corneal Pachymetry

pachymetry and its tecnique

1. KAVITA SHASTRI OPTOMETRIST(TMH) CORNEAL PACHYMETRY

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 ( H elmholtz 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.

12. 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.

13. 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 :

14. 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 p atient to lie supine Various layers of cornea can be identified. Cannot be used intraoperatively.

15. 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.

16. 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.

17. 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

18. 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.

19. 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.

20. 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.

21. 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.

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

23. 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.

24. 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.

25. 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.

26. 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.

27. 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 measurement s 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.

28. 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.

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

30. 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: Preoperative planning for corneal refractive surgery Glaucoma screening IOP modification with regard to corneal thickness Keratoconus detection and quantification.

31. 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 : Non contact. Compact, portable and light weight.

32. 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.

33. CLINICAL UTILITY LASIK GLAUCOMA CORNEAL TRANSPLANT KERATOCONOUS

34. 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.

35. 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.

36. 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.

37. 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 [phone] +6 522 [phone] +6 535 [phone] [phone] [phone] [phone] [phone]

38. 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.

39. 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.

40. 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.

41. 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.