Disorders of the visual pathway

The visual pathway transmits signals from the retina to the visual cortex. It consists of the retina, optic nerve, optic chiasm, optic tract, lateral geniculate nucleus, optic radiations, and visual cortex. Lesions of the visual pathway may lead to visual disturbances or visual loss, the pattern of which can assist in determining the exact location of the lesion. Unilateral visual field loss indicates a prechiasmal pathology, while bilateral visual field loss indicates a chiasmal or retrochiasmal pathology. Retinal diseases mostly cause central or paracentral scotomas. Prechiasmal damage to the visual pathway usually involves the optic nerve (e.g., optic neuritis, optic atrophy, AION, papilledema) and may manifest with anopia or a centrocecal scotoma. Damage in the region of the optic chiasm is most commonly due to compression by a pituitary adenoma or craniopharyngioma and manifests with bitemporal heteronymous hemianopsia (lesions of the crossing fibers) or binasal hemianopsia (lesions of noncrossing fibers). Retrochiasmal visual pathway damage occurs in the region of the optic tract, lateral geniculate nucleus, optic radiation, or visual cortex and is most commonly due to cerebral ischemia/hemorrhage, tumors, or trauma. It usually manifests with homonymous hemianopsia or homonymous quadrantanopsia.

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

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:^[2]

Scotoma

An area within an otherwise normal visual field where vision is temporarily or permanently reduced or absent

• Blind spot: a physiologic scotoma in the visual field formed by axons leaving the eye to form the optic nerve (optic papilla) temporal to the center of the visual field
• Central scotoma: scotoma in the center of an eye's visual field
• Paracentral scotoma: scotoma near the center of an eye's visual field
• Centrocecal scotoma: scotoma running from the center to the blind spot of an eye's visual field
• Arcuate scotoma: an arc-shaped scotoma that starts at the blind spot and courses over the center of an eye's visual field
• Scintillating scotoma: an arch-shaped scotoma that starts centrally and shifts peripherally, that is typically experienced during an migraine aura (especially in retinal migraines)
  • Perception of an arch-shaped area of decreased vision
  • Moves from paracentral to peripheral vision
  • Flickering lights appearing in zigzag patterns
  • Usually bilateral
  • Typically lasts for 15–30 min ^[3]
  • May be followed by headache or, less commonly, occur concomitantly with headache.
  • See “Migraine” for more information about diagnostics and treatment.

Anopia

A larger visual field defect that affects a quarter, half, or the entire vision of an eye

• Unilateral anopia: vision loss in one eye
• Hemianopia: vision loss in one half of the visual field separated by the vertical midline
  • Bitemporal heteronymous hemianopia: vision loss at the outer (temporal) half of the visual field of both eyes
  • Binasal hemianopia: vision loss at the inner (nasal) half of the visual field of both eyes
  • Homonymous hemianopia: vision loss of one half of the visual field on the same side in both eyes (e.g., right homonymous hemianopia is a vision loss in the temporal visual field of the right eye and the nasal visual field of the left eye)
  • Macular sparing: macular vision is preserved despite adjacent visual field loss
• Quadrantanopia: vision loss in a quarter of the visual field of both eyes

Unilateral vision field loss indicates a disorder of a structure anterior to the optic chiasm. Bilateral visual field loss indicates a pathology at or posterior to the optic chiasm.

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 (e.g., tuberculosis, syphilis, Lyme disease, viral infections)
  • Toxic (e.g., ethambutol, methanol poisoning)
• 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 gray, 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 (e.g., central scotoma)
• Diagnostics
  • Fundoscopy
    • Primary form: pale devitalized papilla, sharply demarcated
    • Secondary form: pale devitalized papilla, prominent with a blurry margin
  • Perimetry
  • Optical coherence tomography: thinning of the retinal nerve fiber layer

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.

Traumatic optic neuropathy (TON)

• Definition: vision loss due to injury of the optic nerve from a penetrating or blunt force head trauma
• Etiology
  • Blunt force head trauma (most common), e.g., due to motorcycle and bicycle collisions, sports injuries, physical assault
  • Penetrating orbital trauma, e.g., stab wound
• Pathophysiology ^[4]
  • Indirect TON: blunt force head trauma → force transmitted to the optic canal through cranial bones → swelling and ischemia within the optic canal → contusion of the optic nerve
  • Direct TON: penetrating trauma (e.g., orbital and/or facial fracture) → bone fragments and/or hemorrhage within the optic canal → transection or avulsion of the optic nerve
• Clinical features
  • Findings consistent with
    • Recent head trauma (e.g., headache, nausea)
    • Sudden unilateral vision impairment (e.g., blurry vision, sudden vision loss, color blindness, central scotoma)
  • Physical examination: signs of orbital trauma and/or fracture (e.g., ecchymosis, hematoma, and/or edema of soft tissues)
• Diagnostics: All patients with suspected TON require fundoscopy, a CT scan of the head, and an ophthalmological examination to assess visual acuity, visual fields, and pupillary reflex.
  • Swinging-flashlight test: relative afferent pupillary defect
  • Fundoscopy
    • Indirect TON
      • Initially normal findings
      • Optic atrophy may be present 3–6 weeks after injury. ^[4]
    • Direct TON: optic disc swelling and hemorrhage
  • CT scan of the head: optic canal fracture, optic nerve hematoma, and/or bony fragment
• Treatment ^[5]
  • Observation for mild symptoms
  • Corticosteroids and/or surgical decompression of the optic nerve

Papilledema

• Definition: swelling of optic disc due to elevated intracranial pressure
• Etiology
  • Hypertensive emergency
  • Idiopathic intracranial hypertension (pseudotumor cerebri)
  • Intracranial tumors (e.g., medulloblastoma)
  • Hydrocephalus
  • Cerebral abscess
  • Hypervitaminosis A
• Pathophysiology: increase in cerebrospinal fluid pressure causes a disruption of the axoplasmic flow in the optic nerve ^[6]
• 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
  • Sometimes chiasmal syndrome: triad of bitemporal visual field defects, unilateral or bilateral reduction in visual acuity, and optic atrophy
• Treatment
  • Depending on the cause
    • Neurosurgical
    • Pharmacological
• Prognosis: Complete regression of impaired vision/visual field defects is possible with timely treatment.

• Definition: damage in the region of the optic tract, lateral geniculate nucleus, optic radiation, or visual cortex
• Etiology
  • Cerebral ischemia/hemorrhage (e.g., posterior cerebral artery occlusion)
  • Tumors
  • Trauma
  • Other causes (e.g., aneurysms of the posterior communicating artery, abscesses, meningitis, vasospasms)
• Clinical features
  • Visual field defects/impaired vision: most commonly homonymous hemianopsia and homonymous quadrantanopsia
  • Optic atrophy in cases with a CN III lesion
• Diagnostics
  • Perimetry
  • Imaging: CT, MRI
• Prognosis: usually no regression of visual field defects