tonometry and tonography

1. -Dr samarth mishra

2.  Measurement of IOP is known as tonometry.
 A true measurement of IOP requires a direct fluid connection to
the anterior chamber.(=manometry)
 Cannulation of the anterior chamber for measurement of IOP is
used frequently in the laboratory and occasionally during
surgery.
 This approach entails too many hazards for the routine clinical
management of glaucoma.

3.  Therefore, we generally use indirect measurements of IOP—
tonometry.
 It is of 2 types:
-indentation tonometry.
-applanation tonometry.
 Schiotz developed first device that quantifies IOP with relative
reproducibility.

4.  Used quite often d/t its :-
a) Simplicity
b) Easy transportability.
-schiotz tonometer is based on the principles
of indentation tonometry.

6.  Schiotz tonometer is an example of indentation tonometry.
 Consists of 3 parts:-
a)footplate (radius of
curvature=15mm )
b)plunger
c)scale
 A series of known weights of 5.5gm,7.5gm, 10.0gm and 12.5gm are
applied to the plunger.
 With the smallest plunger weight (5.5 g), the total weight of the
instrument on the eye is 16.5 g.
 The plunger is kept on the cornea.

7.  The plunger indents the cornea which results in the deformation
of the globe.
 This deformation is measured by the scale attached to the
plunger.
 The reading on the scale is converted to IOP with the help of a
nomogram k/a friedenwald nomogram.
 it was published after modification in 1955.
 The formula requires a constant ‘k’ which is the cofficient of
ocular rigidity.

8.  It is the measure of the resistance of the eye to the distending
force.
Clinical technique:
 Pt lies supine:
 Procedure is explained to pt.
 Topical anaesthetic drops are instilled.
 Pt is asked to look at a fixed target
 Footplate is kept on cornea and reading taken.
 It is then correlated with the nomgram.

9. Limitations:
 ‘k’ value of ocular rigidity is kept average for all eyes.
 The tables that are commonly used to estimate IOP based on a
Schiøtz scale reading assume an eye with normal scleral rigidity.
 The tables give inaccurate estimates of IOP in eyes that do not have
normal scleral rigidity.
 so,in eyes where ocular rigidity is high or low,the value becomes
unreliable.
 HIGH OCULAR RIGIDITY SEEN IN : hypermetropia,chronic
glaucoma and chronic vasoconstrictor therapy.
 So, the recorded iop will be higher in these eyes.

10.  Low ocular rigidity is seen in :-
a)high myopia
b)mitotic therapy
c)after retinal detechment surgery
d)intravitreal injection of gas
e)vasodilator therapy.
 false high readings are obtained with very thick and steep corneas.
 Unreliable readings are found in scarred corneas with significant
pathology.

11.  Based on Imbert fick’s law.
 States that in an ideal round,dry,elastic,infinitely thin walled
sphere
P=F/A
p=pressure
F=force required to
flatten the cornea
A=area

12.  In applanation tonometry 3mm of cornea is flattened.
 This minimally displaces fluid of 5 micro litre.
 The force required to flatten a circle of 3.06 mm is in grams a
tenth(1/10) of IOP in mmHg.
 Therefore, a force of 1.6 g is required to flatten this circular area
when IOP is 16 mmHg.
The 3.06-mm diameter circle of applanation was chosen because of
this simple 10:1 relation between IOP and grams of force.

13.  This area is within the range in which the natural bending force
of the cornea is canceled by the capillary attraction created by
the tear film between the tonometer head and the cornea.
 Flattening so small an area of the cornea creates little fluid
displacement within the eye.
 Therefore,
scleral rigidity is not a factor in Goldmann applanation
tonometry.

15.  In goldmann applanation tonometry, when the cone tip is pressed
against the anesthetized cornea, a small circular area is flattened.
 The applanated area appears as a dark circle surrounded by a narrow
ring of fluorescent tear film.
 Opposing prisms in the tip of the cone split the image, so that the
viewer sees two dark half circles, each with a narrow fluorescent outer
border.
 The force on the cone is adjusted until the inner corners of the
fluorescent half rings just touch.
 At this optical end point, the applanated area is correct: a circle with
diameter = 3.06 mm.

17.  Applanation tonometry can be divided into two subtypes:
-variable area
-variable force.
Variable area:
 These measure the area
of the corneas flattened by
a known amount of force.
 E.g maklakov tonometer.

18. Variable force:-
 These measure the force that is needed to flatten a standard area
of the cornea.
 E.g goldmann applanation tonometer. ( it is the gold standard )

20.  It is mounted on a standard slit lamp.
 It has a plastic biprism,which is used to flatten the cornea after
anaesthetizing.
 The prism is mounted on a rod.
 Before touching the cornea with biprism, sodium fluorescin dye
is instilled & cobalt blue filter is switched on.

21.  When the observer views from the slit lamp uniocularly,two
semi-circles are seen.
 The knob of the tonometer is adjusted so that the inner margins
of both the semi-circles meet and start pulsating.
 This is the end point where reading is taken.

23.  Hand held tonometer.
 Based on same principle as goldmann tonometer.
 Advantage is that no slit lamp is required.
 IOP can be recorded with the patient in supine position
or when under anaesthesia,uncooperative patients and in
children.

25.  Measures the IOP by flattening the cornea with graded flow of
gas against a flexible diaphragm.
 Principle is similar to that of mackey - marg tonometer ,but the
sensor is air pressure.
 Useful for assessing the IOP in :
a) scarred cornea.
b) edematous cornea.
c) assessment of IOP over soft contact lens.

