Visual pathway lesions | |
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Visual pathway lesions From top to bottom: 1. Complete loss of vision in the right eye 2. Bitemporal hemianopia 3. Homonymous hemianopia 4. Quadrantanopia 5.& 6. Quadrantanopia with macular sparing | |
Specialty | Ophthalmology, Neuro-ophthalmology, Neurology |
Symptoms | Loss of vision, Visual field defects, and Blindness |
Diagnostic method | Visual field test, Neuro-imaging |
The visual pathway consists of structures that carry visual information from the retina to the brain. Lesions in that pathway cause a variety of visual field defects. In the visual system of human eye, the visual information processed by retinal photoreceptor cells travel in the following way:
Retina→Optic nerve→Optic chiasm a (here the nasal visual field of both eyes cross over to the opposite side)→Optic tract→Lateral geniculate body→Optic radiation→Primary visual cortex
The type of field defect can help localize where the lesion is located (see picture given in infobox).
The optic nerve, also known as cranial nerve II, extends from the optic disc to the optic chiasma. Lesions in optic nerve causes visual field defects and blindness.
Causes of optic nerve lesions include optic atrophy, optic neuropathy, head injury, traumatic avulsion, acute optic neuritis etc. [1] [2]
The optic chiasm, or optic chiasma is the part of the brain where both optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. [7] Signs and symptoms associated with optic chiasm lesions are also known as chiasmal syndrome. Chiasmal syndrome has been classified into three types; anterior, middle and posterior chiasmal syndromes. [1] Another type is lateral chiasmal syndrome. [8]
Causes of chiasmal syndromes may be classified into intrinsic and extrinsic forms. [9] Intrinsic causes are due to thickening of the chiasm itself and extrinsic implies compression by another structure(gliomas, multiple sclerosis etc. [10] ). Other less common causes of chiasmal syndrome are metabolic, toxic, traumatic, inflammatory or infectious in nature (eg. lymphoid hypophysitis, sarcoidosis.) [1] Compression of the optic chiasm is associated with pituitary adenoma, [11] Craniopharyngioma, [12] Meningioma [13] etc.
The optic tract is a continuation of the optic nerve that relays information from the optic chiasm to the ipsilateral lateral geniculate nucleus (LGN), pretectal nuclei, and superior colliculus. [14] The optic tract represents the first stage in the visual pathway in which visual information is transferred in a homonymous nature. [15] Main characteristic feature of lesion involving whole optic tract is homonymous hemianopsia. A lesion in the left optic tract will cause right-sided homonymous hemianopsia, while a lesion in the right optic tract will cause left-sided homonymous hemianopsia.
The optic tract syndrome is characterized by a contralateral, incongruous homonymous hemianopia, contralateral relative afferent pupillary defect (RAPD), and optic atrophy due to retrograde axonal degeneration. [16] Causes of optic tract lesions are also classified into intrinsic and extrinsic forms. Intrinsic lesions include demyelinating diseases and infarction. Such lesions produce optic tract syndrome type II. [1] Extrinsic or compressive lesions are caused by pituitary craniopharyngioma, [17] tumours of optic thalamus. Other causes include syphilitic meningitis, gumma and tubercular meningitis etc. [1]
The lateral geniculate nucleus (LGN) is the nucleus in the thalamus that receives visual information from the retina and sends it to the visual cortex via optic radiations. A lesion of this nucleus produces moderately to completely congruent visual field defects. [20] Isolated lesions of the lateral geniculate nucleus are rare, it may be diagnosed by distinctive patterns of visual field loss. [15]
Pituitary adenoma compression may cause LGN degeneration. [21] Lesions affecting the anterior or lateral choroidal arteries may affect the lateral geniculate nucleus. [22]
The optic radiation are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex. [22]
Middle cerebral artery and posterior cerebral artery infarcts (including cerebral palsies) may affect the optic radiations, and can cause quadrantanopias. Also vascular occlusions, tumors, trauma, and temporal lobectomy for seizures. [23] [24]
The visual cortex located in the occipital lobe of the brain is that part of the cerebral cortex which processes visual information. [26] Cortical blindness refers to any partial or complete visual deficit that is caused by damage to the visual cortex in the occipital lobe. Unilateral lesions can lead to homonymous hemianopias and scotomas. Bilateral lesions can cause complete cortical blindness and can sometimes be accompanied by a condition called Anton-Babinski syndrome. [26]
Stroke, head injury or gunshot injuries, infection, eclampsia, encephalitis, meningitis, medications, and hyperammonemia can cause cortical blindness. [26]
Measurements of visual field defects can be done by visual field testing. It can be performed by various methods, including confrontation technique, amsler grid, tangent screen, kinetic perimetry, or static perimetry. Cost common is automated perimetry.
