Friday, March 8, 2013

Visual Field Determination

What is Visual field?

Visual field is the area seen by one eye when it is foxed at a certain point. 

Normal ranges of a visual field include:
1.On the medial or nasal side=60-65. It is less on the    
   nasal due to presence of bridge of nose.
2.On lower side=75 cheeks obstruct it.
3.On the upper side=55 superciliary arches obstruct it
4.On lateral or temporal side=90-100 as these is no  
   abstuction on the lateral side.

Note: Since you probably use both your eyes at all times, you may not notice any changes in one eye’s visual field 
unless you close the other eye.
The normal human visual field extends to approximately 60 degrees nasally (toward the nose, or inward) from the vertical meridian in each eye, to 100 degrees temporally (away from the nose, or outwards) from the vertical meridian, and approximately 60 degrees above and 75 below the horizontal meridian. 

Many factors affecting the visual field:

1.Mobility of the object in the visual field.
2.Illumination of the object.
3.Size of the object.
4.Color of the object. 

The significances of diagnosis for the visual field test:

The visual field examination is important to detect many diseases that affect the eye, the optic nerve or the brain. Visual fields can help diagnose brain tumors, strokes and other conditions.

This eye exam will reveal if you have a loss of peripheral vision and help your doctor diagnose the cause.What Abnormal Results Mean?

Abnormal results may be due to diseases or central nervous system disorders such as tumors that damage or compress the parts of the brain that deal with vision.
Other diseases that may affect the visual field of the eye include:
  • Diabetes 
  • Glaucoma 
  • High blood pressure 
  • Multiple sclerosis 
  • Optic glioma 
  • Overactive thyroid (hyperthyroidism) 
  • Pituitary gland disorders 
  • Stroke 
  • Stroke secondary to cardiogenic embolism 
  • Stroke secondary to carotid dissection 
  • Stroke secondary to cocaine 
How the Test is Performed in Clinic?

Confrontation visual field exam: This is a quick and basic check of the visual field.  
  • You will cover one eye, and stare straight ahead with the other. 
  • You will be asked to tell when you can see the examiner's hand. 
  • Ask the pt to cover one eye
  • Cover your opposite eye
  • Ask the pt to look straight ahead
  • Place one hand in the plane between the patient and the examiner out of your vision
  • Move the hand and ask the patient when he/she can see your hand
  • This is to be completed in all 8 cardinal directions
  • Repeat for the other eye
Automated perimetry
  • You sit in front of a concave dome and stare at an object in the middle. 
  • You press a button when you see small flashes of light in your peripheral vision. 
  • Your responses help determine if you have a defect in your visual field. 

Master the method that mensurates the visual field.
(Goldmann field exam)

APPARATUS We Use:      
a. Perimeter
b. Perimetry marker
c. Perimetry chart 
The perimeter consists of a semicircular concave metallic arc. This arc can be rotated on a pilot in any direction along with the test object. 
On the back of this arc, this arc is marked with the degree from 0 to 100, which will be give the extent of the visual field. 

Tangent screen or Goldmann field exam: 
  • You will sit about 3 feet from a screen with a target in the center. 
  • You will be asked to stare at the center object and let the examiner know when you can see an object that moves into your side vision
  • This exam creates a map of your entire peripheral vision. 
Instructions to Experimenters

Using the perimeter and the colour disks, apply the Method of Limits to map the colour sensitivity of the subject's retina by doing the following.
1. Pick an angle and colour at random. Set the  
    perimeter to the angle of approach to be tested.
2. Put the coloured disk in the wand and show it to 
    the data collector but not the subject.
3. Slowly, steadily move the disk along the arm from 
    90 degrees toward the central point (at 0 degrees).
4. Stop when the subject correctly reports the colour. 
    (The data collector should read from the arm the degrees from centre and record this as the ‘in’ measure.)
5. Repeat this procedure and steadily moving the disk 
    outward until the subject reports no longer being 
    able to see the colour. (Data collector should record  
    the degrees from centre as the ‘out’ measure.) 
6. Test all the angles marked on the data sheet 
    moving both in and out using the blue, white and 
    green disks.
  • Never test more than two colours at given  
  • angle before selecting a new angle.
  •  If the subject shifts his eye or if the difference between the 'in' and the 'out' measures is greater than ten degrees, discard the results and retest that angle later. 
  • Also if the subject is clearly guessing, discard the results for those trials where this happens and retest later.

Retinal Mapping Data
Collect the data by using following table

Analysis Sheet 1
the Norms
the different colours zones on this ‘normative map’.

Analysis Sheet 2
Use means for each colour for each angle from the data collected from your subject to draw his or her map and compare to the normative map.
1. Use the appropriately coloured pencil to place dots  
   on the rays for the angles tested (0, 45, 90, 135, 
   180, 225, 280, 315).
2. Connect the dots using a straight-edge. 
3. Ignore the normative lines on this map.

Subject should not be able to predict stimulus

Normative findings (see Analysis Sheet 1)
Zero angle most extensive
white, Blue then Green zone

Warning about data interpretation

A. Inadequate sample size –can’t generalize from a  
    single subject; don’t be surprised if your subject 
    looks different from the norms
B. Lighting conditions –more light the better; cones 
    need bright light to detect colour
C. Graph distortion –graphing 3D retina on 2D   
    paper; just as an atlas is a distorted view of the 

Here are some important question just for your own review and test

 1.What is the visual field?
 2.Describe the characters of the human 
    visual field. 
 3.What factors affect the field of vision?
 4.What are the significances of diagnosis  
    for the visual field test?

I hope this post will help you.Stay in touch for more knowledge.

1 comment:

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