Central Corneal Thickness and Glaucoma

1
Central Corneal Thickness and Glaucoma
Prof. Shlomo Melamed The Sam Rothberg Glaucoma
Center Shiba M. C.

• How REALLY important is it for Glaucoma
• An Attempt to make some Seder in this issue,
  based on Evidence Based Medicine

2
What do we practically know about CCT ?

• The Thicker the Cornea, the Higher IOP
  measurement by Goldman Tonometry, and vice versa
• Thin Cornea is an independent risk factor for
  conversion of OHT to POAG, among others..(OHTS)

3
And what about other claims

• CCT correlates with Lamina Cribrosa Structure
  and means much more than just altered IOP
  measurements
• Thin cornea is associated with Glaucoma
  Progression
• How important is CCT in other types of
  Glaucoma
• What is the role of other corneal parameters
  like shape, structure, elasticity, harmony ,
  Hysteresis..

4
CCT is a new Modewith a Herd-like
Echo..Everybody talks about CCT

• However, we should all stick ONLY with the
  important conclusions of Evidence Base Medicine
  !!
• Now, Lets see what we have

5
Claim 1 CCT is an independent risk factor for
Conversion of OHT to POAG

• Proof Strong!
• OHTS has shown that the thinner corneas are
  associated with more conversion
• Blacks have higher risk and they have thinner
  corneas

6
However, CCT is not the only important risk
factor

• OHTS has its limitations, especially regarding
  its design to find other risk factors such as
  Family Hx, Myopia and Diabetes
• There is no data regarding the RELATIVE
  importance of CCT among the other factors

7
The European Glaucoma Prevention Study European
OHTS Miglior et al.

• RR for CCT was only 1.3 vs. 1.7 in OHTS!!
• Apparently black population in USA (25)
  contributes to role of CCT

8
Also, In OHTS, Old Age is a risk factor

• But, the good practice is to be more aggressive
  with the YOUNG patient, with longer life span

9
A Clinical ExampleA 55 yo with IOP26 mm Hg,
his father and brother have glaucoma, he is 5.0
myopic but his CCT is 620 microns.

• Would you treat??..
• I certainly would, despite the normal CCT and no
  case per OHTS

10
Claim 2Thick Corneas will give Higher IOP
measurements

• Proof Generally Accepted
• Several studies indicate direct correlation
  between CCT and IOP measurements

11
However, in the only study correlating IOP, CCT
and Direct Intracameral IOP readings No such
correlation was found

• Feltgen, Leifert and Funk , BJO, 2000
• 73 patients studied in OR
• No systematic error of Applanation Tonometry
  with increasing CCT

12
So, are there other physical parameters of the
cornea which may affect IOP measurement?

• The obvious, based on clinical practice
• Irregular Astigmatism (Corn. Graft)
• Lens behind Cornea in Flat AC
• Very Steep and Bulging Cornea
• Irregular Epithelium (H. Simplex)

13
Corneal Biomechanical Properties are more
important to IOP measurement than just Thickness

• Corneal Resistance
• Corneal Structure
• Corneal Elasticity
• Corneal Harmony (String-Like Response)
• Corneal Hysteresis

14
What is Corneal Hysteresis?

• If Cornea is pushed by air impulse, an advanced
  electro-optical system can record 2 applanation
  pressure measurements one while the cornea is
  moving inward and the other as the cornea
  returns.
• The difference between these 2 measurements is
  Corneal Hysteresis (CH)
• Corneal Resistance Factor (CRF)- Overall
  Resistance of the Cornea

15
Corneal Hysteresis
16
Comparison of Corneal Hysteresis distribution
of normal, keratoconic, and Fuchs subjects
17
Corneal Hysteresis of 15 eyes pre- and post-LASIK
18
So, Claim 3 will beAre factors other than CCT
like Corneal Biomechanics and Hysteresis
important in IOP measurement?

• Absolutely Yes!!.
• The new Dynamic Contour Tonography provides
  accurate IOP measurements les influenced by
  corneal properties

19
Claim 4CCT is directly related to Lamina
Cribrosa and susceptability to Glaucoma

• Very Intuitive, but absolutely no proof!!.
• In fact, a study by JonasHolbach (IOVS , 2005)
  disproves this concept

20
The JonasHolbach Study

• Histomorphometric study of 111 enucleated eyes
• CCT and Central Lamina Cribrosa Thickness were
  statistically independent of each other
• Lamina Cribrosa Thickness at the Optic Nerve
  border and CCT were also independent of each
  other
• Conclusion No Anatomic Correspondence between
  CCT and ONH

21
Cornea may reflect on Lamina Cribrosa

• It is not necessarily the Corneal Thickness
  which is important
• Corneal Hysteresis may correlate better with
  Glaucoma susceptability

22
Claim 5CCT is important in Glaucoma Progression

• We dont know!!
• 2 conflicting studies reach opposite conclusions
  
• Herndon,Weizer Stinnett, Arch. Ophthalmol.,
  2004
• Jonas et al. , IOVS, 2005

23
Herndon et al. Study

• Objective Is CCT related to level of glaucoma
  severity at the Initial Examination
• Retrospective analysis of 350 eyes
• CCT lower in Blacks VS. Whites
• Lower CCT was associated with worsened AGIS
  score, worsened MD of VF, increased Horizontal
  C/D

24
Jonas et al. Study

• Objective At presentation of patient, is ONH
  damage and rate of VF progression related to CCT
  ?
• A Prospective study of 861 eyes (Normal, OHT
  and POAG), F/U of 5 years
• CCT correlated positively with area of
  neuroretinal rim, but negatively with VF
  loss.Progression of VF defects in 119 eyes (21)
  was independent of CCT in Univariate and
  Multivariate analysis

25
Claim 6Is CCT lower in other types of Glaucoma?

• PXFG Conflicting Reports. Some show
  association with thin cornea and some do not
• NTG More studies show correlation with thin
  cornea, some do not.
• Corneal Hysteresis is apparently more important..

26
Corneal Hysteresis in NTG

• IOPcc Compensated IOP for Corneal Resistance
  Factor
• IOPg Goldman IOP

27
So, in SummaryIt is not that simplistic

• CCT is an independent risk facor for OHT
  conversion, but not the only/most important one
• You should not disregard other factors Age, ON
  asymetry, Myopia, Family Hx of Glaucoma , PXF etc.

28
Overall Biomechanical Properties of the Cornea,
manifested by Corneal Hysteresis , are more
important

• CCT is only one factor in Corneal Hysteresis
• Cornea can be Thin, but Rigid, and vice versa
• Thick Cornea does not necessarily mean High
  Corneal Hysteresis

29
Although intuitively it is tempting to correlate
CCT with Lamina Cribrosa susceptability..

• Anatomical Studies Disprove this correlation
• Only Conflicting, partial reports on
  significance in Glaucoma Progression

30
So, we should definitely add Pachymetry and
Dynamic Contour Tonography to our Armamentarium

• But
• Use CCT in the appropriate clinical setting
• Know its limitations
• Apply Common Sense..