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Understanding the flow of visual information allows for the localization of visual deficits to particular regions of the visual pathway (see Fig 6–1).
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A visual deficit limited to just one eye (i.e., the other eye sees the whole world normally when the problematic eye is closed) must be due to either ocular pathology (e.g., lens, anterior chamber, retina) or pathology of the optic nerve.
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A visual deficit limited to a particular visual field in both eyes (i.e., the same deficit in each eye—a homonymous deficit) localizes posterior to the optic chiasm: optic tract, LGN, optic radiation(s), or occipital lobe (on the side contralateral to the homonymous deficit).
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An optic tract or LGN lesion will cause a contralateral homonymous hemianopia. Lesions of the optic tract may cause incongruous visual field deficits that are on the same side in each eye (homonymous) but of different shapes in each eye. Isolated lesions in the optic tract or lateral geniculate nucleus are rare in practice.
Lesions affecting an individual optic radiation or individual bank of the calcarine cortex will cause a contralateral visual deficit in one quadrant in both eyes (quadrantanopia):
A superior quadrantanopia localizes to the contralateral inferior radiation and/or inferior bank of the calcarine cortex.
An inferior quadrantanopia localizes to the contralateral superior radiation and/or superior bank of the calcarine cortex.
Lesions of one occipital lobe that affect both the superior and inferior portions of the calcarine cortex lead to a contralateral homonymous hemianopia, as would a lesion of the optic tract or lateral geniculate nucleus on that side. Infarction of one occipital lobe (due to stroke in the posterior cerebral artery territory) may produce a homonymous hemianopia with macular sparing (preserved central vision). This may be due to the macula receiving some blood supply from the middle cerebral artery and/or the macula being bilaterally represented since it is at the center of vision and, therefore, does not fall into one “field.”
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The region where a lesion creates a more complex visual deficit is at the optic chiasm. As described above, the chiasm is where information from the medial retina from each eye/optic nerve (representing the lateral visual field in each eye/optic nerve) crosses. Therefore, a lesion affecting this crossing information causes loss of vision in the bilateral lateral (temporal) visual fields, with sparing of the bilateral medial (nasal) visual fields. This pattern of visual loss is referred to as bitemporal hemianopia, and causes loss of peripheral vision on both sides.
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It is important to note that to elicit bitemporal hemianopia at the bedside requires visual field testing of each eye individually. Take the example of an object in the left visual field: It is seen by the left nasal retina (information from here crosses in the chiasm) and the right temporal retina (information from here does not cross in the chiasm). Therefore, if there is a bitemporal hemianopia due to a chiasmal lesion, the visual information in this example (an object on the left) projected on to the medial (nasal) retina of the left eye will not “get through,” and so will not be seen. However, the same left visual field information projected onto the temporal retina of the right eye will “get through,” since this information does not cross at the chiasm. So if the visual fields are tested with both eyes open (i.e., rather than by having the patient cover one eye to test each eye separately) in a patient with a bitemporal hemianopia, an object in the left visual field will still be seen by the right eye/optic nerve and right hemisphere. If the right eye is covered so the left eye is tested in isolation, then the temporal field deficit in the left eye will be revealed (since the right eye information that does not traverse the chiasm is not available to the brain when the right eye is covered). Therefore, a bitemporal hemianopia may be missed if visual fields are tested with both eyes open, but should be evident if each eye is tested separately with one eye covered.
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A junctional scotoma occurs when the optic nerve is affected at its junction with the chiasm. This produces an ipsilateral central scotoma (from optic neuropathy) and contralateral superior temporal quadrantanopia because the inferior nasal fibers (representing the superior temporal quadrant) loop anteriorly into the most distal portion of the contralateral optic nerve after crossing before proceeding posteriorly. The portion of the pathway that loops anteriorly is called Wilbrand’s knee.
