Healthcare and Medicine Reference
In-Depth Information
Efron published a very comprehensive paper on CL induced endothelial
blebs (Efron 1996c). Williams found a correlation to hypoxia, as soft CL
(at that time only low Dk) showed more blebs than RGP and thicker CL
of the same material showed more than thinner ones (lower Dk/t)
(Williams 1986a).
Holden and Williams identifi ed an adaptation process, with the numbers
of blebs decreasing after the initial 8 d of EW (Williams and Williams 1986b).
Inagaki et al. looked at three different (by Dk/t) RGP and soft CLs after 20
min of closed eye (Inagaki et al. 2003). The control (non CL wearing eye)
showed 0.4% area of blebs. Hyper Dk/t rigid and soft CLs showed only
1.7% and 1.2% area, respectively. The medium and low Dk/t CLs showed
up to 5 times more. The response disappeared in Hpyer Dk/t CLs after only
5 min, whereas it took up to 30 min in the low Dk/t ones.
In summary, when fi tting high Dk/t CLs, the inevitable (innocuous)
bleb response can be reduced to very low levels, but not eliminated.
Polymorphism and -megethism are also endothelial conditions. Contrary
to blebs, they do not appear after short term wear, but are rather a sign of
long term oxygen deprivation.
In 1996, Efron published a review of CL induced polymegethism
(Efron 1996d). Polymegethism describes a deviation from the normal
approximately same size and polymorphism a deviation from their regular
hexagonal shape. The two are often observed together. Cell size is usually
expressed as the endothelial cell density (ECD), the number of cells per mm 2 .
The coeffi cient of variation (COV) refers to the variation in apparent cell
size. It is calculated by “dividing the standard deviation of the cell areas by
the arithmetic mean area of all cells in that fi eld”. Due to the body's aging
process, ECD decreases from about “4000 at birth to 2200 at age 80 years”.
COV increases in that period. This implies that observations need to take
the subject's age into consideration. Changes are a mean degree of change
in excess of that expected for a given age. Corneal polymegethism may be
a sign for “corneal exhaustion” and a potential upcoming intolerance to
CLs that cannot be explained by other factors.
To assess the impact and capabilities of the modern materials, Doughty
et al. refi tted Hy wearers (average 5.5 yr of wear) with high Dk/t SiHy CLs
(Doughty et al. 2006). After only 6 mon, they observed a slight (while not
signifi cant) increase in mean endothelial cell area (358 to 363 µm 2 ) and a
slight decrease in the mean COV (30.2% to 29.1%). Mean ECD also decreased
slightly (from 2821 to 2774 cells/mm 2 ). There was a strong correlation
between the ECD and a not signifi cant decrease in central corneal thickness
(CCT). As CCT decreased, the apparent ECD did decrease as well. The
percentage of normal six-sided (“hexagonal”) cells increased slightly (58.3
to 60.1%).
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