Tonometry is the measurement of intraocular pressure (IOP) inside the eye. There are several types of tonometers that use different principles like indentation, applanation, and contour matching. The Goldmann applanation tonometer is considered the gold standard. It works by applanating the cornea with a known force and area to indirectly measure IOP. Tonography provides additional information about aqueous humor dynamics by recording IOP changes over time using an electronic Schiotz tonometer. Newer dynamic contour tonometry matches the probe contour to the cornea to reduce deformation compared to applanation tonometry.

1. TONOMETRY and TONOGRAPHY
Presented by
Loknath Goswami
B.Sc optom 2nd year

2. Important terms
• IOP : The pressure exerted by intraocular
fluids on the coats of the eyeball
• Unit : mm of Hg
• Range : 10 to 21 mm of Hg
• Indentation : Depression
• Applanate : Flatten

3. Tonometer and Tonometry
Tonometer
• Instrument that exploits the
physical properties of the
eye to permit measurement
of pressure without the
need to cannulate the eye
TONOMETRY
• The procedure eye care
professionals perform to
determine the intraocular
pressure, the fluid pressure
inside the eye
• It is a non invasive
measurement of measuring
the IOP

4. History
In 1862 : Von Graefe,
first designed an
indentation tonometer
for measuring IOP
In the late 19th
century, Maklakov
designed an
applanation tonometer
Goldmann's
applanation tonometer
of 1950 began the era
of truly accurate IOP
measurement

5. History
Other devices such as the McKay-
Marg tonometer (or its offspring
the Tono-Pen), the
pneumatonometer, and airpuff
applanation tonometers are gaining
adherents
The dynamic contour tonometer is
the first totally new concept in
tonometry in over 100 years.

6. Tonometer
No TouchTouch
DynamicStatic
Indentation
Constant
force
Constant
indentation
Applanation
Constant
area
Constant
force
Grolman
tonometer
Ballistic tonometer
Maklakov Goldmann Schiotz

7. Maklakov tonometer
• The first practical tonometer was
the Maklakov tonometer
• It has a fixed force and a flat
bottom that was smeared with ink
• When the tonometer first touched
and then flattened the cornea, the
ink was transferred to the cornea
• The tonometer was then printed
on a piece of paper

8. Maklakov tonometer
• The area (as determined by the
diameter) in the center of the
inkblot that was devoid of ink was
proportional to the IOP
• If the eye moved during the time tonometer
was on the eye, more ink was transferred to
the cornea than was necessary due to
applanation alone and the IOP was
underestimated

9. Applanation tonometry
• The theory of applanation tonometry comes
from the Imbert-Fick law which states for an
ideal sphere the internal pressure of a very
thin-walled sphere can be obtained by
knowing the force required to flatten a known
sphere
• The formula is : P = F/A or F = PA
 P = Pressure inside the sphere
 F = Force required to applanate its surface
 A = Area of flattening

10. Ideal sphere

11. • The force of capillary attraction (T) between
the tonometer head and the tear film is
additive in the external force
• A force (C), independent of IOP, is required to
flatten the relatively inflexible cornea
F = PA becomes
F + T = PA + C
=> P = F + T – C/ A

12. • The Goldmann applanation tonometer is
designed such that A is equal to 7.35 mm2
• With this value for A, the opposing forces of
capillary attraction and corneal flexibility
cancel out
P = F / 7.35 mm2

13. Goldmann tonometer
• After anaesthetising the cornea with
a drop of 2% xylocaine and staining
the tear film with fluorescein
patient is made to sit in front of the
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 area of cornea

14. Goldmann tonometer
• 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 IOP is determined by multiplying the dial
reading with 10

15. Conditions

21. SCHIOTZ TONOMETER
• Measures degree of corneal decompensation by a
known weight placed on cornea.
• Weights – 5.5g, 7.5g, 10g, 15g.
• Concave foot assembly rests on cornea
• Forms a reference level from which plunger further
indents
• Degree of indentation is displayed on scale.

22. SCHIOTZ TONOMETER
scale
Needle
plunger
holder
Foot plate
Lever
Additional weights

23. Procedure
• After anaesthetizing the
cornea with paracaine or 2-4
% xylocaine, patient is made
to lie supine on a couch and
instructed to fix at a target on
the ceiling
• We should separate the lids
and lower the tonometer
plate on the cornea so that
the plunger is free to move
vertically

24. Procedure
• The reading on scale is recorded as soon as
the needle becomes steady
• It is customary to start with 5.5 gm weight
• 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

25. Procedure
• In end, tonometer is lifted and a drop of
antibiotic is instilled
• A conversion table is then used to derive the
IOP in mmHg from the scale reading and the
plunger weight

26. Friedenwald conversion table

27. Dynamic contour tonometry
• Dynamic contour tonometry (DCT) uses the
principle of contour matching instead of
applanation
• The tip contains a hollow the same shape as
the cornea with a miniature pressure sensor in
its centre

28. Dynamic contour tonometry
• In contrast to applanation tonometry it is
designed to avoid deforming the cornea during
measurement and is therefore thought to be
less influenced by corneal thickness
• The probe is placed on the pre-corneal tear film
on the central cornea and the
integrated piezoresistive pressure sensor
automatically begins to acquire data, measuring
IOP 100 times per second
• The tonometer tip rests on the cornea with a
constant appositional force of one gm

29. Dynamic contour tonometry
• When the sensor is subjected to a change in
pressure, the electrical resistance is altered
and the tonometer's computer calculates a
change in pressure according to the change in
resistance
• A complete measurement cycle requires about
eight seconds of contact time
• The device also measures the variation in
pressure that occurs with the cardiac cycle

31. Tonography
• Tonography is a clinical test
of aqueous humor dynamics
that was introduced by W.
Morton Grant in 1950
• Grant showed that analysis
of a continuous recording
from an electronic Schiotz
tonometer yielded estimates
the rate of aqueous flow

32. • Grant recorded the output of an electronic
tonometer on a strip-chart recorder and showed
that this data combined with the tonometer
calibration of Friedenwald could be used to
provide a quantitative expression relating the
outflow of aqueous humor to the driving
pressure.
• Grant called this value “the coefficient of aqueous
outflow facility”(C)
• The C-value is expressed as aqueous outflow in
microlitres per minute per millimeter of mercury

33. • For a graphic record the electronic Schiotz
tonometer is used by placing it on the eye for 4
minutes
• C-value is calculated from special tonographic
tables taking into consideration the initial IOP (Po)
and the change in scale reading over the 4
minutes
• 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

34. • A tonogram from a patient with glaucoma
• The initial scale reading of 4.5 with a 7.5-g
weight means that the P0 was 28
• The Pt was initially 44.4 and fell gradually to
39.5 over 4 minutes, yielding an average
pressure during tonograghy (Ptav) of 41.9
• This reflects a low aqueous outflow

35. References
• Pearls of glaucoma management, 2nd edition,
JoAnn A. Giaconi, pg no. 92
• https://www.ncbi.nlm.nih.gov/pubmed/2115
0677
• https://en.wikipedia.org/wiki/Ocular_tonome
try
• http://eyewiki.aao.org/IOP_and_Tonometry
• http://www.oculist.net/downaton502/prof/eb
ook/duanes/pages/v3/v3c046.html