Tonometry

A presentation on various techniques and principle of tonometry.

1. TONOMETRY
Dr Saurabh Kushwaha
Resident Ophthalmology

2. SCOPE
Introduction
Tonometry techniques
Manometry
Digital tonometry
Schiotz tonometry
Goldmann applanation tonometry
Non Contact tonometry
Pascal’s DCT

3. INTRODUCTION
IntraOcularPressure:IOPisthepressurewithinthe
eyeball
IntraOcularTension:IOTisthepressureexertedbythe
intraocularcontentsontheoutercoatsofeye
NormalIOP:Itisthatpressurethatdoesnotleadto
glaucomatousdamageoftheopticnervehead.
NormalmeanIOP=15.5+/-2.57mmHg&2SDabove
meanis20.5mmHg

4. TECHNIQUES FOR MEASURING IOP
Techniques
Manometry
Indentation
1. Von Graefe
2. Schiotz
Instrumental Digital
Direct Indirect (Tonometry)
Applanation
Non ContactContact
1. GrolmanNCT
2. Ocular response
analyzer
Variable force Variable area
Contour
Pascal’s DCT
Maklakov1. Goldmann
2. Perkins

5. FACTORS AFFECTING TONOMETRY

6. MANOMETRY
CatheterisinsertedintoACofeye
Otherendisconnectedtoamanometricdevice
MOST accurate method
NOT feasible in humans (invasive procedure)

7. DIGITAL TONOMETRY
Responseofeyeballtopressureappliedbypulpoffinger
METHOD:restbothhandsonpatient’sforehead&
alternatelyapplyjustenoughpressureontheglobe(above
thesuperiortarsalplate)toindentitslightlywith1index
fingerwhilefeelingthecompliancewiththeother
Indentseasily/firmtotouch
NOTaccurate(subjective)

8. DIGITAL TONOMETRY
Advantages
•Easiesttoperform
•Noequipments
•Noanesthesia
•Nostain
•EstimationofIOPwithirregularcorneas,where
applanationtonometryisnotpossible.
Disadvantages
•Readinginaccurate
•Subjective
•Over-estimationorunder-estimationofIOP

9. AN IDEAL TONOMETER
Accurate & reasonable IOP measurement
Convenient to use
Simple to calibrate
Stable from day to day
Easier to standardize
Free of maintenance problems

10. INDENTATIONTONOMETRY
Shapeofdeformation-
TRUNCATEDCONE
Preciseshape-variable&
Unpredictable
Displacelargeintraocular
Volume
Conversiontablesbased
onempiricaldataused
Prototype-Schiøtz
tonometer
APPLANATIONTONOMETRY
Shapeofdeformation-
FLATTENING
Preciseshape–constant
Displacesmallintraocular
Volume
Mathematicalcalculations
forIOP
Differentiatedonthebasis
ofvariablemeasured

11. SCHIØTZ TONOMETER
Handle:toholdtheinstrument
Footplate:restsonthecornea
Plunger:movesfreelywithina
shaftinthefootplate
Bentlever:shortarmandlong
armactingasapointerneedle
Thedegreetowhichtheplunger
indentsthecorneaisindicatedby
themovementofthisneedleona
scale
Weights:5.5gweightis
permanentlyfixedtotheplunger,
whichcanbeincreasedto7.5and
10gm.

12. PRINCIPLE
Theweightoftonometerontheeyeincreasesthe
actualIOP(Po)toahigherlevel(Pt)
ThechangeinpressurefromPotoPtisanexpression
oftheresistanceoftheeye(scleralrigidity)tothe
displacementoffluid
P(t) = P(o) + E
IOPwithTonometerinposition
Pt = Actual IOP Po + Scleral Rigidity E
BecausethetonometeractuallymeasuresPt,itis
necessarytoestimatePoforeachscalereading&weight.

