Tonometry Flashcards
what patients does tonometry have to be performed on? Why?
any patient with glaucoma or are at risk of glaucoma because reducing IOP helps slow glaucoma progression
what patients are at risk of glaucoma?
those who have:
-large cupping
-large C:D ratio
-family history of glaucoma
-central visual field defect
-narrow anterior angles
what corneas are GAT only appropriate for?
only for corneas with near average thickness of approximately 520 micrometers
what are the most common errors when measuring IOP
-obtaining high IOPs due to patient apprehension so explaining the procedure in non threatening terms can help
-taking a reading when a tear meniscus has formed around the GAT probe leasing to tonometer arcs that are too thick and an invalid high pressure measurement
-not repeating NCT measurements 4 times on each eye
what three factors does intraocular pressure depend on?
-amount of fluid in the eye determined by the balance of aqueous humour production and drainage
-external forces acting upon the eye like tension within the ocular walls
-intraocular volume
name 11 short term factors that affect IOP
Ocular pulse
Breath-holding
Straining
Tight clothing around the neck
Posture
Accommodation
Eye position
Lid squeezing
Opening eyes wide
Eye rubbing
Contact lens removal
what time of day should you avoid measuring IOP?
the first 2 hours of the patient waking up
name 8 medium term factors that affect IOP
-Diurnal variation (where the iop fluctuates over a cycle lasting approx 24 hrs e.g. posture, ambient illumination and circulating cortisol levels
-Eating and drinking
-Smoking
-Systemic medication
-Exercise
-Accommodation/reading
-alcohol
-Optometric techniques like making sure gat is performed as quick as possible to reduce number and duration of contacts with the eye
give 6 long term factors that affect IOP
Age
Lifestyle (e.g. smoking)
General health
Gender
Season
Ocular factors
what are the advantages of GAT?
-steady and secure
-inexpensive
-bright and good mag so easy to see inner margins of mires to align them and take measurements
-we can compensate for the ocular pulse as GAT happens over several secs compared to NCT which happens in less time
what are the disadvantages of GAT?
-It needs a slit lamp, which means it is inappropriate for domiciliary visits or for patients that cannot sit at one.
-The procedure cannot be delegated due to the amount of skill required.
-There is a potential for corneal trauma (usually only when learning)
-patient has to sit up
-anaesthetic is required
-There is a potential for transmission of infection, but much less of an issue now that we use disposable probes
-Its accuracy is affected by corneal and tear film properties, especially if your patient’s eye differs from the population of patients used to develop and test it.
What does the Imbert Fick principle state?
IOP can be estimated indirectly via measuring the force required to applanate or ‘flatten’ a known area of the cornea In its original
form
what are the advantages of perkins tonometry
-it’s accuracy is directly comparable to Goldmann since the operation is essentially the same
-portable and can be operated with the patient sitting or supine
-very easy to hold eyelids open for difficult patients who blink alot
what are the disadvantages of Perkins tonometry?
-magnification and lighting are much less than for goldmann and so can make it less accurate
-clinical guidance is based on sitting IOP
-reduced stability which can affect view of mires and may increase the risk of corneal scratches when learning
how is pascal dynamic contour tonometry different to other contact tonometry methods?
it uses a concave probe that is designed to align with the natural curvature of the cornea instead of flatten it
what are the key measurements provided by pascal dynamic contour tonometry?
-IOP: The primary intraocular pressure measurement, given in mmHg. Remember that this IOP is not likely to be the same as a GAT-measured IOP.
-OPA: Ocular Pulse Amplitude, which indicates the difference in IOP during the cardiac cycle and that could provide additional insights into ocular blood flow. There is some association between abnormal OPA and glaucoma, but more research is needed in this area before it can be useful in clinical practice.
-Quality score (Q): A numerical score indicating the reliability of the measurement. This score helps clinicians assess whether the reading can be trusted or if the measurement should be repeated.
what are the limitations of pascal dynamic contour tonometry?
-Availability and cost: DCT devices are more expensive and less commonly available than GAT, which may limit their use in routine clinical practice. As I have already mentioned, the probe tips are also very expensive.
