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Glaucoma > Lecture 5: Corneal Hysteresis > Flashcards

Lecture 5: Corneal Hysteresis Flashcards

1
Q

What are the 3 main causes of glaucoma?

A
  • Stress/strain and mechanical factors - lamina cribrosa
  • Vascular factors - systemic hypotension, or vasospasm
  • Immune factors - Innate and reactive immune system
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2
Q

low CH correlates with a __ cornea

A

soft

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3
Q

What is corneal hysteresis (CH)?

A
  • ability of cornea to absorb and dissipate energy
  • function viscoelastic damping
  • functional measurement (NOT structural)
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4
Q

What is corneal resistant factor (CRF)?

A
  • reflects overall resistant of cornea
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5
Q

What 2 devices measure corneal hysteresis?

A
  • ocular response analyzer (ORA)
  • Corvis ST
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6
Q

What does the ocular response analyzer (ORA) measures/calculates?

A
  • 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
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7
Q

How does Corvis ST work?

A
  • captures corneal deformation in response to metered puff of air
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8
Q

What are the advantages & disadvantages for ORA & Corvis ST?

A
  • 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
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9
Q

What is the mean value for CH?

A
  • 10.1 mmHg - 10.9 mmHg
    • low (CH <8 mmHg)
    • medium (CH 8-12 mmHg)
    • high (CH >12 mmHg)
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10
Q

CH (decreases/increases) with age

A

decreases

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11
Q

lower CH in ____ vs emmetropes

A

high myopes

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12
Q

Decrease/increase in CH with high IOP

A

Decrease

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13
Q

____ corneal hysteresis is associated with greater corneal deformation of the optic nerve surface as measured with HRT. What does this suggest?

A

lower

  • deformed cornea = more readily damaged ON
  • likely related to lamina cribrosa characteristics
  • has been show to improve with tx of IOP
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14
Q

African americans subjects tended to have ___ CH compared to white subjects

A

lower

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15
Q

CH was ___ in glaucomatous than normal eyes

A

lower

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16
Q

CH was __ in glaucomatous eyes tha in ocular HTN eyes

A

lower

17
Q

a low CH value could be responsible for ____ of IOP. CH could also be a risk factor for ___ independent of IOP.

A
  • under-estimation
  • glaucoma
18
Q

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

A
  • deformation of ON
19
Q

Low CH has been consistently shown to be independently and strongly associated with or predictive of ____ ____

A

glaucoma progression

20
Q

CH is more associated with __ defect than structural markers of glc as measured by SD-OCT

A

VF defects

21
Q

What is the relationship between CH and CCT

A
  • 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
22
Q

What is the relationship between CH and IOP?

A
  • 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
23
Q

(thin/thick) CTT increases medication response

A

thin

24
Q

(low/high) CH markedly increase medicine response

A

low

25
Q

cornea hysteresis may be more strongly associated with….

A
  • glaucoma presence, risk of progression, rate of progression, and effectiveness of glc tx than central corneal thickness
26
Q

what are the challenges of measuring CH?

A
  • 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

Glaucoma (11 decks)

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