pachymetry and its tecnique

1. KAVITA SHASTRI
OPTOMETRIST(TMH)

2. CONTENTS :
Introduction
Corneal Anatomy
Techniques
Clinical utility

3. INTRODUCTION :
 Pachymetry (Greek words: Pachos= thick+metry) is term used for
measurement of corneal thickness.
 It is important indicator of health status of the cornea especially
of corneal endothelial function.
 The thickness of cornea was first reported in ancient textbooks
on physiological optics (Helmholtz and Gullstrand).

4. CORNEAL ANATOMY :

5. CORNEAL THICKNESS IN NORMAL EYES
 It ranges from 0.7-0.9 mm at the limbus and varies
between 0.49-0.56 mm at the center.
 The CCT reading of 0.7 mm or more is indicative of
endothelial decompensation.
 Peripheral corneal thickness is asymmetric so that
temporal cornea is thinnest followed by the inferior
cornea.

6. CORNEAL THICKNESS IN NEWBORNS AND
CHILDREN :
 Is similar to that of adult cornea.
 It has been found that cornea on day one is significantly thicker and
decreases in thickness as child grows older.
 The average corneal thickness in infants is 585±52 microns.
 The superior peripheral cornea is thinnest in newborn.

7. Normal Values
Normal corneal thickness – 500-575 microns.
 Abnormal results
Abnormally thick or thin measurements may indicate –
 corneal thinning.
 Corneal edema
 Lower or higher than suspected IOP.

8. FACTORS AFFECTING CCT :
 Higher in young patients, male patients and diabetic
patients.
 The mean CCT in black children is thinner than that
of white children.
 CCT does not correlate with refraction or systemic
hypertension.

9. TECHNIQUES :
ULTRASONIC OPTICAL ALTERNATIVE
MEASUREMENTS
• Conventional ultrasonic
pachymetry
• Manual • Pentacam
• Ultrasound
biomicroscpy (UBM).
• Specular microscopy • Pachycam
• Optical coherence
Tomography(OCT)
• Ocular response
analyzer (ORA).
• Optical coherence
interferometry
• Confocal microscopy
• Laser Doppler
interferometry
• Scanning Slit
technology

10. 1.ULTRASONIC PACHYMETRY :
 Broadly accepted as method of choice, regarded as the gold
standard.
 PRINCIPLE :
 Instruments functions by measuring the amount of time
(transmit time) needed for ultrasound pulse pass from one end
of transducer to descemet’s membrane and back to transducer.
 Corneal thickness=transmit time × propagation velocity)/2
 Speed of sound in cornea : current standard is 1640 m/sec

11. COMPONENTS :
 PROBE HANDLE - It has piezoelectric crystal that emits
an ultrasonic beam of 20 MHz
 TRANSDUCER – It sends ultrasound rays through the
probe to the cornea and receives echoes from the cornea.
 TIP - The diameter of the tip should not be more than
2mm.

13. ADVANTAGES DISADVANTAGES
• Faster ,simpler and easy to use. • Contact method.
• Consistent and eliminates
interoobserver variation.
• Accuracy is dependent on the
perpendicularity of the probe’s
application to the cornea.
• Portable • Reproducibility relies on the precise
probe placement on the center of the
cornea.
• Dry(no coupling medium
required)
• Low resolution.
• Can be used intraoperatively. • Not accurate in edematous cornea.

14.  High resolution ultrasound machine which images anterior segment of
the eye.
 Using a 50-MHz imaging probe or high-frequency UBM, reproducible
images of the cross-sectional anterior chamber anatomy with high
resolution are achieved.
 12.5–50 MHz probe , the depth of penetration is lesser (4mm)than
conventional ultrasound.
2. ULTRASOUND
BIOMICROSCOPY:

15. ADVANTAGES DISADVANTAGES
• AS+ Corneal thickness • The main limitation of UBM is that it
requires a water-bath coupling media
and a very experienced examiner.
• Useful in cases where cornea is opaque • Contact method and patient to lie
supine
• Various layers of cornea can be
identified.
• Cannot be used intraoperatively.

16. 3.MANUAL OPTICAL PACHYMETER:
 This was the original method to measure
corneal thickness.
 The instrument contains two Plano glass
plates that splits the image of the corneal parallelepiped.
 There are two methods to measure corneal thickness.
 Just touch method.
 Overlap method.
 The corneal thickness is then directly read from the scale.
 Range is 0-1.2mm, with a least gradation of 0.02 mm.

