Tonometry is a procedure to measure intraocular pressure (IOP) using various techniques such as indentation, applanation, and non-contact methods. The document details the history, advantages, and limitations of different tonometry devices, including their operation, calibration, and specific applications for diverse corneal conditions. It also discusses potential sources of error and the importance of disinfection between uses to ensure accurate and safe measurements.

1. TONOMETRY
DR LAVANYA

2. INTRODUCTION
• Tonometry is the procedure performed to determine the intraocular pressure.
• Tonometers measure the IOP by relating a deformation of the globe to the force
responsible for the deformation.

3. HISTORY
1826 William
Bowman
Digital Tonometry
1865 Von Graefe First instrument to measure IOP
1885 Maklakov First Applanation Tonometer
1905 Schiotz First Indentation Tonometer
1954 Goldmann Prototype Applanation Tonometer
1972 Grolman NCT

4. TONOMETRY
Indirect
Direct
Manometry Indentation Applanation
Digital
Tonometry

5. Indirect
Indentation
(Schiotz)
Applanation
Non contact
(Pulsair, NT 3000)
Contact
Dynamic Static
Ocular Response Analyser
Pascal DCT
Fixed Area,Variable Force Fixed force,Variable Area
Maklakov
Goldmann, Perkins,Tonopen, Pneumatonometry

6. MANOMETRY
• Needle inserted into the AC through self sealing bevelled
corneal puncture.
• Needle connected to Manometer
• Not practical in human subjects
• Needs General Anesthesia
• Breakdown of Blood Aqueous Barrier, PG release, False raise in IOP

7. DIGITAL TONOMETRY (PALPATION)
Raised IOP Very low IOP
Absent/Feeble Fluctuation
Eyeball feels firm to hard
Eye feels soft like a
partially filled Balloon

8. • Easiest to perform
• No equipment
• No anaesthesia
• No staining
• Not affected by
corneal status
• Not accurate
• Depends on
examiner(subjective)
• Overestimation or
underestimation

9. INDENTATION TONOMETRY
• SCHIOTZTONOMETRY
• A known weight is placed on the cornea
• IOP is estimated by measuring the
deformation or indentation of the globe

11. PRINCIPLE
• When plunger indents the cornea, the baseline/resting
pressure (Po) is artificially raised to a new pressure(Pt).
• Change in pressure from Po to Pt – resistance offered
by the eye (Scleral rigidity).
• Pt = Po + Scleral rigidity
• Tonometer measures Pt.To find Po, Friedenwald
developed a formula.

12. • For a given weight, the scale reading obtained is correlated with IOP using
Friedenwald Conversion Table.

13. ADVANTAGES DISADVANTAGES
Portable Falsely high/ low IOP depending on ocular rigidity
Sturdy Cannot be used in trauma, early postop, corneal
diseases
Relatively inexpensive Instrumental errors, need regular calibration
Easy to operate Muscular contractions raise IOP
Easy to maintain Accommodation decreases IOP
Does not require slit lamp or power supply Moses effect

14. • STERILISATION
• Tonometer is disassembled and soaked in 70% isopropyl alcohol and dried before
reassembling.
• Between patients, ideal to disinfect with Sodium hypochlorite.
• Can also use UV, steam, Ethylene dioxide, Hydrogen peroxide, Bleach.

15. APPLANATIONTONOMETRY
1. Variable force; measures the force Eg: GAT
2. Variable area; measures the area Eg: Maklakov
3. Non contact tonometers: uses a puff of air, measures the time or force of the air puff
that is required to create a standard amount of corneal deformation

16. GOLDMANN APPLANATION TONOMETRY
• Based on IMBERT FICK LAW :
An external force(W) against a sphere equals the pressure in the sphere(Pt) multiplied by the area
flattened(applanated) by the external force(A).
W= Pt X A
• The sphere should be a) perfectly spherical
b) dry
c) perfectly flexible
d) infinitely thin

17. • Moisture creates surface tension(S)
• Lack of flexibility requires a force to bend the cornea(B)
• As cornea has a central thickness of 550 micrometers,
outer area of flattening(A) is not same as the inner area(A1).
• MODIFIED IMBERT FICK LAW:
W+ S = PtA1 + B

18. • W+ S = PtA1+ B
• When A1= 7.35sqmm, S balances B, then W= Pt
• This internal area of applanation is obtained when the diameter of the external area of
corneal applanation is 3.06mm.
• Then ,Volume of displacement ~ 0.50cumm, so that Pt is very close to Po, ocular rigidity
does not significantly influence the measurement.

