7

CHAPTER 2WHAT IS PERIMETRY?

INTRODUCTION

PERIMETRY – A STANDARD TEST IN OPHTHALMOLOGY

Perimetry is a standard method used in ophthalmol-

It provides a measure of the patient’s visual function -

performed for several reasons: 1) detection of pathol-ogies; 2) evaluation of disease status; 3) follow-up of pathologies over time to determine progression or dis-

Any pathology along the visual pathway usually results -

viations from normal, and consequently the associated -

nose glaucoma, but it is also often used to assess visu-al loss resulting from retinal diseases, as well as optic nerve, chiasmal or post-chiasmal damage due to trauma,

Additionally, perimetry is used regularly for visual ability

quantitative measure of visual function in order to de-termine eligibility for a pension for visual impairment,

In sum, perimetry is a universally available diagnostic

8

TE

MP

OR

AL

(rig

ht

eye)

NA

SA

L

(rig

ht

eye)

SUPERIOR

INFERIOR

SUPERIOR

TEMPORAL

(right eye)

NASAL

(right eye)

INFERIOR

Fixation

Fixation

TE

MP

OR

AL

(rig

ht

eye

)

TE

MP

OR

AL

(left

eye)

SUPERIOR

TEMPORAL

(right eye)

TEMPORAL

(left eye)

INFERIOR

SUPERIOR

INFERIOR

FIGURE 2-1 The monocular visual field of one eye is limited by the eye socket, nose, brow and cheekbones (A). The binocu-

lar visual field of two eyes overlaps in the central area (B).

Chapter 2 | What is perimetry?

SPATIAL EXTENT OF A NORMAL VISUAL FIELD

A) MONOCULAR VISUAL FIELD

B) BINOCULAR VISUAL FIELD

THE NORMAL VISUAL FIELD

SPATIAL EXTENT OF THE VISUAL FIELD

which a person can see at a given moment relative to essential part of one’s visual function, because a con-

9

Sensitivity

to light

Light

intensity

Dim lightHigh

Bright lightLow

The normal visual field

SENSITIVITY TO LIGHT

SENSITIVITY TO LIGHT IN THE VISUAL FIELD

But what is a person’s sensitivity to light? One can

room is dim, with an adjustable light bulb at its lowest

-creased, an increasing number of people will be able to

dim light bulb have a very high sensitivity to light, while the others have a lower sensitivity to light (FIG 2-2

-ular (FIG 2-1B

both eyes, with integration and mapping of information from the two eyes, allowing for stereo acuity and depth

FIG 2-1A

vision is limited by the facial anatomy of the person, with the eye socket, nose, brow and cheekbones, which

--

FIGURE 2-2 This figure illustrates the inverse relationship between light intensity and sensitivity to light. A person who can

perceive a very dim light has a very high sensitivity to light, while a person who can only perceive very bright lights has low

sensitivity to light.

10

FIGURE 2-3 The hill of vision is a three-dimensional representation of the visual field, with the x- and y-axes showing the

spatial extent of the visual field using radial coordinates, and the z-axis showing sensitivity to light. Its name stems from the

fact that normal sensitivity to light is higher at the center than in the periphery, so that normal vision in this representation

resembles a hill.

Chapter 2 | What is perimetry?

90˚80˚

70˚

SUPERIOR

TE

MP

OR

AL

NA

SA

L

INFERIOR

Blind SpotFixation

Sensitivity

to light

THE HILL OF VISION – A VISUALIZATION OF VISUAL FUNCTION

-

illumination, sensitivity is highest in the central area of -

HILL OF VISION

this representation resembles a hill, it is commonly re-

represent areas of seeing, and areas outside the hill of vi-sion represent areas of non-seeing (FIG 2-3

11Measuring sensitivity to light across the visual field

Normal Hill of Vision

Pathological Hill of Vision

Sensitivity

to light

PERIMETRY ALLOWS QUANTIFICATION OF ABNORMAL SENSITIVITY TO LIGHT

Deviations from the normal hill of vision provide valu-

PERIMETRY ALLOWS DETECTION OF ABNORMAL SENSITIVITY TO LIGHT

can be either constrictions of the boundaries of the visual

-

patient’s hill of vision with high accuracy and to identify its deviation from a normal hill of vision (FIG 2-4

MEASURING SENSITIVITY TO LIGHT ACROSS THE VISUAL FIELD

FIGURE 2-4 Pathologies affecting sensitivity to light result in an altered hill of vision for the patient. The deviation from the

normal hill of vision provides valuable information regarding the nature and severity of the pathology.

