Pachymeter

1. Pachymetry
• Pachymetry (Greek words: Pachos = thick +
metry = to measure) is term used for the
measurement of corneal thickness.
• It is an important indicator of health status of
the cornea especially of corneal endothelial
pump function)
• corneal thickness in normal eyes ranges from
0.7 to 0.9 mm at the limbus and varies
between 0.49 mm and 0.56 mm at the centre

2. • The Central corneal thickness (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.

3. • Factors affecting central corneal thickness
• higher in younger patients, male patients and
diabetic patients.
• The mean CCT of black children is thinner than
that of white children
• Central corneal thickness does not correlate
with refraction or systemic hypertension

4. Role in clinical practice
• Glaucoma: for applying correction factor in actual IOP
determination
• Congenital Glaucoma: to assess the amount of corneal
edema.
• Refractive surgeries: preoperative screening and
treatment plan of keratorefractive procedures like
LASIK, astigmatic keratotomy,even prior to radial
keratotomy.
• Post operative follow up of keratoplasty :patients to
determine endothelial cell function and its recovery
and to become alert to early graft decompensation.

5. • Contact lens: To assess corneal edema
• Assessing cornea thinness as in corneal
disorders like Terrien ’s and Pellucid marginal
degenerations, keratoconus, keratoglobus,
post LASIK ectasia.
• Other cases of corneal decompensation as in
herpetic endothelitis

6. correction factor:
• It is recommended that in chronic eye
diseases like glaucoma and glaucoma suspects
for every increase in central corneal thickness
of 50 microns, the correction done is to
decrease the recorded IOP by 2.5mm Hg
• For acute onset diseases it was recommended
to correct by 10 mm Hg for every 50 microns.

7. • Pachymetry in refractive surgery
• preoperative assessment of candidates LASIK.
• It is recommended that before undertaking residual
stromal thickness of 250-300 μm must be ensured.
• Patient with adequate thickness to their corneas may
be candidates for LASIK while those with thinner
corneas may be safer considering PRK / LASEK as their
treatment option
• It is important to evaluate the outcome of laser
refractive surgical procedures, especially in candidates
for enhancement surgery.

8. • Pachymetry in contact lens use
• Moreover assessment of corneal thickness before
contact lens prescription is another important
thing to analyze
• It is recommended that the time period for the
pachymetric readings to stabilize is around 15
days.
• So, one must discontinue contact lens atleast 15
days before surgery to get the accurate
pachymetric readings.

9. Measurements Ultrasonic techniques
• a. Conventional ultrasonic pachymetry
• b. Ultrasound Biomicroscopy (UBM)
• OpticalTechniques
• a. Manual Optical Pachymetry
• b. Specular Microscopy
• c. Scanning SlitTechnology
• d. Optical Coherence Tomography(OCT)
• e. Optical Low Coherence Interferometry
• f. Confocal Microscopy
• g. Laser Doppler interferometry
• Alternative Measurements a. Pentacam b. Pachycam c.
Ocular response analyzer (ORA)

10. ULTRASONIC PACHYMETRY
• most commonly used method, regarded as the
gold standard
• Principle: - Instruments functions by measuring
the amount of time (transit time) needed for
ultrasound pulse pass from the one end
ofTransducer to descemet's membrane and back
to the transducer
• Corneal thickness = (Transit 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 2 mm

12. ADVANTAGES
• Faster,simpler and easy to use
• consistent and repeatable between observers thereby eliminating
interobserver variation
• Portable
• Dry (no coupling medium required)
• Can be used intraoperatively
DISADVANTAGES
• Contact method
• Accuracy is dependent on the perpendicularity of the probe’s application
to the cornea
• Reproducibility relies on precise probe placement on the center of the
cornea.
• Low resolution
• Not accurate in edematous corneas

13. • ULTRASOUND BIOMICROSCOPY (UBM) ▪
• high resolution ultrasound machine which images AS
of eye.
• Useful in cases where cornea is opaque.
• Various layers of cornea can be identified.
Disadvantages
• 1.immersing the eye in acoupling fluid.
• 2.Contact method.
• 3. patient to lie supin2
• 4.cannot be used intraoperatively.
• 5.Difficult to standardize

14. • 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.
• There are 2 types of specular microscope
1.Contact 2.non contact

15. • Advantages
• 1.Operator independent
• 2.Non invasive
• 3.Simultaneous measurement of cell count
• Disadvantages
• 1.The exact point where the reading is taken is not known.
• 2.Time consuming.
• 3.Less reproducible
• 4.Impractical for use in operation room
• 5.Clinical use is limited to corneas free of edema, scarring,
deposits or opacities that may distort light transmission.

16. • SLIT-SCANNING PACHYMETRY ▪ elevation based
system which uses scanning slit technology ▪
accuracy in measuring corneal thickness when
clinically significant haze is present.
• ANTERIOR SEGMENT OPTICAL COHERENCE
TOMOGRAPHY ▪ high- resolution, non- contact
OCT customized for the anterior segment ▪ gives
color coded map of the corneal thickness

17. • OPTICAL LOW COHERENCE REFLECTOMETRY
• The instrument is attached to a slit lamp
• measure corneal thickness to a precision of one micron.
• PRINCIPLE:Based on Michelson interferometer. It uses diode laser
beam.
• Advantages
• Precise 1 micron measurement ▪ Automatic alignment ▪ Non-
contact ▪ Real-time data acquisition and display ▪ Convenient and
easy ▪ Variability of measurements is significantly lower ▪
Intraoperative measurements possible Disadvantages ▪Measures
only central corneal thickness
• CONFOCAL MICROSCOPY
• This unique method offers the ability to examine objects at high
magnification ranging from 20X to 500X.

18. • Advantages
• 1.moderate to good repeatability, particularly for
measurements of thin layers such as epithelial or
Bowman’s layer thickness.
• 2.As well as corneal thickness,epithelial
thickness,Bowman’s layer thickness, and
following LASIK surgery, flap thickness can also be
obtained.
• 3. The z-scan curve can be used to assess the
level and location of corneal haze associated with
the various corneal dystrophies.

19. • Disadvantages 1. Poor agreement between CMTF and
ultrasound pachymetry, the latter apparently ov
erestimating corneal thickness. 2. The precision of
measurements with this technique will vary with
contact lens hydration, post-lens tear film thickness
and observation angle. 3. Slower data acquisition 4.
Poor penetration of corneal opacity
• Pentacam
• It is 3-Dimensional (3D) rotating scheimpflug camera.
• It can perform five functions in 2 sec..

20. PACHYCAM ▪ compact and portable non- contact pachymeter
with built-in keratometer.
It can be mounted on slit lamp. It automatically corrects the
IOP
Image acquisition is done with the help of a 3D alignment
screen.
OCULAR RESPONSE ANALYZER ▪ Newer modality for
measuring bio mechanical properties of cornea
Utilises a rapid air impulse and maeasures delays in inward
and outward applanation of cornea resulting in two
different pressure values.
The difference in two pressure values gives corneal thickness