT

29. TONOPEN
 The most commonly used Mackay-Marg type tonometer
today is the Tono-Pen
 FDA approved in March 2006
 Portable handheld instrument with a strain gauge that
creates an electrical signal as the footplate flattens the
cornea
 Micro-strain gauge technology
 Computerised battery - operated pocket tonometer
 Instrument is 18 cm in length and weighs 60 g
 Converts IOP into electric waves
 Wave form is internally analyzed by a microprocessor
 Average of 3 to 6 readings of IOP

30.  As the area of applanation of the Tonopen is smaller
than GAT (2.36 mm2 Vs 7.35mm2) therefore, theoretically
the difference between applanating pressure & IOP is
reduced due to reduced corneal resistance of a smaller
contact area
 It is particularly useful in community eye camps, on
ward rounds ,children, irregular surfaces, measuring
through an amniotic membrane patch graft, to read from
the sclera
 A disposable latex cover which is discarded after each
use provides infection control

31. PASCAL’S DYNAMIC CONTOUR
TONOMETER (2003)
 First totally new concept in tonometry was described by
Kanngiesser et al. in 2005
 Based on principle of contour matching
 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
 Designed to reduce the influence of biomechanical
properties of cornea on measurement of IOP

33.  Cup-like plastic device with contour matched tip
 Concave surface of radius 10.5 mm, which approx. to
the shape of a normal cornea when pressure on both sides
is equal
 Probe is placed in contact with cornea with constant
force of 1 g
 The integrated piezo-electric pressure sensor
automatically begins to acquire data
 Measures IOP 100 times per second
 A complete measurement cycle requires about 8 sec of
contact time
 The device also measures the variation in pressure that
occurs with cardiac cycle (Ocular Pulse Amplitude)

34.  DCT is more accurate than Goldmann tonometry &
pneumo-tonometer
 Not affected by corneal thickness
 IOP is not altered by corneal refractive surgery that
thins the cornea
 DCT tells us ocular pulse amplitude
 OPA may be indicative of the status of ocular blood flow
(low OPA = low ocular blood flow)
 OPA is increased over normal ( 1.2 – 4 mmHg) in most
forms of glaucoma & may be related to the level of IOP

35. CONCLUSION
 Elevated IOP has been known to be associated with
glaucoma for over a millenium
 All our instruments give us an indirect measure of IOP
 Most of the newer devices can measure higher IOPs
accurately but we are often interested in lower IOPs as
well (which is best measured by GAT)
 Despite all the limitations, GAT remains the GOLD
STANDARD for IOP measurement.

36. THANK YOU