12 Chapter 2 | What is perimetry?

DimStimulus

No

No

No

Yes

Yes

Yes

Yes

BrightStimulus

Stimulus

Do you seethe stimulus?

Fixation

SE

NS

ITIV

ITY

TH

RE

SH

OLD

= Seen

= Not seen

THE PERIMETRIC TEST

-spheric cupola is typically used to project small light

stimuli, and the uniform background onto which the

-ditions project a round, white stimulus on a background,

-

-ate onto a target in the center, and to indicate seeing a

-

the stimulus is increased from the “off” position to the

cannot see the stimulus when it is off or very dim, anoth-er stimulus is shown later, at a higher level of light inten-

be noted that the stimulus is always turned off before the

non-seeing and seeing) (FIG 2-5

method, in perimetry the word threshold is often used,

“sensitivity threshold” is the term used throughout

SENSITIVITY THRESHOLDS

FIGURE 2-5 The sensitivity threshold between seeing and non-seeing for stimuli of different intensity presented against a

fixed background illumination at a given location in the visual field provides one data point on the hill of vision.

13Measuring sensitivity to light across the visual field

Stimulus

Sensitivity thresholdof first location

Do you seethe stimulus?

Fixation

Sensitivity

threshold

Sensitivity thresholdsat all tested locations

Stimulus

Fixation

Do you see here?

Do you see there?

Sensitivity

threshold

Sensitivity thresholdsat all tested locationsStimulus

Fixation

Do you see here?

Do you see there?

Sensitivity

threshold

(FIG

2-6A -mentioned procedure is then repeated at many locations

across the visual field (FIG 2-6B

sensitivity thresholds at all tested locations, a patient’s hill of vision can be drawn (FIG 2-6C

DRAWING THE HILL OF VISION FROM THE SENSITIVITY THRESHOLDS

FIGURE 2-6 The hill of vision can be drawn from the individually determined sensitivity thresholds at each location.

A) SENSITIVITY THRESHOLD OF FIRST LOCATION

B) SENSITIVITY THRESHOLDS AT DIFFERENT LOCATIONS

C) SENSITIVITY THRESHOLDS AT ALL TESTED LOCATIONS

14 Chapter 2 | What is perimetry?

While the process used to determine sensitivity thresh-olds is easy to understand, it would be much too time-con-suming to test each location of the hill of vision in this

perimetry and they will be discussed in depth in Chapters 4,

THE DECIBEL SCALE USED IN PERIMETRY

easy to interpret and should directly correspond to the

the decibel scale, with its unit of measurement being --

means that a patient is not able to see the most intense

perimetric stimulus that the device can display, whereas values close to 32 dB represent normal foveal vision for

to understand and use in clinical practice, the underlying considerations and formulas are less intuitive and of lim-

BOX 2A

DISPLAY OF SENSITIVITYTHRESHOLDS

THE RATIONALE FOR THE USE OF THE DECIBEL SCALE

) or in the

Iwould be impractical in clinical practice for the following reasons:

1. Large number of discrete luminance levels

2. The relationship between visual function and luminance is not linear

3. Inverse relationship between luminance and sensitivity to light

A patient with high sensitivity to light only needs a stimulus with low luminance to be able to see

BOX 2A

15Display of sensitivity thresholds

SENSITIVITY TO LIGHT

(SENSITIVITY THRESHOLD)

LUMINANCESTIMULUS

40

~32.8 dBFoveal normal

sensitivity for

20-year-old

person

30

20

10

0

Apostilb (asb)Decibels (dB)

