Lecture 3 Tonometry Flashcards
How does high axial myopia affect IOP?
IOP is usually less because eyeball is longer (keep in mind >6.00D myopia there is always an axial component)
Describe IOP in females over 40 (post menopausal).
IOP is usually higher. (males equal females between the ages of 20 and 40)
- What is the main risk factor affecting IOP long term?
a) genetics
b) age
c) gender
d) refractive error
a) genetic (#1 risk factor)
which of the 2 causes increase in IOP?
a) supine position (laying on back)
b) prone (laying on stomach)
a) supine (by 3mmHg)
(T/F) exercise decreases IOP
true
(T/F) People tend to have higher IOP in the winter
true. People get fatter, venous flow is more restricted, leading to an increase in IOP
Why is it important for your patient to relax and breathe during tonometry?
holding your breath (valsalva maneuver) can temporarily increase IOP by 5mmHg.
- Match the following:
1) decrease IOP
2) increase IOP
a) alcohol
b) caffiene
c) tobacco
d) heroin and marijuana
a=1 (because alcohol dehydrates), b=2, c=2, d=1
What are the only 2 proven methods of decreasing IOP?
by medical or surgical techniques
What is the only form of treatment for glaucoma?
lowering IOP (high IOP is the most important risk factor)
What type of tonometry is Schiotz?
indentation (only theoretical interest). performed in supine position (when IOP would be higher).
What device do you use for digital palpation?
fingers
What is the Imbert-fick law equation for applanation tonometry?
W=PxA (W=external force, A=area, P=pressure inside sphere)
What are the conditions for the Imbert-Fick law?
1) perfect sphere
2) dry
3) infinitely thin
What is the modified imbert-Fick law for applanation tonometry?
W+S=PxA+B (W=external force, S=surface tension, P=pressure inside sphere, A=area, B=force required to bend the cornea) S cancels out B if cornea is 520um
What is the area of applanation in mm?
a) 2.88
b) 2.96
c) 3.06
d) 3.33
c) 3.06mm
(T/F) Goldman tonometry will overestimate IOP in thicker corneas
true. and it will underestimate in thinner corneas. (analogy: the pressure in a football may be the same in a balloon but its harder to push on a football because its thicker)
- Match these 3 factors affecting IOP measurements:
1) Central corneal thickness
2) corneal curvature
3) biomechanical properties
A) 1.76 mmHg
B) 17.26 mmHg
C) 2.87 mmHg
1=C, 2=A, 3=B
*in order of greater influence:
biomechanics (affects IOP the most)>thickness>curvature
(example: 1.76mmHg is the amount of error in IOP reading that can be caused by corneal curvature)
*(T/F) the cornea is viscoelastic
true. Many common visco-elastic materials and systems exhibit hysteresis. You push on it and it doesn’t pop back right away like an elastic property.
Which of the following is not a property regarding biomechanics:
a) rigidity
b) opacification
c) hydration
d) elasticity
b) opacification
What are the 2 types of cornea stiffness?
geometric (can be measured) and material (cant be measured)
Match the following:
1) geometric
2) material
A) shape B) age C) size D) hydration E) medical history F) corneal thickness G) radius of curvature H) refractive surgery
A=1, B=2, C=1, D=2, E=2, F=1, G=1, H=2. Cornea gets thinner with age because it gets dehydrated. The cornea is ~78% water. A difference of 4% or 6% change can make a huge difference in thickness. Also corneal edema makes the cornea softer and easier to compress (IOP would be underestimated).
(T/F) 12 microns of cornea=1.00D
true. If someone had 60 microns of cornea removed it would equal a 5.00D change. This would make goldman tonometry no longer accurate.
What is the human cornea thickness range in microns?
a) 300-700
b) 400-700
c) 360-740
d) 420-620
d) 420-620
Match the following:
1) tonopen
2) goldman
A) observer dependent
B) observer independent
A=2, B=1
- Which tonometer requires no anesthetic?
a) Pascal-dynamic contour tonometer
b) rebound tonometer
c) goldman applanation tonometer
d) tonopen XL
b) rebound tonometer
- Which tonometer measures ocular pulse amplitude?
a) Pascal-dynamic contour tonometer
b) rebound tonometer
c) goldman applanation tonometer
d) tonopen XL
a) Pascal-dynamic contour tonometer (higher pressure pt’s will have a lower pulse amplitude)
- Which tonometer isnt affected by corneal thickness?
a) Pascal-dynamic contour tonometer
b) rebound tonometer
c) goldman applanation tonometer
d) tonopen XL
a) Pascal-dynamic contour tonometer
For NCT, a 0.46mmHg IOP drop is equal to ____microns.
