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Corneal Pachymetry

NC

Nick C

Transcript

Corneal Pachymetry

Clinical applications, techniques and accuracy of measurement

AIMS

ACCURACY OF MEASUREMENTS

SUMMARY

Definition

Pachymetry: The measurement of corneal thickness

1. Definition

2. Clinical Applications

3. The Ideal Pachymeter

4. Techniques/Instruments

5. Accuracy of instruments

6. References

Clinical Applications

Pentacam

• Excellent reproducibility and repeatability (13)

• Can be used interchangeably with USP when measuring normal eyes and mild keratoconus (15)

• Following LASIK, under-estimated corneal thickness in comparison with USP (12)

ASOCT

• Excellent reproducibility (13)

• Conflicting studies

o Overestimates in comparison to USP by around 23 µm (10)

o Underestimates central corneal thickness compared to USP (16)

o Underestimates relative to USP pre-refractive surgery, and overestimates post-refractive surgery (10)

  • Many important clinical applications in ophthalmology
  • Pachymeters employ either ultrasound or optical principles
  • Each instrument has its own advantages and limitations
  • Ultrasound still the gold-standard, however newer techniques provide additional valuable information
  • No clear consensus in literature on accuracy of many of pachymeters

Ultrasound Pachymetry (USP)

• Considered gold standard (11-14)

• Contacts the cornea and displaces tear film

Specular Microscopy

• Thickness readings generally up to 20-30µm lower (11, 15)

• Reproducibility lower than Pentacam and USP (10)

• Unreliable in eyes with keratoconus.(10)

Orbscan

• Overestimates corneal thickness in comparison to ultrasound by up to 28µm (10, 16)

• Inaccurate following LASIK (10)

1. Refractive surgery

2. Diagnosis and monitoring of corneal pathology

a. Corneal oedema

b. Corneal ectasias eg. Keratoconus

c. Endothelial dysfunction

3. Intraocular pressure measurement and risk of glaucoma

Figure 1: Anterior segment OCT of central corneal thickness post-refractive surgery (1)

The Ideal Pachymeter

REFERENCES

1. Accurate

2. Low inter-observer variability / Reliable

3. Quick / Time-efficient

4. Easy to operate

5. Cost effective

6. Minimises complications / Patient discomfort

TECHNIQUES / INSTRUMENTS

Ultrasound

1. Ultrasound pachymetry (10-20 MHz)

2. Ultrasound biomicroscopy (UBM) – 50 MHz

3. Very high frequency ultrasound – eg. Artemis-2

Optical

1. Slit lamp pachymetry

2. Specular microscopy (SM)

3. Confocal microscopy

4. Slit-scanning topography (eg. Orbscan)

5. Pentacam

6. Optical coherence tomography (OCT)

1. Herndon L. Measuring IOP: The Cornea Factor. Review of Ophthalmology2006.

2. Accuvision. Ultrasound Pachymetry Chicago, IL 2014 [cited 2014 May 18]. Available from: http://www.accuvision.com/Content/eye_exam/ultrasound_pachymetry/pachymetry.aspx.

3. California Center for Refractive Surgery. Technology Los Angeles, CA 2012 [cited 2014 May 18]. Available from: http://www.ccrsclearvision.com/technology.html.

4. Silverman RH. High-resolution ultrasound imaging of the eye – a review. Clinical & Experimental Ophthalmology. 2009;37(1):54-67.

5. Bentley E. Pachymetry, specular microscopy and confocal microscopy University of Wisconsin-Madison. Available from: http://www.cvm.ncsu.edu/conted/documents/BentleyPachySpecBSC12Bentley.pdf.

6. Orbis. Specular Microscopy: Specular Microscopy Basics 2014 [cited 2014 May 18]. Available from: http://telemedicine.orbis.org/bins/content_page.asp?cid=1-1581-1614-1615.

7. John Innes Centre. Confocal Microscopy [cited 2014 May 18]. Available from: https://www.jic.ac.uk/microscopy/more/T5_8.htm.

8. Jain R, Grewal SPS. Pentacam: Principle and Clinical Applications. Journal of Current Glaucoma Practice. 2009;3(2):20-32.

9. Liu ZG, Huang AJ, Pflugfelder SC. Evaluation of corneal thickness and topography in normal eyes using the Orbscan corneal topography system. Br J Ophthalmol. 1999;83(7):774-8.

10. Swartz T, Marten L, Wang M. Measuring the cornea: the latest developments in corneal topography. Current Opinion in Ophthalmology. 2007;18(4):325-33.

11. Tai LY, Khaw KW, Ng CM, Subrayan V. Central Corneal Thickness Measurements With Different Imaging Devices and Ultrasound Pachymetry. Cornea. 2013;32(6):771.

12. Ho T, Cheng ACK, Rao SK, Lau S, Leung CKS, Lam DSC. Central corneal thickness measurements using Orbscan II, Visante, ultrasound, and Pentacam pachymetry after laser in situ keratomileusis for myopia. J Cataract Refract Surg. 2007;33(7):1182.

13. Chen S, Huang J, Wen D, Chen W, Huang D, Wang Q. Measurement of central corneal thickness by high-resolution Scheimpflug imaging, Fourier-domain optical coherence tomography and ultrasound pachymetry. Acta Ophthalmologica. 2012;90(5):449-55.

