Tonometry and pachymetry

Question 1

for which 2 reasons is the early detection of glaucoma needed

Answer 1

• Treatment is more effective the earlier you pick it up
  Lowering IOP is the only treatment that is effective in glaucoma as its a modifiable risk factor
• It allows the identification and follow up of “at risk” patients

Question 2

how good are optometrists at detecting glaucoma

Answer 2

• Glaucoma detection rates vary between 17% and 39% for patients referred by Optometrists with a suspected diagnosis of glaucoma
• But a lot of those referred who do not have glaucoma are ocular hypertensives or other “suspects” who the ophthalmologists may wish to treat:
  One third had glaucoma, one third were suspects, and one third were discharged
• more than 95% of those referred for suspected glaucoma are referred by optometrists

Question 3

list the 8 Demographic and clinical factors that can affect IOP (long term effects) giving reasons as to why/how for each and which one out of these is the biggest factor

Answer 3

• Age: a rise of 1 – 2mmHg occurs between 20 and 70 years of age
  vascular - changes with age
• Gender: IOP 1 – 2mmHg higher in older women
• IOP seems to be genetically determined
• Race
  In one study mean IOP was higher in a non-glaucomatous black population compared with a caucasian population
• Myopia: associated with higher IOP
• Systemic disease
  Systemic hypertension and diabetes may lead to higher IOP
• Ocular disease
  Some diseases can raise IOP (e.g. Pigment Dispersion Syndrome) and some can lower it (e.g. some retinal detachments)
• Corneal characteristics - the biggest factor

Question 4

what is contradicting about Gender: IOP 1 – 2mmHg higher in older women

Answer 4

but men are more likely to develop open angle glaucoma than women slightly

Question 5

which 2 corneal characteristics influences IOP measurement

Answer 5

• Corneal thickness (see next slides)
• Corneal curvature
  Steep corneas over - estimate true IOP
  Flat corneas under - estimate true IOP

Question 6

which type of corneal curvature under estimates and which type over estimates the true IOP

Answer 6

• under estimate = flat corneas

- over estimates = steep corneas

Question 7

what is manometry

what advantage does it have

Answer 7

• a measure of IOP which bypasses the cornea but putting a tube into the anterior chamber
• IOP is measured without involving the cornea, so readings cannot be affected by corneal thickness or curvature

Question 8

what was discovered about the corneas of:

• ocular hypertensives
• patients with NTG

Answer 8

• Ocular hypertensives have thicker corneas ((which contributes to high IOP readings)
• Patients with NTG have thinner corneas (true pressure may be higher than that measured)

Question 9

what did the ocular HTN treatment study find was a risk factor for POAG

Answer 9

having a thin cornea

Question 10

what does carrying out pachymetry in practice measure and why

Answer 10

central corneal thickness

it is an important measure to consider when assessing IOP in practice and deciding whether to refer

Question 11

what error range is suggested to be used as a correction factor from an avg CCT of 535um

Answer 11

an error range of ~0.2-0.7mmHg per 10µm difference from an average central corneal thickness (approx 535µm)

Question 12

how does pachymetry measure CCT
how many % of practices is it available in
what is the gold standard instrument to measure pachymetry and give 2 trade names of this device

Answer 12

• Measures corneal thickness by optical, interferometric or ultrasound techniques
• Pachymetry is available in about 18% of practices
• Gold standard for corneal thickness determination is the hand-held ultrasound pachymeter
• Pachmate
• PachPen

Question 13

what do ultrasound pachymeters:

• operate at
• what is their mechanism of taking the measurement
• how is the CCT then calculated
• what are the 2 advantages to this technique

Answer 13

• Operate at frequencies of 20 to 50 MHz
• Emit short acoustic pulses and detect reflections from the anterior and posterior surfaces of the cornea
• Corneal thickness is then calculated from the measured time-of-flight between these reflections and the accepted speed of sound in the cornea of 1636–1640 m/s
• time x speed = thickness
• results are accurate and repeatable

Question 14

list 6 sources of error in pachymetry

Answer 14

• Decentration (thicker values if off-centre)
• Oblique incidence (thicker values if probe not at 90 degrees to corneal surface)
• Possibly corneal compression by the probe (if press too hard, can flatten cornea)
• Possible effects of local anaesthetic on cornea up to 10 μm difference (? Epithelial oedema)
• Variation in the speed of sound between healthy and diseased tissue of cornea
• Inter- and intra-observer variability