20. TECHNIQUE
Cornea anesthetized, tear film stained with sodium fluorescein.
Tonometer tip cleaned
Tension knob is se at 1gm
Widest beam, maximum illumination, cobalt blue filter
Prism is advanced until it just touches the apex of cornea

21. 2 Fluorescent semicircles are viewed through the biprism, one above and below the
horizontal midline
Force against the cornea is adjusted by rotating the
Tension knob until the inner edges overlap
The reading on the dial multiplied by 10 gives the
IOP in mmHg

22. SOURCES OF ERROR
• Inappropriate fluorescein pattern
• Pressure on the globe- anomalous high reading
• CCT- assumed to be 520micrometer; thin cornea- underestimation, thick- overestimation
• Corneal edema- artificial lowering of IOP
• Astigmatism- distorted mires
• Incorrect calibration
• Wide pulse pressure
• Repeated readings over a short period

23. FALSELY LOW IOP
1. Too little fluorescein
2. Thin cornea
3. Corneal edema
4. WTR Astigmatism(1mmHg per 4D)
5. Repeated tonometry
FALSELY HIGH IOP
1. Too much fluorescein
2. Thick cornea
3. Steep cornea
4. ATR Astigmatism(1mmHg per3D)
5. Excessive tearing
6. Pressure on globe
7. Obstruction of venous return

24. ADVANTAGES DISADVANTAGES
Most accurate Needs slit lamp
Unlike indentation, not much pressure is applied
on the cornea
Needs Anaesthesia
Does not get affected by corneoscleral rigidity Staining
IOP readings are derived from the knob Dark room
Can be done in postop/ trauma cases

25. DISINFECTION
• Applanation tip soaked for 5-15min in diluted sodium hypochlorite, 3% H2O2 or 70%
isopropyl alcohol
• Wiping with alcohol, H2O2, povidone iodine or 1:1000 Merthiolate
• Other methods: 10min rinsing in running tap water, wash with soap and water, cover the
tip with disposable film, exposure to UV light

26. PERKINS TONOMETER
• Same prisms as Goldmann Tonometry
• Can be performed in any position
• Force adjusted manually
• Illuminated by battery powered bulbs
• Portable–IOP in infants/children, operating rooms

27. DRAEGERTONOMETER
• Similar to Perkins
• Uses different set of prisms
• It has an electric motor that varies the force

28. MACKAY MARG TONOMETER
• No longer available
• A plunger with diameter 1.5mm surrounded by a rubber sleeve
• The plunger indents the cornea, movement of the plunger electronically monitored by a
transducer, recorded on a moving paper strip
• Specific utility – scarred, irregular & edematous corneas

29. TONOPEN
• Same principle as Mackay Marg
• Hand held instrument
• Built in single chip microprocessor senses the proper force
curves & averages 4-10 readings to give a final digital reading
• Battery operated
• Most accurate between10-50mmHg
• Useful in irregular surface, amniotic membrane graft
• Disposable latex cover

30. PNEUMOTONOMETER
• It is like Mackay Marg tonometer
• Plunger is replaced by a column of air
• It has a sensing device that consists of 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
• Used for continuous monitoring of IOP
• Scarred, irregular, edematous cornea

31. MAKLAKOV TONOMETER
• IOP is estimated by measuring the area of cornea flattened by a known weight
• Here, the volume displacement is large enough to consider ocular rigidity in calculating
IOP – elastometric rise
• Dumb bell shaped metal cylinder, 10mm diameter flat end plate of polished glass on
either end
• A set of 4 such instruments weighing 5, 7.5, 10 and 15gm
• A dye suspension of argyrol, glycerin & water applied to either end plate.

32. • Patient in supine position
• Cornea anaesthetized, instrument rested vertically on the cornea for 1sec
• Resultant circular white imprint on the end plate corresponds to the area of cornea that
was flattened.
• Diameter of white area is measured with transparent plastic measuring scale to 0.1mm
• IOP is read from a conversion table corresponding to the weight used.

34. NON CONTACT TONOMETER
• Introduced by Grolman
• 3 systems
1) Alignment system
2) Optoelectronic applanation monitoring system
3) Pneumatic system
• Corneal apex is deformed by a jet of air
• When reflected light reaches peak intensity, cornea is
presumed to be flattened.
• Time taken to flatten cornea correlates with IOP

35. • Screening procedure
• Can be operated by non
medical personnel
• No anaesthesia required
• No direct contact
between eye and
instrument
• Accuracy
decreases with
increase in IOP &
with poor corneal
fixation

36. PULSAIR
• New handheld portable NCT

37. REBOUND TONOMETER
• Eg: iCare
• 1.8mm plastic ball attached to a wire
• Deceleration of the probe upon contact with the cornea is
proportional to IOP
• Probe bounces faster as the IOP increases
• No anaesthesia required
• For self monitoring of IOP
• For screening in community
• Repeatable & reliable readings

