Download Methods of Intraocular Pressure Measurement

Clinical
Methods of Intraocular Pressure Measurement
Dr. Vidya Anandam, MS, Aravind Eye Hospital, Madurai
Transpalpebral IOP Measurement
1. Digital Palpation
Palpation is the oldest method of rough IOP
evaluation. Johann Zacharias Platner was the first
scientist to state that the glaucomatous eye was
hard. For the examination, the patient is asked
to look in downgaze. The redundant skin of the
upper eyelid is displaced, and the central meridian
of the globe is balloted alternately with the tips of
each index finger. By comparing tactile estimations
of IOP to formal pressure measurements, the
examiner’s sense of touch can be ‘calibrated’ to a
limited extent.
Bowman’s Grading System
Tn = normal tension
T+1, T+2, T+3 = indicates degree of increased tension
T-1, T-2, T-3
= indicates degree of low tension
Although palpation correlates poorly with
Goldmann applanation readings, palpation
may have a limited role in screening for marked
elevations of IOP.
Merits
• Simplest, least expensive
• Instrumentation not required
• Useful when external tonometry is not possible,
for example, after penetrating keratoplasty or
corneal scarring.
• Palpation may be the only feasible technique
in patients who are unwilling or unable to
undergo other methods of IOP measurement.
Demerits
• Least accurate method of IOP measurement
• Palpation is best avoided in eyes with significant
trauma or in certain postoperative conditions.
2. Transpalpebral Tonometers
Transpalpebral tonometers, such as the TGDc-01
and IGD-02 devices. These portable instruments
measure the IOP through the eyelid. The operation
of both instruments is based on determining
the acceleration of freely falling rod after its
interaction with the elastic eye surface. Troost
et al demonstrated an increasing underestimation
of IOP at elevated pressure levels when compared
with Goldmann applanation tonometer.
The preview eye-pressure monitor (Bausch
& Lomb, Rochester, NY USA) was developed as
a psychophysical test for self-tonometry at home.
The pencil – shaped instrument is pressed with
its probe against the upper eyelid with increasing
pressure until visual phenomenon are detected.
These phosphenes should appear opposite to
where the pressure was applied. The position of
probe application can influence the measurement.
Application of the probe to the superonasal part of
upper lid gives the most accurate results.
Merits
By not applanating or indentating the cornea
scarring, edema of this cornea can be prevented.
Variations of the central corneal thickness did not
contribute to the difference.
Demerits
Li et al compared IOP values using the Proview
eye pressure monitor with those measured with the
Goldmann applanation tonometer and with the
TonoPen. The IOPs obtained with the Proview eye
pressure monitor were significantly lower.
Manometry
Manometry is an invasive technique that
precisely measures the pressure inside the eye.
It is the reference pressure by which all other
tonometers should be judged. Manometry is
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used most commonly as a laboratory technique
in performing continuous pressure measurements
over time, evaluating the effect of physiologic
and pharmacologic manipulations on pressure,
and studying aqueous humor dynamics in postmortem eyes. Most widely used tonometers,
such as Goldmann’s applanation tonometry,
Schiotz’s indentation tonometry, Langham’s
pneumotonometry and Kanngiesser’s dynamic
contour tonometry have been calibrated and
validated on human cadaver eyes against a
manometric reference pressure. The ethical use
of manometry in living human eyes is restricted
to eyes undergoing enucleation or intraocular
surgery.
Tonometry
Tonometers are the instruments for performing
tonometry. Their purpose is to obtain an accurate
measurement of the IOP with least disturbance to
the eye. So far, cornea is the only structure of the
eye that is accessible to external tonometry. Each
technique has its advantages and disadvantages
and none is ideal.
The ideal tonometer must record accurate,
reproducible measurements, without affecting
the pressure or harming the eye. In addition the
tonometer should be portable, simple to calibrate
easy to maintain and standardize.
