COT Biometry and A-Scans 2 — Questions and Answers
Question 1: Which IOL formula is generally preferred for eyes with axial lengths greater than 26 mm?
- SRK/T
- Holladay 1
- Barrett Universal II (Correct answer)
- Hoffer Q
Correct answer: Barrett Universal II
Barrett Universal II performs well across a wide range of axial lengths, including long eyes over 26 mm.
Question 2: In immersion A-scan biometry, the probe is:
- Placed directly on the cornea
- Submerged in a saline-filled shell over the eye (Correct answer)
- Placed on the sclera
- Held 10 mm from the eye
Correct answer: Submerged in a saline-filled shell over the eye
Immersion technique uses a saline-filled scleral shell to eliminate corneal compression artifacts.
Question 3: What does the term 'applanation' refer to in contact A-scan biometry?
- Flooding the eye with saline
- Compressing the cornea with the probe (Correct answer)
- Dilating the pupil before measurement
- Aligning the probe to the optic axis
Correct answer: Compressing the cornea with the probe
Applanation biometry involves the probe touching and slightly compressing the cornea, which can shorten axial length readings.
Question 4: The typical ultrasound frequency used in ophthalmic A-scan biometry is:
- 1 MHz
- 8 MHz
- 10 MHz (Correct answer)
- 20 MHz
Correct answer: 10 MHz
Ophthalmic A-scans use 10 MHz ultrasound to achieve adequate resolution for ocular structures.
Question 5: Which structure produces the posterior lens spike in an A-scan waveform?
- Anterior corneal surface
- Posterior corneal surface
- Posterior lens capsule (Correct answer)
- Internal limiting membrane
Correct answer: Posterior lens capsule
The posterior lens capsule generates the second large spike after the anterior lens spike in A-scan.
Question 6: A patient has a short axial length of 20.5 mm. Which IOL formula is most appropriate?
- SRK II
- Holladay 2 or Hoffer Q (Correct answer)
- SRK/T
- Haigis with default constants
Correct answer: Holladay 2 or Hoffer Q
Hoffer Q and Holladay 2 are preferred for short eyes (axial length <22 mm) to minimize prediction errors.
Question 7: What artifact can cause a falsely low axial length reading on contact A-scan?
- Posterior staphyloma
- Corneal compression by the probe (Correct answer)
- Dense posterior subcapsular cataract
- High refractive error
Correct answer: Corneal compression by the probe
Probe compression indents the cornea and shortens the measured axial length, leading to IOL power errors.
COT Biometry and A-Scans 2
Which IOL formula is generally preferred for eyes with axial lengths greater than 26 mm?