14. AlFarhan HM. Measurements of central corneal thickness using two immersion ultrasound techniques and optical technique. Journal of the Pakistan Medical Association. 2014;64(3):41.

15. Ucakhan OO, Ozkan M, Kanpolat A. Corneal thickness measurements in normal and keratoconic eyes: Pentacam comprehensive eye scanner versus noncontact specular microscopy and ultrasound pachymetry. 2006;32(6):977.

16. Li EYM, Mohamed S, Leung CKS, Rao SK, Cheng ACK, Cheung CYL, et al. Agreement among 3 methods to measure corneal thickness: Ultrasound pachymetry, Orbscan II, and Visante anterior segment optical coherence tomography. Ophthalmology. 2007;114(10):1847.

Figure 2: Ultrasound pachymetry (2)

Figure 3: Pentacam (3)

SPECULAR & CONFOCAL MICROSCOPY

PENTACAM & ORBSCAN

ULTRASOUND PACHYMETRY

OPTICAL SLIT-LAMP PACHYMETRY

ANTERIOR SEGMENT OCT (ASOCT)

Specular Microscopy

• Reflection of light from anterior and posterior corneal surfaces.

• Specular microscope captures light reflected from surfaces.

• Difference in refractive indices proportional to amount of reflection.

Confocal Microscopy

• Normal microscopes have single sharp image and superimposed blurry images.

• Confocal eliminates superimposed blurry images, >50% increase in sharpness of image.

• Reflected light captured through narrow focal plane, blocking light rays that are out of focus.

Pentacam

• Pentacam uses rotating Scheimpflug camera to provide 3D scanning of whole anterior segment.

• Uses Scheimpflug principle in which image plane not parallel to lens plane.

  • Gold Standard
  • Uses piezoelectric crystal to generate ultrasound waves at 10-20 MHz
  • Ultrasound probe contacts the corneal surface perpendicularly.
  • Ultrasound biomicroscopy (UBM) uses higher frequency 50-70 MHz ultrasound waves for higher resolution images.

Figure 10: Illustration of Scheimpflug principle (8)

Figure 9: Rotating Scheimpflug camera (8)

Principles

  • Purkinje-Sanson images formed by anterior and posterior corneal surfaces.
  • Optical doubling used (eg. Jaeger pachymeter)

o Doubles the observer’s image of the cornea.

o Image from the cornea passed through two glass plates, splits the image horizontally.

o Lower glass plate is fixed but upper glass plate can be tilted to change the upper half of the image.

o Thickness is determined by aligning anterior surface of cornea with posterior surface, and then read off a dial.

• Eg. Visante

• Images anterior segment using low-coherence interferometry

• Cross section of cornea obtained by reflection and scattering of light from cornea.

• Infrared spots that move across corneal surface

• Multiple axial scans put together for form 3D cross-sectional images.

Orbscan (slit-scanning topography)

• Evaluates corneal topography and measures corneal thickness by analysing images of anterior and posterior corneal reflecting surfaces.

• 40 light slits projected onto cornea at 45 degree angle.

• Uses both projective slit-scanning and reflective Placido disk principles.

Figure 12: ASOCT of a corneal flap (10)

Principles:

  • Amount of time taken for reflected ultrasound wave to pass back to transducer from posterior corneal surface.
  • Corneal thickness = [Time taken (s) × Speed of ultrasound in cornea (m/sec⁡)] / 2
  • Requires a standard speed of ultrasound in the cornea = 1640 m/sec

Figure 4: UBM image of a corneal scar with corresponding pachymetric maps below (4)

Figure 5: Representation of a Jaeger pachymeter (5)

Figure 11: Orbscan pachymetric maps (9)

Figure 7: Specular microscopy based upon differing amounts of reflected light from irregular surfaces (6)

Figure 8: Confocal microscope. Pinhole does not allow out of focus images to reach detector. (7)

ASOCT

OPTICAL SLIT-LAMP PACHYMETRY

PENTACAM & ORBSCAN

ULTRASOUND PACHYMETRY

SPECULAR & CONFOCAL MICROSCOPY

Disadvantages

1. Requires clear reflection of epithelial and endothelial surfaces.

Advantages

1. Non-contact

2. Can attach to slit lamp

Disadvantages:

1. Requires clear reflection of epithelial and endothelial surfaces

Disadvantages

1. Inter-observer variability

2. Lack of fixation target

3. Based on equations using assumptions of refractive index and anterior curvature of cornea

Advantages

1. Assessment of corneal thickness across entire corneal surface

2. Non-contact

3. Able to determine sublayer measurements

Advantages

1. Operator independency

2. Non-contact

3. Pachymetry over entire cornea

4. Provides anterior and posterior elevation maps

5. Pentacam tracks eye movements

Disadvantages

1. Limited use with corneal opacities

2. Time consuming

3. Confocal requires direct contact with cornea

Advantages

1. Specular microscopy used for endothelial cell count

2. Confocal allows detailed measurement of corneal layers

3. Both can be used to identify corneal pathology

Disadvantages

1. Requires corneal contact and topical anaesthesia

2. Patient discomfort, possible epithelial damage

3. Inter-observer variability

4. Prone to incorrect probe placement

5. Possible indentation of cornea can produce errors

6. Lack of fixation/gaze control

Advantages

1. Accurate

2. Easy to use

3. Portability

4. Low cost

5. High frequency US can produce thickness maps of all layers

Figure 6: Image doubling principle used to measure corneal thickness (5)