2. HISTORY
• von GRAEFE DEVELOPED THE FIRST
INDENTATION TONOMETER IN 1865.
• THE FIRST REASONABLY
ACCURATE INSTRUMENT
DESIGNED WAS THE MAKLAKOFF
APPLANATION TONOMETER IN 1885.
• SCHIÖTZ IN THE FIRST TWO THIRDS
OF THE 20TH CENTURY DEVELOPED
AN INDENTATION TONOMETER
THAT WAS WIDELY USED.
3. • GOLDMANN'S APPLANATION
TONOMETER IN 1954 BEGAN
THE ERA OF TRULY ACCURATE
INTRAOCULAR PRESSURE
MEASUREMENT.
• THE ELECTRONIC TONOPEN XL
WAS INTRODUCED IN 1988 AND
WAS THE FIRST
COMMERCIALLY AVAILABLE
PORTABLE TONOMETER.
• IN 2005, THE NEW HAND HELD
I-CARE REBOUND TONOMETER
A REPRODUCIBLE METHOD OF
DETERMINING IOP IN HUMANS
BECAME AVAILABLE.
7. PRINCIPLE
WHEN THE TONOMETER IS PLACED
ON THE ANAESTHETISED CORNEA,
IT INDENTS THE CORNEA WHICH
DISPLACES SOME VOLUME.
WHICH INCREASES THE BASELINE
IOP.
THIS CHANGE OF PRESSURE IS AN
EXPRESSION OF THE RESISTANCE
THE EYE OFFERS TO THE
DISPLACEMENT OF A VOLUME OF
FLUID.
THE SCALE READING MEASURES THE
ARTIFICIALLY RAISED IOP WHICH
IS CONVERTED TO THE RESTING
BASELINE FROM CONVERSION
TABLES DEVELOPED BY
FRIEDENWALD (1948)
8. TECHNIQUE
• ANAETHETISE THE CORNEA USING TOPICAL XYLOCAINE 2-4%,
PATIENT IS ASKED TO LIE SUPINE AND FIXATE AT A TARGET ON
THE CEILING.
• SEPARATE THE LIDS, GENTLY REST THE FOOTPLATE
VERTICALLY AGAINST THE CENTER OF THE CORNEA.
• THE TEST IS DONE INITIALLY WITH 5.5 gms AND THE
DEFLECTION OF THE LEVER IS NOTED. IF THE LEVER
DEFLECTS LESS THAN 3 THEN 7.5 gms OR 10 gms WEIGHTS ARE
ADDED TO THE PLUNGER.
• THE GREATEST ACCURACY IS ATTAINED WHEN THE
DEFLECTION OF THE LEVER IS BETWEEN 3-4.
• ONCE THE PRESSURE READING HAVE BEEN TAKEN A
STANDARDIZED FORMAT FOR RECORDING IS PRESCRIBED
WHICH INCLUDES THE SCALE READING, TONOMETER WEIGHT,
IOP, CONVERSION TABLE AND EYE MEASURED.
9. STERILIZATION
• THE INSTRUMENT IS DISSEMBLED AND
THE BARREL IS CLEANED WITH TWO
PIPE CLEANERS, FIRST SOAKED IN
ALCOHOL, SECOND DRY.
• FOOTPLATE IS CLEANED WITH
ALCOHOL SWAB.
• IT IS REASSEMBLED WHEN ALL
SURFACES BECOME DRY.
10. ERRORS
• INHERENT: TO PREVENT THIS THE AMERICAN
ACADEMY OF OPHTHALMOLOGY AND
OTOTLARYNGOLOGY HAS STRICT
STANDARDISATION NORMS.
• CONTRACTION OF EOM: IOP INCREASES DUE TO
REFLEX CONTRACTIONS OF THE EOM.
• ACCOMODATION: IOP DECREASES DUE TO
PULLING OF THE CILIARY MUSCELS ON THE TM.
• OCULAR RIGIDITY
• CORNEAL CURVATURE AND THICKNESS
• MOSES EFFECT
• REPEATED IOP MEASUREMENTS
• BLOOD VOLUME ALTERATION
11. GOLDMANN APPLANATION
TONOMETRY
• IT IS THE REFRENCE STANDARD FOR TONOMETRY.
• IT IS BASED ON THE MODIFIED IMBERT-FICK LAW.
• THIS LAW STATES THAT AN EXTERNAL FORCE (W)
AGAINST A SPHERE EQUALS THE PRESSURE IN THE
SPHERE (Pt) MULTIPLIED BY THE AREA FLATTENED
BY THE EXTERNAL FORCE (A).{W= Pt A}.
