A-Scan Biometry.pptx

Giagnostic procedure

1. A-Scan Biometry By: Saif ullah Email ID: [email protected] FIACLE, M.PHIL OPTM, MPH, BS OPTM Assistant Professor Optometry PIO, Al- Shifa Trust Eye Hospital

2. INTRODUCTION Ultrasound --- a vibrational form of energy, used in medical imaging. Formats for display of ultrasound signals --- include A-SCAN (A for Amplitude): B-SCAN (B for Brightness): C-SCAN (C for Coronal): D-SCAN (D for Deflection): M-SCAN (M for Motion): DOPPLER: The ultrasound display modalities used in Ophthalmological practice – A- and B-scans

3. INTRODUCTION A-scan: is a time-amplitude display. is one dimensional. echoes are displayed as vertical deflections from a base-line. the strength of the echo is indicated by the height of the spike. is a real-time display. B-scan: is a brightness intensity-modulated display. is two dimensional. echoes are displayed as dots. the strength of the echo is indicated by the brightness of the dots. is a real-time display.

4. Ultrasound Principles Sound is a vibratory disturbances in a solid or liquid medium with frequency of 20-20000Hz Ultrasound frequency of >20KHz Ophthalmic A & B scan 10 MHz Higher the frequency==  less penetration==  High resolution

5. Velocity of sound Medium Meter/second Cornea 1641 Aqueous/vitreous 1532 Lens 1641 Phakic eye 1550 Aphakic eye 1532 Pseudophakic eye 1532

6. Principles A thin parallel wave of sound is emitted from the probe tip At each interface sound wave is reflected back and received by the probe This is converted into a spike by the biometer The height of the spike depends upon the deference in the density of interfaces and orientation of the probe

7. Instrumentation Gain setting High gain → wide sound beam, more penetration, high spikes, less resolution Low gain→ narrow beam, less penetration, low spikes, more resolution Resolution is the ability of the machine to display two interfaces that lie close together as separate spikes. High gain shows retina and sclera as one thick spike with a wide flattened peak

8. High gain Low gain

9. Instrumentation Gates are electronic calipers placed on the spikes. Distances is measured between the two gates Most machines place 4 gates automatically on corneal, anterior lens spike, posterior lens spike, and retinal spike Distance between each pair of gates is measured automatically

10. Callipers placement

11. Accuracy Average axial length of eye 23.5 mm Anterior chamber depth 3.24 mm Average lens thickness 4.63 mm Average keratometry 43-44 D 1mm of error in axial length will produce 2.5 D error in post-op refraction

12. IOL formulas SRK II SRK/T Hoffer Q Holladay Binkhorst II Haigis

13. SRK II P = A -2.5AL - 0.9K + C AL <20 mm C = +3 AL =>20 & <21 mm C = +2 AL =>21 & <22 mm C = +1 AL =>22 & <2 4.5 mm C = 0 AL => 24.5 C = -o.5

14. A constant of some IOLs Type PC IOL AC IOL Rayner 118.0 116.0 Cilco 118.7 115.3 Domi 118.4 114.8 FHF 118.3 Acreysof 118.9 CeeOn 118.3

15. Contact technique The tip of the placed gently over the vortex of the cornea with less corneal compression Five high amplitude spikes are displayed Probe/corneal spike Anterior lens spike Posterior lens spike Retinal spike Scleral spike

16. Contact technique

17. Immersion technique In immersion technique 6 spikes are displayed. The extra spike is of the probe tip which no longer in contact with the cornea Corneal compression is avoided

18. Immersion

19. Biometry through various IOLs Biometry of pseudophakic may be required for IOL exchange The exact material of IOL must be known to have an accurate IOL power Correction factor is added to the axial length of aphakic settings IOL material Velocity of sound Correction factor PMMA 2718 m/s +0.4mm Acrylic 2120 m/s +0.2mm Silicon 980-1090 m/s -0.8mm

20. Common errors in biometry Corneal compression Misalignment Fluid meniscus between tip of the probe and cornea Dense cataract Posterior staphyloma

21. Misalignment

22. Misalignment Small posterior lens spike Through optic nerve absent scleral spike

23. Biometry following refractive surgery These patients require special attention Their K reading is no obtainable due to corneal flattening It is calculated by the following formula K = Preoperative Av K – Chang in pre-refractive and post-refractive surgery If preoperative record is not available then use this formula K = Base curve + difference in refractive error with and without contact lens using a known base curve hard contact lens

24. Postoperative Refractive Surprises Sources of error Keratometry Axial length Estimated lens position A constant Labelling by manufacturer Viscoelastic bag entrapment Choice of IOL power calculation formulae Use SRK/T, Hoffer Q or Holladay II formula

25. Keratometry 1.00 D error in keratometry will produce 1.00 D error in IOL power calculation Gonioscopy and tonometry should not be done immediately prior to biometry Soft gas permeable and hard contact lenses should be discontinued 2, 4, and 6 weeks prior to biometry respectively In case of scarred cornea the fellow cornea is measured

26. Axial length Compression of cornea during biometry Posterior staphyloma Silicon oil in vitreous cavity

27. Management Hypermetropic surprise Multiply the postoperative hyperopic refractive by factor 1.5 Add that figure to the power of IOL already implanted. For example for refractive error of +2 , a 21D IOL is exchanged with 24D IOL in the bag or 23.5D IOL in the sulcus.

28. Management Myopic surprise For early intervention exchange at the ratio of 1:1. For example for refractive error of -2 D, a 21D IOL is exchanged with 19D IOL. For late intervention consider PRK, LASIK or LASEK For monovision we need a residual refraction in the non dominant eye of -1.75D and emmetropisation in the dominant eye

29. Management Piggyback IOLs In case of well fixated IOL and mislabelled IOl this may be more appropriate technically less demanding For piggy back IOL calculation with +4D refractive error +6D IOL and for -2D refractive error -2D IOL is added

30. Astigmatic surprises Donor rush Wait for at least one year for magnitude and axis stability Sources of error Lenticular astigmatism neutralizing corneal astigmatism Keratoconus Thermal effect