17. ADVANTAGE DISADVANTAGE
• Non contact method. • Lack of accuracy in measurements,
the usual range of error with an
optical pachymeter is ±2%
• Lack of repeatability.
• Requires slit lamp and therefore has
poor portability and cannot be used
in operating room.

18. 4. SPECULAR PACHYMETRY :
 This is the oldest method to measure corneal thickness.
 PRINCIPLE : This measures the distance between the
anterior and the posterior surfaces of cornea and depends
on the focusing of light rays through front back cornea
unlike sound waves in ultrasound pachymeter.
 2 types 1.contact
2.Non contact

20. ADVANTAGES DISADVANTAGES
• Operator independent. • The exact point where reading is taken is
not known.
• Non invasive. • Time consuming.
• Simultaneous measurement of
cell count.
• Less reproducible.
• Impractical to use in operation room.
• Clinical use is limited to corneas free of
edema, scarring, deposits or opacities
that may distort light transmission.

21. 5.SLIT LAMP PACHYMETER:
 Elevation based system which uses scanning slit
technology.
 Capable of assessment of multiple functions thickness
profile, anterior and posterior topography, elevation,
and anterior chamber depth.
 PRINCIPLE : It measures anterior and posterior
elevations by comparing it to a best fit sphere.

22. Demonstration of slit-lamp adapted ultrasonic pachymetry. The ultrasound probe is
inserted into the Goldmann tonometry mount, and under joystick control, it can be
softly and precisely used to touch the surface of the cornea.

23. ADVANTAGES DISADVANTAGES
• It gives wide field pachymetry
measures across the entire cornea.
• It overestimates corneal thickness by
5%
• It also identifies the thinnest part of
cornea.
• Tendency to underestimate corneal
thickness in keratoconic, post PRK,
and post-Lasik eyes.
• Corneal alignment is not required • The measurements are adjusted for
normal prolate shape of cornea.
Change of shape may interfere with
the reconstruction algorithms
• Can be used to calculate optical
zones in corneal refractive surgeries.
• Decreased accuracy in measuring
corneal thickness when clinically
significant haze is present.

24. 6. ANTERIOR SEGMENT OPTICAL
COHERANCE TOMOGRAPHY :
 High resolution, non-contact OCT customized
anterior segment.
 Gives color coded map of corneal thickness.

26. ADVANTAGES :
 Non contact
 Rapid acquisition during the pachymetry scan ensures an
accurate and repeatable pachymetry map.
 High resolution
 It measures and documents both corneal flap thickness and
residual stromal thickness immediately following LASIK
surgery.
 Measures through corneal opacity.

27. 7. OPTICAL LOW COHERENCE REFLECTOMETRY:
 The instrument is attached
to a slit lamp.
 Measure corneal thickness
to a precision of 1micron.
 PRINCIPLE:
Based on Michelson interferometer. It uses diode laser
beam.

28. ADVANTAGES DISADVANTAGES
• Precise 1 micron measurement. • Measures only central corneal
thickness.
• Automatic alignment.
• Non-contact.
• Real-time data acquisition and
display.
• Convenient and easy.
• Variability of measurement is
significantly lower.
• Intraoperative measurements
possible.

29. 8. CONFOCAL MICROSCOPY:
 This unique method offers the ability to examine objects at
high magnification ranging from 20 X to 500 X.
 It also measures thickness of each layer by using
computerized scanning system providing the total corneal
thickness in studied area.
 Beside endothelium examination also measures endothelial
cell count (density) which is comparable to specular
microscopy.
 It offers the possibility to visualize structures posterior to
haze, scars or edema in the cornea.

31. ADVANTAGES DISADVANTAGES
• Moderate to good repeatability,
particularly for measurements of thin
layers such as epithelial and bowman’s
layer thickness.
• Poor agreement with ultrasound
pachymetry, it apparently
overestimates corneal thickness.
• Following LASIK surgery, flap
thickness can also be obtained.
• The precision of measurements with
this technique will vary with contact
lens hydration, post-lens tear film
thickness and observation angle.
• The Z-scan curve can be used to assess
the level and location of corneal haze
associated with the various corneal
dystrophies.
• Slower data acquisition.
• Poor penetration of corneal opacity.

32. 9. LASER DOPPLER INTERFEROMETRY :
 It is a noncontact technique that uses a dual-beam
infrared laser Doppler interferometry to measure
corneal thickness.
 There is limited data available in the literature
regarding this technique.

