Central Corneal Thickness Flashcards
Types of Glaucoma
1) Normal tension glaucoma
2) angle-closure glaucoma
3) primary open angle glaucoma (most common)
4) acute angle closure glaucoma
5) secondary glaucoma (from eye trauma, eye disease, diabetes, or meds)
6) childhood glaucoma
7) pigmentary glaucoma
IOP and CCT
average thickness - 520 um
fails when cornea is not average thickness
fails when cornea has disease
IOP measurements become less reliable with variable CCT
normal IOP
upper limit of normal = 21 mmHg
ocular hypertension
IOP > 21 in one or both eyes in more than 2 occaisions
does not have defect VF testing
normal ONH and NFL
anatomically normal
absence of ocular conditions such as narrow angles, neovascular conditions, and uveitis
mean CCT
mean CCT of normal eyes is 544 +/- 34um
OHTS study
goal: determine reducing elevated eye pressure delayed or prevented glaucoma
OHTS 6 predictive factors
1) older age
2) african descent
3) higher eye pressure
4) cup to disc ratio of ONH
5) thinness of the cornea
6) VF pattern std dev.
OHTS - CCT
pt with thinner cornea at greater risk
cct < 555um and iop > 25.75 has 36% risk of glaucoma
cct > 588 and iop > 25.75 has a 6% risk of glaucoma
goldmann underestimate tru IOP in thinner cornea and overestimates in thicker cornea
applanation IOP sources of error
corneal variable
thin cornea -> falsely low
thick cornea -> falsely high
OHTS summary
HIGH RISK:
IOP > 25.75 mmHg
CCT < 555 um
Vertical C/D > 0.50
mod risk:
IOP 23.75 - 25.75
CCT 555 - 588 um
vertical C/D 0.3 - 0.5
low risk
IOP < 23.75 mmHg
CCT > 588 um
vertical C/D <0.30
Pachymetry
- Determine CCT
- 2 forms: optical and ultrasound (more reliable)
Optical Pachymetry
1) optical slit lamp technique to measure corneal thickness
2) specular microscopy - contact based
3) orbscan - non contact
4) OCT - noncontact based on optical interferometry
5) confocal microscopy - contact through confocal microscope through thickness
6) LDI - noncontact using infrared laser doppler interferometry
7) pentacam scheimpflug camera - noncontact technique using camera to rapidly capture images of ant. segment of eye
Ultrasound pachymetry (used in lab)
low frequency of 10-20 MHz
accuracy dependent of perpendicularity of probe
preferred method
meausres transit time needed for pulse pass from one end to descemet membrane and back
determined by density and compressibility of cornea
ctt =( transient time x propagation velocity_ / 2
pachymeter reading: reading, std dev of cct
if in orange -> std dev is greater than 10