38. NEWER TONOMETERS
1. Transpalpebral Tonometer
2. Disposable Tonometer: Tonosafe(acrylic biprism),Tonojet
3. Dynamic ContourTonometer
4. Ocular Response Analyser
5. ImplantableTonometer

39. TRANSPALPEBRAL TONOMETRY
• Pressure on the eyelid required to induce retinal phosphene is proportional to IOP
• Young children
• Demented or
developmentally
challenged patients
• Home monitoring
of IOP
• Thickness of eyelids
• Higher effect of scleral
rigidity
• Tone of orbicularis
• Potential intrapalpebral
scarring

40. DIATONTONOMETER
• Estimates IOP by rebound tonometry through the upper eyelid
• It calculates pressure by measuring the response of a free falling rod, based on Newton’s
second law, as it rebounds against the tarsal plate of the eyelid.
• The patient is positioned so that the tip of the device and lid are overlying the sclera

41. DYNAMIC CONTOURTONOMETRY (PASCAL)
• Based on PASCAL’S LAW OF PRESSURE
• Concave contour of probe tip generate minimal
corneal distortion, thereby eliminating errors arising
due to ocular rigidity when the cornea is applanated
with Flat Tip Tonometer
• Operated like GAT

42. • Pascal DCT utilises the principle that when the contours of
the cornea & tonometer tip match, the pressure measured at
the surface of the eye equals pressure inside the eye.
• Microchip enabled, solid- state sensor embedded within the
tip records 100 IOP measurements per second and averages
them.

43. • Digital display shows the final average IOP
and Q value(ie. Ocular pulse amplitude -
difference between diastolic& systolic IOP)
• Studies comparing DCT & GAT IOP
readings reveal that DCT provides more
physiological measurements.

45. DRAWBACKS OF DCT
• More time consuming than GAT as 5 cardiac cycles need to be recorded
• Doesn’t seem as useful in diseased corneas or after corneal transplant
• Tends to read a little higher, in general than AT in people with thin or average thickness
corneas
• Recurrent cost of disposable tip

46. OCULAR RESPONSE ANALYSER
• Eg: ReichertTonometer
• Non contact tonometer
• Utilises air-puff technology to record two Applanation
measurements: one while the cornea is moving inward and
one when the cornea returns to its normal position.
• Average of these two gives Goldmann correlated IOP
• Difference between the two – Corneal Hysteresis

47. • Corneal Hysteresis is a measure of the viscous dampening and hence the biochemical
properties of the cornea.
• Biochemical properties of cornea are related to corneal thickness and include elastic and
viscous dampening attributes
• The machine uses this value to correct for the effects
of cornea on the IOP measurement

48. • Provides IOP measurement less
affected by factors such as previous
Laser Refractive Surgery
• Low Hysteresis value associated with
greater risk of Glaucoma Progression
• Biomarker to aid Glaucoma case detection

49. CONTINUOUS IOP MONITORING
• Contact lenses like SENSIMED TRIGGER FISH measure IOP by detecting changes in the
corneoscleral curvature induced by IOP changes
• Used for 24 hour IOP recording including sleeping hours
• It uses a sensor- soft hydrophilic single use contact lens, containing passive & active strain
gauges embedded in the silicone- fluctuations in diameter of corneoscleral junction.
• Output signal sent wirelessly to the antenna
• Adhesive antenna, worn around the eye is connected to a portable recorder through a thin
flexible data cable.

51. TONOMETRY ON IRREGULAR CORNEAS
• Scarred or edematous corneas, the Mackay MargTonometer is considered to be the
most accurate
• Pneumatic tonometers are also useful in patients with diseased corneas
• Tonopen also compared favourably with Mackay Marg on irregular corneas

52. TONOMETRY OVER SOFT CONTACT LENSES
• The Mackay Marg tonometer, Pneumotonometer &Tonopen can measure IOP through
bandage contact lenses with reasonable accuracy.
• Applanation tonometers are affected by the power of the contact lens with high water
content & correction tables are developed to compensate it.

53. TONOMETRY WITH GAS FILLED EYES
• Intraocular gas significantly influences scleral rigidity rendering indentation tonometry
unsatisfactory.
• Pneumotonometer underestimates GAT readings in gas filled eyes whileTonopen
compared favorably with GAT readings.
• Both instruments significantly underestimated the IOP at pressures greater than
30mmHg

54. TONOMETRY WITH FLAT ANTERIOR CHAMBER
• IOP readings from the GAT, Pneumotonometer andTonopen do not correlate well with
manometrically determined pressures

55. TONOMETRY IN EYESWITH KERATOPROSTHESES
• In patients at high risk for corneal transplant rejection, implantation of a keratoprosthesis
is now viable option for vision rehabilitation
• Most keratoprosthesis 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.

56. THANKYOU…