0.4

4.0

40

400

4,000

4. Lack of definition of complete visual field loss

THE DEFINITION OF SENSITIVITY TO LIGHT USING THE DECIBEL SCALE

dB = 10 * log (Lmax/L)

where dB is the sensitivity threshold, L

he logarithmic scale is used to address the large range of luminance values and to relate this range

Si

RELATIONSHIP BETWEEN SENSITIVITY TO LIGHT AND LUMINANCE

recent Octopus perimeter models, is 4,000 asb. It is a logarithmic scale and is inversely related to the linear luminance scale in apostilbs (asb). Note that the maximum stimulus brightness might be

16 Chapter 2 | What is perimetry?

GRAPHIC DISPLAY OF SENSITIVITY THRESHOLDS

to appropriately display all aspects of a patient’s visual Cartographers face similar challenges when displaying three-dimensional mountains or hills, and have used

10 30 40 50 60 70 80 90

0m

600m 1200m

2400m

3000m

Numerical altitude map Numerical sensitivity threshold map

Color altitude map Color sensitivity threshold map

Altitude lines map Sensitivity threshold lines map

Values

Grayscale of Values

Kinetic Perimetry

3D map 3D map

0m

0m

0m

0m0m

30 dB30 dB

20 dB

20 dB

20 dB

20 dB

10 dB

10 dB

10 dB

10 dB

10 dB

10 dB

10 dB

600m

600m600m

600m

1200m

1200m

1200m

1200m

1800m

2400m

1800m

1800m

1800m

1800m

2400m3000m

1800m

2400m

2400m3000m3600m

2727

27

28

2527

28

27

25

26

28

2729

30

2829

31

30

28

26

27

2629

29

21 2931

29

31

31

31

30

333131

30

30

323128

26

332928

MOUNTAIN – Geographical display HILL OF VISION – Perimetric display OCTOPUS REPRESENTATIONS

No 3D map availableon Octopus perimeters

GRAPHIC DISPLAY OF SENSITIVITY THRESHOLDS

FIGURE 2-7 As in cartography, there are different ways to display the three-dimensional hill of vision in two dimensions.

Sampled altitude levels can be displayed numerically, a color code can be used to represent different altitude levels, or altitude

lines can show the different altitude levels.

17

90

270

0180

90

270

0180

Ideal Practical Ideal Practical

Challenges in visual field testing and interpretation

strategies are used to display the hill of vision in two

As in geographical maps (FIG 2-7), the various sensitivity -

pling certain altitudes to give a feel for the overall shape

Last but not least, lines of the same altitude level can

perimetry, these lines of equal altitude are referred to as

It should be noted that whichever display form is used,

PERIMETRIC TESTING HAS LOW RESOLUTION

So far, this book has presented perimetry as a very accu-rate way of continuously showing the stimuli of increasing

thresholding is performed at all locations across the (spatial resolution) using each possible light intensity

CHALLENGES IN VISUAL FIELD TESTING AND INTERPRETATION

IDEAL VERSUS PRACTICAL PERIMETRIC TESTING

SPATIAL RESOLUTION RESOLUTION OF SENSITIVITY THRESHOLDS

FIGURE 2-8 Ideally, the hill of vision would be drawn from an infinite number of test locations and from a continuously

changing stimulus luminance. In reality, the time constraints do not allow for this kind of testing, and only sampling at some

locations and some luminance levels is possible.

18

20-year-old

Sensitivity

threshold

85-year-old

Chapter 2 | What is perimetry?

has a set number of discrete levels, such as high, medi-um and low, and there are only a few bulbs to illuminate the room (FIG 2-8)

-

still be able to receive the information necessary for good clinical decision-making, a number of elaborate process-

-mation and offers a good trade-off between testing time

NORMAL SENSITIVITIES DEPEND ON AGE AND TEST LOCATION

As already illustrated in the section about the hill of vision, normal sensitivity thresholds depend on the test location and are higher at the center than in the pe-

1-3

person (FIG 2-9

to interpret directly in the clinic, because the representations of normal and abnormal values depend on testing- and

of sensitivity thresholds, a clinician would have to keep normal reference values in mind for all age groups and

HILL OF VISION IS AGE- AND LOCATION-DEPENDENT

FIGURE 2-9 The normal hill of vision shows the highest sensitivity thresholds at the center, with decreasing sensitivity thresh-

olds towards the periphery. Similarly, there is also a decrease in sensitivity thresholds with increasing age at all test locations.

