This document discusses intraocular pressure (IOP) measurement. It begins by defining IOP as the pressure exerted by intraocular fluids on the eyeball coats. The normal IOP range is then provided as 10.5-20.5 mm Hg. Factors that can affect IOP levels are described such as aqueous secretion, outflow resistance, and episcleral venous pressure. Common methods of measuring IOP are then outlined, including digital assessment, manometry, indentation tonometry using the Schiotz tonometer, and applanation tonometry using the Goldmann tonometer. Advantages and limitations of each method are summarized.

1. IOP MEASUREMENT
DR. PREETIILAL
MBBS, MS, FLVPEI
DIRECTOR,
DR. NANDLAL HOSPITAL AND RESEARCH CENTRE,AJMER

2. IOP MEASUREMENT
INTRA OCULAR PRESSURE(IOP)
• Pressure exerted by the intraocular fluids on the coats of the
eyeball
• Normal range : any pressure that doesn’t induce
glaucomatous changes in the optic nerve head.
• 10.5 – 20.5 mm of Hg (15.5+/- 2SD)

3. IOP MEASUREMENT
FACTORS AFFECTING LEVEL OF IOP:
• Rate of aqueous secretion
• Resistance encountered in outflow channels
• Level of Episcleral Venous Pressure (N 8-10 mmHG)
 AQUEOUS FORMATION:
▪ Aqueous humor is secreted by the Nonpigmented
epithelium of the ciliary processes of ciliary body from a
substrate of blood plasma.
• Flow rate is 2-3 microlitre/ min.

4. IOP MEASUREMENT
Significance of Aqueous humor
1. Nutrition to lens, cornea and iris
2. Medium for removal of metabolic toxic products
3. Has an RI of 1.33
4. Inflates the globe and maintains IOP
5. Facilitates cellular and humoral response of eye
towards inflammation & infection.

5. IOP MEASUREMENT
The aqueous enters the posterior chamber by:
• ACTIVE : Energy dependent secretion of certain ions and
substrates
• PASSIVE: Diffusion & Ultrafiltration

6. IOP MEASUREMENT
DRAINAGE
• CONVENTIONAL OUTFLOW: 75-90%
Trabecular Meshwork Schlemm’s Canal Collector Channels
(Intrascleral Plexus- Indirect) Episcleral and Conjunctival Veins
Cavernous Sinus
• UVEOSCLERAL OUTFLOW: 10-25%
Ciliary Body Supra-Choroidal Space Venous Circulation of Ciliary
Body Choroid Sclera Orbit
The outflow is approx. 0.3 ml/min and is independent of IOP changes

7. IOP MEASUREMENT
Why IOP measurement?
 Because ….
- IOP is the only modifiable risk factor in Glaucoma
- most frequently examined parameter in the follow up
of a glaucoma patient.

8. IOP MEASUREMENT
CONDITIONS THAT INFLUENCE THE IOP:
 1. Diurnal variation
 2. Postural variation
 3. Exertional influences
 4. Lid and eye movement
 5. Intraocular conditions
 6. Systemic conditions
 7. Environmental conditions
 8. General anesthesia
 9. Food and drugs

9. IOP MEASUREMENT
DIURNAL VARIATION:
• The most common pattern is that the IOP is maximum in the morning
and decreases as day progresses and becomes minimum in the
evening because of endogeneous catecholamines.
It once again starts to rise as the night progresses.
• The variation is about 3 to 6 mm Hg in normal individuals and about 10
mm Hg or more in a glaucomatous eye.
• It is due to cyclic fluctuation of blood levels of adrenocortical
steroids. Maximum IOP is reached 3 to 4 hours after the peak of plasma
cortisol.

