GLAUCOMA 2 - tonometry + pachymetry Flashcards
why is early detection of glaucoma needed?
treatment is more effective
allows for identification and follow up of at risk px’s
which is the only treatment that is effective in glaucoma?
lowering IOP
demographic and clinical factors that can affect IOP in long term
- age
- race (african)
- genetics
- gender (women)
- myopia
- systemic HTN and diabetes
- ocular disease
- corneal characteristics
which ocular disease can raise IOP? (an example)
pigment dispersion syndrome
which ocular disease can lower IOP? (an example)
retinal detachment
what are the 2 factors that affect IOP measurement?
corneal thickness
corneal curvature
steep corneas…
over estimate true IOP
flat corneas…
under estimate true IOP
according to ocular HTN treatment study, does a thick or thin cornea pose as a risk factor for OAG?
thin
on average, do people with ocular HTN have thin or thick corneas?
thick
on average, do people with NTG have thin or thick corneas?
thin
how do you measure CCT?
pachymetry
what is the suggested correction factor for IOP measurement using CCT?
~0.2 - 0.7 mmHg per 10um difference from an average CCT
what is the average CCT?
535um
what are the 3 ways pachymetry measures CCT?
- optical
- interferometric
- ultrasound techniques
what is the gold standard for CCT measurement>
hand-held ultrasound pachymeter
what are 2 examples of hand-held ultrasound pachymeter?
pachmate
PachPen
why can’t you use hand-held ultrasound pachymeter on everyone?
need to use anaesthetic and touch the px’s eye
what frequencies do ultrasound pachymeters operate at?
20-50MHz
how do ultrasound pachymeters work?
they emit short acoustic pulses and detect reflections from anterior and posterior surfaces of the cornea
CCT is calculated from the measured time-of-flight between the reflections and the accepted speed of sound
what is the accepted speed of sound in the cornea?
1636-1640 m/s
name 6 sources of error in pachymetry?
- decentration
- oblique angle
- central compression by the probe
- effects of local anaesthetic on cornea up to 10um difference
- variation in the speed of sound between healthy and diseased tissue
- inter and intra observer variability
what are some short term factors that affect IOP (there are BARE)
- time of day
- cardiac cycle
- body position
- blinking, force blinking or hard squeezing
- fluid intake
- px holding their breath
- accommodation
what is the normal diurnal range in IOP?
3-5 mmHg
what is the glaucomatous average diurnal range?
13mmHg
what diurnal range is considered pathological?
10 mmHg or greater
why should you repeat IOP measurements at diff time of day?
most people’s IOP higher in morning, but some people can have afternoon peak instead
what is phasing?
measuring IOP over 24 hours
at what time of day is IOP the lowest?
middle of the night (lines up with BP)
what is the average variation in cardiac cycle?
2-3 mmHg
how many readings of blood pressure should you take?
3-4
what is the effect of body position on IOP?
erect to supine = 0.3 - 6 mmHg increase
when you are upside down, what is the increase in IOP in normals AND glaucomatous px’s
30-35 mmHg
why wouldn’t you get glaucoma from yoga (being upside down)?
blood pressure in your head also increases therefore provides some sort of protection
how does regular blinking affect IOP?
increases IOP by 3 mmHg
how does forced blinking affect IOP?
increases IOP by 10 mmHg
how does hard squeeze blinking affect IOP?
IOP can go up to more than 50 mmHg
how does REPEAT hard squeeze blinking affect IOP?
potentially lower IOP
how does drinking water affect IOP ?
increases IOP by up to 3 mmHg with max effect at 20 min
how does drinking alcohol affect IOP?
decreases IOP by up to 3 mmHg with max effect at 5 mins
how does dirnking coffee affect IOP?
increases IOP by up to 3 mmHg with max effect at 20 min
how does holding your breath affect IOP?
increases IOP by roughly 5-20mmHg
IOP CAN DOUBLE !
how does sustained accommodation affect IOP?
reduction by about 3 mmHg
how does the onset of accommodation affect IOP?
can increase IOP
what are the advantages of NCT?
- not operator dependent
- repeat measures do not affect IOP
- no anaestheic
- can be used by para-professionals
- little (if any) risk of cross infection
- can be quicker
disadvantages of NCT
- accuracy
- need multiple measures
- initial cost
- opthalmologists don’t like NCTs but optoms still use them LOOL
NICE 2017 guideline on NCT and referral
if IOP is >= 24 mmHg is the only sus finding - DO GAT before referral
what is the regression to mean effect?
the second reading will be more likely to measure a lower IOP when the px is examined in hospital
how does the reichart ocular response analyser work?
determines the total corneal resistance and includes combines effects of CCT, rigidity and hydration
how does Dynamic Contour Tonometry work?
used on slit lamp like GAT
IOP sampled continuously
said to produce readings unaffected by CCT
how does iCare “rebound” tonometry work?
magnetised probe is launched towards the cornea
probe consists of a magnetized steel wire shaft covered with round plastic tip at the end
probe hits the cornea and bounces back
how many readings does the iCare tonometer take?
6 measurements
how does iCare tell the IOP ??
a solenoid inside the probe is used to detect motion and impact, the probe slows down faster as the IOP increases
THE HIGHER THE IOP, THE SHORTER THE DURATION OF IMPACT
what 3 questions were asked in the iCare tonometer questionnaire?
1 - how comfortable did you find this test
2 - how anxious did the test make you feel
3 - could you descrive the test in 3 words?
what should patients be told before performing NCT?
loosen neck ties and dont hold your breath
how many readings should you take for NCT>
4 per eye
what is the normal inter-eye difference in IOP?
<4mm
what is suspect inter-eye difference in IOP ?
5-7 mmHg
what is considered abnormal inter-eye difference in IOP?
> 8 mmHg
what is the referral speed for suspect glaucoma?
routine
what is the referral speed for IOP >40 mmHg
emergency (within 24 hours)
what is the referral speed for IOP between 35-40 mmHg with visual field loss?
urgent (1 week)
