This document discusses different types of tonometers used to measure intraocular pressure. It describes indentation tonometers like the Schiotz tonometer which measures indentation depth from a known weight. Applanation tonometers like the Goldmann tonometer flatten a circular area of the cornea to measure pressure. Non-contact tonometers use an air puff to detect inward and outward corneal movement at different pressures. Accuracy can be affected by corneal properties, accommodation, and proper technique.

1. TONOMETERS
MAAZ UL HAQ
OPTOMETRIST

2. INTRODUCTION
• Intraocular Pressure & its significance
• Factors influencing IOP level
• WHAT IS A TONOMETER???

3. TYPES OF TONOMETERS
TONOMETER
CONTACT TONOMETER
1.Indentation Tonometer
2.Applanation Tonometer
• Variable force
• Variable area
NON-CONTACT TONOMETER
• Air Puff Tonometer

4. Indentation Tonometer
• Indentation tonometer measures the depth of the impression
or indentation by a small plunger carrying a known weight
• Principle: the degree of indentation is inversely proportional to
intraocular pressure
• The degree of indentation is depend on the flexibility of the co
rnea, surface tension and elasticity of the walls of the eyeball

5. SCHIOTZ TONOMETER
Invented in 1905 & modified by Schiotz in 1924.
Parts:-
1. Handle 5. Scale with markings
2. Cylinder 6. Weights
3. Plunger assembly 7. Metal sphere
4. Bend Lever

7. PARTS
1. HANDLE:-
• For holding the instrument in vertical position
on cornea

8. 2. CYLINDER:-
• At lower end forms a concave foot plate
• Has a free floating barrel with a footplate
of 10.1mm diameter
• Radius of curvature of foot plate is 15mm
• whereas that of cornea is 7.8mm
• When foot plate is placed on cornea the c
entral part of concavity only touches the c
ornea
• The periphery of foot doesn’t touch the c
ornea

9. 3. PLUNGER ASSEMBLY:-
• Solid cylinder
• 1.5mm in diameter
• It passes through hole in centre of cylinder wit
h foot plate & can be moved up & down
• Lower end of plunger has a slightly convex surf
ace which indents the cornea when placed ov
er it

10. 4. BEND LEVER:-
• Has a shorter arm resting on upper edge of
plunger
• Long arm that acts as a pointer moves agai
nst scale

11. 5. SCALE WITH MARKINGS:-
• Pointer glides against scale according to mo
vement of plunger
• Interval between consecutive marking on sc
ale represent displacement 0.05mm of top
of plunger relative to foot plate

12. 6. WEIGHT:-
• Three types of weights are used
• Additional weights are 7.5 & 10gms
• Plunger itself weighs about 5.5gms which is marked on circ
ular disc above upper part of plunger

13. 7. METAL SPHERE:-
• Used as dummy cornea
• Radius of curvature is 1.5mm

14. PROCEDURE
1. The tonometer should be sterilized
2. The patient is informed about the procedure
3. Topical anesthetic (0.5 % Proparacaine or 0.4%
Benoxinate) is instilled
4. Patient is seated in semi supine position with f
ace turned upwards
5. Instruct the patient to keep his both eyes open
6. Place 7.5g on the plunger and hold the instrum
ent with thumb and first finger

15. PROCEDURE CONT..
7. Hold patient’s lids apart gently, not applying press
ure on the globe
8. Place the tonometer so that the foot plate rests c
entrally on cornea and the instrument is vertical
9. The pointer should show slight fluctuations and t
he mean is recorded
10.Convert the reading in mmHg
11.Repeat the procedure for the other eye
12.Inspect the cornea for any abrasion

16. • Errors inherent in instrument:-
- Difference in weight of different part of instru
ment
- Friction arising in working of plunger
- Shape & curvature of foot plate
- Smoothness of gliding movement of the point
er on the scale
ERRORS

17. - During accommodation there is contraction of
ciliary muscles thus increasing aqueous flow d
ue to pull on trabecular meshwork at angle of
A/C & thus causing lowering of IOP
• Errors due to contraction of EOM’s
• Errors due to rigidity of outer coats of eyeball
• Errors due to corneal curvature
Errors due to accommodation

18. Advantages of Schiotz Tonometer
• Availability (portable)
• Can measure IOP of the eye with scarred cornea
• Simplicity of the design
• Can be used on opaque corneas
• low cost