Indentation Tonometry
The shape of deformation is truncated cone. There
is no precise shape and these type of tonometers
displace a relatively large intraocular volume
in response to a standard weight applied to the
cornea. As a result conversion tables based on
empirical data from in vitro and in vivo studies
must be used to estimate IOP.
The prototype of this group is the Schiotz
tonometer (1905). Because of its simplicity,
reliability and relative accuracy, it is the only
mechanical indentation tonometer in use today.
Other types of indentation tonomers include
• Von Graefe (1962)
• Donders (1863), Snellen (1968), Monnik
(1868), Dor (1869)
AECS Illumination
• Layerat (1885), Smith (1887), Nicati (1900),
Romer (1918)
• McLean (1914), Many (1919), Bailliart (1923)
• Recording Tonometer by Maurice (1958)
• Electronic Tonometer by Mueller (1960)
Schiotz Tonometer
The body of the tonometer has a
• Footplate which rests on cornea
• Plunger which moves freely within a shaft in
the footplate
• Indicator needle
• Scale
• Additional weights 7.5g, 10g, 15g. A 5.5g
weight is permanently fixed to the plunger.
Basis concept of indentation tonometry:
When plunger indents the cornea
• Baseline pressure P0 is raised to Pt. Tonometer
measures Pt. The change from P0 to Pt is an
expression of resistance an eye offers to the
displacement of a volume of fluid (Vc). P0 is
estimated from conversion tables.
• Friedenwald – developed an empirical formula
for linear relationship between logarithm of
pressure and volume change in a given eye.
Constant used in the formula is (K) coefficient of
ocular rigidity
Conversion tables use
- K = 0.0245 (1948 tables) or
- K = 0.0215 (1955 tables).
1948 tables agree more closely with
measurements by Goldmann applanation
tonometry.
Technique
• With patient lying supine and fixing on
overhead target
• Under the effect of topical anesthetic drops,
lids are separated by the examiner
• Foot plate is placed perpendicular to cornea
• Needle shows fine movements in response to
ocular pulsations.
Vol. XV, No.2, April - June 2015
• If scale reading is less than 4, additional weight
should be added to the plunger.
• A conversion table is used to derive the IOP
in mm Hg from the scale reading and plunger
weight.
Source of Error
1. Ocular rigidity : conversion tables assume an
average coefficient of ocular rigidity (K)
In eyes which deviate - False value
High K- False high IOP
Low K - False low IOP
• High ocular rigidity seen in
- High hyperopia
- Longstanding glaucoma
- Vasoconstrictor therapy
• Low ocular rigidity seen in
- High myopia
- Elevated IOP
- Osteogenesis imperfect
- Miotic therapy
- Vasodilators
- Retinal detachment surgery
- Intravitreal gas
2. Blood volume alteration during indentation
tonometry
3. Corneal influences : steeper and thicker corneas
– more displacement of fluid, false high IOP
4. Moses effect : cornea moulds into the space
between plunger and the hole – false elevation
of IOP.
Calibration
Placing the tonometer on a steel test block results
in a scale reading of zero.
Applanation Tonometers
The shape of deformation with these tonometers
is a simple flattening and the shape is constant.
Therefore the IOP is derived from mathematical
calculations. The force necessary to flatten a small,
standard area of cornea is calculated.
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The applanation tonometers are further
differentiated on the basis of the variable that is
measured.
Variable Force (Fixed Area)
This type of tonometer measures the force that
is required to applanate a standard area of the
corneal surface. The prototype is the Goldmann
applanation tonometer (1955). Other tonometers
include;
• Weber (1867)
• Fick (1888)
• Schmidt (1955)
• Draeger (1966) 49 –51) and Perkins
(1965)
• Noncontact (air puff ) Tonometers
• Reichert ocular response analyser (2005)
• Dynamic observing tonometer (1996)
Variable Area (Fixed Force)
This type of applanation tonometers measure
area of cornea that is flattened by a known force
(weight). The prototype is the Maklakov-type
(1885), the other being posner (1965).
