Optic Disc Area, Optic Rim Area, and Axial Length in evaluation of Glaucoma' P157, ASCRS 2006

1
Optic Disc Area, Optic Rim Area, and Axial Length
in evaluation of Glaucoma. P157, ASCRS 2006

• Mitsugu Shimmyo, MD,
• Paul Orloff, MD and
• Naomi Hayashi, MD
• New York, New York, USA

2
Introduction 1

• Large optic nerve cup to disc ratio, as well as
  elevated intraocular pressure and advanced age
  (Le), is regarded to be a risk factor for
  glaucoma.
• Cup to disc ratio of 0.7 in an average sized eye
  represent cup area of 49 and rim area of 51 of
  the total disc area.
• It has been shown (Quigley) that visual field
  defect which is the hallmark of glaucomatous
  functional change appears when more than 50 of
  retinal ganglion cells or nerve fibers are lost.

3
Introduction 2

• Cup disc ratio of 0.7 in an average eye may
  correspond to the loss of 50 of axons in the
  optic nerve head.
• However the size of optic nerve disc varies
  widely probably based on the size of the optic
  nerve canal and possibly the globe size.
• It is conceivable that myopic eyes with larger
  globes have greater optic disc diameter and
  therefore greater disc area.
• With the same cup to disc ratio, an eye with
  larger disc may contain more rim area than a
  smaller eye.

4
Introduction 3

• With the same cup to disc ratio, an eye with
  larger disc may contain more rim area than a
  smaller eye.
• If rim area represents size of neuro-fibers,
  diagnostic significance of cup to disc ratio need
  to be modified taking disc size into account.
• Although it is conceivable that larger eyes have
  larger optic disc size, reports to that effect
  have been deficient.

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Purpose

• To find correlation between globe size and optic
  disc area, we reviewed charts of a general
  ophthalmology clinic.

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Materials and Method

• Charts of consecutive patients who had both axial
  length measurements by IOL Master and optic nerve
  imaging by Heidelberg Retinal Tomography II were
  reviewed.
• Randomly selected one eye of each patient was
  reviewed and a total of 1283 eyes were studied as
  summarized in Table 1 Demography of eyes studied.
• Correlations between ocular dimensions and HRT II
  measurement parameters were statistically
  analyzed in total eyes and in subgroups by
  gender, ethnic groups and laterality. Correlation
  coefficient R of each pair of comparison was
  obtained.

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Table 1 Demography of eyes
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Table 2 Refractive and dimensional parameters
average standard deviation, range
9
Result 1

• Parameters of refraction and ocular dimensions of
  each eye including axial length, anterior chamber
  depth and central corneal thickness are
  summarized in Table 2.
• HRT II data including optic disc area, cup area,
  rim area, linear cup to disc ratio, cup to disc
  area ratio, cup volume, rim volume, nerve fiber
  layer thickness are summarized in Table 3.

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Table 3 Heidelberg Retinal Tomography II data
average standard deviation range. (N 1283)
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Result 2

• In the eyes we studied here, the average disc
  area measured by Heidelberg Retinal Tomography II
  was 2.345 0.546mm2, which corresponds to the
  optic disc diameter of 1.7279 mm.
• The maximum area of 4.506 mm2 corresponds to a
  diameter of 2.395 mm and the minimum disc area of
  1.233mm2 corresponds to a diameter of 1.253mm.
• As a benchmark, we compared our data with the
  published data of Burkes study of eyes with
  normal visual field as in Table 4. There is no
  significant difference between Burkes data and
  ours.

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Table 4. Comparison of means of Burks study and
our study

• .

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Result 3

• Correlations between ocular dimensions and HRT II
  parameters were statistically analyzed in total
  eyes, and further analyzed in subgroups by gender
  and ethnic groups.
• Correlation coefficient R of each pair of
  comparison is tabulated in a matrix in Table 6.
• Parameters in Table 6 were designated in Table 5
  as below

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Table 5. Designation of parameters for Table 6.
15
Discussion 1

• In the eyes we studied here, the average disc
  area measured by Heidelberg Retinal Tomography II
  was 2.345 0.546mm2, which corresponds to the
  optic disc diameter of 1.7279 mm.
• The maximum area of 4.506 mm2 corresponds to a
  diameter of 2.395 mm and the minimum disc area of
  1.233mm2 corresponds to a diameter of 1.253mm.
• As a benchmark, we compared our data with the
  published data of Burkes study of eyes with
  normal visual field as in Table 4. There is no
  significant difference between Burkes data and
  ours. Both of these data are comparable.

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Table 6. Coefficients of correlation R in
comparisons of HRT and Ocular dimension
parameters R values are shown in absolute
numbers, p values are not shown to save space.

• .

