Lecture 5: Corneal Hysteresis Flashcards
What are the 3 main causes of glaucoma?
- Stress/strain and mechanical factors - lamina cribrosa
- Vascular factors - systemic hypotension, or vasospasm
- Immune factors - Innate and reactive immune system
low CH correlates with a __ cornea
soft
What is corneal hysteresis (CH)?
- ability of cornea to absorb and dissipate energy
- function viscoelastic damping
- functional measurement (NOT structural)
What is corneal resistant factor (CRF)?
- reflects overall resistant of cornea
What 2 devices measure corneal hysteresis?
- ocular response analyzer (ORA)
- Corvis ST
What does the ocular response analyzer (ORA) measures/calculates?
-
CH
- difference btw 2 measurements
-
Corneal compensated IOP (IOPcc) based on CH
- uses the information of CH to provide an IOP value less affected by corneal properties
-
Goldmann correlated IOP (IOPg)
- avg of the 2 measurements
-
Corneal resistance factor (CRF)
- cumulative effects of both viscous & elastic resistance encountered by the air pulse while deforming the cornea. Increases with higher IOPS
- FDA approved
How does Corvis ST work?
- captures corneal deformation in response to metered puff of air
What are the advantages & disadvantages for ORA & Corvis ST?
-
Pros
- Automated
- Takes into account biomechanical properties of the cornea
- May provide an IOP value that is less affected by corneal properties
-
Cons
- Portability
- Expense
- Decreased accuracy for IOPS >30mmHg (ORA understimate)
- More research/validation needed
What is the mean value for CH?
- 10.1 mmHg - 10.9 mmHg
- low (CH <8 mmHg)
- medium (CH 8-12 mmHg)
- high (CH >12 mmHg)
CH (decreases/increases) with age
decreases
lower CH in ____ vs emmetropes
high myopes
Decrease/increase in CH with high IOP
Decrease
____ corneal hysteresis is associated with greater corneal deformation of the optic nerve surface as measured with HRT. What does this suggest?
lower
- deformed cornea = more readily damaged ON
- likely related to lamina cribrosa characteristics
- has been show to improve with tx of IOP
African americans subjects tended to have ___ CH compared to white subjects
lower
CH was ___ in glaucomatous than normal eyes
lower
CH was __ in glaucomatous eyes tha in ocular HTN eyes
lower
a low CH value could be responsible for ____ of IOP. CH could also be a risk factor for ___ independent of IOP.
- under-estimation
- glaucoma
CH but not CCT or other anterior segment parameter is associated with increased ____________ during transient IOP elevations in glc pt but NOT normal in controls
- deformation of ON
Low CH has been consistently shown to be independently and strongly associated with or predictive of ____ ____
glaucoma progression
CH is more associated with __ defect than structural markers of glc as measured by SD-OCT
VF defects
What is the relationship between CH and CCT
- Avg CH increases w/ increasing CCT
- Corneal viscoelastic behavior influenced by thickness
- CH remains lower in POAG vs OH eyes across the range of CCT
- So.. CH value of 8.0 would suggest POAG in an eyw with thicker CCT, but the more consistent with OH in an eye with thinner CCT
What is the relationship between CH and IOP?
-
inverse relationship
- may be partially related to artifact/measurment error
- Variable in different patients
- increasing IOP is associated with stiffening of the ocular coats (sclera/cornea)
- reduction in the eye’s dampening capacity as IOP increases may play a role in explaining how IOP is related to the glaucomatous disease process
(thin/thick) CTT increases medication response
thin
(low/high) CH markedly increase medicine response
low
cornea hysteresis may be more strongly associated with….
- glaucoma presence, risk of progression, rate of progression, and effectiveness of glc tx than central corneal thickness
what are the challenges of measuring CH?
- decr accuracy >30mmHg (ORA underestimate)
- CH is NOT a reliable indicator for glc risk in pt w/ altered corneas (LASIK, keratoconus, edema)
- consensus guidelines needed for low/med/high values
