CLINICAL CASE | Difficulty Seeing
A 70-year-old woman suddenly developed difficulty seeing. She was taken to the emergency room. A neurological examination was performed in which her vision was tested, using the visual field test (see Figure 7–3). She was asked to look directly at the examiner’s nose. The examiner asked the patient if his face appeared symmetrical. She reported that she did not see the left side of his face. Further testing revealed that she could not see an object brought in from the left side until it was at the midline. She also displayed the left-side visual impairment for each side, when the eyes were tested independently. The patient had an MRI (Figure 7–1A), which showed a lesion of the right medial occipital lobe, both the cortex and underlying white matter, produced by a stroke. The patient subsequently had a more precise examination of the visual fields of each eye (Figure 7–1B). The left diagram is the extent of the patient’s vision for the left eye; and the right diagram, for the right eye. The darker areas correspond to regions where there was no sight.
You should be able to answer the following questions based on your reading of the chapter, earlier readings, inspection of the images, and consideration of the neurological signs.
1. What is the visual field?
2. What is the name for the patient’s visual field impairment and explain the meaning of each component of the name of the impairment?
3. What cerebral artery became occluded in this patient?
4. Why is visual function affected on one side only?
5. What is macular vision and was macular vision spared on the affected side? Key neurological signs and corresponding damaged brain structures Homonymous hemianopsia
This is loss of sight in the contralateral visual fields (see Figure 7–3). It can be due to lesion of the visual pathway proximal to the optic chiasm, on one side: along the optic tract, the lateral geniculate nucleus, optic radiations, and the occipital lobe (see Figure 7–2). The T2 MRI shows a lesion of the medial occipital lobe. The primary and higher-order visual cortical areas are located here (see Figure 7–14). The basic visual loss is attributed to interrupting the pathway from the eyes to the primary visual cortex; both white matter damage and damage to the cortex itself. Lack of macular sparing
Macular vision is mediated by the most posterior part of the occipital lobe, at the occipital pole. After occlusion of the occipital branch of the posterior cerebral artery, a distal artery supplying the visual cortex more selectively than a proximal portion, the occipital pole may be relatively unaffected. This is because of collateral blood supply from the middle cerebral artery. Figure 7–1C shows this overlap schematically. It is important to note that this collateral supply protects the cortex only, not the underlying axons in the white matter. In the patient, ...