Visual pathways and its disturbances
Visual disorders in humans may be as result from lesions following anatomical structures:
- optic nerve,
- optic chiasm,
- optic tract,
- lateral geniculate body,
- geniculate-occipital tract,
- the visual center of the cerebral hemisphere's occipital lobe cortex.
Visual field defects and lesion sites are here illustrated for the left visual pathway: 1 - unilateral lesion of optic nerve, 2 - lesion of optic chiasm, 3 - unilateral lesion of optic tract, 4 - unilateral lesion of optic radiation in Meyer's loop (anterior temporal lobe), 5 - unilateral lesion of optic radiation, medial part, 6 - lesion of occipital lobe, 7 - lLesion of occipital pole (cortical areas).
Lesions of the visual pathway:
Visual field defects
|1. Unilateral lesion of optic nerve||Blindness in affected eye|
|2. Lesion of optic chiasm||Bitemporal hemianopia ("blinders")|
|3. Unilateral lesion of optic tract||Contralateral homonymous hemianopia|
|4. Unilateral lesion of optic radiation in Meyer's loop (anterior temporal lobe)||Contralateral upper quadrantanopia ("pie-in-the-sky")|
|5. Unilateral lesion of optic radiation, medial part||Contralateral lower quadrantanopia|
|6. Lesion of occipital lobe||Homonymous hemianopia|
|7. Lesion of occipital pole (cortical areas)||Homonymous hemianopic central scotoma|
Retinal lesions cause the appearance of arcuate cattle (focal loss of visual fields), defined as the "islands" loss of sight, which are directed towards the blind spot or come out of it. Arcuate (bundle of nerve fibers) defects are clearly border on the horizontal centerline, and its extensive dimensions lead to the loss of half of the visual field. Damage of papillomacular bundle, providing central fixation, leads to the appearance of the central (damage of fixation point) or central cecal (damage of fixation point and the blind spot) scotoma. In case of macular damage, small size central scotoma often leads to impairment of vision, visual perception, characterized by distortion of the shape and size of visible objects, especially straight lines (metamorphopsia), which distinguishes macular damage from the optic nerve damage.
Central cecal scotoma is a frequent feature of the specific lesions of the optic nerve, which causes may be internal (demyelinating, infiltrating and degenerative processes) and external compression (aneurysm, tumor) factors. Toxic effects (methyl alcohol, quinine, some phenothiazine series tranquilizers), nutritional disorders (tobacco and alcohol blindness), lead to the formation of relatively symmetrical bilateral central or central cecal cattle. Progressive generalized constriction of peripheral isopters with relative preservation of central vision may be a consequence of the annular compression by tumor, such as the optic nerve sheath meningioma. Spiral narrowing or "pipe vision" of inorganic origin (hysteria, simulation) are stored in the study of patientís vision from any distance. In the case of organic lesions, outer diameter of the visual field defects will increase with the distance between the eye and the object under study.
Visual field defect localized in one-half of the visual field of each eye - is hemianopsia. At hemianopsia there is a clear border in a vertical line.
Bitemporal hemianopia points to the lesion of crossing fibers of the nasal portion of the retina in the optic chiasm, usually due to compression of the optic chiasm (with pituitary tumors, craniopharyngioma, sellar diaphragm meningioma, suprasellar aneurysm of the arteries of the circle of Willis).
Homonymous hemianopia (loss of left or right half of the visual fields) occurs in lesions of the visual pathway above the chiasm, in case of complete hemianopsia does not allow to determine the exact location. Incomplete homonymous hemianopia more specifically refers to the possible place of lesion:
- if the visual field defects are identical on both sides, it is likely that the lesion is localized in the cortex of calcarine sulcus;
- if there is a mismatch defects (asymmetry), it is likely that there was lesion of the optic tract fibers, lateral geniculate body or parietal or temporal lobe's optic radiations.
Lesions of the optic tract characterized by the development of asymmetric homonymous hemianopsia. Chronic injury tract accompanied by impairments of afferent pupil reaction to light and transverse optic atrophy on the opposite side.
Collected light information transmitted from the retina along the optic nerve to the brain for processing and analysis of the data with subsequent perception.
In the case of lesions of the visual pathway above the lateral geniculate body, pupillary reflexes are preserved.
Nerve fibers of the lower quadrants of the retina project to the temporal lobe, and this lobe lesion can cause upper quadrant homonymous hemianopsia.
Parietal lobes lesions more effect in the lower quadrants than in the upper; it can also lead to hemianopsia due to carelessness.
Total homonymous hemianopia with the destruction of fibers coming from the macular, develops when the cerebral hemispheres cortical areas is damaged in the calcarine sulcus on one side. Macular preservation of is often due to imperfect fixation.
Bilateral homonymous hemianopia is the result of bilateral lesions of the visual cortex, usually ischemic, in the posterior cerebral arteries blood supply areas. May develop persistent cortical blindness. In such patients, observed Anton's syndrome: bilateral blindness, denial of visual loss, normal pupillary reflexes and bilateral infarcts in the occipital-parietal areas.
Other central vision disorders include various types of distortion of the image in which objects appear to be either too small (micropsia) or too large (makropsiya) or crooked. For two-sided symptoms is most likely lesion of the temporal lobes; in this case, visual impairments appear during attacks and accompanied by complex visual hallucinations or other manifestations of temporal lobe epilepsy.
During optic neuritis (neuropathy), inflammatory process is damages retrobulbar myelin sheath along its entire length from the optic disc to the optic nerve junction (optic chiasm).
In addition to the study of the visual fields, for evaluation lesions of the anterior segment of the visual pathway is used functional electrophysiological methods:
- electroretinography (ERG) with photostimulation,
- electroretinography with a checkerboard pattern (P-ERG),
- visual evoked potentials (VEP) to stimulation with a checkerboard pattern.
During the electroretinography (ERG) electrical potentials measured from each layer of the retina that allows to identify lesions of the retina before the appearance of changes in the fundus, as, for example, retinitis pigmentosa. However, electroretinography (ERG) does not detect changes caused by lesions of the retinal ganglion cells and afferent divisions of the visual pathway. Data with a checkerboard pattern electroretinography (P-ERG) provide an indication of the activity of the ganglion cells, which reduced or disappears completely when the optic nerve lesions developing because of ganglion cells retrograde death (Leber's optic atrophy, demyelination).
Visual evoked potentials (VEP) characterized by a macular predominant reaction defined in the occipital of the brain cerebral cortex pole. In the absence of retinal lesions, visual evoked potentials (VEP) provide an opportunity to assess the functioning of the first segment of the visual pathway to the lateral geniculate body, and especially the optic nerve.
Visual evoked potentials (VEP) is extremely helpful in diagnosis of multiple sclerosis, allowing establishing the presence of optic nerve damage even in the absence of other symptoms of visual impairment.
If you have any questions, you can ask them to our neurosurgeon or neurologist: (499) 130–08–09
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