• VALIDITY OF THE LAW REQUIRES THE SPHERE TO
BE:
PERFECTLY SPHERICAL
DRY
PERFECTLY FLEXIBLE
INFINITELY THIN
12. • THE CORNEA FAILS TO SATISFY ANY OF THE
ABOVE REQUIREMENTS. IT IS ASPHERICAL, WET,
NOT PERFECTLY FLEXIBLE NOR IS IT INFINITELY
THIN.
• MOISTURE CREATES SURFACE TENSION (S),
FORCE REQUIRED TO BEND THE CORNEA WHICH IS
INDEPENDENT OF THE INTERNAL PRESSURE (B).
• SINCE THE CENTRAL CONREAL THICKNESS IS
ABOUT 550µm THE OUTER AREA OF FLATTENING
(A) IS NOT EQUAL TO THE INNER AREA (A1).
• IT IS THEREFORE NECESSARY TO MODIFY THE
EQUATION TO ACCOUNT FOR THESE
CHARACTERISTICS OF CORNEA.
W+S=Pt A1+B
13. • WHEN A1 EQUALS 7.35mm2, S BALANCES B
AND W EQUALS Pt. WHEN APPLANATING
THIS AREAA FORCE OF 0.1g
CORRESPONDS TO AN IOP OF 1mm Hg.
• THE INTERNALAREA OF APPLINATION IS
OBTAINED WHEN THE DIAMETER OF THE
EXTERNAL AREA OF CORNEAL
APPLANATION IS 3.06mm.
• THE VOLUME DISPLACED BY THIS AREA
IS APPROXIMATELY 0.50mm3
• THE OCULAR RIGIDITY DOES NOT
SIGNIFICANTLY INFLUENCE THE
MEASUREMENTS.
14.
15. DESCRIPTION OF TONOMETER
• IT IS MOUNTED ON A STANDARD SLIT LAMP IN A WAY
THAT THE EXAMINER’S VIEW IS DIRECTED THROUGH
THE CENTER OF A PLASTIC BIPRISM, USED TO
APPLANATED THE CORNEA.
• TWO BEAM SPLITTING PRISMS WITHIN THE
APPLANATING UNIT OPTICALLY CONVERT THE
CIRCULAR AREA OF THE CORNEAL CONTACT INTO 2
SEMICIRCLES.
• THE PRISMS ARE ADJUSTED SO THAT THE INNER
MARGINS OF THE SEMICIRCLES OVERLAP WHEN 3.06mm
OF CORNEA IS APPLANATED.
• THE BIPRISM IS CONNECTED BY A ROD TO THE HOUSING
WHICH CONTAINS A COIL SPRING AND SERIES OF LEVERS
THAT ARE USED TO ADJUST THE FORCE OF THE BIPRISM
AGAINST THE CORNEA.
16.
17.
18. TECHNIQUE
• THE CORNEA IS ANAESTHETISED WITH A TOPICAL
PREPARATION AND THE TEAR FILM IS STAINED
WITH SODIUM FLUORESCEIN.
• THE COREAAND THE BIPRISM ARE ILLUMINATED
BY A COBALT BLUE LIGHT FROM THE SLIT LAMP
AND THE THE BIPRISM IS BROUGHT IN GENTLE
CONTACT WITH THE APEX OF THE CORNEA.
• FLUORESCENCE OF THE STAINED TEARS IS USED
TO VISUALISE THE TEAR MENISCUS AT THE
MARGIN OF CONTACT BETWEEN THE BIPRISM AND
THE CORNEA.
• FLUORESCENT SEMICIRCLES ARE SEEN THROUGH
THE PRISM AND THE FORCE AGAINST CORNEA IS
ADJUSTED TILL THE INNER EDGES OVERLAP.
19. • THE INFLUENCE OF OCULAR
PULSATIONS IS SEEN WHEN THE
INSTRUMENT IS PROPERLY
POSITIONED, THE EXCURTIONS MUST
BE AVERAGED TO GIVE THE DESIRED
END POINT.
20. SOURCES OF ERROR
• APPROPRIATE AMOUNT OF FLUORESCEIN IS IMPORTANT.
WIDER MENISCI CAUSE FALSE HIGHER PRESSURE
ESTIMATES.
• IMPROPER VERTICAL ALLIGNMENT LEADS TO FALSE
HIGH IOP ESTIMATES.
• VARIATIONS IN CCT, FALSE HIGH PRESSURE READINGS
ARE ASSOCIATED WITH THICKER CORNEAS. AVERAGE
ERROR IN IOP READINGS IS FOUND TO BE 0.7mm Hg per 10µ
OF DEVIATION FROM 520µ. ( EHLER et al)
• CHANGES IN CORNEAL CURVATURE INFLUENCE IOP
MEASUREMENTS, WITH AN INCREASE OF
APPROXIMATELY 1mm Hg FOR EVERY 3D OF INCREASE IN
CORNEAL POWER.
21. • CORNEAL ASTIGMATISM TOO
INFLUENCES THE IOP MEASUREMENTS.
