Lec.3: IOP measurement Flashcards
why does IOP increase with age?
outflow facility decreases over time
is true that IOP for females over 40 decreases?
false. it increases after 40
what type of excercises are known to double IOP?
inversion excercises
how do alcohol, heroin and marijuana affect IOP?
decrease IOP
how is IOP affected with caffeine, tobacco and drinking 2 16 oz water bottles in 15 min?
increases IOP
in glaucoma what is the most important risk factor that is also alterable?
IOP
what type of tonometry is Schiotz?
indentation
what are the types of applanation tonometry?
goldman, perkins, mackay-marg-tonopen, pneumotonometer and non-contact
in what tonometry method does scleral rigidity play big role?
Schiotz. (myopes with weaker sclera give false values)
what is the equation for the modified imbert-fick law?
W+S=PxA+B
S=surface tension
B=force required to bend the cornea
what is the average central corneal thickness?
520 microns
how would a thicker than average cornea affect IOP measurement?
lead to over-estimate of IOP
how would corneal curvature affect the outcome of the IOP measurement?
the steeper the cornea the more force required to flatten it
on average do older people have thicker or thinner corneas?
thinner (with age thickness decreases)
what biomechanical properties contribute to the corneal structure?
rigidity, hydration and elasticity
what characteristic do visco-elastic materials demonstrate?
hysteresis
what kind of parameters can be measured?
geometric parameters
what kind of parameters cannot be measured?
material parameters
if patient has a contagion, what is the best way to clean the tono tip?
autoclave
what type of cornea would lead to low pressure reading?
thin cornea
what type of IOP estimation would an edematous cornea yield?
underestimation of IOP
in refractive surgery how many mircons of cornea are removed to compensate for 1 D?
12 microns
what type of trend do we see in IOP if patients have thicker cornea?
negative trened with IOP being lower
what does a negative residual slope after correction indicate?
over correction (problem)
what are the names of the statistical models that show a negative residual effect?
Orssengo and Pye model
Ehlers Model
can formulas correct for the error in a population?
yes
can formulas correct for errors in an individual?
no
what type of tonometer matches the contour of the cornea so the cornea no longer is a source of error in the measurement?
dynamic contour tonometer (PASCAL)
how many times per second does the dynamic contour tonometer measure the IOP waveform?
100 times/sec
what is the radius of curvature of tip of the dynamic contour tonometer?
10.5 mm (sufficient for any cornea that is physiologically normal)
what is the difference of capillary forces in DCT compared to GAT?
forces much greater in DCT
according to quality index, which values are acceptable for DCT?
1-3
according to quality index, which values are unacceptable for DCT?
4-5
what is the characteristic that results from the difference of inward and outward pressure values?
corneal hysteresis
what type of property does hysteresis describe?
the bending of the cornea
what type of hysteresis is less capable of absorbing (damping) the energy of the air pulse?
low corneal hysteresis
what type of diseases tend to show lower corneal hysteresis?
keratoconus, fuchs and glaucoma (on average)
what tonometry method is not significantly affected by corneal thickness? and which is not?
DCT not significantly affected
GAT significantly affected
how are DCT and GAT affected by corneal curvature?
they are not significantly affected
why is DCT more repeatable than GAT?
DCT has a lower intra/inter observer variability than GAT does
how can fluid pockets acquired from post refractive surgery affect IOP measurement?
cause a gross underestimation
what type of medication is prescribed after refractive surgery and has the ability to elevate IOP?
steroids
was there a significant decrease in IOP after refractive surgery when measured by DCT?
no, however GAT readings decreased (underestimation)
what is the relationship between IOP error and Goldman tonometry?
not linear
how is blood flow affected in patients with normal tension glaucoma?
lower amount of blood getting to the eye
what is the chance that patient has normal tension glaucoma if they have an ocular pulse amplitude (OPA) of less then 2 mmHg?
80% chance of NTG
what does OPA (ocular pulse amplitude) refer too?
overall pressure of the eye?
how are IOP and OPA related?
inversely related
is OPA influenced by both IOP and corneal thickness?
no just IOP
can you diagnose glaucoma based on hysteresis and corneal thickness alone?
no
is corneal hysteresis correlated with lamina cribrosa hysteresis?
yes
what tonopen setup mistake will result in a flawed IOP reading?
if the cap is on too tight (plunger wont have enough space to move)
are tonopen readings reliable?
no (below the standard of care)
how long after removing contact lenses should an IOP reading be taken?
2 hours
what type of tonometer does not come into contact with the cornea?
diaton tonometer
what are the advantages of permanent IOP devices?
not influenced by ocular parameters, direct anterior chamber measurements
what are the disadvantages of permanent devices?
invasive, sub conjunctival devices may not be as accurate
what are the advantages of temporary devices?
non invasive, not permanent
what are the disadvantages of temporary devices?
eye movement may have greater effect when compared to permanent devices, surface tension, light exposure, temperature, change in sleeping posture
for CL IOP measuring devices, what is the repeatability percentage?
35%
is it true that PASCAL and ORA are more accurate than GAT?
yes
when may you want to use a tonopen?
when measuring IOP on scarred or damaged cornea
