| Wizkid 2004-11-14, 11:09 am |
| USAF Studies Offer Insights on Contrast Sensitivity After PRK
The United States Air Force recently completed a longitudinal clinical
study designed to evaluate PRK's long-term effect on aircrew visual
performance, an issue that can be of critical importance during
combat. (PRK was the subject of the study because LASIK still isn't
considered as universal a corrective option for aircrew members; the
flap can be displaced by blunt trauma and windblast, such as might be
encountered when ejecting from an aircraft.)
Col. Douglas J. Ivan‚ MD‚ chief of the ophthalmology branch at the
U.S. Air Force School of Aerospace Medicine and program manager for
the USAF PRK research study project, explains that one purpose of the
study was to evaluate the sensitivity of different contrast
sensitivity testing methods. "We built a variety of quality-of-vision
tests into our study, including charts and electronic contrast
sensitivity testing, which is kind of a gold standard," he says.
"Some contrast sensitivity tests aren't sensitive enough to pick up
subtle change. Yet that change may be operationally significant in the
military."
One section of the overall study, used the Freiburg Acuity and
Contrast Test (FRACT) to monitor the effects of PRK on visual acuity
(VA) and CS under different conditions over a two-year follow-up
period. "We found that the Freiburg test was sensitive enough to show
that there was a contrast sensitivity drop-off that was not picked up
by traditional high contrast visual acuity testing," Dr. Ivan
explains.
As expected, the presence of glare degraded both VA and CS, and the
presence of the windscreen (with its minute optical flaws from wear)
made the impact of glare worse. However, two results were noteworthy.
First, although the tests generally failed to detect significant
changes in VA following PRK, the data showed long-term worsening of CS
in many subjects that did not improve with healing.
Second, despite the equal luminance of the two glare sources, the
laser glare interfered far more with both VA and CS than the broadband
glare source. "We weren't expecting that to be the case," notes
Richard J. Dennis, OD, MS, FAAO, a senior vision scientist at the AF
Laser Research Laboratory.
"Because the military uses lasers for many purposes, this could be a
significant concern." Researchers observed that the laser glare
produced coherent spatial noise—i.e., "laser speckle"—that may account
for the increased visual interference.
In terms of contrast sensitivity, Dr. Ivan notes that not all PRK
patients end up with "permanent" CS loss. "Those who still have a
deficit two years out are a subset of this population. We're trying to
find the right tests to determine who those individuals are, and
ultimately create a set of standards for this that we can get policy
makers to buy into.
"We can't afford to put pilots in the cockpit who may have subtle but
significant loss of contrast sensitivity," he adds. "After refractive
surgery, older, traditional vision standards simply don't tell us all
that we need to know."
Opt & Vis Sci 81;7:516-524
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