IOP IS UNDERESTIMATED FOR WITH
THE RULE AND OVERESTIMATED FOR
AGAINST THE RULE, WITH
APPROXIMATELY 1mm Hg OF ERROR
FOR EVERY 4 D OF ASTIGMATISM.
• CORNEAL EDEMAAND SUSTAINED
ACCOMODATION LEADS TO
UNDERESTIMATION OF IOP.
22. DISINFECTION
• ADENOVIRUS TYPE 8 IS INACTIVATED BY SOAKING THE
APPLANATION TIP FOR 5-15 MINS IN DILUTED SODIUM
HYPOCHLORIDE (1:10 SOLUTION), 3% HYDROGEN PEROXIDE,
70% ISOPROPYL ALCOHOL OR BY WIPING WITH ALCOHOL,
HYDROGEN PEROXIDE, POVIDONE IODINE.
• HSV TYPE 1 IS ELIMINATED BY SWABBING WITH 70%
ISOPROPYL ALCOHOL.
• HBV IS REMOVED BY WASHING THE APPLANATION TIP WTH
RUNNING TAP WATER FOR 10 MINUTES.
• THE APPLANATION TIP CAN BE COMPLETELY DISINFECTED OF
HIV 1 BY WIPING WITH 3% HYDROGEN PEROXIDE OR 70 %
ISPROPYL ALCOHOL.
• IT IS IMPORTANT TO REMOVE THE DISINFECTANTS LIKE
ALCOHOL AND HYDROGEN PEROXIDE FROM THE CONTACT
SURFACE BEFORE THE NEXT USE AS THEY CAN CAUSE
TRANSIENT CORNEAL DEFECTS.
23. TONOPEN
• IT IS BASED ON THE MACKAY MARG TONOMETER.
• IT COMPRISES OF A CENTRAL MOVEABLE
PLUNGER OF DIAMETER 1.02 mm WHICH IS
SURROUNDED BY A LARGER FOOTPLATE.
• PRESSING THE INSTRUMENT TIP AGAINST THE
CORNEAACTIVATES A STRAIN GUAGE THAT
SENSES THE FORCE GENERATED BY THE PLUNGER
TO INDENT THE CENTRAL CORNEA.
• AS THE REST OF THE TONOMETER COMES INTO
CONTACT WITH THE CORNEA, THE FORCE
EXERTED ON THE PLUNGER REDUCES UNTIL THE
PLUNGER IS FLUSH WIT THE FOOTPLATE.
24. • THE EFFECT OF THE CORNEAL RIGIDITY
IS TRANSFERRED TO THE SURROUNDING
FOOTPLATE AND AT THAT POINT THE
FORCE EXERTED ON THE PLUNGER IS
CONSIDERED TO BE ONLY THE IOP.
• THE CHANGE IN FORCE GENERATES A
WAVEFORM TRACING WHICH IS
ANALYZED BY A MICROPROCESSOR.
• IT EXHIBITS HIGH CONCORDANCE WITH
TRANSDUCER PRESSURES AT IOP UPTO 40
mm Hg.
• AT ELEVATED PRESSURES THE MACHINE
UNDERESTIMATES.
25. ADVANTAGES OF TONOPEN
• PORTABLE
• USED IN CASE ON CORNEAL EPITHELIAL
IRREGULARITIES.
• MEASUREMENT OF IOP OVER BANDAGE
CONTACT LENS.
• USEFUL IN EDEMATOUS AND SCARRED
CORNEAS.
• USEFUL IN PATIENTS WITH NYSTAGMUS
AND HEAD TREMORS.
• USED IN OPERATION THEATRE
26. NON CONTACT TONOMETRY
• DEVELOPED IN THE EARLY 1970s, IT USES
A JET OF AIR TO APPLANATE THE CORNEA.
• THE PROTOTYPE WAS INTRODUCED BY
GROLMANN IN 1972.
• THE SYSTEM CONTAINS A CENTRAL AIR
PLENUM FLANKED EITHER SIDE BY
INFRARED LIGHT EMITTER AND
DETECTOR.
• IN THE RESTING STATE, THE CONVEX
CORNEA SCATTERS LIGHT AND NO
SIGNAL IS PICKED UP BY THE DETECTOR.
27.
28. • THE PRESSURE OF THE AIR PULSE IS GRADUALLY
INCREASED TO DEFORM THE CORNEA .
• AT CORNEAL APPLANATION, THE CORNEAL
SURFACE BEHAVES LIKE A PLANE MIRROR AND
REFLECTS LIGHT TO THE DETECTOR.
• THIS SIGNAL IS THE TRIGGER TO SWITCH OFF THE
AIR PRESSURE PULSE.
• EARLY NCTs USED TO DETERMINE THE IOP BY THE
TIME TAKEN FOR THE AIR JET TO APPLANATE THE
CORNEA.
