Gonio, tonometry, US Flashcards
principle of direct gonio
Angle is directly visualised through the contact lens
examples of direct gonion
Koeppe
Richardson
Barkan
Wurst
Swan-Jacob
principle of indirect gonio
Light rays are reflected by a mirror in the contact lens
examples of indirect gonio
Goldmann
Zeiss
Posner
Sussman
scleral-type gonio
Goldmann:
Larger
Require coupling solution
Better stability and eyelid control
Compression will narrow angle by pushing on sclera
corneal type gonio
Zeiss, Sussman:
Smaller
No coupling solution needed
Facilitates dynamic indentation gonioscopy: can discriminate appositional
from synechial angle closure
Compression will open angle by pushing aqueous in
principle of goldman tonometry
Based on Imbert-Fick principle
P=F/A
Force of application is directly proportional to the intraocular pressure when the
area of applanation equals 3.06mm
tonometer scale is in
dynes so is multiplied by ten to give an IOP in mmHg
causes of flasley low goldman readings
corneal oedema
low CCT
previous refractive surgery
too little fluroescein
>3D with the rule astigmatisim
high myopia
causes of flasely high goldman readings
high CCT
digital pressure
corneal scar
too much fluroscien
>3D against the rule astigmatism
perkins tonometer
uses split like goldman and fluroscein
portable
can be used in patiens upright or supine
principles of US
A piezoelectric crystal transducer produces high-frequency (8-100 MHz) sound
waves
The sound waves travel through tissues and echos are generated from changes in
the impedance of a tissue (therefore a homogenous tissue will not generate echoes)
The reflected echo signal is converted into an electrical signal and the amplitude is
measured
frequencies in US
Higher frequencies provide greater resolution but poorer depth of tissue
penetration
Most ocular ultrasound is performed around 10 MHz
A-scan
Plots the intensity of the echo versus time delay: converted to distance
requires US probe to be placed directly on the cornea
amount of compression in a-scan
0.14mm to 0.27mm
peaks in a-scan
Corneal surface
Anterior lens
Posterior lens
ILM of retina
Sclera
orbital fat
types of a-scan
applanation - US on central cornea, may cause compression
immersion - saline filled sclearal shell between eye and probe, takes longer
causes of errors in a-scan
Misalignment
Probe not perpendicular to lens or macular, or aligned to optic nerve
Gain too high
High gain increases sensitivity, but reduces resolution of spikes, causing retina and scleral spikes to merge together
Falsely short reading
Cornea compression as discussed above
Falsely long reading
Fluid meniscus between probe and cornea, posterior staphyloma
Incorrect velocity
Important to consider if the eye is phakic, aphakic, pseudophakic, or if there is silicone oil as this can result in changes the sound velocity. A correction factor should be applied
b-scan
2-dimensional images created from multiple A-scans
b-scan gain
3) Gain
Measure in dB ( decibles)
Affects the amplitude of displayed echoes
4) High gain: better at displaying weak signals, but can pick up unwanted artefacts
5) Low gain: can’t pick up weak signals, but can have higher resolution
b-scan grey scale
Ability to display various scales of brightness
High grey scale display maximum samples from white ( high amplitude) to black ( lowest amplitude)
- Examination tips
Start with high grey scale, once diagnosis is made-lower the grey scale
quadrants in b-scan
Four quadrants of the eye are typically denominated with the following nomenclature based on clock hours
T12 ( superior quadrant)
T9 ( lateral or nasal quadrant)
T3 ( nasal or lateral quadrant)
T6 ( inferior quadrant)