Question 15

list 7 short-term factors affecting IOP (which may affect the accuracy of your IOP measurement) from using either NCT or CT

Answer 15

• Time of day (diurnal range)
• Cardiac Cycle
• Body position
• Blinking, forced blinking and hard squeezing
• Fluid intake
• Patient holding their breath
• Accommodation

Question 16

what is the diurnal range (time of day) for:
normals
glaucomatous avg range
wha value of diurnal variation is considered pathological

Answer 16

• normal range 3 - 5mmHg
• glaucomatous average range ~13mmHg
• greater than 10mmHg is usually considered pathological

Question 17

what are the 2 different groups people can fall into as regards to their peak IOP/diurnal variation, what do both have in common
and because of these 2 different groups, what 3 things should be done

Answer 17

• most higher in mornings with afternoon dip (esp males?)
  or
• some patients have afternoon peaks
• both have lowest pressure in the middle of the night
• you should:
• Repeat IOP measurement at a different time of day
• Always record time of day
• Phasing

Question 18

what is phasing and why is it carried out

Answer 18

• done at hospital = measure IOP over a 24 hour period as much as they can
• because diurnal variation can play a role as it varies across individuals

Question 19

by how much on average can the cardiac cycle vary the IOP
what extreme values can it vary IOP by
what is the cardiac cycle crucial in taking into consideration and why
what is recommended to do because of this

Answer 19

• average variation is 2 - 3mmHg
• can be 1 - 7mmHg
• crucial in non-contact tonometry
• because if just take one reading, you can get a peak or a trough
• so it is recommended that we should always take 4 readings

Question 20

how much can body position affect IOP and by how much in normals and in glaucomatous patients

Answer 20

• erect to supine 0.3 - 6mmHg increase
• inversion - an increase to 30 - 35mmHg in both normals and glaucomatous patients

Inversion occurs in yoga, some back treatments for example
If measuring IOP of someone who is confined to the bed = pressure will be higher
Upside down turns IOP up by high levels as the blood pressure in the head also goes up

Question 21

how and by how much can IOP be affected by: 
blinking 
forced blink 
hard squeeze 
repeated squeezing

Answer 21

• Blinking - increase of 3mmHg
• Forced blinking - increase of 10mmHg
• Hard squeezing - IOP can go up to more than 50mmHg
• Repeated squeezing would potentially lower IOP

Question 22

how does drinking affect IOP, by how much and when is the maximum affect, with:
water
coffee
alcohol

Answer 22

• water increases IOP by up to 3mmHg with maximum effect at 20 minutes
• coffee increases IOP by up to 3mmHg with maximum effect at 20 minutes
• alcohol decreases IOP by up to 3mmHg with maximum effect at 5 minutes

Question 23

what affect does holding breath have on IOP
what is it also called
by how much do the affects have an impact

Answer 23

• increases IOP in the majority of patients
• Valsalva manoeuvre
• Effects are variable
  Estimated at from 5 – 20mmHg
  IOP can double

Question 24

explain how accommodation has an affect on IOP

Answer 24

• When first accommodate it can increase IOP

- But sustained accommodation can cause a reduction (~3mmHg) which is greater in the young

Question 25

list 6 advantages of non contact tonometry

Answer 25

- Not operator dependent (but sometimes pre screener doesn't do a good job) - Repeat measures do not affect IOP - No anaesthetic - Can be used by para-professionals - Little if any risk of cross infection - Can be quicker

Question 26

list 4 disadvantages of non contact tonometry

Answer 26

- NCT accuracy has been challenged But “regression to the mean” effect is a factor - There is a need to take multiple measures (due to cardiac cycle) - Initial cost - Ophthalmologists in general do not like NCTs but most optometrists use them

Question 27

what did a study find about which tonometers optometrists use routinely when a mystery shopper visited 100 optician practices with suspect glaucoma

Answer 27

84% used non-contact tonometer = most used 12% used contact tonometry 4% did not use a tonometer

Question 28

what does Bell and O’Brien (1997) recommend

Answer 28

That Goldmann tonometry is used to confirm original NCT findings before referral Always repeat tonometry readings before referral (as various factors are affecting the measurements)

Question 29

what is meant by the regression to the mean affect

Answer 29

Suppose a patient is referred for suspected POAG because of raised IOP: As a result of the regression to the mean effect the ophthalmologist is likely to measure a lower IOP when the patient is examined in hospital This can be due to the diurnal variation e.g. IOP measured at practice in morning and reach hospital mid day when IOP is lower - so the only way for the ophthalmologist to find the same IOP is if its measured at the same time of the day or px may decide to have a coffee in the morning before they get their pressure taken in practice which can increase the IOP

