Tonometry, IOP, and Central Corneal Thickness Flashcards
Define tonometry
- The indirect estimation of intraocular pressure by measuring resistance of the eye to indentation by an applied force
- A single reading is only an estimate of fluctuations
__ __ is the most important modifiable risk factor for the development and progression of GON
Raised IOP
__ and __ clinical measurement is important
- Validity: how close a measurement is to true value / how accurate it is
- Reliability: how reproducible the measurement is
All current forms of tonometry measure IOP through the ___
- Accuracy is subject to biomechanical properties of the cornea
- IOP fluctuation
- May be due to measurement error
- True IOP fluctuation - recent literature suggested that IOP fluctuation (in addition to raised IOP) may be a risk factor for glaucoma progression
__ is the gold standard of IOP measurement
Goldmann Applanation Tonometry
How is IOP determined via goldmann applanation tonometry?
- IOP determined by measuring the amount of force needed to flatten the constant area of the cornea
- Equals the force inside the sphere
- Uses an adjustable foce to determinepressure
- Assumes negligible effect of corneal rigidity, IOP volume, surface tension changes
- Based on Imbert Fick principle (P = F/A)
- P = dry thin wall sphere the pressure inside the sphere
- F = equals the force necessary to flatten its surface
- A = area of flattening
Describe the pulsatile IOP seen with goldmann applanation tonometry
- Pulsation of the mires seen on GAT
- IOP can pulsate considerably overtime as the choroida fills with blood and empties in concert with the cardiac cycle
- Variation of ~2mmHg
- This phenomenon can be directly observed by viewing pulsation of mires during goldmann tonometry
- Measuement is taken on the downbeat of pulsation
- pulse endpoint is the innermost point of the contact of the mires
- Do not allow mires to cross one another
What is serial tonometry?
- multiple IOP measurements are taken in one day
- Minimum of 3 measurements taken on the same day usually
- Patient returns for multiple visits
- Early AM, mid-day, last appt
- Allows for establishment of diurnal variation range
- Average diurnal range of ~3mmHg (2-5mmHg) throughout the day
- Indicated in patients with suspicion of glaucoma or intermittent angle closure
- Glaucoma patients thought to have larger range of diurnal variation
- _>_5mmHg = higher risk of glucoma
- _>_10 mmHg = higher risk of progressive optic nerve damage
- Glaucoma patients thought to have larger range of diurnal variation
What can cause flase high readings of when measuring IOP with goldmann applanation tonometry?
- Holding breath
- Patient anxiety
- tight collar or neck tie
- Supine position
- Pressing on globe while holding lids
- Too much pressure on applanation
- Thick mires - too much FL, or too much tears
- Thick central corneal thickness
- Steep cornea
What could case falsley low readings when measuring IOP with GAT?
- Thin mires
- Too little FL or too little tears
- Thin central corneal thickness
- Repeated measurements
- Corneal edema
- >3D with the rule corneal astigmatism
What are some common sources of measurement error? (others)
- Obtaining off axis measurements
- Improper alingment of tono tips (increase 1mmHg for every 4.00D of corneal cylinder when not aligned properly)
- Obtaining high IOP’s because of patient apprehension
- Pressing on the globe while holding the lids open
- Using too much or too little fluorescein
- Taking measurements when the patient has too tight a shirt collar
- Taking measurements when the patient is holding their breath
- Repeating applannation too many times successively (reduces IOP)
- Performing goinio before tono will produce lower pressures
- ALWAYS perform tonometry before gonioscopy
What are other types of tonometry? (Indirect/Direct)
What are other types of tono used most commonly?
Accuracy of GAT depends on…?
- Corneal central thickness
- Corneal curvature
- Viscoelastic properties of the cornea (aka corneal “hysteresis)
What are the effects of central corneal thickness and hysteresis?
Describe aqueous flow
What is the average IOP? Normal range? Difference of ___ between the eye is considered abnormal? Diurnal variation highest in __ and lowest in __?
- Avg = 15.5 mmHg
- Normal range = 10-22 mmHg (statistical not clinical norm)
- Approximately symmetrical
- Difference of >2mmHg is considered abnormal
- Diurnal variation in pressure
- highest in am - 7am
- lowst in pm - 5pm
- diurnal variations 3 - 5 mmHg are normal
Factors that increase vs. decrease IOP?
Factors that effect IOP?
What causes IOP fluctuation - ultra short term?
- systolic cardiac cycle
- Change external ocular pressure
- Episcleral venous pressure
- Aqueous flow
- Most important factor determining spike height in ultra short term is scleral rigidity - scleral rigidity sig increase w/ age (eyes as biomechanical structure)
- Clinical sig of ultra short term fluctuation is unclear
- IOP monitoring between __ & __ has 60% of capturing peak IOP
- __ IOP measurement will likely be the highest (Tmax)
- Circadian pattern of aqueous flow rate has been known for many years - marked effect on IOP
- Clinical relevance
- IOP monitoring between 8am and 4pm has 60% chance of capturing peak IOP
- Early morning IOP measurement will likely be the highest (Tmax)
The most obvious difference in diurnal IOP fluctuation between untreated glaucomatous eyes and eyes of normal subjects is the ___________.
greater mean IOP range in glaucomatous eyes