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Discussion 2

• As for the relationship between and among
  parameters, many parameters are derivatives of
  other parameters.
• For example, disc radius (r), disc diameter (2r)
  and disc area (pr2) are derivatives of one
  another and have identical value in statistical
  correlation to other parameters.
• Some parameters are compounds of other
  parameters. Rim area is disc area minus cup area,
  which is p times difference of square of disc
  radius and square of cup radius.
• Those related and derived parameters were found
  to have similar to identical coefficient of
  correlation. Removing redundancy, simplified
  matrix of parameters are shown in Table 8.

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Table 8. Highlight of matrix of correlation
coefficients R in comparisons of HRT II and
Ocular dimension parameters, R shown in absolute
values

• .

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Discussion 3

• As our primary interest was seeking correlation
  with axial length (AL) and the HRT II parameters,
  the coefficient of correlation R between AL and
  HRT II parameters were arranged in descending
  order in Table 7.
• The higher correlation between AL and anterior
  chamber depth (R 0.22) is understandable as
  larger eyes have deeper anterior chambers.
• Then rim area correlates moderately with AL (R
  0.15), followed by reference height (R 0.12),
  rim volume (R 0.11).
• We anticipated higher correlation between AL and
  disc are, but the correlation was relatively
  small (R 0.05).

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Table 7. Coefficient of correlation R of Axial
length to other parameters in descending order
21
Discussion 4

• Although highly myopic eyes have pathological
  features with thinner retina, the anatomical size
  of the optic nerve disc was found to have
  relatively low correlation with AL.
• This signifies that axial length may not be a
  significant indicator in evaluation of glaucoma.
• Nevertheless, the measurement of the total disc
  area, rim area as well as mean NFL thickness is
  important in evaluation of optic nerves in
  diagnosis and progression of glaucomatous optic
  neuropathy.

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Table 9. Comparisons of Cup to disc ratio (CDR),
Cup area ratio (CAR), Rim area ratio (RAR), and
Rim area (RA in mm2)
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Discussion 5

• The theoretical relationship between, linear cup
  to disc ratio (CDR) to cup area ratio (CAR), rim
  area ratio (RAR) and rim area (RA) is shown in
  Table 9.
• It is to be noted that a large disc with 2.4mm in
  diameter with
• CDR 0.9 may have RA of 0.86 mm2 and
• CDR 0.8 and RA 0.84 mm2 in 1.73 mm disc
• CDR 0.6 and RA 0.85 mm2 in 1.3 mm disc
• CDR 0.5 and RA 0.85 mm2 in 1.2 mm disc
• CDR 0.3 and RA 0.86 mm2 in 1.1 mm disc

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Discussion 6

• In larger eyes, there will be enough rim area in
  larger eyes with larger cup disc ratio, but in
  smaller eyes, as the maximum area allow fibers is
  smaller, even eyes with smaller cup disc ratio
  may have less live nerve fibers present.
• For an example a large eye with a disc are of
  4.52mm2 with a diameter of 2.4 mm , Cup to disc
  ratio of 0.9 may be the border line of normal and
  abnormal..

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Discussion 7

• Conversely, a small eye with a disc diameter if
  1.00 mm may have only half the optic nerve fibers
  of an average eye without the existence cupping.
• The concept of cup disc ratio is relative to the
  disc size, and risk assessment of cup to disc
  ratio should be adjusted to the disc size and
  area size.
• As the area is square of its radius, the
  relationship is not linear.

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Discussion 8

• This relationship suggests that one may be
  careful in evaluating the optic disc as smaller
  CDR in a smaller disc may escape detection of
  optic nerve fiber losses in casual
  ophthalmoscopy.
• This discussion is theoretical and speculative.
• This concept should be tested clinically in
  evaluation of individual cases in assessing optic
  nerve status.

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Conclusion

• Axial length correlates mildly with rim area,
  reference height and rim volume.
• Axial length correlates weakly with cup area, cup
  disc area ratio, and linear cup disc ratio and
  disc area.
• Theoretically larger eyes with larger cupping may
  have good size rim area.
• Smaller eyes with smaller cup may have relatively
  smaller rim area and smaller number of
  neurofibers.
• In evaluation of cup to disc ratio of the optic
  nerves, one need to take disc size into
  consideration.

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References 1

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References 2

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References 3

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Authors

• Mitsugu Shimmyo, MD private practice at 345 East
  37th Street, New York , NY 10016-3256 Tel
  212-867-5700 email MShimmyo_at_aol.com Assistant
  Professor, Clinical Ophthalmology, New York
  Medical College (Corresponding author)
• Paul Orloff, MD private practice in New York
  City, Attending Surgeon, Manhattan Eye, Ear and
  Throat Hospital, New York, NY
• Naomi Hayashi, MD Chief Resident, Department of
  Ophthalmology, Cornel University School of
  Medicine, New York-Presbyterian Hospital, New
  York, NY