Question 30

list 3 types of new tonometers

Answer 30

- iCare “rebound” tonometry - Reichert Ocular Response Analyser - Dynamic Contour Tonometry

Question 31

how does the new type of tonometer, the Reichert Ocular Response Analyser measure IOP and why is it considered to have clinical value

Answer 31

- is an air puff machine - determines the total corneal resistance - includes the combined effects of corneal thickness, rigidity and hydration

Question 32

how does the new type of tonometer, Dynamic Contour Tonometry measure IOP and what advantage does it have

Answer 32

- Used on slit lamp like a Goldmann - IOP sampled continuously - Said to produce readings unaffected by CCT

Question 33

how does the new type of tonometer, the iCare tonometer: Dynamic or Rebound tonometry take an IOP measurement

Answer 33

- A magnetised probe is launched towards the cornea - The probe consists of a magnetized steel wire shaft covered with a round plastic tip at the end - Probe hits the cornea and bounces back - A solenoid, inside which the probe moves, is used to detect the motion and impact when the probe collides with the eye and bounces back - The probe slows down faster as the IOP increases So the higher the IOP, the shorter is the duration of the impact

Question 34

what has a clinical study found about the iCare tonometer: Dynamic or Rebound and GAT

Answer 34

the difference in readings between the iCare and GAT was less than 3 mmHg iCare tends to read “high” compared with GAT (mean difference was 0.27mmHg in Jorge et al)

Question 35

list 5 reasons Patients should be referred [for suspected glaucoma] if the optom identifies one or more of the following (CoO and RCOph Guidance)

Answer 35

- There are optic disc signs consistent with glaucoma in either eye - The IOP in either eye exceeds 21mmHg (but note referral in specific scenarios “below”) [OHT?], not just once, but regularly - A visual field defect consistent with glaucoma is detected in either eye - A narrow anterior drainage angle on van Herick testing consistent with a significant risk of acute angle closure within the foreseeable future - Signs often associated with glaucoma (e.g. pigment dispersion or pseudoexfoliation)

Question 36

what 3 points does the CoO and RCOph Guidance make about referring a px based on IOP alone

Answer 36

- When referring a patient on IOP grounds alone, Goldmann applanation tonometry (or Perkins tonometry) is regarded as offering greater accuracy - Practitioners should ensure the patient is prepared for the procedure. For example, they should instruct patients to loosen neck ties and not to hold their breath - Practitioners should take four readings per eye and use the mean as the result. Only when the mean result is >21mmHg should the practitioner consider referring the patient for further assessment if this is the only abnormality found. Reducing the number of readings per eye increases the chance of recording a mean result of >21mmHg in eyes of normal persons with Goldmann IOPs of

Question 37

what does the CoO and RCOph Guidance advise for patients who hasn't had NCT done before

Answer 37

“If a patient has not been subject to non-contact tonometry before, practitioners should undertake four readings per eye as usual from both eyes. If the mean result is >21mmHg for either eye, a new set of readings should be taken for the eye, or eyes, for which this is the case. This is because research in normal eyes shows that the mean of subsequent sets of four readings will often be within the normal range.”

Question 38

what 2 specific scenarios regarding IOP results does the CoO and RCOph Guidance advise shouldn't be referred and why

Answer 38

Practitioners may consider not referring patients at low risk of significant visual field loss in their lifetime:- - Patients aged 80 years and over with measured IOPs less than 26mmHg with otherwise normal ocular examinations (normal discs, fields and van Herick) - Patients aged 65 and over with IOPs of less than 25mmHg (but more than 21mmHg) and with otherwise normal ocular examinations (normal discs, fields and van Herick) These groups do not qualify for treatment under current NICE guidance. Such patients may be advised that they should be reviewed by a community optometrist every 12 months.

Question 39

what inter-eye difference in IOP is regarded as: normal suspect abnormal

Answer 39

IOP difference between right and left eyes: normal = If less than 4mmHg suspect = If 5 – 7mmHg abnormal = If more than 8mmHg

Question 40

what is the speed of referral for suspect glaucoma | when will you refer a patient urgently and why

Answer 40

- Referral for suspect glaucoma is usually “routine” (or “standard”) - But if IOP > 35mmHg then this is an “urgent” referral (within a few days) as there is a risk of central retinal vein occlusion