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Visual pathway lesions

Visual pathway lesions is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Visual pathway lesions rather than just read about it. In short: 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.

Visual pathway lesions — main illustration
Visual pathway lesions — illustration

Key takeaways

  • Visual pathway lesions belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Visual pathway lesions to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Visual pathway lesions from memory before moving on to harder problems.

Reference excerpt

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 chiasma (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).

Optic nerve lesions 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 Causes of optic nerve lesions include optic atrophy, optic neuropathy, head injury, traumatic avulsion, acute optic neuritis etc.

Signs and symptoms

Lesions involving the whole optic nerve cause complete blindness on the affected side, that means damage at the right optic nerve causes complete loss of vision in the right eye. Optic neuritis involving external fibers of the optic nerve causes tunnel vision. Optic neuritis involving internal fibers of the optic nerve causes central scotoma. lf unilateral central scotoma is detected, careful observation of the temporal visual field of other eye is essential to rule out the possibility of compressive lesions at the junction of optic nerve and optic chiasm. Other symptoms include absence of direct light reflex, afferent pupillary defect, defective colour vision, decreased contrast sensitivity, generalized decrease in visual sensitivity etc.

Optic chiasm lesions

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. 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. Another type is lateral chiasmal syndrome.

Causes Causes of chiasmal syndromes may be classified into intrinsic and extrinsic forms. Intrinsic causes are due to thickening of the chiasm itself and extrinsic implies compression by another structure(gliomas, multiple sclerosis etc.). Other less common causes of chiasmal syndrome are metabolic, toxic, traumatic, inflammatory or infectious in nature (eg. lymphoid hypophysitis, sarcoidosis.) Compression of the optic chiasm is associated with pituitary adenoma, Craniopharyngioma, Meningioma etc.

Signs and symptoms

A lesion involving complete optic chiasm, which disrupts the axons from the nasal field of both eyes, causes loss of vision of the right half of the right visual field and the left half of the left visual field. This visual field defect is called as bitemporal hemianopia. Anterior chiasmal syndrome, the lesions that affect the ipsilateral optic nerve fibres and the contralateral inferonasal fibres located in the Willebrand knee produce junctional scotoma, i.e., a combination of central scotoma in one eye and temporal hemianopia defect in the other eye. Middle chiasmal syndrome, the lesions involving the decussating fibres in the body of chiasma produce bitemporal hemianopia. Posterior chiasmal syndrome, the lesions affecting the caudal fibres in chiasma produce paracentral bitemporal field defects. Homonymous hemianopia on the contralateral side may occur when posterior chiasmal lesions involve the optic tract. Lateral chiasmal lesions may produce binasal hemianopia. Lesions at the junction of the optic nerve and chiasm may produce an ipsilateral monocular temporal scotoma known as 'junctional scotoma'. Scottish ophthalmologist Harry Moss Traquair first identified and named this field defect.

Lesions of optic tract 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. The optic tract represents the first stage in the visual pathway in which visual information is transferred in a homonymous nature. 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.

Causes 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. 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. Extrinsic or compressive lesions are caused by pituitary craniopharyngioma, tumours of optic thalamus. Other causes include syphilitic meningitis, gumma and tubercular meningitis etc.

Signs and symptoms

Incongruous homonymous hemianopia. Wernicke's pupillary reaction- No pupillary reaction when light shown to the blind half of retina, but if light is shown to seeing half, pupil shows reaction. Visual acuity is intact. Optic disc changes: descending type of parital optic atrophy is produced characterized by temporal pallor on the side of the lesion and bow tie atrophy on the contralateral side.

Lesions of lateral geniculate nucleus 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. Isolated lesions of the lateral geniculate nucleus are rare, it may be diagnosed by distinctive patterns of visual field loss.

Causes Pituitary adenoma compression may cause LGN degeneration. Lesions affecting the anterior or lateral choroidal arteries may affect the lateral geniculate nucleus.

Signs and symptoms Incongruous homonymous hemianopia. Pupillary reflexes are normal, as fibers for pupillary reflexes from the optic tract are diverted to pretectal nucleus and do not reach the LGN. Optic disc pallor may occur due to partial descending atrophy.

Lesions of optic radiations The optic radiation are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex.

… excerpt ends here. Continue reading the full article.

Illustrations

Visual pathway lesions illustration
Visual pathway lesions: Visual field-tubular vision
Visual field-tubular vision
Visual pathway lesions: Visual field-central scotoma
Visual field-central scotoma
Visual pathway lesions: Visual field-bitemporal hemianopia
Visual field-bitemporal hemianopia
Visual pathway lesions: Visual field-binasal hemianopia
Visual field-binasal hemianopia

Worked examples

Example 1 — a first encounter with Visual pathway lesions

Start with the simplest possible case. Write down what Visual pathway lesions claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Visual pathway lesions before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Visual pathway lesions ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Visual pathway lesions

In research
Visual pathway lesions appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Visual pathway lesions in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Visual pathway lesions is common in secondary-school and first-year university syllabi. It links to neighbouring topics Disorders of optic nerve and visual pathways, Neurology, Ophthalmology, so understanding it makes those chapters shorter.
In everyday life
Look for Visual pathway lesions outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Visual pathway lesions in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Visual pathway lesions means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Visual pathway lesions out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Visual pathway lesions in simple terms?

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.

Why does Visual pathway lesions matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Visual pathway lesions?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Visual pathway lesions.

Tags

  • Disorders of optic nerve and visual pathways
  • Neurology
  • Ophthalmology

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