Chapter 6: The Visual Pathway and Approach to Visual Loss

The optic nerve is the output of the retina. Therefore, each optic nerve carries all of the visual information from the eye from which it emerges. Since the left hemisphere moves the right side of the body and the right hemisphere moves the left side of the body, it makes sense that the left hemisphere should receive the visual information from the right half of the world and the right hemisphere should receive the visual information from the left half of the world. Therefore, some of the information in each optic nerve must cross so that the brain can work with the left and right visual fields rather than merely what is seen by the left and right eyes: The left hemisphere must receive right visual field information from both the left eye and the right eye; the right hemisphere must receive left visual field information from both the left eye and the right eye. The crossing of visual field information to convert the visual world from left eye–right eye organization to left field–right field organization occurs at the optic chiasm. Posterior to the optic chiasm, visual information is organized into fields: the left visual field is processed in the right hemisphere, and the right visual field in the left hemisphere.

Which visual information from each eye needs to cross to convert the organization of visual information from eyes to visual fields? Imagine viewing a simple rectangle that is half tan and half blue (Fig. 6–1). The right visual field (blue) projects onto the medial (nasal) retina in the right eye and the lateral (temporal) retina in the left eye. The left visual field (tan) projects to the medial (nasal) retina in the left eye and the lateral (temporal) retina in the right eye. The medial (nasal) retinas are, therefore, receiving the lateral (temporal) visual fields, and the lateral (temporal) retinas are receiving the medial (nasal) visual fields.

All visual information from the right visual field must end up in the left hemisphere, and all visual information from the left visual field must end up in the right hemisphere. The right visual field information in the left optic nerve (from the lateral retina) is already on the correct (left) side of the brain, but the right visual field information in the right optic nerve (from the medial retina) must cross from the right optic nerve to the left side of the brain to join it. The left visual field information in the right optic nerve (from the lateral retina) is already on the correct side of the brain, but the left visual field information in the left optic nerve (from the medial retina) must cross from the left optic nerve to the right side to join it.

In summary, the information seen by the lateral (temporal) retina of each eye is from the contralateral visual field, and so it is already on the correct side of the brain in the optic nerve and does not need to cross. The information in the medial (nasal) retina in each eye is from the ipsilateral visual field, so that information needs to cross at the chiasm. Therefore, at the chiasm, the information from the medial (nasal) retinas (representing the lateral [temporal] visual fields) crosses, and the information from the lateral (temporal) retinas (representing the medial [nasal] visual fields) remains ipsilateral.

Posterior to the optic chiasm, the visual world is separated into left and right visual fields, with the left visual field represented in the right hemisphere, and the right visual field represented in the left hemisphere. This information travels in the left and right optic tracts from the chiasm to the left and right lateral geniculate nucleus (LGN) of the thalamus. From each LGN, information travels to the visual cortex of the occipital lobes in a superior radiation and an inferior radiation on each side. Just as the left visual field goes to the opposite (i.e., right) side of the brain and the right visual field goes to the opposite (i.e., left) side of the brain, the inferior visual fields travel in the superior radiations to the superior bank of the calcarine cortex in the posterior occipital lobe, and the superior visual fields travel in the inferior radiations to the inferior bank of the calcarine cortex. The superior radiations travel through the parietal lobes to the occipital lobes, and the inferior radiations travel through the temporal lobes to the occipital lobes. The inferior radiations loop anteriorly before proceeding posteriorly, and this anterior loop is called Meyer’s loop.

Understanding the flow of visual information allows for the localization of visual deficits to particular regions of the visual pathway (see Fig 6–1).

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.

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).

• 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.”

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.

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.

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.

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.

Monocular Visual Loss

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.

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.

Hyperacute to acute causes of optic neuropathy include:

• Ischemic:

  • Anterior ischemic optic neuropathy (AION):

    • Arteritic AION: giant cell arteritis or systemic vasculitis

    • Nonarteritic AION: small “crowded” optic disc increases risk; caused by hypotension and/or vascular risk factors

  • Posterior ischemic optic neuropathy (PION): usually caused by hypotension (e.g., postsurgical, especially with prone positioning during surgery)

• Traumatic: severe head trauma can cause shearing of optic nerves

Acute to subacute causes of optic neuropathy include:

• 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)

Subacute to chronic causes of optic neuropathy include:

• 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:

  • Medications: ethambutol, linezolid

  • Toxins: methanol, tobacco-alcohol amblyopia

• Metabolic: vitamin B12 deficiency

• Hereditary: For example, Leber’s hereditary optic neuropathy (a mitochondrial condition)

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.

Bitemporal Hemianopia

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.

Homonymous Visual Field Deficits

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).

Visual information from the primary visual cortex (at the occipital pole) is transmitted superiorly to the parietal lobe for spatial processing (the “where” pathway) and transmitted inferiorly to the temporal lobe for object identification/recognition (the “what” pathway). The left “what” pathway is specialized for processing of visual word forms, so lesions in the left inferior temporo-occipital region can lead to inability to read (alexia), sometimes with preserved ability to write (alexia without agraphia). The right “what” pathway is specialized for processing of faces, so lesions in the right inferior temporo-occipital region (in the fusiform gyrus) can lead to inability to recognize faces (prosopagnosia).

Balint Syndrome

Balint syndrome occurs with lesions of the bilateral parieto-occipital junctions (e.g., due to medial cerebral artery–posterior cerebral artery [MCA-PCA] watershed/borderzone infarcts; see Ch. 7). This syndrome is characterized by a triad of signs that are manifestations of deficits in visual attention: optic ataxia, ocular apraxia, and simultanagnosia.

Optic ataxia is an “ataxia” due to difficulty using visual attention to guide extremity movements. This can be demonstrated on the finger–nose test. Rather than the oscillatory movements seen in cerebellar ataxia, the patient’s movements appear to be misdirected, as if the patient cannot determine how to direct the finger to the target.

Ocular apraxia is an inability to use visual attention to guide eye movements. The patient will not be able to track the examiner’s finger, but may be able to move the eyes appropriately in response to commands such as “look left” and “look right.”

Simultanagnosia is an inability to visually “survey” a scene and see the “forest for the trees.” For example, if a patient is shown a drawing of one large letter or number composed of smaller versions of a different letter or number, the patient may see only the small letters or numbers but not the larger letter or number they create.

Cortical Blindness and Anton Syndrome

If enough of the occipital cortex is damaged bilaterally (e.g., bilateral posterior cerebral artery [PCA] strokes), this can lead to cortical blindness: the eyes and optic nerves still work (as can be proven by normal pupillary light reflexes), but the brain cannot decode visual information. Some patients with cortical blindness are unaware that they are blind and may deny being unable to see, a condition called Anton syndrome.

Charles Bonnet Syndrome

In patients with bilateral visual loss of any cause (most commonly ocular in older adults), patients may develop “release” hallucinations (Charles Bonnet syndrome). These hallucinations are generally of small people, are not threatening to the patient, and the patient usually knows they are not real.