Tonometry, IOP, and Central Corneal Thickness

Question 1

Define tonometry

Answer 1

• 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

Question 2

__ __ is the most important modifiable risk factor for the development and progression of GON

Answer 2

Raised IOP

Question 3

__ and __ clinical measurement is important

Answer 3

• Validity: how close a measurement is to true value / how accurate it is
• Reliability: how reproducible the measurement is

Question 4

All current forms of tonometry measure IOP through the ___

Answer 4

• 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

Question 5

__ is the gold standard of IOP measurement

Answer 5

Goldmann Applanation Tonometry

Question 6

How is IOP determined via goldmann applanation tonometry?

Answer 6

• 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

Question 7

Describe the pulsatile IOP seen with goldmann applanation tonometry

Answer 7

• 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

Question 8

What is serial tonometry?

Answer 8

• 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

Question 9

What can cause flase high readings of when measuring IOP with goldmann applanation tonometry?

Answer 9

• 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

Question 10

What could case falsley low readings when measuring IOP with GAT?

Answer 10

• Thin mires
  
  • Too little FL or too little tears
• Thin central corneal thickness
• Repeated measurements
• Corneal edema
• >3D with the rule corneal astigmatism

Question 11

What are some common sources of measurement error? (others)

Answer 11

• 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

Question 12

What are other types of tonometry? (Indirect/Direct)

Answer 12

Question 13

What are other types of tono used most commonly?

Answer 13

Question 14

Accuracy of GAT depends on…?

Answer 14

• Corneal central thickness
• Corneal curvature
• Viscoelastic properties of the cornea (aka corneal “hysteresis)

Question 15

What are the effects of central corneal thickness and hysteresis?

Answer 15

Question 16

Describe aqueous flow

Answer 16

Question 17

What is the equation for IOP?

Answer 17

• IOP = F/C + PV
  
  • F = aqueous fluid formation rate
  • C = outflow rate
  • PV = episcleral venous pressure
• Typically
  
  • Production of aqueous ~2.5uL/minute
  • Drainage of aqueous ~2.5uL/minute

Question 18

The IOP is dependent what factors?

Answer 18

• RCOVE
• rate of queous humor production
• Circulation of aqueous humor
• outflow of aqueous humor from the eye
• volume of the vitreous humor
• elasticity of the cornea and sclera (ex. ocular rigidity)

Question 19

What is the average IOP? Normal range? Difference of ___ between the eye is considered abnormal? Diurnal variation highest in __ and lowest in __?

Answer 19

• 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

Question 20

Factors that increase vs. decrease IOP?

Answer 20

Question 21

Factors that effect IOP?

Answer 21

Question 22

___ and __ may vary between different types of glaucoma

Answer 22

Diurnal variations and peak IOP

Question 23

What are the most important parameters of long term IOP variations which are important in management of glc are?

Answer 23

• Peak (Tmax)
• Mean range
• Currently no tool to measure variation (normal/spontaneous) of IOP fluctuation
• In studies, they usually need to measure IOP repeatedly during office hours/hospital visits

Question 24

What are the 3 categories of IOP fluctuation?

Answer 24

• Ultrashort term - seconds to minutes
• Short term - hours to days
• Long term - months to years
  
  • Diurnal - daytime
  • Nocturnal - nighttime
  • Circadiant - 24hrs

Question 25

What causes IOP fluctuation - ultra short term?

Answer 25

* 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

Question 26

What is the likely cause for IOP fluctuation - short term?

Answer 26

* Changes in aqueous flow rate * Changes in EVP * Changes in trabecular outflow *

Question 27

* IOP monitoring between __ & __ has 60% of capturing peak IOP * __ IOP measurement will likely be the highest (Tmax)

Answer 27

* 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)

Question 28

In IOP fluctation short term, the __ has a clear effect on episcleral venous pressure. What is a clinical relevance for this?

Answer 28

* Body posture * This affects IOP accordingly * IOP is 4-5 mmHg HIGHER when lying down compared to sitting up position * Clinical relevance - should we recommend our glc patients to sleep with their head elevated?

Question 29

The most obvious difference in diurnal IOP fluctuation between untreated glaucomatous eyes and eyes of normal subjects is the \_\_\_\_\_\_\_\_\_\_\_.

Answer 29

**greater mean IOP range in glaucomatous eyes**

Question 30

What are the 2 means of assessing "true" long term IOP fluctuations?

Question 31

Describe what studies have suggested with IOP asymmetry as a risk factor?

Answer 31

* Population studies indicate that the avg IOP is 15.5 mmHg +/-2.6 mmHg * Some studies suggest that there is a relationship between the amount of IOP asymmetry and the likelihood of having glc * Difference in IOP of 0 mmHg correlates with a 0.5% chanceof having glc * A difference in IOP of 3 mmHg correlates with a 10% chance of having glc * A difference in IOP of 6 mmHg correlates with a 50% chance of having glc * A difference of IOP of more than 6 mmHg correlates almost 100% of the time with patients having glc

Question 32

GAT was most accurate when CCT was \_\_\_um

Answer 32

520um

Question 33

What is the average corneal thickness?

Answer 33

550um * Thinnest centrally & thickens peripherally * Thinnest point located ~1.5mm temporal to central cornea * High variable among patients * Relationship between zones should be consistent - ex. thinner cornea in mid periphery highly supscious of endothelial compromise

Question 34

Describe the ocular hypertension treatment study (OHTS)

Answer 34

* Brought importance of CCT in the management of glc * esp important for those with OHTN * Found that african-american pt has THINNER corneas compared to caucasions * Multivariate model of baseline characteristics found to be predictive of conversion from OHTN to glc * CCt was the most potent variant * Findings confirmed EGPS (european glaucoma prevention study) * Places CCT as major component glc risk * Showed central cornea corneal thickness to be a powerful predictor of devel of glc * eyes w/ CCT \<555 microns or had a threefold greater risk of developing glc than those with CCT \>588 microns * CCT \<555 microns are an independent risk factor for glc * Thin corneas have recently been show to be correlated with higher O2 levels in the anterior chamber angle * Oxidative stress may lead to damage in TM