Disorders of the visual pathway

The visual pathways transmit signals from the retina to the visual cortex (striate cortex, brodmann area 17).

Structure	Characteristics	Lesions	Causes
Retina	Cones and rods transform light into visual signals, which are projected to the brain via the optic nerve.	Central scotoma occurs in case of macula involvement Paracentral scotoma Occurs in case of macula involvement with foveal sparing (e.g., hydroxychloroquine/chloroquine maculopathy) Seen as concentric rings of hypopigmentation and hyperpigmentation on the macula (“bull's eye maculopathy”)	Diseases of the retina Retinal detachment Retinal vessel occlusion
Optic nerve	Transmits visual information from the ipsilateral retina to the brain Afferent limb of pupillary reflex and accomodation reflex	Ipsilateral blindness (anopia),scotoma Centrocecal scotoma: loss of vision in the fovea centralis region and a blind spot Pupillary light reflex: loss of afferent limb → absent direct pupillary light reflex	Optic neuritis (MS) Optic atrophy Retinal vessel occlusion Anterior ischemic optic neuropathy Papilledema
Optic chiasm	Fibers from nasal part of the retina cross to contralateral side Fibers from the temporal part of the retina do not cross to the contralateral side	Middle lesions (crossing fibers) → bitemporal heteronymous hemianopsia Bilateral lesions (non-crossing fibers) → binasal hemianopsia	Pituitary adenoma Craniopharyngioma Parasellar aneurysm of the internal carotid artery Meningioma
Optic tract	Transmits information of the ipsilateral, nasal visual field contralateral, temporal visual field Projects to lateral geniculate nucleus, pretectal area, superior colliculi, suprachiasmatic nuclei	Contralateral homonymous hemianopsia	Middle cerebral artery (MCA) infarction
Lateral geniculate nucleus (LGN)	Transmits information of the ipsilateral, nasal visual field contralateral, temporal visual field Projects to primary visual cortex via optic radiations	Contralateral homonymous hemianopsia	MCA infarction
Optic radiations	Anterior/lateral bundle in Meyer loop Passes through the temporal lobe in close proximity to the inferior horn of the lateral ventricle Transmits information of the superior visual field from the lower retina Projects to the lingual gyrus Posterior/medial bundle Located in the parietal lobe, passes through the internal capsule Transmits information of the inferior visual field from the upper retina Projects to the cuneus gyrus	More common than optic tract or LGN lesions Anterior/lateral bundle (Meyer loop) Contralateral upper homonymous quadrantanopsia (pie in the sky) Posterior/medial bundle Contralateral lower homonymous quadrantanopsia (pie on the floor)	MCA infarction Upper quadrantanopsia indicates temporal lobe lesion Lower quadrantanopsia indicates parietal lobe lesion
Visual cortex	Located in the calcarine sulcus of the occipital lobe	Contralateral homonymous hemianopsia with macular sparing	Posterior cerebral artery (PCA) infarction

Unilateral visual field loss → pathology mainly in front of the optic chiasm. Bilateral visual field loss → pathology mainly in or behind the optic chiasm

Remember that the Meyer Loop transmits the signal from the Lower retina, Looping around the inferior horn of the Lateral ventricle.

References:^[1][2]

Prechiasmal damage to the visual pathway mainly involves the optic nerve.For retinal diseases see table above.

Optic nerve

Optic neuritis

• Definition: inflammation of the optic nerve
  • Retrobulbar neuritis: inflammation of the retrobulbar part of the optic nerve
  • Papillitis: inflammation of the intrabulbar part of the optic nerve
• Etiology
  • Most common cause: multiple sclerosis
  • Infections
  • Toxic
• Clinical features
  • Vision impairment: blurry vision, sudden vision loss, color blindness, visual field defects (e.g., central scotoma)
  • Retrobulbar pain (increased pain caused by eye movements)
• Diagnostics
  • Ophthalmoscopy
    • Retrobulbar neuritis: normal ophthalmoscopic finding
    • Papillitis: poorly defined papilla, hyperemia, hemorrhage at the border of the papilla
  • Visual evoked potential (VEP)
  • Neurological examination
• Treatment
  • High-dose treatment with glucocorticoids
  • Corresponding treatment in the case of multiple sclerosis

Multiple sclerosis is the most common cause of optic neuritis!

Optic atrophy

• Definition: irreversible loss of axons in the optic nerve
• Etiology
  • Most common causes: optic neuritis, glaucoma
  • Vascular
  • Hydrocephalus, orbital/intracranial lesion
  • Hereditary: Leber optic atrophy, Krabbe disease, autosomal-dominant optic atrophy
• Clinical features:
  • Vision impairment: blurry vision, color blindness, visual field defects (e.g., central scotoma)
• Diagnostics
  • Fundoscopy
  • Perimetry

Anterior ischemic optic neuropathy (AION)

• Definition: segmental or generalized infarction of the anterior part of the optic nerve
• Etiology
  • Nonarteritic form due to atherosclerosis (NAION)
  • Arteritic form due to giant cell arteritis (AAION)
• Clinical features
  • Sudden unilateral loss of vision
  • Wedge-shaped and altitudinal visual field defects
  • AAION: other symptoms of giant cell arteritis
• Diagnostics
  • Fundoscopy
  • Laboratory findings
    • AAION: ↑ ESR and CRP
    • NAION: normal ESR and CRP
  • AAION: temporal artery biopsy
• Treatment
  • AAION: immediate high-dose systemic glucocorticoids
  • NAION: trial of hemodilution and systemic glucocorticoid therapy
• Prognosis
  • Typically poor: permanent loss of vision
  • Involvement of the other eye is common during the course of the disease.

Immediate high-dose glucocorticoid treatment is essential in AAION!

Papilledema

• Definition: swelling of optic disc due to elevated intracranial pressure
• Etiology:
  • Hypertensive emergency
  • Idiopathic intracranial hypertension (pseduotumor cerebri)
  • Intracranial tumors (e.g., medulloblastoma)
  • Hydrocephalus
  • Cerebral abscess
  • Hypervitaminosis A
• Clinical features
  • Acute: usually asymptomatic, can present with loss of visual acuity due to enlargement of blind spot
  • Chronic (rare): impaired vision, can cause visual field defects and blindness
• Diagnostics
  • Fundoscopy
    • Acute:
      • Edematous, poorly defined, prominent optic disc with blurry margins
      • Widened blind spot
      • Radial hemorrhage around the disc margin
    • Later stages of disease: optic atrophy possible
  • Cranial CT or MRI to detect cause of increased intracranial pressure

• Neovascularization of retina and uvea

• Etiology
  • Most commonly: compression by pituitary adenoma (prolactinoma), craniopharyngioma, meningioma, aneurysms of the internal carotid artery
  • Demyelination, multiple sclerosis
  • Trauma
• Pathophysiology
  • Medial lesion (e.g., pituitary adenoma) → temporal visual field defects
  • Lateral lesion (e.g., aneurysm of the internal carotid artery) → nasal visual field defects
• Clinical features
  • Visual field defects/impaired vision: most commonly bitemporal heteronymous hemianopsia
• Treatment
  • Depending on the cause
    • Neurosurgical
    • Pharmacological

• Definition: damage in the region of the optic tract, lateral geniculate nucleus, optic radiation, or visual cortex
• Etiology
  • Cerebral ischemia/hemorrhage
  • Tumors
  • Trauma
  • Other causes
• Clinical features
  • Visual field defects/impaired vision: most commonly homonymous hemianopsia and homonymous quadrantanopsia
• Diagnostics
  • Perimetry
  • Imaging: CT, MRI