Corneal Pachymetry

D E F I N I T I O NI N D I C A T I O N S

C O R N E A L A N A T O M YT 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 YE X A M P L E

C L I N I C A L U T I L I T Y

Content

Corneal Pachymetry

Non-invasive process of measuring the thickness of the cornea

Central Corneal Thickness (CTT)

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

Corneal Anatomy

Corneal Anatomy

Picture of cornea

Corneal Pachymetry

Normal ValuesNormal corneal thickness

500 to 575 microns.

Abnormal ResultsAbnormally thick or thin measurements may indicate

Corneal thinning

Corneal edema

Lower or higher than suspected intraocular pressure

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).

Ultrasound Pachymetry

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

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

Steps

Video

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

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

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.

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

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.

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.

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.

Keratoconus

Keratoconus is associated with corneal thinning

Available evidence indicates that ultrasonic corneal pachymetry is not as accurate as videokeratographyin diagnosing keratoconus.

The investigators concluded that pachymetry should not be relied on to exclude or diagnose keratoconusbecause the false-negative and false-positive rates are unacceptably higher than those obtained by videokeratography

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

Conclusion

Measure Central Corneal Thickness

Contact and non contact techniques

Ultrasound pachymetry prefered

Easy, reproducible

Keratoplasty, glaucoma

References

Dr Ben O’Keeffe

Ophthalmology SHO, Wellington, NZ