Ophthalmology Update - Cleveland Clinic

Study Conclusively Links Haze After PRK
to Stromal Surface Irregularity
Despite advances in laser technology, clinically
significant haze still develops in 2% to
4% of eyes undergoing PRK for the correction
of higher levels of myopia. Various
hypotheses have been put forth regarding
the pathogenesis for haze formation, but no
studies have produced definitive evidence to
support any one theory.
now, however, results of a study undertaken by steven
e. Wilson, m.d., and colleagues at the cole eye institute
are providing new understanding about the basic
mechanisms for the development of haze after prK.
in an elegantly designed experiment using a rabbit
model, the researchers demonstrated conclusively
that haze development after prK was related to
levels of stromal surface irregularity and anterior
stromal myofibroblast generation. in addition, they
showed haze formation could be mitigated by
performing phototherapeutic keratectomy (ptK)
smoothing after prK and provided evidence that
defective basement membrane regeneration in eyes
with surface irregularity likely plays a role in the
development of haze.
“it has always been suspected that haze develops after
prK because of the rough stromal surface that
remains after the ablation, and there has been some
indirect evidence to support that concept. the
results of our study demonstrate unequivocally that
it is critical to leave a smooth surface at the end of
any surface ablation procedure, and they also support
the use of ptK-smoothing to achieve that goal.
Based on another study we will be reporting that investigated
the long-term effects of mitomycin-c on
stromal cells, ptK-smoothing is certainly an option
to adjunctive mitomycin-c from a safety standpoint
as a method for minimizing haze when performing
prK for higher corrections,” says dr. Wilson.
the study divided rabbits into eight groups to receive
no treatment, –4.5 d prK, –4.5 d prK with a fine mesh
screen positioned in the path of the laser to create
surface irregularity by blocking 10%, 30% or 50% of
followed by ptK smoothing, –9.0 d prK or –9.0 d prK
with ptK smoothing. haze was graded at the slit lamp
after 4 weeks using a scale of 0 to 4.
consistent with previous studies, the results of those
evaluations showed haze was trace or absent in the
eyes treated with the –4.5 d ablation whereas severe
haze developed after the -9.0 d prK. in the groups
where the –4.5 d prK was performed with the mesh
screen in place for part of the procedure, the amount
of haze present increased proportionally as the
percentage of surface irregularity increased.
the effect of surface irregularity on haze development
was also demonstrated by the findings in eyes that
underwent ptK smoothing. in eyes that had a –4.5 d
ablation with 50% irregularity, use of the ptK smoothing
technique essentially mitigated haze development,
the terminal pulses, –4.5 d prK with 50% irregularity Continued on page 14
i n v e s t i g a t i O n s
Basement membrane defects and
myofibroblasts in corneas with
irregular surfaces after PRK.
Triple staining of the central
cornea for α-smooth muscle
actin-expressing myofibroblasts
(red) and integrin beta-4 (green),
along with DAPI (blue).
A) Note the more homogeneous
basement membrane regeneration
in a cornea without haze
after –4.5 D PRK.
B) Basement membrane with
obvious disruptions (arrows)
adjacent to an area with more
uniform basement membrane
(arrowheads) Red α-smooth
muscle actin-positive cells can
be noted below the defects in
the basement membrane.
Magnification is 400x.
Reprinted from Experimental Eye
Research; Vol 82; Marcelo V.
Netto, Rajiv R. Mohan, Sunilima
Sinha, Ajay Sharma, William
Dupps, Steven E. Wilson; Stromal
haze, myofibroblasts, and surface
irregularity after PRK; Pages 788
– 797; Copyright 2006, with
permission from Elsevier
c O l e e y e i n s t i t U t e c l e v e l a n d c l i n i c . O r g / e y e //