CLINICAL TECHNIQUES - Biometry and Pentacam and Microscopy

Question 1

What is the difference between K1 and K2 readings in pentacams? What is Km? What is Kmax?

Answer 1

K1 - flattest meridian
K2 - steepest meridian
Km - mean power in diopters

Kmax - maximal K reading - its location tels you the apex of the cone

Question 2

What is the Q-value in pentacam? What is normal Q-value range?

Answer 2

Q value - corneal shape
Normal is between -1 and 0

> 0 = oblate cornea
<-1 = prolate cornea (keratoconus)

Question 3

Which unit assesses change in corneal thickness of the whole cornea of pentacams? What is it used for clinically?

Answer 3

Used to assess change in corneal thickness of the whole cornea - useful for ectatic disease.

Question 4

What principal is used for confocal microscopy?

Answer 4

Principle of confocal single point of tissue illuminated by a point source of light whilst simulatenously imaged by a camera in the same plane

Question 5

What is specular microscopy? What does it measure?

Answer 5

Provides objective measurement of corneal endothelial cells

1. Parameters
2. Density
3. Coefficiency of Variation (>0.4 increased variation)
4. Percentage of hexagonal cells (>50% pleomorphism, may not tolerate cataract surgery)

Question 6

How wide is the field of view in colour fundus photography?

Answer 6

30-35 degrees

Question 7

How wide is the field of view in colour fundus photography?

Answer 7

30-35 degrees

Question 8

What are the causes of post-operative HYPERMETROPIC surprise (5)

Answer 8

Hypermetropic suprise –> greater axial length and/or reduced lens power.

1. Previous unrecognised myopic refractive surgery
2. 2. Unrecognised staphyloma in a highly myopic eye (can occur if an A-scan is used instead of optical biometry) - overestimates the AL of the eye, leading to underestimation of the IOL power
3. Use of a lower A constant than intended
4. Post-operative cystoid macular oedema
5. Use of biometry in keratoconus (as corneal power is typically overestimated)

Question 9

What are the causes of MYOPIC refractive surprise? (4)

Answer 9

1. Capsular block syndrome (accumulation of fluid between IOL and posterior capsule causing anterior displacement)
2. AC shallowing
3. Previously unrecognised hyperopic refractive surgery
4. Errenously using higher A constant value –> higher IOL power than required for eye.

Question 10

What is the difference between OCT and optical biometry in mechanism?

Answer 10

OCT: low coherence inferometry
Optical biometry: partial coherence inferometry where two co-axial partially coherent laser beams are used (infrared)

Question 11

When should biometry be repeated (5)

Answer 11

1. Axial length < 21.2 or > 26.6
2. Difference in axial length >0.7mm
3. Mean keratometry < 41D or >47D
4. Difference in mean keratometry > 0.9D
5. DeltaK (corneal astigmatism) > 2.5D

Question 12

Which patients may not tolerate intraocular surgery in regards to specular microscopy findings?

Answer 12

1. Corneas with low endothelial cell density
2. Significant polymegathism (>0.40) - relates to coefficient of variation (increases in contact lens wear)
3. high pleomorphism (>50% non-hexagonal cells)

Question 13

What is the typical corneal compression from A scan biometry?

Answer 13

0.1-0.3mm

Question 14

What strategy is used to detect the threshold stimulus intensity?

Answer 14

Bracketing