Intro to IOP and Tonometry Flashcards
what does tonometry measure
indirect estimation of IOP at one point in time by measuring resistance of eye to deformation by applying force to anterior surface
when should tonometry not be used
- red eye of infectious origin (exception is herpetic eye disease)
- severely traumatized cornea
- open globe
- hyphema
- eyes w/ keratoprosthesis (artificial cornea)
what are the structures involved in aqueous humor dynamics
- limbus
- main route of aq humor outflow
- ciliary body (site of aq humor production and other outflow route of unconventional or uveoscleral outflow)
- iris and lens
what dynamic factors does IOP depend on
- rate of aq humor production
- circulation of aq humor
- outflow of aq humor
- volume of vit humor
5 elasticity of cornea/scerla (ex. ocular rigidity)
flow of aq humor against resistance generates ____ necessary for shape and optical properties of eye
IOP
what is corneal hysteresis
the difference btwn inward and outward applanation pressures
- difference in first/repeated applanation measures
- indication of biomechanical properties of cornea
what is ocular hypotension
what is ocular hypotony
what can you get hypotony from? (2) what are examples
oculular hypotension: IOP below normal range
ocular hypotony: IOP < ~5mmHg
hypotony from aq. loss
-surgery, trauma, RD
hypotony from decreased production of aq:
-infl, medications, proliferative vitreworetinopathy
what is ocular hypertension
IOP above normal range (>21mmHg) w/ no detectable changes in vf or damage to optic nerve
who is at risk for ocular hypertenstion
african americans age 40+ \+family history high myopia diabetes
what is glaucoma
group of ocular diseases w/ various cuases that ultiately are associated w/ a progressive optic neuropathy leading to loss of vision function
-atrophy of the optic nerve and loss of retinal ganglion cells and their axons
what kind or glaucoma patients are usually asymptomatic
POAG
what are some signs of glauc
- loss of retinal rim-consequent enlargement of C/D ratio
- elevated IOP
- thin corneas
- thinning/damage of NFL
- drepeatable vf loss
- peripapillary atrophy, flame hem
what are the 2 methods of tonometry
- indentation
- indentation of cornea - applanation
- flattening of cornea
how is IOP determined in goldman applanation tonometry
IOP determined by amount of force needed to flatten a small corneal surface
what is the imbert-fick principle (goldman is based on this)
for dry thin walled sphere, presure inside is equal to the force necessary to flatten the surface divided by the area flattened
P=F/A
-but the cornea is aspheric, wet, and not perfectly flexible, and not infinetely thin soooooo
P+S(surface tension) = F*Ai(inner area cornea) + B (force required to bend cornea)
the 2 beam-splitting prisms w/in the applanating unit optically convert circular area of corneal contact into ____
semicircles
the prisms of the GAT are adjusted so the ______ of semicicles overlap when 3.06mm of cornea is applanated
inner margins of semicircles
what is the idea thickness of the mires
1/10 diameter of semicircles
what happens if the tonometry mires are too thick
what happens if they are too thin
too thick: IOP will be overestimated-blow some fluid from tips/lids (decrease fluroesine)
too thin: IOP will be underestimated: add more fluorescein
what do you do if the mires are overly pulsating
apply very slightly more pressure, just enough to stop it
if the space w/in (inside) semi-circles are splotchy and indistinct, what is happening
too much pressure is being applied
-pull back slightly on joystick
what irregularly shaped corneas, what would you use to take pressure
use tonopen or NCT
bc irreg shaped corneas will distort mires and have inaccurate measures
if there is high astig how do you do tonometry
if >3D of astig: align markings on tonometer tip to match minus cyl axis of astig
thicker corneas result in _____ IOP
thinner corneas result in ____ IOP
what is the exception
ticker=falsely high IOP
thinner=falsely low IOP
exception: thick corneas due to edema are easier to indent due to high water content and result in falsely low IOP