10. IOP MEASUREMENT
• The night time elevated IOP is due to the supine position along with
the fluctuating cortisol levels.
There are four types of diurnal variation curves.
Falling type: maximal at 6–8 am followed by a continuous decline
Rising type: maximal at 4–6 pm
Double variation type: with 2 peaks 9–11 am and 6 pm
Flat type of curve

11. IOP MEASUREMENT
• POSTURE: The IOP rises (0.3–6 mm Hg) when a person is lying down.
This may be because of increase in the episcleral venous pressure in
the supine posture.
• EXERTION: Valsalva maneuvers increases IOP (by increasing
episcleral venous pressure) while prolonged exercise decreases IOP
(by metabolic acidosis and increased colloid osmotic pressure)
• HARD LID SQUEEZING: Increases IOP because of increased
orbicularis tone

12. IOP MEASUREMENT
INTRAOCULAR CONDITIONS:
• Acute anterior uveitis causes a slight reduction in IOP because of
decreased aqueous humour production.
• Rhegmatogenous retinal detachment also causes a reduction
because of reduced aqueous humor production as well as shunting
of aqueous humour from the posterior chamber through the vitreous
and retinal hole into the subretinal space.

13. IOP MEASUREMENT
SYSTEMIC CONDITIONS CAUSING
INCREASED IOP:
 1. Systemic hypertension
 2. Systemic hyperthermia
 3. ACTH and growth
hormone stimulation
 4. Hypothyroidism
 5. Diabetes
SYSTEMIC FACTORS CAUSING
DECREASED IOP:
 1. Pregnancy
 2. Hyperthyroidism
 3. Myotonic dystrophy

14. IOP MEASUREMENT
• ENVIRONMENTAL CONDITIONS:
Exposure to cold decreases IOP (because of lowered episcleral
venous pressure)
Reduced gravity increases IOP
• ANESTHETIC AGENTS: In general,
general anesthetic agents reduce IOP.
However, tricholoroethylene, ketamine, succinylcholine and
suxamethonium increase IOP

15. IOP MEASUREMENT
• FOOD AND DRUGS:
Factors increasing IOP: Factors decreasing IOP:
i. Caffeine i. Alcohol
ii. Tobacco smoking ii. Heroin and marijuana
• HEREDITY: IOP tends to be higher in individuals with enlarged cup-
disk ratio and in relatives of open-angle glaucoma

16. IOP MEASUREMENT
 Also known as tonometry
 Device used to measure intra ocular pressure is tonometer.
 In 1865, Donders designed the first tonometer intended for
use against the sclera and
 Priestley Smith in 1884, came up with something similar.

17. IOP MEASUREMENT
FACTORS INFLUENCING TONOMETRY:
 CENTRAL CORNEAL THICKNESS:
Increase in corneal rigidity increased IOP reading
 ASTIGMATISM
 CORNEAL CURVATURE
 OCCULAR RIGIDITY:
• resistance to deformation of the ocular coats.
• It is decreased in Acutely elevated IOP, Osteogenesis imperfecta,
Miotic therapy, Vasodilator therapy, Myopic eyes.
• The increase in ocular rigidity is seen in ARMD, Hypermetropic eyes
and long standing glaucoma

18. IOP MEASUREMENT

19. IOP MEASUREMENT

20. IOP MEASUREMENT
Digital assessment
 It is the response of the intact eyeball to the pressure exerted
on it by the pulp of finger.
 Alternately just enough pressure is applied on the eyeball
to indent it with the index finger of one hand while feeling
the compliance with the other.

21. IOP MEASUREMENT
Advantages:
 It is easy to perform
 no equipment required
 No anesthesia is required
 Helps to estimate the IOP in irregular corneas,
where applanation tonometry isn’t possible.
Disadvantages :
 It is inaccurate and often subjective;
 leads to over or under estimation of the IOP.

22. IOP MEASUREMENT
IDEAL TONOMETER:
• Must be accurate in its measurement
• Should be convenient to use
• Calibration should be simple
• Easy Standardization
• Maintenance should be hassle-free

23. IOP MEASUREMENT
MANOMETRY:
▪ The only direct measurement of IOP.
▪ A needle is introduced into the AC or the vitreous.
▪ And then connected to a mercury or
water manometer

24. IOP MEASUREMENT
 Uses:
Continuous measurements of IOP;
Used in animal eyes for research and experimental purposes
 Disadvantages:
An impractical method for use on humans;
Needs general anesthesia;
Introduction of needle produces breakdown of blood aqueous
barrier and release of prostaglandins which alter the IOP

