Biomechanical Properties of the Cornea in Normal-Tension Glaucoma

1
Biomechanical Properties of the Cornea in
Normal-Tension Glaucoma

• Authors
• Leonidas Traipe
• Ines Cayuqueo
• Fabiola Cerfogli
• Claudia Goya
• Allister Gibbons

The authors have no financial interest
Universidad de Chile. Fundación Oftalmológica los
Andes.
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Purpose/Objectives

• To evaluate corneal Hysteresis (CH), Corneal
  resistance factor (CRF), Corneal Compensated IOP
  (ccIOP) and Goldman type IOP (gIOP), in
  patients with the diagnosis of normal-tension
  glaucoma.

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Materials/Methods

• We analyzed 49 eyes of 26 patients seen in our
  center. 22 (84.6) of patients were female. The
  total sample had on average 68 9.9 years.
• We identified the patients through the
  retrospective revision of clinical charts, and
  posteriorly examined them between March and July
  of 2008.
• The patients were examined with the Reichert
  Ocular Response Analyzer and Ultrasonic
  Pachimetry (CCT).

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• Inclusion Criteria
• Confirmed Diagnosis of Normal-Tension Glaucoma
  (NTG)
• IOP lt 21 mmHg with the Goldman aplanation
  Tonometer in all examinations without treatment
• Open Angle determined by gonioscopy
• Typicall glaucomatous optic disc damage,
  excavation size, notch, etc.
• Visual Field Defect compatible with the Optic
  Nerve Defect
• No previous intraocular surgery
• Exclusion Criteria
• IOP gt 21 mmHg with the Goldman aplanation
  Tonometer in any examination without treatment
• Corneal ectasia
• Contact lens user
• Pregnancy

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Results / CH
R 0.763 plt0.0001
R 0.554 plt0.0001
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Results / CRF
R 0.761 p lt0.0001
R 0.649 p lt0.0001
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R 0.407 p lt 0.004

• Practically no correlation (r 0.407).
• This means aproximately only 16.6 of ccIOP
  could be explained by the CRF (R2 0,166)

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Results / gIOP
R 0.902 p lt0.0001
R 0.434 p lt 0.002
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R 0.182 p lt 0.19

• ccIOP and CCT do not seem to be governed by a
  linear relantionship. Either
• There is no relationship
• The relationship is more complex

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• ccIOP was significantly higher than the gIOP.

The average difference was 2.01 mmHg
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Conclusions

• In our sample of NTG patients, Corneal
  Compensated IOP was higher than goldman simulated
  IOP - Approximately 2 mmHg.
• Corneal Resistance Factor
• Correlated positively with gIOP (R0,76)
• Correlated poorly with ccIOP (R0,41)
• There was a strong correlation between Corneal
  Hysteresis and Corneal Resistance Factor.

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Conclusions

• In our study population, our results differed
  from expected in that
• We found some correlation between Corneal
  Hysteresis and Corneal Resistance Factor with the
  Central Corneal Thickness.
• There seems to be none or poor correlation
  between gIOP or ccIOP with Central Corneal
  Thickness.
• This could be explained in part because
• We used a diseased population.
• We studied a population (chilean) for which we
  have no normal values calculated.