19. Applanation Tonometer
• The first applanation tonometers were designed by Makla
kow(1885) and Weber(1867)
• It is based on Imbert Fick law
• It states that pressure within a sphere(P) is equal to the ex
ternal force(F) needed to flatten a portion of the sphere di
vided by the area(A) of the sphere which is flattened
• IOP = tonometer weight (g)/ applanated area (mm )
• P = F/A
• P = Pressure
• F = Force
• A = Area of sphere

20. • It is correct only for spherical container, with a thin li
miting membrane, perfectly flexible, elastic and dry
• Cornea satisfies none of these criteria
• It is thick and has some structural rigidity and is wet a
llowing tear meniscus to form between the tonomete
r head and the cornea
• Two types of Applanation tonometers
• Variable weight tonometer
• Variable area tonometer

21. EXAMPLES
Variable force
• Goldmann
• Perkins
• Draeger
• Mackay-marg
Variable area
• Maklakow
• Applanaometer
• Tonomat
• Barraquer

22. Two parts
• Mechanical device - It exerts force against the
cornea
• Contact element-It indicate the degree of flatte
ning of cornea

23. Goldmann applanation tonometer
Bi-prism
Light source

24. Goldmann Applanation Tonometer
• It measures the forces required to applanate (fl
atten) the cornea over a circular area of diame
ter 3.06 mm
�Why 3.06 mm?
- amount of aqueous displacement with such a
small applanated area is negligible
- corneal rigidity & capillary attraction cancel e
ach other when the flattened area has dia.of 3.
06mm
- simple conversion
Area applanated on the cornea is 7.35mm2

25. • Its is attached to slit lamp
• Compsed of an applanating head
and a spring-loaded lever
• Prisms with their based in
opposite directions are placed
within the head of the
tonometer
• These prisms split the
applanated area into two and
separate the two half images by
3.06mm

26. Procedure
1. The dry, clean tonometer probe with
its zero axis aligned to the reference
mark on the holder is inserted into t
he holder
2. The patient is informed about the pr
ocedure
3. A mild topical anesthetic is instilled a
nd allow 30sec
4. Place fluorescein into the lower forni
x
5. The room lights should be dimmed &
slit lamp illumination system is wide
opened with cobalt blue filter

27. 6. The illumination system should be a
pprox. 60° to the lateral side of the
eye to be measured
7. Instruct the patient to keep the eyes
wide open & look in the direction of
the adjustable target
8. The wheel is adjusted until the fluor
escein rings apposition each other
9. The IOP is measured by multiplying t
he reading with 10
10.Repeat the measurement for the oth
er eye
11.The cornea is examined for any dam
age
12.The tonometer is to be sterilised

28. Low IOP
Normal
High IOP

29. ERRORS
• Lids touching the probe
• Surface tension of the tears altered by the fluids other than wa
ter to wet fluorescein
• Improper cleaning of the head
• Corneal astigmatism
• Repeated tonometry
• Elevating eyelids more then 15°
• Inadequate fluorescein in precorneal T.F

30. ADVANTAGES
• It displaces only 0.56 ul of aqueous humour
and hence the factor of scleral rigidity is
almost negligible
• Displacement of 0.56 ul aqueous increases
IOP by about 2.5 %

31. Checking the calibration of
applanation tonometer

32. PERKINS
TONOMETER

33. MACKAY-MARG TONOMETER

34. TONOPEN
■ Mackay marg principle

35. PNEUMO TONOMETER

36. (constant force)
Maklakov Tonometer
▪ Principle:
▪ Impression – AT
▪ IOP estimated by measuring
the diameter of corneal area flattened by a known
weight of tonometer.
▪ P=W/∏(d/2)2 grams/cm2 - divide this value by 1.3
6, a conversion to mm hg.

37. NON-CONTACT TONOMETER

38. OTHER TYPES OF TONOMETERS
• Dynamic contour tonometer
• Electronic indentation tonometer
• Rebound tonometer
• Non-corneal and Transpalpebral tonometry
• Ocular response analyzer
• OCT Tonometer

39. Transpalpebral(through eyelid) diaton
tonometry
• It measures IOP through eyelid
• Requires no contact with cornea, therefore ste
rilizationRequires no contact with cornea, ther
efore sterilization of the device and topical ane
sthetic drops are not required and there is very
little risk of infection
• It is useful in post operatives, childrens,
• and in corneal pathology cases

40. Ocular response analyzer
The difference between the pressures at which the
cornea flattens inward and outward is measured by
the machine