principles of tonometry Flashcards
what is IOP?
intra ocular pressure is largely determined by balance between aqueous production and drainage
what does positive IOP do?
positive IOP maintains the shape of the eye
what is the mean IOP?
mean IOP=15.7mmHg
what is the standard deviation of IOP?
standard deviation of IOP=2.5mmHg
2SD=5mmHg
21mmHG is by 16+2SD(5)
what does IOP rise with?
–rises with age
rise of 1-2mmHg between 20 -70 years of age
–raise with systolic BP
–Gender in older women
-race higher in black population, less in caucasion population
-Myopia have high IOP
How is IOP determined?
IOP is genetically determined
How does IOP range during the day?
- Normal range 3-5mmHg
- glaucotamous average range 13mmHg
- greater than 10mmHg is considered pathological
- some with high IOP in morning with afternoon dip and some have afternoon peaks
- repeat at different times of day to get average
how does fluid intake effect IOP?
water increases IOP by 3mmHg with max effect 20 mins
alcahol decreases IOP by 3mmHg with max effect 5 minutes
Coffee increases IOP by 5mmHg with max effect 20 mins
How does provoctive test for glaucoma?
drink 1 litre of water and measure IOP after 15 mins and lying down in dark for 1 hour. a rise in IOP of 8mmHg suggests a risk of glaucoma due to decrease in aqueous outflow
why do we worry about raise IOP?
raise IOP is one major risk factor of glaucoma
what does glaucoma cause
glaucoma causes optic nerve fibre atrophy
optic neauropathy
visual field defects
possible blindness
why do we worry about reduced IOP?
reduced IOP slows down progression of optic neuropathy and early detection is beneficial
how do you measure IOP?
manometry
pressure sensor inserted into the eye but not popular with px
what is indentation tonometry ?
force required to flatten (applanate) a constant area of the cornea, for the Imbert-Fick law.
amount of indentation is proportional to pressure inside the eye.
how does schiotz tonometer work?
rigidity of the globe must be taken into account
-Pt=F/a + b.S
a and b are constants and S is scale of reading
conversion tables supplied within instruments and translate scale reading into estimate IOP
give disadvantage of using schiotz tonometer?
less accurate than applantation of IOP
Explain about dynamic and rebound tonometry?
-a magnetised probe is launched toward the cornea
-magnetised probe consists of steel wire shaft covered with round plastic tip at the end.
probe hits the cornea and bounces back
what is a tonopen?
tonopen another instrument to measure IOP, has transducer senses indentation produced by probe
- digital readout
- portable accurate and easy to use
Explain the imberts ficks law?
for a spherical container which has and infinitely thin, perfectly elastic and dry surfaces, equilibrium is achieved for an object placed in contact when
pressure = force/area
Pt=F/A.
what if the eyes/cornea does not full fill the required criteria
;cornea is not thin
;cornea not perfectly active
;corneal surfaces moistened by the tear film
what does Pt=F/A-k+t
imbert ficks law
when the diameter of circle area of applantation is between 3mm and 4mm then k and t are approx equal so they cancel each other out
Pt=F/A
what is the applanatation tonometry method A?
apply a known force and measure areas applanated
What is the applanatation tonometry method B?
increase force until known area is applanated
what is the goldmann applanation tonometer ?
- circle area of applanation is 7.35mm squared
diameter = 3.06mm - as diameter is between 3mm- 4 mm we can assume that surface tension of fluid and rigidity of the cornea cancel out
-gives huge advantage 1g force=10mmHg IOP and no conversion tables are needed
How does goldmann applanation tonometer work?
=use disposable probe
= position patient
=anaesthetise the cornea
=apply fluorescein using the fluorescein paper strip
=set 1.0gm or 1.5gm force
=insert cobalt blue filter
=bring probe in contact with cornea by moving the slit lamp forward each side and above to keep probe centred on cornea
what are the non contact methods of tonometry?
=same as applanation tonometry but puff of air is used instead as applanation force.
=puff of air gradually increases force , can measure time to achieve applantation or force used when applanation achieved.
what are the advantages of non contact tonometry?
no contact with cornea
no anesthetics required
can be performed by non optometric staff
name one non contact method ?
reichert non-contact tonometer
- has a IR emitter
- IR detector
- air puff nozzle
what does the non contact tonometer do?
time take to flatten the cornea (iIR signals piked up by detector increases) measured
calibrated against goldmann to give output in mmHg
name another non contact tonometer?
keeler pulsair
how does keeler pulsair work?
handheld
similar to principle to reichert but uses transducer to sense air pressure at moment of applanation
what is IOP influenced by?
Corneal thickness
corneal curvature
—–steep corneas over estimate true IOP
——Flat cornea under estimate true IOP
what is the average CCT ?
Average corneal thickness is 0.54mm
but can range 0.44mm to 0.64mm
what measures corneal thickness in practice?
pachymetry
what is use of corneal anaesthesia?
necessary for contact tonometry
sting on instillation
anaesthesia within 1 min last around 25 mins
mustn’t rub eyes damage cornea
cause allergic reaction hence must check with the px.
what are the ocular anaesthetics?
tetracaine hydrochloride (amethocaine) 0.50%
oxybuprocaine hydrochloride (benoxinate) 0.40
proxymetacaine hydrochloride 0.50%
available in minims
