Module 11, Section 2

	Module 11 Section 2
Home	Catalog	Section 1
Contact Us	Post-test	Section 3

Module 11:	The Visual Pathways and Visual Field Defects
Section 2:	Visual Field Defects as they relate to the Pathways


Damage to the pathways that are anterior to optic chiasm Damage to the optic nerve chiasm Damage to the pathway behind the optic chiasm

The visual pathway is a "wiring diagram" showing how the visual signal reaches the brain from the eye. The peculiarities of the diagram, particularly the crossing of some of the nerve fibers at the optic chiasm, provide important clues to the ophthalmologist when diagnosing diseases of the eye. In some disease processes of the eye a defect is caused in our field of vision. This defect can be in the form of a field cut or a scotoma. A field cut is a defect that moves the border of our visual field inward. A scotoma is a defect within the border of our visual field. Field defects can be in one eye or both eyes, large or small, dense or slight, of various shapes, and in various locations. Because of the characteristics of the "wiring" of the eye, the characteristics of visual field defects give clues as to the location of the problem. The diagram of the visual pathways from Section 1 is presented here again for you to refer to.


Figure B: 1 visual field, 2,3 retina, 4 optic nerve, 5 optic chiasm, 6 optic tract, 7 lateral geniculate body, 8 optic radiations and occipital cortex.

Damage to the pathways that are anterior to optic chiasm Damage to the pathways that are anterior to optic chiasm produce monocular defects. An example would be retinal damage such as macular degeneration that produces a central scotoma (figure below).



Another example would be a retinal detachment that produces a "curtain" defect or field cut as pictured below.



Another example of a disease process anterior to the chiasm would the monocular central field defect produced by optic nerve atrophy in one eye.



Nerve fiber layer defects such as those caused by glaucoma (figure below) are also monocular in nature. The arcuate defects are many times seen in both eyes because the disease often affects both eyes.



Arcuate defects may start out as an extension of the blind spot or as a nasal step as depicted below.



Swelling of the optic nerve head due to papilledema or optic nerve head drusen will produce an enlarged blind spot, either monocularly or in both fields if both eyes are involved. In the figure below, the right eye field has an enlarged blind spot.



Damage to the optic nerve chiasm Damage to the optic nerve chiasm may result in a bitemporal defect because it is the crossing fibers that are the most vulnerable to damage. It is the nerve fibers from the nasal half of each eye that cross at the chiasm, affecting the temporal visual field areas. Chiasmal damage is often caused by pituitary gland enlargement. The chiasmal defect pictured below is termed a bitemporal hemianopsia. "Bitemporal" means that it affects the temporal area of both eyes, and "hemianopsia" is the term for a defect affecting half of the eye.



Damage to the pathway behind the optic chiasm Any damage to the pathway behind the optic chiasm most often produces either field defects that affect the right side of the patient’s vision in both eyes, or field defects that affect the left side of the patient’s vision in both eyes. Remember that because of the fiber crossings at the chiasm, the brain perceives the right half of the visual field in the left side of the brain and vice-versa. The field defect pictured below is termed a right complete homonymous hemianopsia. It affects the right side of both fields (homonymous) and it is a complete loss of half the visual field (hemianopsia). Such a defect is often caused by a stroke. The stroke in this case would be in the left side of the brain.



Homonymous (affects the same half of the field in each eye) field defects that are incomplete or incongruous give the ophthalmologist clues as to the location of the damage in the nerve pathway. The field defect pictured below would be termed a left superior incongruous homonymous quadrantanopia. left – on the left side of each field superior – on the superior half of each field incongruous – not exactly the same in each eye homonymous – on the same side of each eye quadrantanopia – affects a quadrant of each eye Defects that are identified as inferior or superior are sometimes referred to as altitudinal defects. Pre-chiasmal altitudinal defects generally respect (do not cross) the horizontal meridian and post-chiasmal altitudinal defects generally respect the vertical meridian (as pictured below).



Altitudinal defects can be described according to visual quadrant, as pictured below. If only describing the defect in the left eye pictured above, it can be termed a superior temporal field cut or a sector defect in the left superior visual field.



Large, sector type defects can be detected by any of the conventional methods of measuring the visual field: automated, Goldmann, or tangent screen. Small macular defects can be mapped with the Amsler Grid. The confrontation visual field test is meant to be a screening device for the larger field defects such as a hemianopsia , a generalized constriction, or a sector defect such as would be produced by a retinal detachment. In some cases the map produced by a confrontation field exam is sufficient documentation of the defect. Most of the time a patient with a large defect will be acutely aware of the problem because of the obvious visual limitations, and he will be able to tell you were the defect is. Optic nerve and nerve fiber layer defects (glaucoma) are very subtle and are not noticed by the patient until much vision is lost. The ophthalmologist uses the precision of an automated field test to follow the progress of these defects.

Back to top Go to Section 3