For GAT, a 0.71mmHg IOP drop is equal to ____microns
NCT–>10 microns
GAT–>10 microns
(T/F) Ehlers correction factor of 5mmHg for 70 microns of change is the appropriate correction factor?
False. Turns out this is not useful and in fact the Ehlers correction factor overcorrects as evidence with the negative slope after correcting.
Which of the following is NOT true regarding the Pascal-dynamic contour tonometer:
a) It is better than Golman because it can obtain 100 IOP readings per second where as GAT only gets one IOP reading
b) It can measure OPA (ocular pulse amplitude)
c) It matches the contour of the cornea eliminating the cornea as a source of error.
d) The pressure on both sides of tip that presses on the cornea are slightly different to adjust for different cornea shapes
d) The pressure on both sides of the tip that presses on the cornea are slightly different to adjust for different cornea shapes. Nope, not true–> the pressure on both sides must be the same with contour matching.
With the Pascal-dynamic tonometer, there is a quality index (1 thru 5). Match the following
1) (1-3)
2) (4-5)
A) acceptable
B) not acceptable
A=1, B=2 (1-3 is acceptable, 4-5 is not acceptable)
Which of the following is NOT true regarding the Pascal-dynamic tonometer:
a) It has a digital output
b) It has maximal corneal deformation
c) the tip radius of curvature is 10.5mm and the pressure sensor is 1.5mm
d) it is accurate despite bio-mechanic properties of the cornea
e) it requires sensor caps to protect the pt and the tip
b) It has maximal corneal deformation. Nope, it has minimal corneal deformation.
Which of the following is not true about ORA (ocular response analyzer)
a) it can measure hysteresis (the bending property of the cornea)
b) it is accurate despite bio-mechanic properties of the cornea
c) it requires sensor caps to protect the pt and the tip
d) It uses an IR light emitter and IR light detector
e) It is a non-contact tonometer (NCT)
c) it requires sensor caps to protect the pt and the tip. No, that’s the Pascal-dynamic tonometer
(T/F) Hysteresis is the measurement between the 2 applanation events of the ORA.
True. Air pushes the cornea in until its flat (1st applanation) then air continues to push cornea inward, air stops. The cornea comes back out from its concave state to 1st another applanation state (2nd applanation) then back to its original state.
(T/F) Corneal disorders correlate with lower hysteresis values
True. Glaucoma also has lower corneal hysteresis measurements.
What is a normal corneal hysteresis measurement?
normal=11.5, Fuch’s dystrophy is lower, keratocous is 8.5-9 on avg but can have a wide range, In corneal disorders, the cornea is less capable of absorbing the energy of the air pulse in the ORA.
How does corneal thickness affect dynamic contour tonometer (DCT) vs Golman applanation tonometry (GAT)
DCT not significantly affected
GAT is significantly affected
How does corneal curvature affect dynamic contour tonometer (DCT) vs Golman applanation tonometry (GAT)
DCT not significantly affected
GAT not significantly affected
How does corneal bio-mechanics affect dynamic contour tonometer (DCT) vs Golman applanation tonometry (GAT)
DCT not significantly affected
GAT is significantly affected
(T/F) It is better to have an observer-independent device that can read the pressure. Inter-observer measurements can be different up to 2.5mmHg
True. Also, DCT is more repeatable than GAT
(T/F) Corneal bio-mechanics are not the same after refractive surgery and GAT is not accurate for post-lasix patients.
True. Corneal rigidity, hysteresis, and hydration changes. Fluid pockets between the flap and stroma can cause gross underestimation of IOP. GAT is significantly affected by corneal thickness, DCT is better for measuring lasix pt’s.
What is manometry?
Insertion of a needle in the eye to obtain the most accurate pressure. Rarely used, has many risks.
Describe the relationship of GAT and IOP error
As IOP increases, the error for GAT increases. IOP underestimation for GAT is greater at higher pressures.
Describe the relationship between DCT and IOP error
DCT slightly overestimated true pressure (comparing to manometry as true). However was still relatively constant even in higher pressures, unlike GAT.
Pulse amplitude is the difference between what?
Systolic and diastolic
(T/F) A low OPA (ocular pulse amplitude) correlates to a higher probability of NTG (normal tension glaucoma)
True. NTG pt’s have a lower amount of blood getting to the eye
(T/F) Glaucoma pts tend to have lower hysteresis
True
(T/F) The pulse is the total amount of blood into the eye. Most goes to choroid and some goes to retina.