-Learning curve: While the procedure is similar to GAT in many ways, further training is required to ensure accurate measurement with DCT, and to learn how to interpret its measurements.
name an applanating non-contact tonometry technique
-Ocular Response Analyzer
name 2 non-applanating contact tonometry techniques
-ICare
-Pascal dynamic contour tonometry
name 3 applanating contact tonometry techniques
-Goldmann tonometry
-Perkins tonometry
-TonoPen
name 2 alternatives to van herrick
-shadow test
-gonioscopy
what kind of cornea is associated with an increased risk of glaucoma? why could this be?
thin as thick appear to protect against glaucoma. Thinner corneas theoretically cause an increased amount of stress around the optic nerve for a given level of IOP. This means that eyes with thinner corneas may be mores sensitive to IOP and thus more susceptible to damage than a more robust eye, even if the level of IOP is the same.
quantify a thin and a thick cornea
thin = ≤555μm
thick = >588μm
what is the gold-standard device for measuring CCT?
an ultrasound pachymeter
why may you use alternatives to pachymetry?
as they are non-invasive asnd so are indicated for patients who cannot be anaesthetised
what are the alternatives to pachymetry?
-pentacam
-orbscan IIz
-anterior segment OCT (AS-OCT)
-standard OCT with an anterior eye attachment
where can you find more info about alternatives to pachymetry?
in further tonometry file in things to do before week 5 folder
in what patients should central corneal thickness be measured?
- In all patients with ocular hypertension
- When IOP is measured
- Prior to referral for refractive surgery
- Screening and/or monitoring corneal oedema
- Screening and monitoring keratoconus and corneal dystrophies
what is the range of average central corneal thickness in healthy eyes?
520-580 micrometres
what is the formula for the imbert fick principle
IOP = contact force / area of contact
what does the imbert-fick principle assume?
the cornea is:
-spherical
-dry
-perfectly flexible
-infinitely thin
even though its none of these so tonometers are calibrated to take this into account
what is the mean IOP of people with African descent?
18.7mmHg (they also have thinner corneas)
what is the mean IOP in people of east Asian descent?
13mmHg
what are the advantages of non contact tonometry in general?
-Non-invasive
- No contact minimises corneal compromise /
CJD risk
- No need for anaesthesia
Repeat measurements do not change IOP
- Override facility for scarred/irregular cornea although accuracy questionable for this
-some are portable
what are the disadvantages of non-contact tonometry?
-not a gold standard
-sensitive to the ocular pulse
-need to take 4 readings as single are more variable than GAT
-may end up overestimating IOP when it’s high
-expensive (3x cost of goldmann)
-occupy a lot of valuable consulting room space
what are the steps of doing GAT?
- Setup GAT; mount to tonometer, cobalt blue
- wide beam at max brightness, wide angle, probe horizontally unless >3D astigmatism
- Explain GAT and obtain consent
then apply one drop of topical anaesthetic and add fluorescein - Approximately align probe over the pupil by looking around the slit lamp; check set at 10mmHg (or below a known IOP)
- Ask patient to blink few times, then to hold their blink
- Move forward slowly and steadily until contact is made with cornea
- Align mires
- Three readings each eye
how does GAT get over the assumptions of the Imbert Fick principle?
As tonometers were calibrated by comparing intracameral IOP to applanation IOP using a limited number of eyes so the accuracy of tonometry readings depends on how similar the eye is to the eyes of the test population
in GAT, what should you do if the mires are unequal in size?
move probe towards larger circle
in GAT, what should you do if mires are not centered?
move probe towards where the bulk of the image is
in GAT, what should you do if mires are too thin
add more fluorescein
in GAT, what should you do if the mires are too thick?
remove the probe from the eye, blot with a tissue, and then try
again
what should you record after GAT?
-Name of device
- Time of day and date
- Eye tested
- Each valid reading that you obtained: n = 3 for
GAT
- Average of valid readings
- Units
- Details of anaesthetic used (more on that
soon)
- Name of drug
- Concentration
- Number of drops
- Batch number and expiry date