26.  It is an electronic applanation tonometer.
 It functions by applanating the cornea with a probe which has a
1.5mm fused quartz plunger that records the IOP.
 Recording is through an attached stylus that documents the
pressure curve.
 In this method, the applanation force is sensed electronically
through a sensor attached to the central cylinder
As observation of the mires on the patient’s cornea is not a
prerequisite for the assessment of IOP, it can be used in scarred and
irrregular cornea.
 the major advantage is its portability.

28.  Puff of compressed air is blown through a nozzle towards the
patient cornea.
 invented by Bernard Grolman of Reichert, Inc (formerly
American Optical).
 IOP is measured based on the physical relationship of the
flattening of cornea of a required measure to the amount of
compressed air blown through the nozzle as per a
predetermined pressure-time charecteristic curve.
 The moment of flattening is recorded optoelectronically and
converted into an estimate of IOP by a computer in the machine
 This is the physical basis of non contact tonometer.

29.  It is a digital tonometer.
 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.
 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 and other
biomechanical properties of the cornea .
 Because the tip shape is designed for the shape of a normal
cornea, it is more influenced by corneal curvature.

30.  The probe is placed on the pre-corneal tear film on the central
cornea and the integrated peizoresistive pressure sensor
automatically begins to acquire data, measuring IOP 100 times per
second.
 A complete measurement cycle requires about 8 seconds of contact
time.
 The device also measures the variation in pressure that occurs with
the cardiac cycle
 Sensitive enough to detect the ocular pulse amplitude(OPA)due to
patient’s heartbeat.
 Provides direct trans corneal measurement of IOP.
 Eliminates the systematic errors inherent in all previous
tonometers, such as the influence of corneal thickness and rigidity.

32.  Rebound tonometers determine intraocular pressure by
bouncing a small plastic tipped metal probe against the cornea.
 The device uses an induction coil to magnetise the probe and
fire it against the cornea.
 As the probe bounces against the cornea and back into the
device, it creates an induction current from which the intraocular
pressure is calculated.
 Simple ,portable & cheap device.
 Of use in children, uncooperative patients.

33.  This tonometer utilizes the principle of air-puff tonometery.
 It uses an air pulse to deform the cornea into a slight concavity.
 provides additional information on the biomechanical
properties of the cornea.
 It corrects the effect of corneal hysteresis on the IOP
measurment.

34.  Measures the IOP through the eyelid, overlying the sclera.
 The response of free falling rod,rebounding against the tarsal
plate, gives the measure of IOP.
 The patient is positioned so that the tip of the device and lid are
overlying sclera.
 Non-corneal and transpalpebral tonometry does not involve
contact with the cornea.
 Does not require topical anesthetic during routine use
 This is of use in children & uncooperative patients.

36.  Palpation (also known as digital tonometry) is the method of
estimating intraocular pressure by gently pressing the index
finger against the cornea of a closed eye.
 This method is unreliable

38. Ocular/Periocular Anomalies
 Lid, muscle, orbit malformation, infiltration, or congestion.
 Corneal anomalies: thickness, scarring, edema.
 Absence of “aqueous free space” behind cornea.
 Abnormal scleral rigidity (indentation tonometry)

39. Patient-induced
 Lid squeezing
 Breathholding, constrictive clothing
 Eye/head movement
 Unsuspected/unreported drug effects (usually → lower IOP)
(e.g., recent ethanol ingestion, marijuana, systemic beta
blockers)
 Recent exercise .

40. Instrument Error
 Poor maintenance, cleaning
 Out of calibration
Operation Error
 Failure to consider/observe any of the above
 Applying pressure to the lids
 Using inappropriate fluorescein concentration
 Failure to establish steady state through patient observation, repeat
measurement
 Failure to record time of day.

41.  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 Schiøtz tonometer yielded estimates of aqueous
outflow and rate of aqueous flow.
 The principle of the test may be traced to the massage effect,
whereby pressure on the eye leads to a softening of the globe
due to an increased outflow of aqueous humor induced by the
higher pressure.
 Grant recorded the output of an electronic tonometer on a strip-
chart recorder .

42.  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 coefficient of aqueous outflow facility is calculated from Grant's
formula:
[ episcleral venous pressure rises an average of 1.25 mmHg during Schiøtz
tonometry; therefore, the formula is usually corrected by adding 1.25 to
P0. ]
C = Δ VT (Ptav - P0)
 Δ V=change in ocular volume
 T=time interval
 Ptav = average pressure during tonography.

43.  The output of the electronic tonometer is traced on a strip chart.
 A good test shows a gentle downward trend in the scale
reading, with fine oscillations of the ocular pulse superimposed
on the tracing.
 Scale readings at 0 and 4 minutes are read from a smooth pencil
line, which is drawn through the tracing to make a good visual
approximation of the average slope.
 When the tracing is a good one, this average slope is easy to
recognize and draw.
If the tracing is of poor quality, the approximation is difficult to
draw and should be a signal to the examiner that the record is
probably not reliable.

44.  The calculation assumes that the pressure change that results
from placing the tonometer on the eye does not induce a change
in the rate of production of aqueous humor or in the resistance
of the outflow channels.
 There is evidence that increased IOP results in some decrease in
aqueous formation.
 In standard tonography, this effect is indistinguishable from
true outflow facility and has therefore been called
“pseudofacility.
 Also increased IOP causes some increase in the resistance to
aqueous outflow.

45.  The units for the outflow facility are given as μL/minute/mmHg.
A tonogram from a patient with glaucoma
C = Δ VT (Ptav - P0

47. .