True. 95% to choroid and 5% to retina. Also, the retina is suppose to auto-regulate IOP in normal pt’s and it doesn’t do that in glaucoma pt’s. Choroid doesn’t autoregulate.
(T/F) In glaucoma pt’s, when IOP goes up, blood flow goes down. In normal pts, when IOP increases, blood flow stays the same
True
(T/F) The Reichert 7 (glaucoma tonometer) compensates for central cornea thickness. A study shows IOPcc is not influenced by the thickness of the cornea and has a good linear relationship (when IOPcc and CCT were graphed)
True
(T/F) The tonopen is very reliable
False.
ADVANTAGES: small contact area does allow for use with scarred corneas, peripheral measurements can be obtained. it is portable, and uses sterile caps.
DISADVANTAGES: it requires anesthetic, it has calibration issues, its expensive over time, and when compared to manometry values it was quite incorrect.
Which one of the following is NOT true in regards to the Icare Rebound tonometer?
a) It requires an anesthetic
b) It is more reliable than Tonopen XL
c) It has a disposable magnetized probe
d) Studies show that the readings are all over the place and that it is fairly unreliable
a) It requires an anesthetic. It actually does not require and anesthetic. Both Tonopen and Icare are deemed fairly unreliable in match ups against GAT and ORA.
Which one of the following is NOT true regarding the Diaton tonometer?
a) It doesn’t come in contact with the cornea, just the eyelid
b) It cannot measure through the tarsal plate
c) Studies show that it is variable in measurements making it fairly unreliable.
b) It cannot measure through the tarsal plate. (No, it does measure through the tarsal plate but probe has to be in the right area.)
How is intraocular telemetry used?
It gives 24-hour IOP measurements. It can be non-invasive or invasive.
(T/F) Non-invasive forms of IOP telemetry are temporary, such as contact lenses or continuous applanation or indentation devices.
True
(T/F) Invasive forms of IOP telemetry are permanent, such as subconjunctival, anterior chamber implants, and posterior chamber implants
True.
Which one of the following is NOT a problem in regards to IOP telemetry?
a) Data linear but not reproducible
b) Data better for rabbits and not dogs
c) Data affected by ocular rigidity
d) all of the above are correct
d) all of the above are correct
(T/F) Invasive IOP telemetry devices have a capacitive circuit-pair of spiral coils in a gas filled capsule. As pressure changes, distance b/w the coils change as gas is compressed. Change in capacitance and resonant frequency is changed, external sensor measures the oscillations in the coil.
True
What are the advantages and disadvantages of permanent IOP telemetry devices?
ADVANTAGES: Not influenced by ocular parameters. Direct anterior chamber measurements more accurate than estimates obtained from external devices.
DISADVANTAGES: Invasive-all known problems with surgeries. Sub-conjunctival devices many not be as accurate as intraocular devices. Unknown long term safety, best location of implant, best sampling rate is unknown.
How does a temporary IOP telemetry device, such as the contact lens, measure IOP?
It measures peripheral changes in curvatures with IOP changes
What are the advantages and disadvantages of temporary IOP telemetry devices?
ADVANTAGES: non invasive. not permanent, can be used on an as needed basis
DISADVANTAGES: eye movement had greater effect when compared to permanent devices (moving eye left to right can cause a pressure spike of 15mmHg). surface tension, light exposure, and temperature play a role, not reproducible. Can cause SPK (irritable)
Which one of the following is NOT true regarding Triggerfish?
a) Accuracy of predicting day 2 from day 1 is 35%
b) It is a continuous 24-hr IOP monitoring device similar to a contact lens
c) Its main adverse effect was SPK
d) It revealed a nocturnal acrophase (nighttime spike in IOP)
c) Its main adverse effect was SPK. No, its main adverse effect was blurry vision–>82%. Hyperemia–>80%, SPK–>15%
(T/F) The pascal measurement of OPA (ocular pulse amplitude) may be an indicator of glaucoma
True
(T/F) Pascal and ORA are more accurate than GAT
True
(T/F) ORA gives a wealth of corneal bio-mechanical parameters.
True. Corneal hysteresis is the most interesting parameter for ORA
(T/F) Tonopen’s best advatage is its use in scarred or damaged cornea
True
(T/F) Rebound and Diaton need more research to make it more accurate
True
(T/F) Contact lens tonometry may give insight into pathogenesis of certain types of glaucoma because it takes 24-hour IOP measurements.
True