Confrontation visual field testing is a simple and quick visual field assessing method. A confrontational field test requires little or no special equipment and can be performed in any room, which is well illuminated.
Patient sitting straight in front of the examiner, is asked look directly at the examiner's eye during the test. The target eye should be the one directly across from the patient's eye. When the patient's right eye is being tested, closing the other eye, patient is instructed to look directly at the examiners left eye. Examiner closes his/her left eye, and then conduct finger movements, bringing his/her fingers or any other into your visual field from the sides. Since the test is basically comparison of the patient's visual field with the examiner's visual field, [28] it is not an accurate measurement of visual field.
Visual field assessment encompasses various perimetry techniques used in ophthalmology to evaluate how well someone can see in different areas of their vision. These techniques include flicker perimetry, which assesses temporal visual function and spatial resolution targeting the M pathway; Frequency Doubling Technology (FDT), which utilizes an optical illusion to evaluate ganglion cell damage within the M pathway; Short-Wavelength Automated Perimetry (SWAP), isolating the S cone system to detect early glaucoma-related damage; High-pass Resolution Perimetry, focusing on resolution over the central visual field; Saccadic Vector Optokinetic Perimetry (SVOP), using eye tracking for assessing natural eye movements; and various standard and automated perimetry methods like Goldmann, Humphrey field analyzer, and Octopus, each employing different techniques for visual field assessment. These diverse methods aid in diagnosing and monitoring ocular conditions, offering valuable insights into visual function and pathology. [5] [29] [30]
Modern computerized perimeters like humphrey field analyser (HFA) give more comprehensive and accurate reports than finger testing methods.
MRI of the brain and orbit helps to find the exact site of a lesion. [31]
CT scan is also used for investigating cause of visual pathway lesions. [31]
Tumours and other compressive lesions could often present with visual impairment and/or visual field defects. Careful clinical assessment could aid in accurate diagnosis of the cause of the visual field defect and loss of vision. Compressive lesions of the visual pathway, especially lesions affecting optic nerve require a multi-disciplinary approach involving neurosurgeon, physician as well as the ophthalmologist. [32] Treatment is given according to the cause.
In neuroanatomy, the optic chiasm, or optic chiasma, is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrates, although in cyclostomes, it is located within the brain.
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived from optic stalks during the seventh week of development and is composed of retinal ganglion cell axons and glial cells; it extends from the optic disc to the optic chiasma and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus.
The visual system is the physiological basis of visual perception. The system detects, transduces and interprets information concerning light within the visible range to construct an image and build a mental model of the surrounding environment. The visual system is associated with the eye and functionally divided into the optical system and the neural system.
In neuroanatomy, the lateral geniculate nucleus is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projection of the thalamus where the thalamus connects with the optic nerve. There are two LGNs, one on the left and another on the right side of the thalamus. In humans, both LGNs have six layers of neurons alternating with optic fibers.
The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict, whereas a lower intensity of light causes the pupil to dilate. Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.
In neuroanatomy, the optic radiation are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex. The optic radiation receives blood through deep branches of the middle cerebral artery and posterior cerebral artery.
A retinal ganglion cell (RGC) is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells. Retina amacrine cells, particularly narrow field cells, are important for creating functional subunits within the ganglion cell layer and making it so that ganglion cells can observe a small dot moving a small distance. Retinal ganglion cells collectively transmit image-forming and non-image forming visual information from the retina in the form of action potential to several regions in the thalamus, hypothalamus, and mesencephalon, or midbrain.
