The Visual System Flashcards
The left brain receives visual information from ___, and the right brain receives visual informaton from ___.
The left brain receives visual information from the right visual fields of both eyes, and the right brain receives visual informaton from the left visual fields of both eyes.
Movement of visual information through the optic tract (diagram)
Lateral geniculate nucleus
Region of the thalamus that receives information from a visual field.
One on each side, carrying contralateral visual field information.
Monocular visual deficit
Due to either ocular pathology or optic nerve (pre-chiasm) pathology
Homonymous visual deficit
ie, one visual field in both eyes
Posterior to the optic chiasm, in the optic tract, LGN, optic radiation(s), or occipital lobe (on the side contralateral to the homonymous deficit
Quadrantanopia
A visual deficit in one quadrant in both eyes
Caused by lesions affecting an individual optic radiation or individual bank of the calcarine cortex on the contralateral side
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.
Homonymous hemianopia
Incongruous visual field deficits that are on the same side in each eye (homonymous) but of different shapes in each eye
Caused by a contralateral optic tract lesion, contralateral LGN lesion, or lesion in the contralateral occipital lobe
Isolated lesions in the optic tract or lateral geniculate nucleus are rare in practice.
Homonymous hemianopia with macular sparing
Due to infarction of one occipital lobe from stroke in the posterior cerebral artery territory
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.”
Bitemporal hemianopia
Loss of both lateral visual fields.
This is the result of damage to the optic chiasm, causing loss of peripheral vision on both sides.
Ipsilateral central scotoma and contralateral superior temporal quadrantanopia
This combination is called a “junctional scotoma,” and it 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.
In general, monocular vision loss is classified as . . .
In general, monocular vision loss is classified as acute or nonacute, and painful or painless.
Sudden and painless monocular visual loss is generally ___ in etiology
Sudden and painless monocular visual loss is generally vascular in etiology
Painful monocular visual loss often occurs with. . .
. . . acute angle closure glaucoma and optic neuritis.
Signs of optic neuropathy
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.
Optic neuritis sign
Usually on top of optic neuropathy signs
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 ___ on exam
Chronic optic neuropathies cause optic nerve pallor on exam
Anti-CRMP-5 antibodies
May be produced as a part of a paraneoplastic syndrome in small cell lung cancer
Known to cause paraneoplastic neurological syndromes, including encephalitis, myelitis and neuropathy, including of the optic nerve. May cause acute to subacute optic neuropathy.
Anti-HMGCR antibodies
Anti-HMG-CoA-Reductase
The cause of autoimmune statin-induced rhabdomyolysis.
Occurs often about ~3 years into statin usage and is more likely with atorvastatin and more likely in patients with type II DM. Hip flexor weakness often observed at presentation.
Vision problems caused by vitamin deficiency
Vitamin A deficiency may cause vision loss due to its role in retinal function, and vitamin B12 deficiency may cause vision loss due to optic nerve pathology.
Vitamin A deficiency is likely to present bilaterally at onset while b12 deficiency is likely to present sequentially, one eye at a time.
In general, ischemic, neoplastic, and inflammatory causes of optic neuropathy present ___.
In general, ischemic, neoplastic, and inflammatory causes of optic neuropathy present unilaterally.
Vascular territories of structures important for vision
- 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
Visual information from the primary visual cortex (at the occipital pole) is transmitted . . .
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).
Alexia and alexia without agraphia
The left “what” pathway (connection between occipital lobe and left temporal lobe) 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).
Prosopagnosia
The right “what” pathway (connection between the occipital lobe and right temporal lobe) 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
- Occurs with lesions of the bilateral parieto-occipital junctions (e.g., due to MCA-PCA watershed/borderzone infarcts)
- Syndrome is characterized by a triad of signs that are manifestations of deficits in visual attention:
- optic ataxia,
- ocular apraxia,
- and simultanagnosia.
Optic ataxia
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
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
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
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.
Anton syndrome
Some patients with cortical blindness are unaware that they are blind and may deny being unable to see
Charles Bonnet syndrome
In patients with bilateral visual loss of any cause (most commonly ocular in older adults), patients may develop “release” hallucinations
These hallucinations are generally of small people, are not threatening to the patient, and the patient usually knows they are not real.
The lingula and cuneus
The lingula, inferior to the calcarine fissure, accepts fibers from the inferior radiations from the temporal lobe, and thus processes superior visual field information.
The cuneus, superior to the calcarine fissure, accepts fibers from the superior radiations from the parietal lobe, and thus processes inferior visual field information.