19

Sensitivity

threshold

Measured Valuesof a 20-year-old

Comparisons

Normative Values

of 20-year-olds

NORMATIVE VALUES (MEASURED) VALUES COMPARISONS (TO NORMAL)Normal sensitivity threshold Measured sensitivity threshold Sensitivity loss

- =

Challenges in visual field testing and interpretation

-oped for most modern perimeters and these databases

for each age group and test location (BOX 2B

used to compare any measured sensitivity threshold

Due to their ease of use, most representations in the Octopus perimeters are based on the Comparisons to

to the respective normative value for someone of that

meaningful, as they relate directly to sensitivity loss (FIG

2-10

COMPARISONS SHOW THE DEVIATION FROM NORMAL

FIGURE 2-10 The difference between a normal and a measured visual field point is commonly called ‘Comparison to

normal’ (also referred to as deviation from normal or defect) and its interpretation is independent of a patient’s age or the

visual field location.

20 Chapter 2 | What is perimetry?

NORMATIVE DATABASES IN OCTOPUS PERIMETERS

DESIGN OF A NORMATIVE DATABASE

B

typical process to comply with the standards is to perform a clinical study that includes a thorough eye

DISTINCT NORMATIVE DATABASES FOR DIFFERENT DEVICES AND EXAMINATION PARAMETERS

Si

PERIMETRY HAS OBJECTIVE AND SUBJECTIVE COMPONENTS

NORMAL FLUCTUATION DEPENDS ON TEST LOCATIONS AND DISEASE SEVERITY

In the interest of simplicity, perimetry has been treated as

However, there is a subjective element to perimetry, due

As a result, there is always a certain amount of normal

well as between different measurements of the same

the test results is highly dependent on several factors, including the cooperation of the patients, their cognitive

If the

FIG 2-11 -

patient does not understand the test, does not pay atten-tion or does not focus continuously on the central target,

Additionally, some patients may be very conservative in their judgements, requiring a more intense stimulus for detection, while other patients may be liberal and accept

Chapter 3 focuses on potential sources of unreliable and -

periphery and is also smaller in areas of good vision than

BOX 2B

21Challenges in visual field testing and interpretation

Average Hill of Vision

Normal fluctuation

Sensitivity

threshold

Abnormal

THE FREQUENCY-OF-SEEING (FOS) CURVE

A“unseen” to “seen” responses, so that the probability that a patient will perceive a stimulus changes

--

In areas of normal sensitivity,

high probability of seeing stimuli that are slightly more intense than the luminance at the threshold, and

In

BOX 2C

NORMAL FLUCTUATION IN PERIMETRY

FIGURE 2-11 Since perimetry contains a subjective, patient-related component, there is always normal fluctuation. Its magnitude

depends on both the test location and disease severity.

BOX 2C

22 Chapter 2 | What is perimetry?

Fluctuation Fluctuation

Normal

sensitivity

threshold

Pro

ba

bilit

y o

f se

ein

g t

he

sti

mu

lus

0%

Dim stimulus Bright stimulus

50%

100%

Abnormal

sensitivity

threshold

FREQUENCY-OF-SEEING (FOS) CURVE

CLINICAL STANDARD FOR VISUAL FUNCTION TESTING

Even though perimetry has low resolution and contains subjective, patient-related components resulting in normal

-

patient’s quality of life and ability to perform activities of

daily living, which are the most important factors for the

glaucoma can be followed accurately through all stages

23References

REFERENCES

urv Ophthalmol

Invest Ophthalmol Vis Sci

Invest Ophthalmol Vis Sci