Gonio, tonometry, US

Question 1

principle of direct gonio

Answer 1

 Angle is directly visualised through the contact lens

Question 2

examples of direct gonion

Answer 2

 Koeppe
 Richardson
 Barkan
 Wurst
 Swan-Jacob

Question 3

principle of indirect gonio

Answer 3

Light rays are reflected by a mirror in the contact lens

Question 4

examples of indirect gonio

Answer 4

 Goldmann
 Zeiss
 Posner
 Sussman

Question 5

scleral-type gonio

Answer 5

Goldmann:
 Larger
 Require coupling solution
 Better stability and eyelid control
 Compression will narrow angle by pushing on sclera

Question 6

corneal type gonio

Answer 6

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

Question 7

principle of goldman tonometry

Answer 7

 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

Question 8

tonometer scale is in

Answer 8

dynes so is multiplied by ten to give an IOP in mmHg

Question 9

causes of flasley low goldman readings

Answer 9

corneal oedema
low CCT
previous refractive surgery
too little fluroescein
>3D with the rule astigmatisim
high myopia

Question 10

causes of flasely high goldman readings

Answer 10

high CCT
digital pressure
corneal scar
too much fluroscien
>3D against the rule astigmatism

Question 11

perkins tonometer

Answer 11

uses split like goldman and fluroscein
portable
can be used in patiens upright or supine

Question 12

principles of US

Answer 12

 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

Question 13

frequencies in US

Answer 13

 Higher frequencies provide greater resolution but poorer depth of tissue
penetration
 Most ocular ultrasound is performed around 10 MHz

Question 14

A-scan

Answer 14

Plots the intensity of the echo versus time delay: converted to distance
requires US probe to be placed directly on the cornea

Question 15

amount of compression in a-scan

Answer 15

0.14mm to 0.27mm

Question 16

peaks in a-scan

Answer 16

 Corneal surface
 Anterior lens
 Posterior lens
 ILM of retina
 Sclera
orbital fat

Question 17

types of a-scan

Answer 17

applanation - US on central cornea, may cause compression
immersion - saline filled sclearal shell between eye and probe, takes longer

Question 18

causes of errors in a-scan

Answer 18

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

Question 19

b-scan

Answer 19

 2-dimensional images created from multiple A-scans

Question 20

b-scan gain

Answer 20

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

Question 21

b-scan grey scale

Answer 21

Ability to display various scales of brightness
High grey scale display maximum samples from white ( high amplitude) to black ( lowest amplitude)

8. Examination tips
   Start with high grey scale, once diagnosis is made-lower the grey scale

Question 22

quadrants in b-scan

Answer 22

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)