33. 10. PENTACAM :
 It is3-dimensional (3D) rotating scheimpflug.
 It can perform five functions in 2 sec.
1. Scheimpflug image of anterior segment.
2. Pachymetry
3. 3-D anterior chamber analyzer
4. Corneal topography.
5. Cataract analyzer.

34. ADVANTAGES DISADVANTAGES
• Non invasiveness, non contact. • It underestimates the corneal
thickness in comparison to
ultrasonic pachymetry.
• Even minute eye movement are
captured and corrected
simultaneously.
• Gives precise representation and
repeatability.
APPLICATIONS:
1. Preoperative planning for corneal refractive surgery
2. Glaucoma screening
3. IOP modification with regard to corneal thickness
4. Keratoconus detection and quantification.

35. 11. PACHYCAM :
 Compact and portable non-contact pachymeter with built in keratometer. It
can be mounted on slit lamp. It automatically corrects IOP.
 Image acquisition is done with help of a 3-D alignment screen.
ADVATAGES :
1. Non contact.
2. Compact, portable and light weight.

36. 12. OCULAR RESPONSE
ANALYZER :
 Newer modality for measuring biomechanical properties of cornea.
 Utilizes a rapid air impulse and measures delays in inward and outward
applanation of cornea resulting in two different pressure values
 The difference in two pressure values gives corneal thickness.

37. CLINICAL UTILITY
 LASIK
 GLAUCOMA
 CORNEAL TRANSPLANT
 KERATOCONOUS

38. USE IN LASIK
Essential prior to a Lasik procedure for ensuring
sufficient corneal thickness to prevent abnormal
bulging of cornea ( ectasia). Differences in CCT
between pre op and intra op readings.
Differences in central corneal thickness between
pre op and intra op reading. Intra op readings are
always used for setting blasé length and depth of
incision.

39. USE IN GLAUCOMA
 CCT statistically significant predictor of development of glaucoma.
 In congenital glaucoma to assess amount of corneal edema.
 For applying correction factor in actual IOP determination.
 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 thick
cornea.
• Eyes with thick corneas have a true IOP that is lower than the
measured IOP.
• Thus individuals with thicker cornea may be mis- classified as having
ocular hypertension.

40. FACTS ABOUT CCT IN GLAUCOMA :
 Bechmann in the yr. 2000 found following association of CCT with
different forms of glaucoma.
 Increased CCT measurement are found in patients with ocular
hypertension, which can lead to falsely elevated IOP readings.
 Decreased CCT is found in patients with low tension glaucoma, resulting
in falsely reduced IOP measurements.
 CCT is found to be lower in patients with Pseudoexfoliation syndrome
(PXS) and in POAG.
 There is no difference in corneal thickness in individuals with Pigmentary
glaucoma and PACG.

41. CORRECTION FACTOR FOR IOP BASED CCT
MEASUREMENT:
 The adjustment factor is applied to the measured
IOP.
CCT
(micro
meters)
IOP
adjustment
(mmHg
CCT
(micro
meters)
IOP
adjustment
(mmHg)
CCT
(micro
meters)
IOP
adjustment
(mmHg)
445 +7 515 +2 585 -3
455 +6 522 +1 595 -4
465 +6 535 +1 605 -4
475 +5 545 0 615 -5
485 +4 555 -1 625 -6
495 +4 565 -1 635 -6
505 +3 575 -2 645 -7

42. CORNEAL TRANSPLANTS GRAFTS
Assessing candidates for penetrating
keratoplasty(corneal transplant)
Post operative follow up of keratoplasty patients to
determine endothelial cell function and its
recovery and to become alert to early graft
decompensation.

43. KERATOCONOUS
Keratoconous is associated with corneal thinning.
Available evidence indicates that ultrasonic
corneal pachymetry is not as accurate as
videokeratography in diagnosing keratoconus.
False +ve and false –ve rates are unacceptably
higher than those obtained by videokeratography.

44. CLINICAL UTILITY
ASSESSING CORNEA THINNESS:
as in corneal disorders like Terriens and Pellucid
marginal degenerations , keratoconus,keratoglobus ,
post LASIK ectasia.
Other cases of corneal decompensation as in herpetic
endothelitis.
CONTACT LENS CARE:
To access corneal edema.

45. TAKE HOME MESSAGE :
 The methodologies used in these techniques are based on either ultrasonic or
optical principles.
 While each of the methods have a peculiarity of their own, all have been described
as reliable.
 Keep in mind that systematic differences exist between the different techniques
and result in different values.
 As a result, the measurements cannot simply be substituted between the different
modalities.