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New visual field testing strategies
to banish patient boredom and facilitate earlier detection
Laszlo Dosa
in Miami, Florida
THE initials SITA, SWAP and FDT have come to characterise the current
status of new strategies in visual field testing methods.
A usually laborious procedure, visual field testing is tiring for
the patient who has to sit for 45 minutes and respond whenever they
see a light flash very quickly in different places. And it is not
so much the patient than the neurological system that fatigues,
Douglas R Anderson MD noted at a symposium on controversies in glaucoma
management.
Swedish Interactive Thresholding Algorithm (SITA) is an effort to
perform field testing in a way that is easier on the patient. Developed
by a group of Swedish mathematicians and ophthalmologists, SITA
is aimed at making the testing more efficient.
It is an interactive thresholding algorithm in the sense that it
makes use of the information it is gathering as it goes along to
enable it to decide what to do next in the test, as opposed to simply
having the test algorithm as it proceeds.
He said perhaps the greatest merit of SITA- standard is that the
test is fast, (half the time of the previous standard) and is as
acurate as other tests.
The SITA-standard test is now the routine method used by many glaucoma
specialists. SITA also has a fast version, SITA-FAST. The SITA fast
version should not be a practice standard, but can be used for patients
who do not do field tests well. The SITA-fast version is difficult
to handle for inexperienced field takers which can be a problem
when used for screening, said Dr Anderson.
It should not be a practice standard, but can be used for patients
who do not do field tests well. Even though the five-year-old test
is already quite widely used, it is difficult to handle for inexperienced
field takers, which can be a problem when used for screening, said
Dr Anderson.
Short Wavelength Automated Perimetry (SWAP) was initially developed
to highlight visual pathways thought to be particularly involved
in glaucoma.
It is employed to try to look for very early subtle defects by testing
only one part of the visual system, namely the part that sees blue.
When a white light is shone, which is done in standard polarimetry,
SWAP sees the light if the patient’s blue, red and green systems
are working.
If only one of those three systems is not operating properly, the
person still sees the white light. But if you show him a blue light,
if the blue system is out, then it won’t show. In so doing
the test bypasses redundancy of visual system neurons, Dr Anderson
explained.
Frequency Doubling Technology (FDT) is used to isolate a particular
pathway, just like the SWAP does. It is a new technique, still under
evolution and improvement, and is used mostly for detection but
it is also capable of thresholding.
“Frequency doubling is a phenomenon in which if you alternate
bars of white and black so that if the white goes off at the same
time the white goes on, you actually see twice as many bars as there
really are.
“That may not be relevant but at least it stimulates one of
the systems and it is just a different system. It’s not the
blue system, just a different one. By isolating one system, presumably
it can be more effective,” he said.
The new field testing systems offer some hope of earlier detection
of glaucoma. Recent clinical research indicates that the systems
are starting to live up to their promise.
For example, a study at the University of California in San Diego
showed that SWAP screening could detect filed defects two to five
years earlier than standard visual field testing. Another study
showed that FDT could identify 70% of patients with obvious glaucomatous
optic neuropathy. Standard perimetry only identifies 6% of these
patients.
Earlier detection of glaucoma offers clinicians the chance to intervene
earlier in the course of glaucoma, possibly reducing the rate of
progression of the disease.
These same systems could prove useful in determining the effects
of treatment over time, a boon for those conducting clinical trials.
The new screening systems also offer a wealth of useful information
to researchers who are studying the pathology of the disease.
Douglas
R Anderson MD
Bascom Palmer Eye Institute, Miami, Florida, US
Email: danderson@med.miami.edu
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