25. IOP MEASUREMENT
Indentation Tonometry
•The shape of deformation is a truncated cone.
•It is based on the principle that the plunger would indent a soft eye
more than the hard one
•SCHIOTZ TONOMETER
It was devised in 1905
The most popular,

26. IOP MEASUREMENT
Indentation Tonometry
•When the tonometer is placed on the cornea, the following
forces become relevant :
W- Weight of the tonometer
A- Area of indentation
Vc- Volume of fluid displaced after indentation
T- Tensile forces of the outer coats of the eyeball

27. IOP MEASUREMENT
 The resting intraocular raises to a new artificial value.
 The scale reading of tonometer actually measures the
artificially raised Intraocular pressure.
 The conversion ofread pressure to resting pressure is elicited
from the conversion tables developed by Friedenwald.
 The calibration was carried by experiments in cadaveric
eyes connected with manometer through cannula.
 The observation were plotted on semi log scale, which
serve as Friedenwald nomogram where 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.

28. IOP MEASUREMENT

29. IOP MEASUREMENT
COMPONENTS OF SCHIOTZ
 Handle to hold the instrument in vertically
onto the cornea
 Footplate that rests on the corneal surface
 A Plunger that moves freely within a shaft in
footplate
 A Bent lever whose short arm rests on upper
end of plunger.
 A long arm which acts as pointer needle
 Weights of 5.5 gm that is permanently fixed to
the plunger and can be increased to 7.5 and
10gm.

30. IOP MEASUREMENT
Schiotz Tonometry
 Patient should be anesthetised with 4% lignocaine or 0.5%
proparacaine.
 With the patient in supine position, looking up at a fixation target,
while the examiner would separate the lids and lower the tonometer
plate to rest on the cornea so that plunger becomes free to move.
 5.5 gm is the initial weight that is used.
 If the scale reading is 4 or less, additional weight is added.
 IOP measurement is repeated until 3 consecutive readings agree
within 0.5 scale units.
 Conversion table is used to derive IOP in mmHg. from scale reading
and plunger weight.

31. IOP MEASUREMENT
Scope of Error
 In the instrument,
Might be due to the difference in shape, weight, size and
curvature of footplate
 Contraction of extra ocular muscles tend to increase IOP
 Due to accommodation,
With the patient looking at the tonometer resulting in
accommodation causes lowering of IOP because of
contraction of ciliary muscle

32. IOP MEASUREMENT
Scope of Error
 Moses effect:
In low scale reading, the cornea might mold into the space
between the Plunger and hole, pushing the plunger up and giving
false high IOP reading.
► Due to ocular rigidity
► Due to variation in corneal curvature:
Steep and thick cornea will cause greater displacement of fluid
causing high IOP readings

33. IOP MEASUREMENT
ADVANTAGES
 Simple Technique
 Elegant design
 Portable
 No need for slit lamp or power supply
 Economical
 Widely used tonometer
DISADVANTAGES
 Falsely high/low IOP in Ocular rigidity
 Cannot be used in traumatic cases and corneal
pathologies

34. IOP MEASUREMENT
APPLANATION TONOMETRY
 It was introduced by Goldmann in 1954.
 The shape of deformation is flattening
 It is based on IMBERT FICK LAW.
 The prism applanates the cornea in an area of 3.06 mm diameter.
 Variable force, Fixed area of applanation
 For a very thin walled sphere, perfectly elastic and dry,
the internal pressure is equivalent to the force per unit
area of applanation.

