Topic 4 The Visual Fields

Overall yield:

Clinical exam:




What is the visual field? What is an isopter? And what is a scotoma? _

"The visual field (VF) is one of the functional components of vision."

"It is defined as the area that is perceived simultaneously by a fixating eye."

Visual field basics 1. Definition

• Area that is perceived simultaneously by a fixating eye

• "Island of vision in a sea of darkness" (Traquair's definition)

03 Exam tips:

• See also the visual field examination in neuroophthalmology (page 255)

2. Limits

3. Isopter

60 degrees nasally, 60 degrees superiorly, 110 degrees temporally, 70 degrees interiorly Blind spot 15 degrees temporal to fixation

• Line in VF connecting points with the same visual threshold

• Encloses an area within which a target of given size and intensity is visible

4. Scotoma and VF defect

• Absolute or relative decrease in retinal sensitivity within the VF, bounded by areas of normal retinal sensitivity

• Absolute or relative decrease in retinal sensitivity extending from the edge of the VF

5. Luminance and visual threshold

• Intensity of light

• Apostilb (asb) is an absolute unit of luminance

• Normal human range: 2 to 9000 asb

• Humphrey VF can measure form 0.08 to 10,000 asb

• Decibel (dB) is a relative unit of luminance

• Inverse log scale

• Visual threshold

• Luminance of stimulus which is perceived 50% of time

• The brighter the stimulus needed to be perceived, the lower the visual threshold

• Therefore, bright stimulus = high asb = low dB = low visual threshold

<§>What is perimetry? What are the types and advantages of each?

"Perimetry is the quantification of the VF." "It can be divided into ..."

• Comparison between Goldman and HVF is a common question

Perimetry basics

1. Classification

• Campimetry (flat surface)

• Tangent screen

• Manual and kinetic

• Test central 30 degrees

• Subject seated 1 or 2 meters from black screen

• Target is presented by examiner

• Perimetry (curved surface)

• Manual and kinetic

• Extend beyond 30 degrees (peripheral fields)

• Goldman bowl perimeter

• Manual and kinetic or static

• Hemispherical bowl with radius of 33cm (subject at 33cm)

• Stimuli has different intensities (1-4) and size (l-V)

• Extend beyond 30 degrees (peripheral fields)

• Humphrey visual field analyzer (HVF)

• Automated and static

• Test central 30 degrees

2. Advantages of automated (over manual)

• More quantitative

• No examiner bias

• Constant monitoring of fixation

• Automated re-testing of abnormal points

• Computer software for analysis

3. Advantages of static (over kinetic)

• More objective and quantitative

• More sensitive to shallow scotomas

• Random presentation of stimuli (less anticipation of subject)

<2> What are the uses of visual field in ophthalmology?

"VF are used for diagnosis and follow-up of ophthalmic conditions."

Uses of visual field

1. Diagnosis of

• Optic nerve diseases (optic neuritis, anterior ischemic optic neuropathy, toxic neuropathy)

• Unexplained visual loss

• Malingering patients

2. Follow-up of

• Tumors (pituitary adenoma)

^S^ Tell me about the Humphrey visual field analyzer

"Humphrey visual field analyzer is an automated static perimetry."

"Maps the VF by quantifying the visual threshold at predetermined locations."

Humphrey visual field analyzer 1. Basic

• Automated static perimetry

• Stimuli (size = Goldman size III with duration of 0.2 s)

• Background illumination = 31.5 asb 2. Test strategies

• Full threshold strategy

• Uses the "4-2 bracketing" algorithm at each retinal point

• Stimuli intensity increases in 4 dB steps until threshold is crossed (patients see stimuli)

• Threshold is recrossed with stimuli Intensity decreasing in 2 dB steps.

• Test central 24 degrees of fixation and on either side of meridian (24-2) as opposed to tests on meridians as well (24-1)

• 30-1 or 30-2 (test central 30 degrees of fixation)

• Related threshold strategies

• Full threshold with prior data

• Faster, uses prior VF data, presents each point at 2 dB brighter than patient's previous threshold values and tests each point in 2 dB decrement

• Fast threshold

• Even faster, presents entire field at 2 dB brighter than patient's previous threshold values and then tests only abnormal points

• Suprathreshold test strategy

• Fast screening test

• Presents stimuli at 6 dB higher than expected threshold

• Each point recorded as normal versus abnormal

How do you read the Humphrey visual field?

"This is a HVF for the left and right eyes respectively." "Done on January 2nd 1999 using the 24-2 threshold test pattern ..." "First, the reliability indices are ..."

Evaluating the HVF

1. Reliability indices

• Fixation loss

• Positive response to blind spot stimulation

• "Moving eyes around"

• False positive

• Positive response but no stimuli

• False negative

• Negative response with brighter than threshold stimuli

• "Falling asleep"

• Other clues of unreliability

• Short term fluctuation significantly raised

• "Clover leaf pattern" on greyscale (inattentiveness with time)

• Increased eye (upstroke) or lid (downstroke) movement on eye tracker line

2. Global indices

• Average deviation of each point from age-corrected normal (e.g. -5 dB MD means that on average, each point has a 5 dB lower threshold than normal)

• Pattern standard deviation (PSD)

• Standard deviation of each point from age-corrected normal

* You may be given a HVF printout to read. You need to be systematic and not jump at the obvious VF defect seen

• Remember Mean deviation = Minus is bad. Pattern standard deviation = Pius is bad

What are the newer VF techniques?

What are the newer VF techniques?

Newer perimetry techniques

1. SITA (Swedish Interactive Thresholding Algorithm)

• Aims to increase speed without losing accuracy

• SITA Standard

• Full version comparable to standard threshold algorithm in sensitivity and accuracy but twice as

• Similar to suprathreshold algorithm in sensitivity and accuracy but twice as fast

• Smart questions and smart pacing

• All factors considered as test occurs, producing estimate of threshold at each point

• Uses normal age-corrected threshold values as starting point

• Real time calculation and re-calculation of threshold values as test proceeds

• Knows when to quit when standardized amount of information obtained

• Use all information from every point

2. SWAP (Short Wavelength Automated Perimetry)

• Blue on yellow perimetry

• Blue-yellow ganglion cells lost first in glaucoma

• SWAP detects abnormal VF 2-5 years before white on white VF tests become abnormal

3. Frequency doubling perimetry

• Low spatial frequency sinusoidal grating undergoing high temporal frequency flicker

• Tests magnocellular pathway, which appears to be lost first in glaucoma

• Possible screening tool for the future fast

Was this article helpful?

0 0

Post a comment