14. On the basis of,
Friedenwaldformula,asetof
conversiontablesforIOPwas
made
Theindicatedscalereading
andtheplungerweightare
convertedtoan IOP
measurement
Moretheplungerindents
thecornea,higherthescale
readingandlowertheIOP
Eachscaleunitrepresents
0.05mmprotrusionofthe
plunger

15. GOLDMANN APPLANATION
TONOMETER (1954)
Goldmannbasedhisconceptoftonometryonthe
ModifiedImbert-FickLaw
W + S = PA1 + B
WhenA1=7.35mm2SbalancesB&WequalsP
Thisinternalareaofapplanationisobtainedwhendiam.
ofexternalareaofapplanationis3.06mm
Volumeofdisplacementproducedbyapplanatingthis
areaisapprox.0.50mm3
SoPisveryclosetoactualIOP&ocularrigiditydoes
notsignificantlyinfluencemeasurements

16. GOLDMANN APPLANATION
TONOMETER (1954)
ApplanationtonometryisbasedontheImbert-Fick
principle,whichstatesthatthepressure(p)insideanideal
dry,thinwalledsphereequalstheforce(F)necessaryto
flattenitssurfacedividedbytheareaoftheflattening(A).
Corneabeingaspherical,wet,andslightlyinflexiblefails
tofollowthelaw.
Moisturecreatessurfacetension(S)orcapillaryattraction
oftearfilmfortonometryhead.
Lackofflexibilityrequiresforcetobendthecornea(B)
whichisindependentofinternalpressure.
Thecentralthicknessofcorneaisabout0.55mmandthe
outerareaofcornealflatteningdiffersfromtheinnerareaof
flattening(A1).Itisthisinnerareawhichisofimportance.

17. Thepressure(P)ofabodyoffluidencapsulatedwithin
asphereisdirectlyproportionaltotheforce(W)required
toapplanateanarea(A)ofthesphere:
W = PA
IMBERT –FICK PRINCIPLE (1885)

18. W + S = PA1 + B
W = tonometer force
S = surface tension of pre-corneal tear film
P = intra-ocular pressure
A1 = inner corneal area of applanation
B = corneal rigidity
MODIFIED IMBERT-FICK’S LAW

19. PARTS OF GOLDMANN TONOMETER

20. Thetwobeam-splittingprismwithinthe
applanatingunitopticallyconvertthecircularareaof
cornealcontactinto2semicircles
BIPRISM PROBE

21. Thefluorescentsemicircles
areviewedthroughthebiprism
andtheforceagainstthecornea
isadjusteduntiltheinneredges
overlap.

22. FALSELY LOW IOP
Inadequate flourescein
thin cornea
corneal edema
with the rule astigmatism
(1mm Hg per 4 D)
prolonged contact
Repeated tonometry
FALSELY HIGH IOP
Excess flourescein
thick cornea
steep cornea
against the rule astigmatism
(1mm Hg per 3D)
wider meniscus
Widening the lid fissure
excessively
Elevating the eyes more
than 15°

23. POTENTIAL ERRORS
Thin cornea
Thick cornea
Astigmatism > 3D
Irregular cornea
Inadequate fluorescein
Too much fluorescein
Tonometer out of calibration
Repeated tonometry
Elevating eyes > 15°
Pressing on the eyelids or globe
Squeezing of the eyelids
Observer bias (expectations and even Numbers)

24. PERKINS TONOMETER (1965)
DevelopedbyESPerkins
Hand-heldapplanationtonometer
UsessameprismtipsasGAT
Theprismisilluminatedbybatterypoweredbulbs
AdvantagesOverGAT:
•Portable&canbeusedinanypositionofpt.
•Infants/children
•intheO.T.
•atthebedsidefornon-ambulatorypt.

