Tonometry

this was prepared for a seminar.

1. TONOMETRY

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.

4. CLASSIFICATION A] INDENTATION SCHIOTZ TONOMETER HERRINGTON TONOMETER GRANTS TONOMETER MAURICE TONOMETER B] APPLANATION 1) VARIABLE AREA MAKLAKOV- KALFA TONOMETER (PROTOTYPE) APPLANOMETER (CERAMIC ENDPLATES) TONOMAT ( DISPOSABLE ENDPLATES) HALBERG TONOMETER (TRANSPARENT ENDPLATE) BARRAQUER TONOMETER (PLASTIC TONOMETER) OCULAR TENSION INDICATOR (SCREENING TONOMETER) GLAUCOTEST (SCREENING TONOMETER)

5. 2 ) VARIABLE FORCE GOLDMANN APPLANATION TONOMETER PERKINS APPLANATION TONOMETER DRAEGER APPLANATION TONOMETER TONOPEN PNEUMATONOMETER C] NON CONTACT TONOMETRY OCULAR RESPONSE ANALYZER CORVIS ST TONOMETER

6. SCHIÖTZ TONOMETER PROTOTYPE INDENTATION TONOMETER . PARTS: SCALE LEVER WEIGHT HANDLE PLUNGER FOOTPLATE

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 (P t ) MULTIPLIED BY THE AREA FLATTENED BY THE EXTERNAL FORCE (A).{ W= P t 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 (A 1 ). IT IS THEREFORE NECESSARY TO MODIFY THE EQUATION TO ACCOUNT FOR THESE CHARACTERISTICS OF CORNEA. W+S=P t A 1 +B

13. WHEN A 1 EQUALS 7.35mm 2 , S BALANCES B AND W EQUALS P t . WHEN APPLANATING THIS AREA A FORCE OF 0.1g CORRESPONDS TO AN IOP OF 1mm Hg. THE INTERNAL AREA 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 0.50mm 3 THE OCULAR RIGIDITY DOES NOT SIGNIFICANTLY INFLUENCE THE MEASUREMENTS.

14. 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.

15. TECHNIQUE THE CORNEA IS ANAESTHETISED WITH A TOPICAL PREPARATION AND THE TEAR FILM IS STAINED WITH SODIUM FLUORESCEIN. THE COREA AND 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.

16. THE INFLUENCE OF OCULAR PULSATIONS IS SEEN WHEN THE INSTRUMENT IS PROPERLY POSITIONED, THE EXCURTIONS MUST BE AVERAGED TO GIVE THE DESIRED END POINT.

17. 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.

18. 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 EDEMA AND SUSTAINED ACCOMODATION LEADS TO UNDERESTIMATION OF IOP.

19. 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.

20. 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 CORNEA ACTIVATES 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.

21. 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.

22. 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

23. 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.

24. 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.

25. 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.

26. AFTER REACHING A PEAK, THE AIR PRESSURE STEADILY REDUCES UNTIL IT IS COMPLETELY REMOVED. THE INSTRUMENT TAKES TWO MEASUREMENTS: THE FORCE REQUIRED TO FLATTEN THE CORNEA AS THE PRESSURE RISES (FORCE-IN APPLANATION, P1) THE FORCE AT WHICH THE CORNEA FLATTENS AGAIN AS THE AIR PRESSURE FALLS (FORCE-OUT APPLANATION, P2)

27. 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.

28. 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.

29. 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.

30. 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.

31. AS THE PROBE IMPACTS CORNEA IT DECELERATES AND REBOUNDS FROM THE SURFACE. iCare , BECAME AVAILABLE IN 2003.

32. 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.

33. CONTINUOUS TONOMETRY SENSIMED TRIGGERFISH SENSOR IS THE ONLY DEVICE WHICH IS AVAILABLE COMMERCIALLY.

34. TONOMETRY FOR SPECIAL CIRCUMSTANCES TONOMETRY ON IRREGULAR CORNEA TONOMETRY OVER SOFT CONTACT LENS TONOMETRY IN A GAS FILLED EYE TONOMETRY IN EYES WITH KERATOPROSTHESES

35. THANK YOU