Pachymetry in glaucoma evaluation presentation

Presentation

1. PACHYMETRY IN GLAUCOMA EVALUATION

2. INTRODUCTION 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 The thickness of the cornea was first reported in ancient textbooks on physiological optics (Helmholtz and Gullstrand )

3. TYPES OF PACHYMETRY Contact methods Ultrasound(mean CCT 544 μ m ) optical such as confocal microscopy (CONFOSCAN )( mean CCT 530 μ m )

4. TYPES OF PACHYMETRY 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)

5. CORNEAL THICKNESS IN NORMAL EYE It ranges from 0.7 to 0.9 mm at the limbus and varies between 0.49 mm and 0.56 mm at the centre 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

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

7. FACTORS AFFECTING CORNEAL THICKNESS Higher in younger patients, male patients and diabetic patients The mean CCT of black children is thinner than that of white children The PITX2/Pitx2 mutation seen in Axenfeld-Rieger malformations results in reduced corneal thickness Central corneal thickness does not correlate with refraction or systemic hypertension

8. 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

9. ROLE IN CLINICAL PRACTICE 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

10. FACTS ABOUT CCT IN GLAUCOMA Bechmann in 2000 found following association of CCT with different forms of glaucoma 1)Increased CCT measurements are found in patients with ocular hypertension, which can lead to falsely elevated IOP readings 2) Decreased CCT is found in patients with low tension glaucoma, resulting in falsely reduced IOP measurements

11. FACTS ABOUT CCT IN GLAUCOMA 3) CCT was found to be lower in patients with Pseudoexfoliation syndrome (PXS) and in Primary open angle Glaucoma (POAG) 4) There is no difference in corneal thickness in individuals with Pigmentary Glaucoma(PG) and Primary angle Closure Glaucoma (PACG)

12. EVIDENCE BASED GUIDELINE FOR CORNEAL PACHYMETRY IN GLAUCOMA Patients who have risk factors for developing Primary Open Angle Glaucoma Elevated intraocular pressure repeatedly measured >24 mm Hg African descent Advancing age (>65 years old) Family history of glaucoma

13. ROLE IN GLAUCOMA Applanation tonometry is based on Imbert Fick’s law, which assumes that cornea is a perfect flexible, dry, sphere which is infinitely thin Therefore increase in the tissue in thicker cornea makes it less compliant and subsequently leading to overestimation of IOP and Viceversa Ocular Hypertension Treatment Study (OHTS) group published that central corneal thickness (CCT) was an important independent risk factor for progression from ocular hypertension to early glaucoma

14. OCULAR HYPERTENSION TREATMENT STUDY The Ocular Hypertension Treatment Study ( OHTS) highlighted the prognostic significance of CCT in identifying patients with ocular hypertension (OHT ) who would progress to glaucoma A multivariate model which included the baseline characteristics of 1618 OHT individuals showed that thinner CCT was the most important predictive factor for the development of POAG

15. OCULAR HYPERTENSION TREATMENT STUDY The routine measurement of CCT should be included in the initial assessment of all glaucoma patients The prognostic value of CCT is well recognized in OHT patients , but its predictive value in other glaucoma suspects and patients with established glaucoma is less certain

16. Tonometry artefact IOP is the principal modifiable risk factor for the progression of established glaucoma and for the development of glaucoma from OHT A 1mmHg decrease in IOP was associated with a 10% decreased risk of both perimetric progression in the EMGTS and the development of glaucoma in the OHTS It was found that found that the most accurate GAT reading was obtained in eyes with CCT of approximately 520 μ m

17. Tonometry artefact Every 100 μ m deviation from this value would result in an error in IOP measurement of 7 mmHg A positive correlation between CCT and IOP has been verified T hick cornea would result in an artefactually increased IOP reading, whereas a thin cornea would result in an artefactually reduced IOP reading

18. CORRECTION FACTOR I n 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 2.5mmHg For acute onset diseases it was recommended to correct by 10 mmHg for every 50 microns

