On the regional variability of averaged cell area estimates for the human corneal endothelium in relation to the extent of polymegethism
On the regional variability of averaged cell area estimates for the human corneal endothelium in relation to the extent of polymegethism
M. J. Doughty 0
Morphometry Non-contact 0
0 M. J. Doughty (&) Department of Vision Sciences, Glasgow-Caledonian University , Cowcaddens Rd, Glasgow G4 OBA , UK
Purpose To assess variability in the coefficient of variation (COV) in cell area estimates when using different numbers of cells for endothelial morphometry. Methods Using non-contact specular microscopy images of the corneal endothelium, 4 sets of 20 cases were selected that included 200 cells and had overall (global) COV values of less than 30 (group 1), 31-40 (group 2), 41-50 (group 3) and over 50% (group 4). Subjects could be normal, or had ophthalmic disease (such as diabetes), a history of rigid or soft contact lens wear or were assessed after cataract surgery. A stepwise analysis was undertaken, 20 cells at a time, of the variability in cell area estimates when using different numbers of cells for the calculations. Results Variability in the average cell area values was higher if only 20-60 cells were used in the calculations and then tended to decrease. The standard deviation values on these average cell area values and the calculated COV showed the same overall trends and were more than twice as large for endothelia with marked polymegethism. Using more than 100 cells/ image in markedly polymegethous endothelia only increased the variability in the calculations.
Corneal endothelium; Cell areas; Human; Polymegethism specular microscopy
Introduction
As viewed in vivo by specular microscopy, the corneal
endothelium of young healthy adults appears as a
mosaic of cells having uniform size and shape [
1, 2
].
The cell size, as reported in the most endothelial
assessments, is assessed as the endothelial cell density
(ECD), in cells/mm2, and provides a very useful
indicator of the status of the endothelial cell layer [3].
However, when even some of this uniformity is
reduced, then actual considerations of the variation in
cell size (area) has been considered important. A
specific term was introduced to describe the
nonuniformity (i.e. heterogeneity) to the endothelial
mosaic, namely polymegethism. This estimates the
increased variation in cell areas, reported as the
coefficient of variation or COV [
3
].
In early studies [
4
], it was noted that substantial
differences in cell area variation could exist and that any
estimates of ECD could be very much dependent on the
overall (global) COV assigned to an endothelial cell
layer (assessed by wide-field specular microscopy). For
the COV estimates themselves, a later retrospective
analysis of published endothelial images indicated that
the reliability of any COV calculations would be
predictably less if cell area heterogeneity appeared to
be present, i.e. whether images were subjectively
considered to be normal (homogeneous) or showing
some evidence of heterogeneity (polymegethism) [
5
].
The analysis was, however, limited by the fact that
relatively few images were available for analysis and
some included somewhat fewer cells than others, often
less than 100/image. As a result it was not possible to
systematically assess how much the reliability in COV
calculations might be reduced according to the extent
(or severity) of the perceived polymegethism.
The essential basis of determining the extent of
polymegethism is to measure the areas of the cells and
then to calculate the average value of the cell area and
the standard deviation (SD) of this average area value.
It is this SD value that is then used to estimate the area
variability as a standardised variance, generally
known as the coefficient of variation (abbreviated as
CV or COV) based on SD/average cell area
calculation. This relative variance can be presented as a
fraction (e.g. 0.5 for a moderately polymegethous
endothelium) or (more usually) as a percentage (e.g.
50%). In general terms, an increased COV could result
from the presence of even a few rather larger cells or
small cells, or a combination of both [
6
]. Stated
another way, from a theoretical perspective it could be
that parts of an endothelial image could be largely
normal with only small regions (or portions) of the
mosaic showing larger or smaller cells. The overall
estimates of COV have been reported to be dependent
on the number of cells measured and therefore
included in the calculations [
5, 7
].
Small field endothelial images taken from normal
corneas of young adults with modern day instruments
can be expected to include over 100 cells [
8–11
], while
in evaluation of corneas after surgical interventions it
has been noted that a good quality image should
contain at least 75 cells [6]. Assessments of published
images indicated that measuring this number of cells
(i.e. 75/image) should give reasonably reliable data in
terms of predicted variability in cell areas [
5
].
Notwithstanding, a systematic analysis of the region (...truncated)