Goldmann Applanation Tonometer
Goldmann applanation tonometer is based on
Imbert – Fick law which states that the pressure
inside an ideal dry, thin walled sphere equals the
force necessary to flatten its surface divided by the
area of the flattening.
Where
Pt = pressure within a sphere
W = external force required for flattening
A = total area flattened
Applies to a sphere which is
• Perfectly spherical
• Dry
• Perfectly flexible
• Infinitely thin
But cornea fails as it is
• Aspherical
• Wet
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• Not perfectly flexible
• Not thin
The moisture creates a surface tension(S) and
the lack of flexibility requires a force to bend the
cornea (B) which is independent of the internal
pressure. The outer area of flattening (A) is not
the same as the inner area (A1). To account for
these characteristics of the cornea, Imbert - Fick
law was modified to
Where A1 = internal area of applanation
When A1 = 7.35mm2, S balances B and W = Pt
This happens when the diameter of external
area of applanation (A) = 3.06 mm.
Description of Tonometer
• The instrument is mounted on slit lamp in
such a way that the examiner’s view is directed
through the centre of the plastic biprism, which
is used to applanate the cornea. The biprism
is attached by a rod to a housing to adjust the
force of the biprism against the cornea.
• Two beam splitting prisms convert the circular
area of corneal contact into two semicircles.
• Prisms are arranged so that inner margins of the
semicircles overlap when 3.06 mm of cornea
is applanated.
Technique
• Biprism is placed in the holder - 180° marking
aligned with white line on the holder and
tension nob is set at 1g.
• Cornea is anesthetized with a topical
preparation.
• Tear film is stained with sodium fluorescein –
paper strip or fluorescein solution of 0.25%
• The tonometer tip is cleaned with disinfecting
solution
• Cornea and biprism illuminated by a cobalt
blue light from the slit lamp, the angle between
the biprism and the illumination is 60 deg.
• Low magnification and slit beam is opened
maximally.
• Patient looks straight. Lids are held against the
bony orbit.
AECS Illumination
• Gentle contact of biprism is made with the
corneal apex while observing through the slit
lamp – Mono ocular view.
• Two semicircles of equal size are seen.
• The rings should be approximately 0.25 to
0.30 mm in thickness
• The tension knob is rotated until the inner
borders of the fluorescein ring touch each other
at the midpoint of their pulsations.
• Reading on the dial is multiplied by 10 to get
the IOP in millimetres of mercury.
• IOP is measured first in one eye until three
successive readings are within 1 mm Hg and
then measured in the other eye
Methods of disinfecting Goldmann
applanation
The biprism should be rinsed and dried
immediately after use. Soaking the tonometer head
for 5 minutes in 3% hydrogen peroxide, 0.5%
sodium hypochlorite or 70% isopropyl alcohol
meets the guidelines published by the Centers for
Disease Control and Prevention (CDC) and the
American Academy of Ophthalmology (AAO).
However, wiping the tip with a 70% isopropyl
alcohol swab is also described to be as effective
in virus elimination as disinfectant immersion.
Alternatively, disposable tonometer tips or silicone
shield over the Goldmann tonometer tip can be
used.
The goldmann applanation should be
caliberated once a month. If the Goldmann
tonometer is not within 0.1g of the correct
calibration, the instrument must be repaired.
Causes of Errors in measurement of IOP
with Goldmann Applanation
• Inappropriate fluorescein pattern : The
fluorescein ring is too wide (overestimation)
or too narrow (under estimation).
• Inadequate staining causes an underestimation
of IOP.
• Elevation of eyes more than 15° above the
horizontal causes an overestimation of IOP.
Vol. XV, No.2, April - June 2015
• Widening of lid fissure excessively causes an
overestimation IOP.
• Repeated tonometry reduces IOP.
• A scarred, irregular cornea distorts the
fluorescein rings and makes it difficult to
estimate the IOP.
• The thickness of cornea affects. If the cornea is
thick because of odema IOP is underestimated.
If it is thick because of the additional tissue,
IOP is overestimated.
• Pressure on the globe by the examiner or lid
squeezing by the patient, IOP is overestimated.