25. GROLMAN NCT (1972)
Introduced by Grolmanin 1972
NCT has 3 subsystems:
1. Alignment system: It aligns patient’s eye in 3
dimensions (axial/ vertical/ lateral)
2. Optoelectronic applanation monitoring system:
a. Transmitter directs a collimated beam of light at
corneal apex
b. Receiver & detector accept only parallel coaxial
rays of light reflected from cornea
c. Timer measures from an internal reference to
the point of peak light intensity
3. Pneumatic system: It generates a puff of room air
directed against cornea

26. GROLMAN NCT (1972)

27. GROLMAN NCT (1972)
Advantages
•Screeningprocedure
•Canbeoperatedbynon-medicalpersonnel
•Noanesthesiarequired
•Nocontamination
•Nochanceofcornealabrasion
Limitations
•IOPisnearnormal,accuracydecreaseswithincrease
inIOP&ineyeswithabnormalcorneaorpoorfixation
•Sub-epithelialairbubblesafterrepeateduseofNCT
(rare)

28. PULSAIR TONOMETER (1986)
NewNonContactTonometer
KeelerPulsairIntellipuffisaportablehand-heldtonometer
LaunchedinEuropeanmarketinApril2007
BasedonthesameprincipleastheGrolmanNCT

29. TONOPEN
ThemostcommonlyusedMackay-Margtypetonometer
todayistheTono-Pen
FDAapprovedinMarch2006
Portablehandheldinstrumentwithastraingaugethat
createsanelectricalsignalasthefootplateflattensthe
cornea
Micro-straingaugetechnology
Computerisedbattery-operatedpockettonometer
Instrumentis18cminlengthandweighs60g
ConvertsIOPintoelectricwaves
Waveformisinternallyanalyzedbyamicroprocessor
Averageof3to6readingsofIOP

30. AstheareaofapplanationoftheTonopenissmaller
thanGAT(2.36mm2Vs7.35mm2)therefore,theoretically
thedifferencebetweenapplanatingpressure&IOPis
reducedduetoreducedcornealresistanceofasmaller
contactarea
Itisparticularlyusefulincommunityeyecamps,on
wardrounds,children,irregularsurfaces,measuring
throughanamnioticmembranepatchgraft,toreadfrom
thesclera
Adisposablelatexcoverwhichisdiscardedaftereach
useprovidesinfectioncontrol

31. PASCAL’S DYNAMIC CONTOUR
TONOMETER (2003)
Firsttotallynewconceptintonometrywasdescribedby
Kanngiesseretal.in2005
Basedonprincipleofcontourmatching
Principle:Bysurroundingandmatchingthecontourof
asphere(oraportionthereof),thepressureontheoutside
equalsthepressureontheinside
Designedtoreducetheinfluenceofbiomechanical
propertiesofcorneaonmeasurementofIOP

33. Cup-likeplasticdevicewithcontourmatchedtip
Concavesurfaceofradius10.5mm,whichapprox.to
theshapeofanormalcorneawhenpressureonbothsides
isequal
Probeisplacedincontactwithcorneawithconstant
forceof1g
Theintegratedpiezo-electricpressuresensor
automaticallybeginstoacquiredata
MeasuresIOP100timespersecond
Acompletemeasurementcyclerequiresabout8secof
contacttime
Thedevicealsomeasuresthevariationinpressurethat
occurswithcardiaccycle(OcularPulseAmplitude)

34. DCTismoreaccuratethanGoldmanntonometry&
pneumo-tonometer
Notaffectedbycornealthickness
IOPisnotalteredbycornealrefractivesurgerythat
thinsthecornea
DCTtellsusocularpulseamplitude
OPAmaybeindicativeofthestatusofocularbloodflow
(lowOPA=lowocularbloodflow)
OPAisincreasedovernormal(1.2–4mmHg)inmost
formsofglaucoma&mayberelatedtothelevelofIOP

35. CONCLUSION
ElevatedIOPhasbeenknowntobeassociatedwith
glaucomaforoveramillenium
AllourinstrumentsgiveusanindirectmeasureofIOP
MostofthenewerdevicescanmeasurehigherIOPs
accuratelybutweareofteninterestedinlowerIOPsas
well(whichisbestmeasuredbyGAT)
Despiteallthelimitations,GATremainstheGOLD
STANDARDforIOPmeasurement.

36. THANK YOU