35. IOP MEASUREMENT
APPLANATION TONOMETRY
 It states that the pressure inside an ideal sphere (P) is equal to
force (W) required to flatten area(A)
P=W/A
 GAT is influenced by corneal thickness,
corneal curvature,
structure of the cornea

36. IOP MEASUREMENT
Modified Imbert-Fick’s Law
W + S = P A1 + 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

37. IOP MEASUREMENT
Goldmann Applanation Tonometer
 Most popular and accurate(Gold Standard)
 Double prism mounted on slit lamp
 Measures the force required to applanate the cornea over
a circular area of 3.06mm².
 Significance of 3.06 mm² area is
* amount of fluid displacement is negligible(0.5μl)
* surface tension of precorneal film and force required to
counteract corneal rigidity act opposite to each other
* tonometer force becomes equal to the force in mmHg
❑ Area applanated on the cornea is 7.35mm²

38. IOP MEASUREMENT
Goldmann Applanation Tonometer

39. IOP MEASUREMENT
Technique
 Topical anesthesia is given, correct eye height is set via
the chin rest.
 The tear film is stained with fluorescein.
 The patient should blink the eyes once or twice to
spread the fluorescein stained tear film over the cornea.
 Keep eyes open wide

40. IOP MEASUREMENT
Technique
 The cornea and biprisms are illuminated with cobalt blue light.
 Meticulously the apex of cornea is just touched by the biprism.
 Applanation force against cornea is adjusted until inner edges of
two semicircles just touches.
Applanation tonometry rings viewed through Goldmann biprism

41. IOP MEASUREMENT
Scope of Error
Falsely low IOP Falsely high IOP
Too little flourecein Too much fluoresce
Thin cornea Thick cornea
Corneal edema Steep cornea
WTR astigmatism ATR astigmatism
Prolonged contact Putting pressure on the lids while
Repeated tonometry separating
Observer bias

42. IOP MEASUREMENT
Advantages
 Highly accurate
 Do not requires supine position
 Portable
Disadvantages
 Not portable as it needs a slit lamp to view
 Costly
 Reading error if the cornea is scarred

43. IOP MEASUREMENT
Perkins Tonometer
 It uses the same biprism as the Goldmann applanation.
 The light source is powered by battery.
 The readings are consistent and compared quite well with
the Goldmann applanation.
ADVANTAGES OVER GAT:
 Portable & counter-balanced, so it can be used
in any position.
 Useful in examining Infants and children
 For bedside examination for non-ambulatory
patients who cannot be examined at the slit lamp.

44. IOP MEASUREMENT
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.

45. IOP MEASUREMENT
Pneumatic tonometer
 High displacement tonometer
 Can also be used to measure outflow facility
 Printouts of IOP over time
 measuring IOP fluctutations possible.
 Measurements independent of CCT
 Post LASIK

46. IOP MEASUREMENT
Tonopen
 This is small, handheld Mackay Marg type
computerised pocket tonometer
 The instrument is 18 cm in length and weighs 60 g.
 It converts IOP into electric waves
 Wave form is internally analyzed by a
microprocessor
 Average of 3 to 6 readings of IOP

47. IOP MEASUREMENT
Tonopen
 As the area of applanation of the Tonopen is smaller
than GAT 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

48. IOP MEASUREMENT
PASCAL DYNAMIC CONTOUR TONOMETER
 It is a slit lamp mounted device.
 The contact surface is contoured to compliment the
average corneal profile.
 Contour matching- Independent of force applied
or the contact area
 Hence, the device is insignificantly affected
by corneal thickness, curvature, optical
aberrations, or surface irregularity.
 Maintains the corneal touch over the course of
the cardiac cycle (5-8 secs)
 The measured OPA is also displayed

49. IOP MEASUREMENT
NON-CONTACT TYPE
 Air puff tonometer :- In this central part of cornea is
flattened by a jet of air. This tonometer is very good for
mass screening as there is no danger of cross infection and
local anesthetic is not required.
 Pulse air tonometer :- It is a non-contact tonometer that
can be used with the patient in any position.

50. IOP MEASUREMENT
GROLMAN NON-CONTACT TONOMETER
 Introduced by Grolman in 1972 and works
on the principle of a time interval.
 Measuring the time from initial generation
of the puff of air where cornea gets flattened
(in milliseconds) to the point where the
timing device stops

51. IOP MEASUREMENT
The NCT has 3 sub-systems:
1. Alignment system:
It aligns patient’s eye in 3 dimensions (axial/vertical/lateral)
2. Optoelectronic applanation monitoring system:
Comprises a transmitter, a receiver&detector and a timer
• Transmitter directs a collimated beam of light at corneal apex
• Receiver & detector accept only parallel coaxial rays of light
reflected from cornea
• 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

52. IOP MEASUREMENT
 A puff of air jet creates a constant force that
momentarily
flattens the cornea.
 The force of air jet 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 & correspondingly to
IOP
 The time from an internal reference point to the
moment of flattening is measured & converted to IOP.

