Defects in the Visual Fields

The pattern of a visual field defect gives useful localising information for lesions in the visual pathway. The right half of each retina is linked by nerves to the right occipital cortex and the splitting of nerve fibres from each half occurs at the chiasm. For this reason, lesions in the optic nerve anterior to the chiasm tend to cause unilateral defects, whereas those posterior to the chiasm produce hemianopic or quadrantianopic defects (Figure 22.5). Cortical lesions tend to be more congruous. That is to say, the blind areas on each side tend to be similar in shape and size. Cortical lesions also show better preservation of central vision ("macular sparing"). A special type of field defect is seen with expanding pituitary tumours, the resulting pressure on the centre of the chiasm producing a bitemporal defect. Localised defects in the retina produce equivalent localised defects in the visual field on the affected side. Defects because of ocular disease are relatively common as, for example, those seen in the elderly with glaucoma. Care must be taken to interpret field defects with this possibility in mind. Notice from the diagram in Figure 22.5 that the right half of the visual field is represented in the left half of each retina and thus, the left half of the brain. This complies with the general rule that events occurring on the right side of the body are represented on the left side of the brain. It is surprising how patients might be unaware of a severe visual field defect, especially in hemi-anopia, providing that the macula is spared (Figure 22.6).

My car keeps knocking my gate post. (Hemianopes should never drive)

Figure 22.6. The effects of hemianopia.Q

My car keeps knocking my gate post. (Hemianopes should never drive)

Figure 22.6. The effects of hemianopia.Q

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