principles of tonometry Flashcards
what is intraocular pressure ?
. pressure exerted by fluid inside the eye in the walls of globe
. positive intraocular pressure required to maintain the shape of the eye
what is the pressure determined by ?
. pressure is largely determined by the balance between aqueous production and outflow §
what is the distribution of IOP within the population ?
. mean of IOP = 15.7mmHg
. standard deviation = 2.5mmHg
. 95% of population within 2SD of mean , so < or equal 21mmHg
. so 11-21 is normal range
when should Goldmann applanation tonometry be used ?
optometrist should recheck IOPs in excess of 21mmHg using GAT
is IOP constant ?
. IOP is determined by 3 factors , which are all fluctuating independently over differing timescales
what are the 3 factors that IOP is determined by?
1 . the amount of fluid within the eye
2 . external forces acting upon the eye, including the tension within the ocular walls
3 . the intraocular volume
what are demographic and clinical factors that can affect IOP ?
1 . age : elevation with increasing age, especially in people of African descent. Related to other factors changing with age e.g. systemic hypertension , diabetes
2 . race ( higher in non glaucomatous black population )
3 . genetics
4 . gender : IOP 1-2mmHg higher in older women
5 . myopia : is associated with higher IOP
what are non-demographic and non- clinical factors that can affect IOP?
- IOP pulsates with cardiac cycle , so from one moment to next IOP can vary by as much as 3-4mmHg
- also affected by lid squeezing , apprehension , breath holding , posture , accommodation , caffeine ( lowers ) , alcohol ( lowers) and water ( higher )
- time of day
what is normal time of day (diurnal range) ?
normal range 3-5mmHg
what is glaucomatous average
diurnal range?
glaucomatous average range is 13mmHg
what is considered pathological diurnal range?
anything greater than 10mmHg
what are the 2 groups of patients in terms of IOP peak?
. most higher in morning but some patients have afternoon peaks
. repeat IOP measurement at a different time of day
. always record time of day
why is IOP clinically important ?
. Raised IOP is one of the major risk factors in glaucoma
. reducing IOP can slow down the progression of optic neuropathy
. early detection is beneficial
what does Glaucoma cause?
. glaucoma causes optic nerve fibres to atrophy
- optic neuropathy
- visual field defect
- possible blindness
what is manometry ?
. it’s an intraocular measurement
. pressure sensor inserted into the eye
. not popular with patients
what is indentation tonometry ?
1 . probe is inserted into surface of eye
2 . the amount by which the globe is indented is proportional to the intraocular pressure inside eye and ocular rigidity
. Schiotz tonometer is mostly used
how does Schiotz tonometer work?
. patient lies down
. anesthesie the cornea
. lower plunger into eye ball
. footplate rests gently onto the cornea
. conversion tables , supplied with the instrument, translate scale readings into estimates of IOP
what are the draw backs of schiotz tonometry ?
. assumes everyone has same ocular rigidity
. tonometer weighs > 16.5g, therefore raises IOP during measurement
. anaesthetic required
what is tonopen ?
. uses a combination of applanation and indentation
. transducer senses indentation produced by probe
. digital readout
. portable ,accurate and easy to use
. anaesthetic required
what is Icare tonometer ?
. uses dynamic or rebound tonometry
. a magnetised probe with a round plastic tip at the end is launched towards the cornea
. probe hits the cornea, decelerates and bounces back
. probe decelerates more rapidly if IOP is high
. no anaesthetic is required
what is Imbert-Fick principle ?
. all applanation tonometry is based on this principle
. principle states that for a spherical container which has an infinitely thin , perfectly elastic and dry surface, equilibrium is achieved for an object placed in contact when
pressure = force/area
why does the Imbert -Fick principle not apply to the cornea ?
. the cornea is not infinitely thin
. the cornea is not perfectly elastic
. the corneal surface is moistened by the tear film
how can we make the Imbert-Fick principle apply to eye ?
. forces acting downwards is force by tonometer and surface tension (t)
. force acting against probe is rigidity ( K ) and pressure ( Pt)
. when the diameter of the circular area of applanation is between 3mm and 4mm then K and t are approximately equal and cancel each other
.Pt=F/A
what is method A of applanation tonometry ?
. apply a known force
. measure the area applanated
. e.g. tonomat
what is method B of applanation tonometry ?
. increase force until
. known area is applanated
what is the circular area of GAT ?
. the circular area of applantion is 7.35mm square ( dimater = 3.06 mm )
. as diameter is between 3-4 mm , we can assume that surface tension of fluid and rigidity of the cornea cancel out
. 3.06 mm gives the huge advantage that 1g force = 10 mmHg IOP and no conversion tables are needed
what is the principle of GAT?
- instil fluorescein
- tonometer head is pushed forward and makes contact with the cornea
- around the edge of tonometer head , there will be a circle of fluorescein where tonometer head touches the cornea this is the applanated area
- keep pushing forward on tonometer head until size of applanated area is 3.06mm
- biprism in tonometer head splits fluorescein ring of applanated area into two semi circles known as mires separated by 3.06mm
- full applanation is when the diameter of circular flattened area = 3.06mm
what is the clinical technique of GAT ?
- use disposable probe
- position patient
- anaesthetise cornea
- apply fluorescein
- set 1.0gm or 1.5gm force
- inset cobalt blue filter
- bring probe in contact with cornea by moving the slit lamp forward ( looking from each side and above to keep probe centred on cornea)
what happens if mires are overlapping ?
. reduce pressure
what happens if mires are thin ?
add more fluorescein
what to do if one semi-circle is bigger?
. if one semi-circle is bigger than the other , remove probe from eye and raise or lower slit lamp in direction of bigger circle
. if mires are off to the side, move horizontally towards them to centre up
what is perkins tonometer ?
. hand held
. look through viewing hole
what is the the theory behind non-contact tonometers?
. theory is the same as applanation tonometry, but puff of air is applanating force
. air puff of gradually increasing force applied. either measure time to achieve applanation, or force of air when applanation achieved
what are advantages of non-contact tonometers?
. no physical contact with cornea
. no anaesthetic required
. can be performed by non optometric staff
how does keeler Pulsair work ?
. handheld
. uses transducer to sense air pressure at moment of applanation
. has an infrared emitter to one side of device and an infrared detector to the other side
. when infrared light shines onto the cornea , light bounces in all directions
. when a puff of air is applied to flatten , infrared light bounces of this flattened surface to hit infrared detector
. area applanated is 3.06mm
disadvantages of non-contact tonometer ?
. require multiple readings
what is IOP measurement influenced by?
. corneal thickness
. corneal curvature
- steep corneas over-estimate true IOP
- flat corneas under-estimate true IOP
what is average corneal thickness ?
. average CCT is approximately 0.54mm
. but CCT can range from around 0.44mm to 0.64mm
what is the relationship between corneal thickness and IOP?
. thin corneas - underestimate IOP
. thick corneas - overestimate
when is GAT closest to true IOP ?
. when corneal thickness is around 0.52 mm
how to measure corneal thickness?
. pachymetry
how to decide who to refer for primary open angle glaucoma?
. there is optic nerve head damage on stereoscopic slit lamp
. there is a visual field defect consistent with glaucoma
. IOP is 24mmHg or more using GAT
. need to check IOP ( GAT ) , fields and evaluate disc
