Tonometry

Tonometry

1. Tonometry

2. The intraocular pressure (IOP)
The measurement of IOP (ocular
tension) should be made in all
suspected cases of glaucoma and in
routine after the age of 40 years

3. •Normal IOP range is 10-21 mm of Hg with an
average tension of 16 ± 2.5 mm of Hg.
• When IOP is less than 10 mm of Hg, it is
called hypotony. An IOP of more than 21 mm
of Hg should always arouse suspicion of
glaucoma and such patients should be
thoroughly investigated.

4. TONOMETER
The intraocular pressure (IOP) is measured with
the help of an instrument called Tonometer.

Two basic types of Tonometers available
are:-
1.indentation or Impression Tonometer.
2. Applanation Tonometer

5. Indentation Tonometry
•Indentation or (impression)
Tonometry is based on the
fundamental fact that a plunger
will indent a soft eye more than
a hard eye.
•The indentation tonometer in
current use is that of Schiotz.

6. Schiotz Tonometer

Because of its simplicity, reliability,
low price and relative accuracy, it is
the most widely used tonometer in
the world.

7. Schiotz Tonometer

8. It consists of:-
•Handle for holding the instrument in vertical
position on the cornea;
• Footplate which rests on the cornea;
• Plunger which moves freely within a shaft in
the footplate;

9. •Bent lever whose short arm rests on the upper
end of the plunger and a long arm which acts 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: a 5.5 g weight is permanently fixed to
the plunger, which can be increased to 7.5 and 10
gm.

10. Technique of Schiotz Tonometry
•Before Tonometry, the footplate and lower end
of plunger should be sterilized.
Sterilized By
• Dipping the footplate in ether, absolute
alcohol,acetone or by heating the footplate in
the flame of spirit.

11. CONT…..
•After anaesthetising the cornea with 2-4
per cent topical xylocaine, patient is made
to lie supine on a couch and instructed to
fix at a target on the ceiling. Then the
examiner separates the lids with left hand
and gently rests the footplate of the
tonometer vertically on the centre of
cornea. The reading on scale is recorded
as soon as the needle becomes steady .

12. It is customary to start with 5.5 gm weight.
However, if the scale reading is less than
3, additional weight should be added to the
plunger to make it 7.5 gm or 10 gm.

13. • In the end, tonometer is lifted and a drop
of antibiotic is instilled.
•A conversion table is then used to
derive the intraocular pressure in mm of
mercury (mmHg) from the scale
reading and the plunger weight.

14. CONT………..

15. Advantages of Schiotz tonometer
•cheap, handy and easy to use.
Disadvantage
•It gives a false reading when used in eyes with
abnormal scleral rigidity. False low levels of IOP
are obtained in eyes with low scleral rigidity seen
in high myopes and following ocular surgery.

16. ERRORS OF INDENTATION
TONOMETRY
•Error inherent in the instrument
•Error due to contraction of extraocular muscles
•Error due to accommodation
•Error due to ocular rigidity

17. •Error due to variation in corneal curvature
•Errors in scale reading
•Blood volume alteration

18. Applanation Tonometry
•The concept of applanation tonometry was
introduced by Goldmann is 1954.
It is based on Imbert-Fick law which states that
the pressure inside a sphere (P) is equal to the
force (W) required to flatten its surface divided
by the area of flattening (A); i.e., P =W/A.

19. Cont…….

20. The commonly used applanation
tonometers are
Goldmann tonometer.
• Currently, it is the most popular and
accurate tonometer. It consists of a double
prism mounted on a standard slit-lamp.
The prism applanates the cornea in an
area of 3.06 mm diameter.

21. Technique
•After anaesthetising the cornea with a drop of 2
per cent xylocaine and staining the tear film
with fluorescein patient is made to sit infront of
slit-lamp.
•The cornea and biprisms are illuminated with
cobalt blue light from the slit-lamp.

22. •Biprism is then advanced until it just touches the
apex of cornea. At this point two fluorescent
semicircles are viewed through the prism.
• Then, the applanation force against cornea is
adjusted until the inner edges of the two
semicircles just touch. This is the end point.
•The intraocular pressure is determined by
multiplying the dial reading with ten.

23. Technique of applanation tonometry

24. Perkin’s applanation tonometer
1.This is a hand-held tonometer utilizing the
same biprism as in the Goldmann applanation
tonometer.
2. It is small, easy to carry and does not require
slit lamp.
3. It requires considerable practice before,
reliable readings can be obtained.

25. Perkin’s applanation tonometer

26. 3. Pneumatic Tonometer
• In this, the cornea is applanated by touching its
apex by a silastic diaphragm covering the
sensing nozzle (which is connected to a central
chamber containing pressurised air).
• In this tonometer, there is a pneumatic-to-
electronic transducer, which converts the air
pressure to a recording on a paper-strip, from
where IOP is read.

27. End point of applanation tonometry. (A) too
small; (B) too large; (C) end point

28. 4. Pulse air tonometer
Tonometer that can be used with the patient in
any position.
5. Tono-Pen
is a computerised pocket tonometer. It employs a
microscopic transducer which applanates the
cornea and converts IOP into electric waves.

29. Digital tonometry
•A rough estimate of IOP can be made by digital
tonometry.
•For this procedur patient is asked to look down
and the eyeball is palpated by index fingers of
both the hands, through the upper lid, beyond
the tarsal plate.

30. Cont….
• One finger is kept stationary which feels the
fluctuation produced by indentation of globe by
the other finger .
•It is a subjective method and needs experience.
•When IOP is raised, fluctuation produced is
feeble or absent and the eyeball feels firm to
hard. When IOP is very low eye feels soft like
a partially filled water Bag.

31. Technique of digital tonometry

33. Thank u