Download Visual fields in General Practice

Visual fields for General Practice
Dr Mark Donaldson
Introduction
• This is a presentation designed to give you a
feel for the place of the visual field test in
current glaucoma management and a taste of
how these modern tests might work in the
primary care setting.
• The introductory case which follows is a true
story.
FDT perimeter
• This instrument is made
by Zeiss.
• It is about the size of a
printer.
• Patient puts his head up
to it like it is a pair of
binoculars.
• There is a clicker to press
each time you see the
stimulus.
• It costs $NZ18000 new.
FDT : frequency doubling perimetry
– The stimulus is displayed on a black & white video
monitor and is a pattern of alternating vertical
black & white bars.
– The stimulus appears to be moving. It is easy to
see… i.e. you don’t imagine you see it as is
possible with the conventional static white spot.
– The results are not affected by blur, i.e. having the
right spectacles on is not at all critical to the
outcome.
True Case
A 37 year old General Practitioner
Volunteered for a field test…
Further patient details
• She attended an educational evening at my
ophthalmology practice in Auckland.
• She was fit and well.
• There was no ophthalmic history.
• There was no family history of glaucoma or
blindness.
N-30-5 FDT Screening test
– On the result printout you can see that the test was
completed in under 50 seconds per eye.
– The reliability indices are high: no fixation errors and
no false positive errors.
– N-30-5 refers to the field of vision – 30 degrees from
fixation tested. The stimulus is a 5 degree square.
– The stimulus is suprathreshold: 95% of normal
individuals will observe the stimulus.
– Both the right and the left field have abnormalities.
Optic disc photographs
Corresponding optic disc photos.
• The vertical Cup/Disc ratio is in excess of 0.8
for both eyes.
• Optic disc haemorrhage left inferotemporal
disc. (Pathognomonic of glaucoma)
• Nerve fibre layer defect – best seen on right
inferior to the disc.
• The nerve fibre defect corresponds to the
position of the field defect on the screening
printout.
Further clinical examination
• Intra ocular pressure (IOP): 16 A 16
• Central corneal thickness: 545 CCT 535
• Open angles, normal trabecular meshwork
Normal tension glaucoma
• This doctor’s Intraocular Pressure is 16 mmHg.
• 16 mmHg is a normal IOP
• The central corneal thicknesses (CCT) are
likewise normal.
– A thin cornea can give rise to a falsely low IOP on
standard tonometry.
• IOP is a very poor screening test for glaucoma
• 30% of glaucoma patients have normal IOP at
presentation
Further field analysis needed
• In this patient I requested two further visual
field tests.
– Standard white on white 24-2 threhold Visual field test.
• This has been used for years in glaucoma practice to
monitor worsening of visual field
– FDT 24-2 threshold visual field test
• This is a threshold test using the FDT stimulus
• The FDT test has been shown in studies to be positive
for glaucoma defects up to 5 years prior to the standard
White on white threshold test.
Standard automated perimetry
• This is an illuminated bowl.
• The instrument is
manufactured by Zeiss and
costs $40000.
• The stimulus is a static white
spot of varying intensity.
• Having the correct refraction is
important.
• The threshold is measured at
each test location.
– Hence it takes longer to test
compared to a suprathreshold
testing strategy
Standard automated perimetry
(Refer to separate files provided to view clearer versions of the images below)
Standard perimetry
• The reliability indices are good.
– The machine counts the times the subject looks
away from the central fixation target
• The test takes 5 minutes per eye in this young
and intelligent patient.
• There is no consistent pattern of abnormality
seen
• The hemifield test is a statistical analysis
which determines if there is a difference
between the superior and inferior field.
FDT 24-2 threshold visual field test
FDT threshold
• Threshold field determined using the FDT
stimulus.
• As with the suprathreshold test abnormalities
are detected.
• The FDT stimulus is said to be detected by Mganglion cells which are preferentially
damaged in glaucoma and convey motion
information to the CNS.
Retinal nerve fibre analysis
Retinal nerve fibre analysis.
• The ocular coherence tomography device is used
to measure the retinal nerve fibre layer around
the optic disc.
• This provides an objective measure of nerve fibre
loss and complements the functional information
provided by a field test.
• The nerve fibre layer defect observable in the disc
photo is measured in the OCT scan and
compared, and comparison to a normal database
displayed.
Case Conclusions
• Advantages of the FDT stimulus highlighted in
detecting glaucoma visual field defects.
• Opportunistic screening with a modern and
quick suprathreshold test has saved this
doctor many years of sightedness.
• IOP measurement is a very unreliable
screening tool for glaucoma detection.
• Could screening be carried out systematically
in your practice using the small FDT
instrument?
The Visual Field
Visual field
• The term applied to the surround vision
capability of the eye.
• Visual field is different from visual acuity.
– the standard eye chart measures acuity at the
centre of the visual field.
• Outside central vision the visual field has poor
acuity but is very sensitive to movement.
Methods of measuring visual fields
• Direct confrontation
– Useful when assessing neurological visual fields.
• Bitemporal hemianopia
• Homonymous hemianopia
– In general practice or at bedside
• Detects absolute scotomata
Clinical Importance of Visual Field
• CNS pathology
– Hemianopia (stroke)
– Bitemporal hemianopia (pituitary tumour)
(Gross field defects can be identified with simple
confrontation screening)
• Glaucoma detection
– Subtle defects will be missed by confrontation
screening.
• Glaucoma management
This sequence of threshold visual fields using a standard white on white static stimulus follows
the progress of a glaucoma patient’s left visual field over several years. After many test
repetitions it is apparent that the visual field is worsening. Decisions on increasing therapy or
going to surgery are made on the basis of these accumulated data.
(Refer to separate files provided to view clearer versions of the images below)
Methods of measuring visual fields
• Visual field analyser (= automatic perimetry)
• Threshold testing
– On a standard automatic perimeter the threshold is the
intensity of the target spot (stimulus) that is detected
by the observer above the intensity of the background
illumination.
– Glaucomatous scotomata are not absolute until endstage.
– This is used for following glaucoma progression
– This test takes 5-12 minutes to perform
Methods of measuring visual fields
• Suprathreshold testing
– The stimuli presented are above what the normal
population would observe at each point tested in the
visual field.
– This is a fast test strategy used for screening.
Automated Field Testing
in General Practice
• Is there a place for visual field analysers in NZ
general practice?
GP perimetry
• Advantages of automated perimetry in General
Practice
– Auditable ( printout of the results of each visual field
test )
– Repeatable ( arguably more reliable than the standard
GP eye chart test)
– Abnormality of field occurs in several important
diseases
•
•
•
•
Glaucoma
Cerebrovascular disease
Intracranial tumours
Diabetes and macular degeneration
General Practice Experience in
Auckland
• FDT perimeter has been
used in an Auckland
General Practice for 4
years.
• The results are currently
being collated.
Value of FDT field test
in General Practice
• Avoid missing serious and advanced sight or
life threatening pathology.
• Enhance your practice by providing
“preventative care” through screening for
asymptomatic disease.
• Better and more enjoyable than current visual
field testing in general practice.