The visual field is "that portion of space in which objects are visible at the same moment during steady fixation of the gaze in one direction"; in ophthalmology and neurology the emphasis is on the structure inside the visual field and it is then considered “the field of functional capacity obtained and recorded by means of perimetry”.
In neuroanatomy, the optic tract is a part of the visual system in the brain. It is a continuation of the optic nerve that relays information from the optic chiasm to the ipsilateral lateral geniculate nucleus (LGN), pretectal nuclei, and superior colliculus.
Bitemporal hemianopsia, is the medical description of a type of partial blindness where vision is missing in the outer half of both the right and left visual field. It is usually associated with lesions of the optic chiasm, the area where the optic nerves from the right and left eyes cross near the pituitary gland.
In neuroanatomy, the pretectal area, or pretectum, is a midbrain structure composed of seven nuclei and comprises part of the subcortical visual system. Through reciprocal bilateral projections from the retina, it is involved primarily in mediating behavioral responses to acute changes in ambient light such as the pupillary light reflex, the optokinetic reflex, and temporary changes to the circadian rhythm. In addition to the pretectum's role in the visual system, the anterior pretectal nucleus has been found to mediate somatosensory and nociceptive information.
The posterior cerebral artery (PCA) is one of a pair of cerebral arteries that supply oxygenated blood to the occipital lobe, part of the back of the human brain. The two arteries originate from the distal end of the basilar artery, where it bifurcates into the left and right posterior cerebral arteries. These anastomose with the middle cerebral arteries and internal carotid arteries via the posterior communicating arteries.
Macular sparing is visual field loss that preserves vision in the center of the visual field, otherwise known as the macula. It appears in people with damage to one hemisphere of their visual cortex, and occurs simultaneously with bilateral homonymous hemianopia or homonymous quadrantanopia. The exact mechanism behind this phenomenon is still uncertain. The opposing effect, where vision in half of the center of the visual field is lost, is known as macular splitting.
A relative afferent pupillary defect (RAPD), also known as a Marcus Gunn pupil, is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils dilate when a bright light is swung from the unaffected eye to the affected eye. The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.
Optic neuropathy is damage to the optic nerve from any cause. The optic nerve is a bundle of millions of fibers in the retina that sends visual signals to the brain. [1].
Hemianopsia, or hemianopia, is a visual field loss on the left or right side of the vertical midline. It can affect one eye but usually affects both eyes.
Chiasmal syndrome is the set of signs and symptoms that are associated with lesions of the optic chiasm, manifesting as various impairments of the affected's visual field according to the location of the lesion along the optic nerve. Pituitary adenomas are the most common cause; however, chiasmal syndrome may be caused by cancer, or associated with other medical conditions such as multiple sclerosis and neurofibromatosis.
Bonnet–Dechaume–Blanc syndrome, also known as Wyburn-Mason syndrome, is a rare congenital disorder characterized by arteriovenous malformations of the brain, retina or facial nevi. The syndrome has a number of possible symptoms and can, more rarely, affect the skin, bones, kidneys, muscles, and gastrointestinal tract. When the syndrome affects the brain, people can experience severe headaches, seizures, acute stroke, meningism, and progressive neurological deficits due to acute or chronic ischaemia caused by arteriovenous shunting.
The contralateral organization of the forebrain is the property that the hemispheres of the cerebrum and the thalamus represent mainly the contralateral side of the body. Consequently, the left side of the forebrain mostly represents the right side of the body, and the right side of the brain primarily represents the left side of the body. The contralateral organization involves both executive and sensory functions. The contralateral organization is only present in vertebrates.
Henry (Harry) Moss Traquair, FRSE, PRCSE was a Scottish ophthalmic surgeon who made important contributions to the science of perimetry and the use of visual field testing in the diagnosis of disease. He was President of the Royal College of Surgeons of Edinburgh in 1939/40 and President of the Ophthalmological Society of the United Kingdom.