Tonometry Flashcards
(19 cards)
What determines IOP in the eye?
ciliary body produces aq (1-2ul/min), 90% through pupil to drain at trabecular meshwork
10% back into ciliary body/iris BVs/choroid sclera (significant uver-scleral outflow)
CBody keeps producing, cant expand so IOP increases
Define glaucoma in short
Conditions with IOP-related optic nerve damage
Explain the features of IOP as a screening tool and 3 associated disorders
Good specificity/Poor sensitivity
Spots healthy px well not glaucomatous px
CAG ~ 1 eye raised IOP
RetDet ~ 1 eye low IOP (<8), IOP difference ~ more uveo-scleral outflow through tear
CRVO ~ higher IOP higher risk
Explain manometry
Cannulation ~ hollow needle with H2O reservoir pierces globe, H2O psi adjusted so no aq escapes/no water flows into eye
Explain digital palpitation
Index finger compares eyes through lids
CAG eye is hard like a stone
Explain Imbert-Fick law and why it’s theoretical
Effort to flatten (applanate) spherical container surface ~ P=W/A
Relies on container being: perfect sphere, infinitely thin, perfectly flexible, elastic, dry
Why does the probe diameter in GAT need to be bigger than 3.06mm?
7mm to protect cornea if too much force applied or probe misaligned
Explain Goldmann Applanation Tonometry
Fluorescein stain, the anaesthetic (lidocaine)
Fresh probe avoids iatrogenic infection (consistent readings) set dial force to 1g (10mmHg)
Align probe with cornea and increase dial force to align mires (semicircles)
Record probe reading x10 to mmHg, time of day and drugs used
Explain portable Perkins applanation tonometry
Portable Goldmann’s
Probe on central cornea prevents corneal thickness variability
Explain operator error in contact tonometry
Probe position (Mire width/thickness (ideal 0.2mm, thinner gives bigger M vice versa)
Px anxiety
Massage effect (aq displacement) - gives lower IOP if probe on eye for too long
Explain astigmatism error in contact tonometry
Oval mires instead of circular
Keratometry finds exact meridians, ensure astigmatism isn’t lenticular
Average of horizontal/vertical probe readings (>3.00DC)
Explain corneal structure variation error in contact tonometry
Central Corneal Thickness (GAT calibrated for 520um CCT)
Higher or lower affects N (corneal rigidity) aswell as IOP (under/overestimate)
Cornea curvature/elasticity/health (keratoconus/oedema)
Explain Px factors in errors for contact tonometry
Make px at ease
Holding breath, lid squeezing, accommodation
IOP slightly higher after waking
Explain contact I-care tonometers
Ballistic/Rebound
Magnetic coil propels probe forward
Sensor coil measures motion parameters (deceleration, contact time with eye, rebound velocity) to gauge IOP
Explain how a tono-pen works
Force measured from protruding plunger position (1mm diameter applanated involves 2.5mm diameter of cornea)
Plunger measures IOP/force needed to bend cornea
Quick small but requires anaesthetic
Explain A/O Reichert NCT
IR (red wavelength) light scattered off curved cornea, IR light from applanated cornea reflected to detector
IR light from concave cornea scattered (too much force)
Distance needs to be correct for detector
Alignment system of invisible IR beam and light target to bring into focus
Explain pulsair tonometers
Same principles as A/O Reichert
Shorter Puff stoped at 1st applanation point (px more comfort)
Transducer measures air force at applanation
Look for black cross on red alignment bars
Explain the accuracy disadvantages of NCT
More variability that contact, less accurate at high IOP
Short time sample so pulse variations not averaged out without multiple readings
What are the main disadvantages of CT?
Harder to master (operator error)
Potential infection from probes, allergic reactions
Corneal abrasion risk
