Visual fields for General Practice Dr Mark Donaldson.

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.

– 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.

FDT : frequency doubling perimetry

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.

– 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.

N-30-5 FDT Screening test

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 M-ganglion 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 end-

stage. – 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.