Chapter 7: The Visual System

CLINICAL CASE | Homonymous Hemianopsia

A 70-year-old woman suddenly developed difficulty seeing on the left side. She was taken to the emergency room. She reported that when she looked directly at her husband, she only saw the right side of his face. On testing in the emergency room, she was found to have a homonymous hemianopsia field defect. The patient did not have left hemineglect or difficulty drawing shapes or in describing the spatial relationships between objects. Figure 7–1A is an MRI showing damage to the right medial occipital lobe and underlying white matter. This was produced by occlusion of the posterior cerebral artery distal to the thalamus.

Answer the following questions based on your reading of the chapter, inspection of the images, and consideration of the neurological signs.

1. Draw the visual field impairment.

2. Explain why the patient does not have macula sparing.

3. Why is it that the patient's visual spatial aptitude is not impaired and that she does not have hemineglect?

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 occipital lobe. The MRI shows a lesion of the medial occipital lobe. The primary and higher-order visual cortical areas are located here (see Figure 7–15). The basic visual loss is attributed to primary visual cortex damage.

After occlusion of the occipital branch of the posterior cerebral artery, a distal artery supplying more selectively the visual cortex, the occipital pole may be relatively unaffected. This is because of collateral blood supply from the middle cerebral artery. Figure 7–1B shows this overlap schematically. In the patient, the infarction was not limited to the visual cortex gray matter; there was some involvement of the optic radiations.

Damage to the region of the right posterior parietal and occipital lobes can produce hemineglect and disorders of visuospatial aptitude. This could be produced by occlusion/hemorrhage of superficial branches of the middle cerebral artery, which supplies the posterior parietal lobe, or the deep branches of these two arteries, which supply parts of the underlying white matter. These impairments are notably absent in this patient. This is because the lesion spares the posterior parietal lobe and the lateral occipital lobe.