53. IOP MEASUREMENT
Non contact tonometer
 A fast and simple way to screen for high IOP
 Modern non contact tonometers have been shown to
correlate well with Goldmann applanation tonometery
measurements.
 Particularly useful
in children and other non compliant patient groups
 Reduces the potential for disease transmission.

54. IOP MEASUREMENT
REBOUND TONOMETER
 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.
 The rebound tonometer is a handheld portable
instrument with distinct advantages for the pediatric
population because the light-weight probe makes
only momentary contact with the cornea
(i.e. 0.25 to 0.30 meters/seconds)

55. IOP MEASUREMENT
REBOUND TONOMETER
 Its main advantage
▪ Topical anesthesia is not required.
▪ It is particularly useful in children who do not
tolerate Goldmann applanation tonometry.
▪ It has also been used in home settings
▪ In situations when patients are unable to
follow-up for frequent IOP assessments
▪ To monitor diurnal variations
(Icare HOME; Icare USA)

56. IOP MEASUREMENT
OCULAR RESPONSE ANALYSER
 It measures the corneal response to indentation by a rapid
air pulse.
 A fully automated alignment system positions an air tube to a
precise position relative to the apex of the cornea.
 Once aligned, a 25 millisecond air pulse applies pressure
to the cornea. The air pulse causes the cornea to move
inward, past applanation and into a slight concavity before
returning to normal curvature.
 Corneal deformation is recorded via an electrooptical
infrared (IR) detection system
 (similar to the classical air-puff tonometer).

57. IOP MEASUREMENT
 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.

58. IOP MEASUREMENT
 The first is the resting intraocular pressure. The difference between
the first and the second applanation pressure is called corneal
hysteresis.It is the "energy absorption capability" of the cornea
 This is because of the speed at which the cornea is deformed
during the dynamic bi-directional applanation process in ORA

59. IOP MEASUREMENT
TRANSPALPEBRAL TONOMETER
 Diaton tonometer (BICOM Inc )
 Measuring intraocular pressure through the eyelid.
 Transpalpebral tonometry does not involve contact with the
cornea and does not require sterilization of the device or topical
anesthetic during routine use
 Moderate correlation with those provided by applanation
tonometer.

60. IOP MEASUREMENT
TRANSPALPEBRAL TONOMETER
 Diaton tonometer calculates pressure by
measuring the response of a free falllling rod.
 The principle is 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 sclera.

61. IOP MEASUREMENT
SPECIAL SITUATIONS
 In eyes with corneal edema, significant surface irregularity,
scarring and recent corneal grafts:
• PNEUMOTONOMETRY
• TONOPEN,
• DCT (It gives more accurate measurement than the GAT)
 In children:
• PERKIN’S
• I-CARE (Rebound tonometry)
• TONOPEN

62. IOP MEASUREMENT
 Post keratorefractive surgery:
• DCT
• Ocular Response Analyser
 In Supine position:
• PERKIN’s
• PNEUMOTONOMETER
• TONOPEN: Less accurate but more practical and commonly used
 Tonometry over Soft Contact Lenses
• PNEUMOTONOMETER
• TONOPEN can measure with reasonable accuracy the IOP through bandage
contact lenses.

63. IOP MEASUREMENT
 Tonometry in Gas-Filled Eyes
• Intraocular gas significantly affects scleral rigidity.
In a study with irregular corneas after vitrectomy and air-gas
fluid exchange, readings with the TONOPEN and
PNEUMOTONOMETER were highly correlated.
 Tonometry in Flat Anterior Chamber
In human autopsy eyes with flat anterior chambers,
IOP readings from the GAT, PNEUMOTONOMETER and TONOPEN did
not correlate well with manometrically determined pressures

64. THANKYOU