• If corneal astigmatism is greater than 3D,
IOP is underestimated for which the rule
astigmatism, and overestimated for against the
rule astigmatism. The IOP reading is inaccurate
by 1mm Hg for every 3D of astigmatism.
Perkins Tonometer
It uses the Goldmann principle, is handheld,
portable, battery – powered, and usable in the
supine as well as sitting positions. This brought
applanation tonometry to screening clinics, the
bedside, and the operating room. The perkins
applanation tonometer is particularly useful in
measuring the IOP in young children, elderly
and in obese patients, permitting measurements
without having to position the patient at a slit
lamp.
Draeger Tonometer
It is similar to the Goldmann and Perkins
tonometers except that uses a different biprism.
The force for applanation is supplied by an electric
motor. Like the Perkins instrument, the Draeger
tonometer is portable and counterbalanced, so it
can be used in a variety of positions and locations.
Noncontact Tonometer
NCT was introduced by Grolman, its unique
advantage being that is does not touch the cornea,
other than with a puff of air.
A puff of room air creates a constant force
which momentarily deforms the cornea. It is
postulated that the central cornea is flattened at
the moment the pressure measurement is made.
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Description of the instrument
NCT machine is mounted on the table and
consists of three subsystems:
a. An alignment system: allows the operator to
optically align the patient’s cornea in three
dimensions (axial, vertical and lateral).
b. An optoelectronic applanation monitoring
system which consists of
1. A transmitter, which directs a collimated
beam at the corneal vertex
2. A receiver and
3. Detector which accepts only parallel,
coaxial rays reflected from the cornea
c. A pneumatic system which generates a puff of
room air, which is directed against the cornea.
Schematic Representation of the working
principle of NCT
A: Light source from transmitter (T) is reflected from undisturbed cornea toward receiver (R), while cornea is aligned by optical
system (O).
B: Air puff (1) from pneumatic system (P)
deforms cornea, which increases the number of
light rays (2) received and detected by R. Time
(t) from internal reference point to moment of
maximum light detection (which presumably
corresponds to applanation of the cornea) is
converted to intraocular pressure (IOP) (based
on calibrations with Goldmann applanation
tonometer) and displayed on digital readout.
C: Continued air pulse produces momentary
concavity of cornea, causing sharp reduction
in Right rays received by R.
Technique
• The patient observes an internal target, the
operator aligns the cornea and superimposes a
reflection of the target from the patients cornea
on a stationary ring.
• During this time, light from the transmitter is
reflected from the undisturbed cornea which
allows only a small number of rays to enter the
receiver. When the cornea is properly aligned,
the operator depresses a trigger which causes
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a puff of air to be directed against the cornea,
and the IOP is displayed on a digital read out.
• At the moment the central cornea is flattened,
the greatest number of reflected light rays
are received, which is recorded as the peak
intensity of light detected. The time from
an internal reference point to the moment
of maximum light detection is converted to
IOP based on prior comparison with readings
by Goldmann tonometry. In the subsequent
versions air puff is automatically triggered
when the alignment criteria are satisfied and
the force of air to achieve peak light detection
is the measured variable
• The time interval for an average NCT
measurement is 1 to 3 ms and is random with
respect to the phase of the cardiac cycle so that
the ocular pulse becomes a significant variable,
it cannot be averaged as with some tonometer.
Glaucomatous eyes have a significantly greater
range of momentary fluctuation in IOP.
Therefore it is recommended that a minimum
of three readings within 3 mm Hg be taken
and averaged as the IOP.
Advantage of Noncontact Tonometer
• The greatest benefit of NCT is the absence of
mechanical contact with the cornea.
• Elimination of a need for topical anaesthesia
thereby avoiding drug sensitivities and toxic
reactions.
• No potential complications of corneal
abrasions, spread of infection (epidemic
keratoconjunctivitis, hepatitis B virus).
• Can be performed by paramedical personnel
with minimal training and in children.
