TONOMETRY

The intraocular pressure (IOP) is measured with the
help of an instrument called tonometer. Two basic
types of tonometers available are: indentation and
applanation.
Indentation tonometery
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, who devised it in 1905
and continued to refine it through 1927. Because of
its simplicity, reliability, low price and relative
accuracy, it is the most widely used tonometer in the
world.
Schiotz tonometer. It consists of (Fig. 21.12):
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.
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 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.
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.
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.
The commonly used applanation tonometers are:
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.
Technique (Fig. 21.14). 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 (Fig.
21.15). This is the end point. The intraocular pressure
is determined by multiplying the dial reading with
ten. 2. Perkin’s applanation tonometer (Fig. 21.16). 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.
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.
4. Pulse air tonometer is a hand-held, non-contact
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.
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.
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.