Intraocular pressure (IOP) is the fluid pressure inside the eye, which is normally between 10-22 mm Hg. IOP can be measured using indentation tonometry like the Schiotz tonometer or applanation tonometry like Goldmann tonometer. The Schiotz tonometer uses weighted plungers to indent the eye, while Goldmann tonometer uses two prisms to flatten a small area of the cornea to measure pressure. Other techniques include pneumatic tonometry and air puff tonometry. Accurately measuring IOP is important for diagnosing and managing glaucoma.

1. INTRAOCULAR PRESSURE

2. • Intraocular pressure (IOP) is the fluid pressure inside the eye.
• Intraocular pressure is determined by the production and drainage of aqueous
humour by the ciliary body and its drainage via the trabecular meshwork and
uveoscleral outflow.
• Normal intraocular pressure ranges from 10 to 22 mm Hg, a unit of measurement based
on mercury.

3. • Intraocular pressure can change due to various factors, such as heart rate, respiration,
exercise, fluid intake, alcohol, caffeine, coughing, vomiting, or straining.
• High intraocular pressure can cause vision problems and eye disease if not treated.

4. • 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.

5. MEASUREMENT OF 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.

6. DIGITAL TONOMETRY
• A rough estimate of IOP can be made by digital tonometry.
• For this procedure 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.
• 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

8. TONOMETER
• The exact measurement of IOP is done by an instrument called tonometer.
• Indentation (Schiotz tonometer) and applanation (e.g., Goldmann’s tonometer)
tonometers are frequently used

9. 1. INDENTATION TONOMETRY
• Indentation (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.
• Because of its simplicity, reliability, low price and relative accuracy, it
is the most widely used tonometer in the world.

10. SCHIOTZ TONOMETER.
• 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;
- 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; and
- Weights: a 5.5 g weight is permanently fixed to the plunger,
which can be increased to 7.5 and 10 gm.

11. TECHNIQUE OF SCHIOTZ TONOMETRY.
• Before tonometry, the footplate and lower end of plunger should be
sterilized.
• For repeated use in multiple patients it can be sterilized by dipping
the footplate in ether, absolute alcohol, acetone or by heating the
footplate in the flame of spirit.
• 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

13. • 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, as
indicated; since with Schiotz tonometer the greatest accuracy is
attained if the deflection of lever is between 3 and 4.
• 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. ADVANTAGE AND DISADVANTAGE
• The main advantages of Schiotz tonometer are that it is cheap, handy and easy to use.
• Its main disadvantage is that 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.

15. 2. APPLANATION TONOMETRY
• The concept of applanation tonometry was introduced by Goldmann in 1954.
• It is based on ImbertFick 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.

16. THE COMMONLY USED APPLANATION
TONOMETERS ARE:
• 1. goldmann tonometer
• 2. Perkin’s applanation tonometer
• 3. Pneumatic tonometer
• 4. Air- puff tonometer
• 5. Pulse air tonometer
• 6.Tono pen

17. 1. 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.

18. 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 in front of slit-lamp.
• The cornea and biprisms are illuminated with cobalt blue light from the slit-lamp.
• 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

19. 2. PERKIN’S APPLANATION TONOMETER
• This is a hand-held tonometer utilizing the same biprism as in the Goldmann
applanation tonometer.
• It is small, easy to carry and does not require slit lamp.
• However, it requires considerable practice before, reliable readings can be obtained.

20. 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

21. • 4. Pulse air tonometer is a hand-held, non-contact tonometer that can be used with the
patient in any position.

22. • 5.Tono-Pen is a computerised pocket tonometer. It employs a microscopic
transducer which applanates the cornea and converts IOP into electric waves.
• 6. air puff tonometer: non contact tonometer based on the principle of goldmann
tonometry. In this, the central part of cornea is flattened by a jet of air.

23. TONOGRAPHY
• Tonography is a non-invasive technique for determining the facility of aqueous
outflow (C-value).
• The C-value is expressed as aqueous outflow in microlitres per minute per millimetre
of mercury.
• It is estimated by placing Schiotz tonometer on the eye for 4 minutes.
• For a graphic record the electronic Schiotz tonometer is used.
• C-value is calculated from special tonographic tables taking into consideration the
initial IOP (P0) and the change in scale reading over the 4 minutes.

24. • Clinically, C-value does not play much role in the management of a glaucoma patient.
• Although, in general, C-values more than 0.20 are considered normal, between 0.2
and 0.11 border line, and those below 0.11 abnormal