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.