• WITH THE INTRODUCTION OF THE PRESSURE
TRANSDUCER IN THE LATE 1980s, IOP WAS
MEASURED FROM THE ACTUAL AIR JET PRESSURE
REQUIRED TO APPLANATE THE CORNEA.
29.
30. OCULAR RESPONSE ANALYZER
• IT IS A NCT THAT MEASURES THE DYNAMIC
ASPECTS OF CORNEAL DEFORMATION BY AIR
PULSE.
• A METER AIR PULSE IS DIRECTED AT THE
CORNEA UNTIL APPLANATION IS ACHIEVED.
• THIS ACTS AS A TRIGGER TO SWITCH OFF THE
AIR PULSE.
• A SMALL TIME DELAY RESULTS IN A FURTHER
INCREASE IN AIR PRESSURE WHICH CAUSES A
DEGREE OF CORNEAL INDENTATION.
31. • AFTER REACHING A PEAK, THE AIR
PRESSURE STEADILY REDUCES UNTIL
IT IS COMPLETELY REMOVED.
• THE INSTRUMENT TAKES TWO
MEASUREMENTS:
1) THE FORCE REQUIRED TO FLATTEN THE
CORNEA AS THE PRESSURE RISES
(FORCE-IN APPLANATION, P1)
2) THE FORCE AT WHICH THE CORNEA
FLATTENS AGAIN AS THE AIR PRESSURE
FALLS (FORCE-OUT APPLANATION, P2)
32. • THE FORCE-OUT APPLANATION
OCCURS AT A LOWER PRESSURE
THAN THE FORCE-IN
APPLANATION, THIS HAS BEEN
ATTRIBUTED TO THE
VISCOELASTIC DAMPENING
EFFECTS OF THE CORNEA.
• THE PRESSURE DIFFERENCE
BETWEEN THE TWO
APPLANATION EVENTS IS
TERMED CORNEAL HYSTERESIS.
• CORENAL HYSTERESIS IS A
DIRECT MEASURE OF THE
BIOMECHANICAL PROPERTIES OF
CORNEAL.
33. CORVIS ST TONOMETER
• NCT WHICH ALSO
MEASURES THE
DYNAMIC ASPECTS OF
CORNEAL DEFORMATION
BY A SYMMETRICALLY
METERED AIR PULSE.
• THE CORNEAL
DEFORMATION RESPONSE
TO THE AIR PULSE IS
VISUALISED BY AN
ULTRA HIGH SPEED
SCHEIMPFLUG CAMERA.
34. PASCAL DYNAMIC CONTOUR
TONOMETER
• INTRODUCED IN 2002
• NON APPLANATING, SLIT LAMP
MOUNTED, CONTACT
TONOMETER.
• IT IS BASED ON THE PRINCIPLE
OF CONTOUR MATCHING.
• IT ASSUMES THAT IF THE EYE IS
ENCLOSED BY A CONTOURED,
TIGHT FITTING SHELL, THE
FORCES GENERATED BY IOP
WOULD ACT ON THE SHELL
WALL.
• REPLACING PART OF THE SHELL
WALL WITH PRESSURE SENSOR
WOULD ENABLE
MEASUREMENT OF IOP.
35. REBOUND TONOMETRY
• IT USES A DYNAMIC ELECTROMECHANICAL
METHOD FOR MEASURING IOP.
• THE DEVICE CONSISTS OF A SOLENOID
PROPELLING COIL AND A SENSING COIL
POSITIONED AROUND A CENTRAL SHAFT
CONTAINING A LIGHT MAGNETIZED PROBE.
• TRANSIENT ELECTRIC CURRENT TO THE
SOLENOID COIL PROPELS THE PROBE TO THE
CORNEA.
• MOVEMENT OF THE MAGNETISED PROBE
INDUCES A VOLTAGE WHICH IS MONITORES BY
THE SENSOR.
36. • AS THE PROBE
IMPACTS
CORNEA IT
DECELERATES
AND REBOUNDS
FROM THE
SURFACE.
• iCare, BECAME
AVAILABLE IN
2003.
37. HOME TONOMETRY
• ZEIMER & Co. DEVELOPED THE FIRST
HOME TONOMETER IN 1983.
• PROVIEW PHOSPHENE TONOMETER
DEVELOPED IN THE LATE 1990s. IT USES
THE ENTOPTIC PHENOMENON OF
PRESSURE PHOSPHENES.
39. TONOMETRY FOR SPECIAL
CIRCUMSTANCES
• TONOMETRY ON IRREGULAR CORNEA
• TONOMETRY OVER SOFT CONTACT
LENS
• TONOMETRY IN A GAS FILLED EYE
• TONOMETRY IN EYES WITH
KERATOPROSTHESES
