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VISUAL FIELD DEFECTS |
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The neuroanatomy of the visual system allows for precise localization
for many visual field defects. In order to understand how a given lesion results
in a visual field defect, it is imperative to have a firm grasp of the
neuroanatomy of vision.
An important rule to remember when assessing visual field defects is that
a visual field defect involving only one eye, is from a
lesion anterior to the optic chiasm. This is due to the fact that
none of the fibers of the visual pathways originating from the ipsilateral eye
cross until they reach the optic chiasm. On the other hand,
if both visual fields are affected, then the lesion is
either at or beyond the optic chiasm.
The diagram below shows the visual field defect resulting from a given
lesion. Note, that the visual field defects are seen from the patient’s
perspective. Thus, when viewing them, imagine that you are looking from behind
the patient’s eyes. Try to figure out what visual field defect will result from
each lesion. Place the cursor over the lesion,
the location of the lesion will appear. Then click the lesion, and the visual
field deficit will appear with an explanation. |
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Differential diagnosis for given lesion of the visual
pathways:
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Optic nerve |
Demyelinating (2nd to
multiple sclerosis), inflammatory (sarcoidosis, lupus), tumor (menigioma,
glioma), vascular (temporal arteritis, anterior ischemic optic
neuropathy), toxic |
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Optic chiasm |
Tumor (pituitary adenoma,
craniopharygioma, menigioma), aneurysm (ICA), infectious (cystercercosis
cyst) |
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Optic tracts |
Stroke, tumor, aneurysm |
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Lateral geniculate
body |
Stroke (anterior choroidal artery),
demyelination |
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Optic radiations |
Stroke, tumor, s/p temporal lobe
epilepsy surgery, infectious (abscess) |
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Occipital cortex |
Vascular (stroke, AVM), tumor, trauma,
infectious (abscess) |
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The device above performs Humphrey
visual field testing, which is an automated visual field test. The
disadvantage to this type of testing is that it requires significant patient
cooperation. The patient sits in front of the concave dome seen on the left and
stares at a central target. The computer program flashes small lights at
different locations. The patient presses a button each time a flash of light is
detected. The responses are compared to age-matched controls and a print-out is
made showing the visual field as seen by the patient. |
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