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 Pupillary light reflex: loss of afferent limb → absent direct pupillary light reflex	Optic neuritis (MS) 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 MCA → anterior choroidal artery Posterior cerebral artery → lateral choroidal artery
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 infromation 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	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

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
  • Swinging-flashlight test: relative afferent pupillary defect
  • Ophthalmoscopy
    • Retrobulbar neuritis: normal ophthalmoscopic finding
    • Papillitis: poorly defined papilla, hyperemia, hemorrhage at the border of the papilla
  • Visual evoked potential (VEP)
    • Assess conduction disorders of the optic nerve
    • In acute retrobulbar neuritis: mainly potential attenuation or deficit (by a conduction block)
    • After previous retrobulbar neuritis: possible delay in latency
  • Cranial magnetic resonance imaging (cMRI)
  • Neurological examination
• Treatment
  • High-dose treatment with glucocorticoids
  • Corresponding treatment in the case of multiple sclerosis
• Prognosis
  • Visual improvement usually starts after 2 weeks.
  • Visual acuity is usually good after 1 year.
  • Frequent persistence of deficits in color vision and contrast sensitivity.
  • Further prognosis depends on the potential existence of an underlying disease.

Multiple sclerosis is the most common cause of optic neuritis!

Optic atrophy

• Definition: irreversible loss of axons in the optic nerve
• Forms: morphological differentiation
  • Primary/simple optic atrophy: the optic disc is pale or chalky white and sharply demarcated.
  • Secondary/complex optic atrophy: the optic disc is pale or grey, poorly defined, and prominent.
  • Glaucomatous optic atrophy (see glaucoma)
• 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
• Diagnostics
  • Fundoscopy
    • Primary form: pale devitalized papilla, sharply demarcated
    • Secondary form: pale devitalized papilla, prominent with a blurry margin
  • 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)
• Epidemiology
  • NAION: peak incidence between 50 and 70 years of age, but also observed in younger patients
  • AAION: almost always after 60 years of age
• Clinical features
  • Sudden unilateral loss of vision
  • Wedge-shaped and altitudinal visual field defects
  • AAION: other symptoms of giant cell arteritis
• Diagnostics
  • Fundoscopy
    • Acute: edematous, poorly defined optic disc, usually sectoral; pale or hyperemic; radial hemorrhage around the disc margin
    • Late course of the disease: optic atrophy
  • 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; radial hemorrhage around the disc margin
    • Later stages of disease: optic atrophy possible
  • Cranial CT or MRI to detect cause of increased intracranial pressure

• 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
• Prognosis
  • Complete regression of impaired vision/visual field defects is possible with timely treatment.

The triad of bitemporal visual field defects, unilateral or bilateral reduction in visual acuity, and optic atrophy is referred to as chiasmal syndrome!

• 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
  • Optic atrophy, in cases with a third neuron lesion
• Diagnostics
  • Perimetry
  • Imaging: CT, MRI
• Prognosis
  • Usually no regression of visual field defects