19. BIOLOGICAL RISK FACTORS E xtent of lamina movement was increased in individuals with thin corneas compared with those with thick corneas(Lesk et al) After IOP reduction, the improvement in neuroretinal rim blood flow was also smaller compared with patients with thicker corneas This supports the hypothesis that eyes with thinner CCT may have an increased risk of developing glaucomatous damage due to laminas that are more susceptible to being displaced in response to IOP changes

20. BIOLOGICAL RISK FACTOR Physical structures of the optic disc, including depth of cup and disc area, have been reported by some studies to be correlated with CCT Another less likely hypothesis is that a thin cornea increases trabecular exposure to oxidative damage

21. GENETIC ASSOCIATION CCT is one of the most heritable human traits and may explain some aspects of the genetic predisposition to glaucoma Genome-wide association studies and candidate gene analyses have uncovered associations between CCT and novel loci. These may provide candidate genes for the interrogation of POAG

22. GENETIC ASSOCIATION Studies that confirmed a genetic link between CCT and open angle glaucoma may enable us to determine the mechanisms underlying this association Two missense mutations in a subtype of collagen-8 , which if inactivated results in thinning of the corneal stroma , were detected in a group of White patients with very thin CCT and advanced POAG

23. GENETIC ASSOCIATION Ulmer et al. found that single nucleotide polymorphisms in two cell adhesions molecules , identified from a CCT genome-wide association study, may increase the susceptibility to POAG

24. GLAUCOMA TREATMENT Ocular hypotensive medications have been shown to alter corneal thickness latanoprost and tafluprost and a significant decrease in CCT possibly by modifying the corneal extracellular matrix

25. GLAUCOMA TREATMENT Topical b blockers have been associated with a reversible increase in CCT T opical carbonic anhydrase inhibitors can cause irreversible corneal decompensation and increased corneal thickness in eyes with underlying cornea diseases but they do not significantly alter CCT in healthy corneas Thicker CCT was associated with a less significant decrease in IOP after starting ocular hypotensive medications in the OHTS

26. GLAUCOMA TREATMENT T he percentage reduction in IOP after laser trabeculoplasty was significantly greater in eyes with thinner corneas (CCT<555 μ m )

27. VARIABILITY IN CENTRAL CORNEAL THICKNESS CCT measurements vary with race, sex, age and other environmental factors, which may confound the relationship between glaucoma and corneal thickness Most epidemiological studies have found that men have thicker corneas compared with women CCT also varies with age, with a small but significant inverse relationship between CCT and age (range 2–10mm per decade) being reported in the majority of cross-sectional studies

28. VARIABILITY IN CENTRAL CORNEAL THICKNESS Hence, it may take up to 20 years for the change in CCT to be clinically significant and to warrant remeasuring CCT is also influenced by environmental factors . Dry eyes and long-term contact lens wear are associated with a decrease in CCT A n indoor occupation and sleep are associated with an increase in CCT

29. CORNEAL BIOMECHANICS CCT is regarded as a correlate of corneal rigidity, but this is only applicable to structurally normal corneas An edematous cornea , though thicker, may result in lower GAT readings because of reduced corneal rigidity . Conversely,scarred cornea, though thinner, may have increased corneal rigidity leading to artefactually higher IOP readings

30. CORNEAL BIOMECHANICS Corneal cross-linking results in a significant increase in IOP 12 months after the procedure despite CCT remaining unchanged. This is presumably because of increased corneal rigidity

31. CONCLUSION The prognostic value of CCT is well recognized in OHT patients, but its predictive value in other glaucoma suspects and patients with established glaucoma is less certain Nevertheless , the routine measurement of CCT should be included in the initial assessment of all glaucoma patients, as it aids in the interpretation of IOP measurements, risk stratification and the setting of a target IOP

32. CONCLUSION An adequately validated correction algorithm for GAT measurements does not exist and better methods of tonometry which are less influenced by corneal properties, should be developed and adopted into clinical practice Most studies investigating the significance of CCT have included patients with POAG, and the role of CCT in PACG and secondary glaucoma is still poorly understood and warrants further investigation