Perimetry or Visual Field Analysis- An Overview.

Perimetry vs Perimeter
Perimetry vs Perimeter

Perimetry vs Perimeter:

  • Measurement of the extension of visual field or degree of visual field defect is called Perimetry.
  • The instrument used to examine visual field is called Perimeter.
  • With this technique, loss of visual function can usually be detected and quantified before the patient becomes aware of it.
  • Perimetry helps us to assess visual integrity between the retina and visual cortex, understanding functional abilities of Visual pathway & diagnose a vision-Brain condition.

Visual Field & It's types
Visual Field & It’s types

What is Visual Field?

Visual Field:

  • Visual Field is a three-dimensional area of subjects’ surroundings that can be seen at any one time around an object of fixation when eyes are stationary and looking straight ahead.

Types Of Visual Field:

Central Visual Field:

  • From a central fixation point to a circle of 30 degree is called central visual field.
  • It consists of blind spots formed by the optic nerve head.

Peripheral Visual Field:

  • Rest of the area beyond 30 degrees to outer extent.

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Extension of Visual Field
Extension of Visual Field

Visual Field- Range:

Superior: 50 deg

Nasal: 60 deg

Inferior: 70 deg

Temporal: 90 deg

What is Threshold in Perimetry?

  • The minimum amount of light required to see an object is called threshold.
  • In our retina fovea has the finest threshold even with very dim light able see object.
  • Threshold increases from fovea to periphery and when threshold increases, we need brighter light to see object.
  • Fovea is most sensitive due to the presence of more cone cells than peripheral retina.
Hill of Vision in Perimetry
Hill of Vision in Perimetry

Hill of Vision of Visual Field:

  • Retinal light sensitivity can be represented as “Hill of Vision”
  • The peak of the hill represents the fovea which has the highest sensitivity.
  • Two slopes represent the nasal and temporal retina where sensitivity to light is decreasing gradually.

What is Scotoma in Perimetry?

Scotoma:

  • When particular point in the retina is required brighter condition to see an object or not able see at all is called scotoma.
Relative Scotoma:
  • When in a particular area in retina, the threshold is increased or need brighter light than normal to see object, then it is called Relative Scotoma
Absolute Scotoma:
  • When in a particular area in retina, can’t detect the target even with the brightest (maximum) light is called Absolute Scotoma.

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Manual Perimetry
Manual Perimetry

How Perimeter Works:

  • Perimetry can be of:
    • 1. Manual Perimetry.
    • 2. Automated Perimetry.

How Manual Perimetry works:

  • In manual perimetry, we generally measure the extension of visual field.
  • We bring a target from periphery to central fixation.
  • Patient is asked to tell at which position he is able to see the target while patient is fixating at central point.
  • Based on patient response examiner documents the visual field extension.

Automated Perimetry– How does a computer assisted software detect visual field or visual field defect?

How automated Perimeter Works:

  • In our retina fovea has the finest threshold even with very dim light able see object.
  • That means in our retina in different point we have different threshold to see an object.
  • What software does, they flash different intensity of light (0 to 50 db) in a particular point to identify the threshold of that point.
  • When they identify the threshold of that point, they compare this threshold with the data they have for the same age for this particular point.
  • If the threshold is higher than the normal data, they have for the same age group then they show scotoma or visual field defect in that particular point.
  • This is how they identify threshold of different points in the retina and then compare with the normal threshold for the same age group and give interpretation.

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Indications of Perimetry/Visual Field Analysis:

Indication of Perimetry
Indication of Perimetry

a. Perimetry in Glaucoma:

  • Early detection and a wider damage spectrum of glaucoma can be diagnosed and monitored over longer periods of time.
  • For the management of advanced stages of glaucoma, perimetry is the method of choice.

b. Visual Field Analysis in Certifications:

  • Certification of the existing visual performance, for example, for driver license testing and for legal blindness tests.

c. Perimetry in Neurological issues:

  • Strokes causes characteristic Visual Field loss that can be identified with perimetry.
  • Neurologist sometimes refers such patient to ophthalmologist for visual field examination.

d. Perimetry in Retinal Diseases:

  • Progression of retinal diseases can be monitored by regular checking visual fields. Such as:
    • Retinitis Pigmentosa,
    • Diabetic Retinopathy etc.

Methods of Perimetry/Visual Field Analysis:

Kinetic vs Static Perimetry
Kinetic vs Static Perimetry

Kinetic Perimetry:

  • Stimulus of known luminance are moved from periphery to center to identify visual field extension.

Example:

  • Confrontation perimetry 
    • Lister’s perimetry 
    • Tangent screen Scotometry
    • Goldmann perimetry.

Static Perimetry:

  • Presenting a stimulus at a predetermined position for a preset duration with various luminance to identify threshold and suprathreshold.

Example:

  • Automated Goldmann perimetry.
  • Friedmann Visual Field Analyzer.
  • Automated Visual Field Analyzer.

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Central vs Peripheral Perimetry:

Central vs Peripheral Perimetry
Central vs Peripheral Perimetry

Central Visual Field Analysis:

  • Asses only Central visual field or visual field defect.

Example:

  • Scotometry.
  • Goldmann perimetry.
  • Automated Perimetry.

Peripheral Visual Field Analysis:

  • Assess only peripheral visual field defect

Example:

  • Confrontation.
  • Lister’s perimeter.

Manual vs Automated Perimetry

Manual vs Automated Perimetry
Manual vs Automated Perimetry

Manual Perimetry:

  • Manual procedure to identify the extension of visual field.

Example:

  • Confrontation.
  • Lister’s perimeter.
  • Goldmann Perimeter.

Automated Perimetry:

  • Computer based system to identify visual field defect.

Example:

  • Humphrey Visual Field Analyzer.
  • Friedmann Visual Field Analyzer.
  • Octopus Visual Field Analyzer.

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