A concise lecture on the principles and application of corneal pachymetry

1. Corneal Pachymetry

2. D E F I N I T I O N
I N D I C A T I O N S
C O R N E A L A N A T O M Y
T Y P E S O F P A C H Y M E T R Y
U L T R A S O U N D P A C H Y M E T R Y
E X A M P L E
C L I N I C A L U T I L I T Y
Content

3. Corneal Pachymetry
 Non-invasive process of measuring the thickness of
the cornea
 Central Corneal Thickness (CTT)

4. Corneal Pachymetry
 Helps to assess the function of the corneal
endothelium
 Corneal refractive surgery or Corneal transplant
 Glaucoma suspects
 Bullous keratopathy
 Corneal edema
 Posterior polymorphous dystrophy
 Fuchs' endothelial dystrophy
 Keratoconus screening

5. Corneal Anatomy

6. Corneal Anatomy
 Picture of cornea

7. Corneal Pachymetry
 Normal Values
Normal corneal thickness
 500 to 575 microns.
 Abnormal Results
Abnormally thick or thin measurements may
indicate
 Corneal thinning
 Corneal edema
 Lower or higher than suspected intraocular pressure

8. Types of Pachymetry
 Contact methods
 Ultrasound
 (mean CCT 544 μm)
 optical such as confocal microscopy (CONFOSCAN)
 (mean CCT 530 μm)
 Non-contact methods
 Optical biometry with a single Scheimpflug camera (SIRIUS or
PENTACAM)
 Dual Scheimpflug camera (GALILEI)
 Optical Coherence Tomography (Visante)
 Optical Coherence Pachymetry (ORBSCAN).

9. Ultrasound Pachymetry

10. Ultrasound Pachymetry
 Ultrasound pachymetry is now broadly accepted as
the method of choice
 Good reproducibility
 Easy
 Portable and used intra-op
 Precise measurement
 Eliminates variation from one observer to the other
 Little intersession variation

11. Ultrasound Pachymetry
 Ultrasonic waves are emitted and bounce back from
the posterior cornea
 Proper speed of sound is set corneal sound velocity
1640-1620m/sec in the pachymeter
 Time take to bounce back detected by the probe and
the computation of (t) the corneal thickness

12. Steps
 Video

13.  Video of how it is done
 Topical anesthetic (numbing) eye drops are
administered.
 The tip is solid and is -2mm in diameter whch allows
the user to place it precisely on the cornea.
 Prope tip is placed perpendicularly and the benefits
are that it is easy for paramedic staff as it requires
simple training to gain proficiency.
 It is based on an average of readings computed by
the instrument. Readings are accurate to 5-10 um

14. Ultrasound Pachymetry
 Problems
 CCT changes overtime and with age or as a result of the use of
topical medications
 CCT decreased approximately 0.3 μm per year
 Weizer et al
 0.7 ± 3.6 μm per year
 OHTS investigators
 Magnitude of change would not have an impact on tonometry or
clinical decision making
 Indentation of cornea
 Spread of infection

15. Corneal Waveform Technology
 Technology used in the newer generation
pachymeters to capture an echogram scan of the
cornea
 Give the user the ability to display, store, recall, analyse and
superimpose corneal waveforms.
 More accurately measures the corneal thickness
 measure changes in corneal thickness over time
 Measure structures within the cornea such as micro bubbles
that are generated by the femtosecond laser during LASIK eye
surgery.

16. L A S I K
G L A U C O M A
C O R N E A L T R A N S P L A N T
K E R A T O C O N U S
Clinical Utility

17. Use in LASIK
 Essential prior to a LASIK procedure for ensuring
sufficient corneal thickness to prevent abnormal
bulging of the cornea, a side effect known as ectasia
 Differences in central cornea thickness between pre
op and intra-op readings
 intraoperative measurements are always used for setting blasé
length and the depth of incision
 Most surgeons measure the thickness peripheral to
the clear zone and mark infero-temporally which is
the thinnest part of the cornea to be incised.

18. Use in Glaucoma
 CCT statistically significant predictor of development
of glaucoma
 Results suggested that IOP measurements need to be
adjusted for abnormally thick or thin corneas.
 The target IOP is lower for a thin cornea and higher
for a thick cornea.
 Eyes with thick corneas have a true IOP that is lower than the
measured IOP.
 Thus, individuals with thicker corneas may be mis-classified as
having ocular hypertension.

19. Corneal Transplant Grafts
 Assessing candidates for penetrating keratoplasty
(corneal transplant)
 Assessing graft failure and the need for regrafting in
corneal transplant recipients by aiding in the early
diagnosis and treatment of graft rejection.
 Assessing the response to treatment of corneal
transplant rejection.

20. Keratoconus
 Keratoconus is associated with corneal thinning
 Available evidence indicates that ultrasonic corneal
pachymetry is not as accurate as videokeratography
in diagnosing keratoconus.
 The investigators concluded that pachymetry should
not be relied on to exclude or diagnose keratoconus
because the false-negative and false-positive rates
are unacceptably higher than those obtained by
videokeratography

21. Marfan’s syndrome
 Demonstrated an association with corneal thinning
and described confocal microscopy findings in MFS.
 Lack of evidence to support the use of ultrasound
pachymetry in the diagnosis of MFS

22. Conclusion
 Measure Central Corneal Thickness
 Contact and non contact techniques
 Ultrasound pachymetry prefered
 Easy, reproducible
 Keratoplasty, glaucoma

23. References
 Dr Ben O’Keeffe
 Ophthalmology SHO, Wellington, NZ