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Another bilateral syndrome of the optic nerves is Foster Kennedy syndrome. In this syndrome, an enlarging mass lesion compresses the optic nerve on one side leading to optic neuropathy (with optic nerve pallor on examination due to chronicity), and the elevated intracranial pressure caused by the mass results in papilledema on the contralateral side.
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Monocular Visual Loss
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The causes of monocular visual loss include problems of the lens (e.g., cataract), anterior chamber (e.g., uveitis), retina (e.g., retinal ischemia, diabetic retinopathy), and optic nerve (i.e., optic neuropathy [see below for differential diagnosis]). In general, monocular vision loss is classified as acute or nonacute, and painful or painless. Sudden monocular visual loss is generally vascular in etiology (e.g., central retinal artery occlusion, central retinal vein occlusion, ischemic optic neuropathy), and all other causes are generally subacute to chronic in presentation. Vascular causes of monocular visual loss such as retinal ischemia and ischemic optic neuropathy are generally painless. (Patients with visual loss due to giant cell arteritis often have headache, although they do not typically have eye pain.) Painful monocular visual loss occurs with acute angle closure glaucoma and optic neuritis.
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Optic neuropathy generally causes blurred vision centrally (central scotoma or cecocentral scotoma if it extends to the blind spot), decreased color vision, decreased visual acuity, and an afferent pupillary defect (see Chapter 10 for explanation of afferent pupillary defect). If optic neuropathy is due to an inflammatory cause (optic neuritis), optic nerve swelling may be visible on fundoscopy (unless the inflammation is more posterior in the optic nerve (retrobulbar), in which case the optic nerve may appear normal). Chronic optic neuropathies cause optic nerve pallor.
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Hyperacute to acute causes of optic neuropathy include:
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Acute to subacute causes of optic neuropathy include:
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Elevated intracranial pressure with papilledema
Inflammatory (optic neuritis; see Ch. 21):
Demyelinating disease: multiple sclerosis, neuromyelitis optica
Systemic inflammatory disease: sarcoidosis, vasculitis, paraneoplastic (anti-CRMP-5 antibodies)
Infectious: syphilis, Bartonella (cat-scratch disease)
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Subacute to chronic causes of optic neuropathy include:
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Neoplasm or treatment of neoplasm:
Intrinsic: optic nerve glioma
Extrinsic: compression from optic nerve sheath meningioma, skull base meningioma, pituitary tumor, leptomeningeal metastases
Radiation-induced optic neuropathy
Toxic:
Metabolic: vitamin B12 deficiency
Hereditary: For example, Leber’s hereditary optic neuropathy (a mitochondrial condition)
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In general, ischemic, neoplastic, and inflammatory causes of optic neuropathy present unilaterally. Although demyelinating disease may affect both optic nerves, this generally occurs sequentially rather than simultaneously. Infectious causes of optic neuropathy can present unilaterally or bilaterally. Toxic, metabolic, and hereditary causes generally present bilaterally. Optic neuritis is typically painful (with worsening ocular pain with eye movement), whereas other etiologies of optic neuropathy are typically painless. Optic neuritis is discussed in Chapter 21.
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Bitemporal Hemianopia
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The most common cause of chiasmal lesions is pituitary pathology, but sellar menigiomas, craniopharyngiomas, and aneurysms (e.g., of the distal carotid) can also occur in this region. Figure 6–2 shows an MRI from a patient who presented with bitemporal hemianopia and was found to have a pituitary macroadenoma compressing the optic chiasm.
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Homonymous Visual Field Deficits
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The causes of lesions of the lateral geniculate nucleus (LGN), optic radiations, and occipital lobes include any diseases that can affect the cerebral hemispheres: e.g., vascular, neoplastic, infectious, inflammatory. The LGN is in the territory of the anterior choroidal artery (a branch of the internal carotid artery), the superior radiation is in the territory of the middle cerebral artery, the inferior radiation is in the territory of the middle and posterior cerebral arteries, and the occipital cortex is in the territory of the posterior cerebral artery (see Ch. 7).