principles of tonometry

Question 1

what is IOP?

Answer 1

intra ocular pressure is largely determined by balance between aqueous production and drainage

Question 2

what does positive IOP do?

Answer 2

positive IOP maintains the shape of the eye

Question 3

what is the mean IOP?

Answer 3

mean IOP=15.7mmHg

Question 4

what is the standard deviation of IOP?

Answer 4

standard deviation of IOP=2.5mmHg
2SD=5mmHg
21mmHG is by 16+2SD(5)

Question 5

what does IOP rise with?

Answer 5

–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

Question 6

How is IOP determined?

Answer 6

IOP is genetically determined

Question 7

How does IOP range during the day?

Answer 7

• 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

Question 8

how does fluid intake effect IOP?

Answer 8

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

Question 9

How does provoctive test for glaucoma?

Answer 9

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

Question 10

why do we worry about raise IOP?

Answer 10

raise IOP is one major risk factor of glaucoma

Question 11

what does glaucoma cause

Answer 11

glaucoma causes optic nerve fibre atrophy
optic neauropathy
visual field defects
possible blindness

Question 12

why do we worry about reduced IOP?

Answer 12

reduced IOP slows down progression of optic neuropathy and early detection is beneficial

Question 13

how do you measure IOP?

Answer 13

manometry

pressure sensor inserted into the eye but not popular with px

Question 14

what is indentation tonometry ?

Answer 14

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.

Question 15

how does schiotz tonometer work?

Answer 15

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

Question 16

give disadvantage of using schiotz tonometer?

Answer 16

less accurate than applantation of IOP

Question 17

Explain about dynamic and rebound tonometry?

Answer 17

-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

Question 18

what is a tonopen?

Answer 18

tonopen another instrument to measure IOP, has transducer senses indentation produced by probe

• digital readout
• portable accurate and easy to use

Question 19

Explain the imberts ficks law?

Answer 19

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.

Question 20

what if the eyes/cornea does not full fill the required criteria

Answer 20

;cornea is not thin
;cornea not perfectly active
;corneal surfaces moistened by the tear film

Question 21

what does Pt=F/A-k+t

Answer 21

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

Question 22

what is the applanatation tonometry method A?

Answer 22

apply a known force and measure areas applanated

Question 23

What is the applanatation tonometry method B?

Answer 23

increase force until known area is applanated

Question 24

what is the goldmann applanation tonometer ?

Answer 24

• 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

Question 25

How does goldmann applanation tonometer work?

Answer 25

=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

Question 26

what are the non contact methods of tonometry?

Answer 26

=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.

Question 27

what are the advantages of non contact tonometry?

Answer 27

no contact with cornea
no anesthetics required
can be performed by non optometric staff

Question 28

name one non contact method ?

Answer 28

reichert non-contact tonometer

• has a IR emitter
• IR detector
• air puff nozzle

Question 29

what does the non contact tonometer do?

Answer 29

time take to flatten the cornea (iIR signals piked up by detector increases) measured
calibrated against goldmann to give output in mmHg

Question 30

name another non contact tonometer?

Answer 30

keeler pulsair

Question 31

how does keeler pulsair work?

Answer 31

handheld

similar to principle to reichert but uses transducer to sense air pressure at moment of applanation

Question 32

what is IOP influenced by?

Answer 32

Corneal thickness
corneal curvature
—–steep corneas over estimate true IOP
——Flat cornea under estimate true IOP

Question 33

what is the average CCT ?

Answer 33

Average corneal thickness is 0.54mm

but can range 0.44mm to 0.64mm

Question 34

what measures corneal thickness in practice?

Answer 34

pachymetry

Question 35

what is use of corneal anaesthesia?

Answer 35

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.

Question 36

what are the ocular anaesthetics?

Answer 36

tetracaine hydrochloride (amethocaine) 0.50%
oxybuprocaine hydrochloride (benoxinate) 0.40
proxymetacaine hydrochloride 0.50%
available in minims