Limitations of Noncontact Tonometer
• The main limitation is decreasing reliability in
higher pressure ranges
• Limited use in irregular corneas or poor visual
acuity
• Less portable than many other tonometers
• Noncontact tonometers are rather expensive
and require regular calibration.
AECS Illumination
Ocular response analyser
The ocular response analyser (ORA) is a non
contact (air puff ) tonometer that does not require
topical anaesthesia and provides additional
information on the biomechanical properties
of the cornea. It uses an air pulse to deform the
cornea into a slight concavity. The difference
between the pressures at which the cornea flattens
inward and outward is measured by the machine
and termed corneal hysteresis (CH). The machine
uses this value to correct the effects of the cornea
on measurement.
Applanation – Fixed Force (Variable
Area) Tonometers
Maklakow Applanation Tonometer
IOP is estimated by measuring the area of cornea
that is flattened by a known weight. Conversion
tables are used as in Schiotz tonometry because
of the volume displacement and therefore ocular
rigidity is considered in computing the IOP.
Combined indentation – Applanation
Tonometers
• MacKay – Marg and TonoPen
• Pneumatic tonometer
MacKay – Marg Tonometer
The MacKay – Marg tonometer uses a microplunger
connected to a sensitive transducer, which
converts plunger displacement into an electrical
signal that is recorded on a paper chart, much
like an electrocardiogram. The MacKay – Marg
tonometer may be a relatively more accurate
tonometer in scarred or edematous corneas
because the IOP reading is less independent of
corneal rigidity and elasticity and the endpoint is
reached electromechanically not optically.
TonoPen
The TonoPen is a handheld, battery operated
version of the MacKay – Marg tonometer. The
tip is covered by a disposable latex cover and
applied perpendicularly to gently indent and
anesthetized cornea. Each measurement requires
Vol. XV, No.2, April - June 2015
several applanations. An acceptable applanation
is indicated by an audible click after contact with
the cornea. A microprocessor averages several
acceptable waveforms and gives a digital readout
of IOP on a liquid crystal display, with an estimate
of the variability between the component readings.
Pneumatic Tonometers
(Pneumotonometer)
The concept of this tonometer is similar to
that of the Mackay-Marg. The sensor in this
case, however, is air pressure, rather than an
electronically controlled plunger.
Rebound Tonometer
Rebound tonometers (1997) determine intraocular
pressure by bouncing a small 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. The device is simple, cheap and easy
to use. It is portable, does not require the use of
eye drops and is particularly suitable for children.
Dynamic Contour Tonometry
Dynamic contour tonometry (DCT) is a novel
method which uses principles of contour matching
instead of applanation. This is designed to
reduce the influence of biomechanical properties
of the cornea on measurement. These include
corneal thickness, rigidity, curvature, and elastic
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properties. It is less influenced by corneal thickness
but more influenced by corneal curvature than the
Goldmann tonometer.
The PASCAL tonometer is currently the only
commercial DCT tonometer available. It uses
a miniature pressure sensor embedded within a
tonometer tip contour matched to the shape of
the cornea. The tonometer tip rests on the cornea
with a constant appositional force of the one
gram. When the sensor is subjected to a change in
pressure, the electrical resistance is altered and the
PASCAL’s computer calculates a change in pressure
in concordance with the change in resistance.
F = Forces ‘F’ are equal on both sides
of the cornea; P = Intraocular pressure, d =
diameter of contour matching area; RC = relaxed
corneal radius; RC' = unrelaxed corneal radius.
RC' = RC + ∆R.
The contour matched tip has a concave surface
of radius 10.5mm, which approximates to the
shape of a normal cornea when the pressure on
both sides is equal. The probe is placed adjacent to
the central cornea and the integrated piezoresistive
pressure sensor automatically begins to acquire
data, measuring IOP 100 times per second. A
complete measurement cycle requires about 8
seconds of contact time. During the measurement
cycle, audio feedback is generated, which helps the
clinician maintain proper contact with the cornea.
The device also measures the variation in pressure
that occurs with the cardiac cycle.