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 help determine the exact location of the lesion. Unilateral visual field loss indicates a prechiasmal pathology, while bilateral visual field loss indicates a systemic, 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 or 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 “Diseases of the retina.”

Definition ^[4]

Optic neuritis is inflammation of the optic nerve; subtypes include retrobulbar neuritis and papillitis.

• Retrobulbar neuritis: inflammation of the retrobulbar part of the optic nerve
• Papillitis: inflammation of the optic disc

Etiology ^[4][5]

• Most common cause: multiple sclerosis (MS) ^[6]
• Infections (e.g., tuberculosis, syphilis, Lyme disease, viral infections)
• Neuromyelitis optica spectrum disorders
• Other inflammatory and autoimmune conditions (e.g., sarcoidosis, systemic lupus erythematosus)
• Idiopathic

Multiple sclerosis is the most common cause of optic neuritis. ^[6]

Clinical features ^[5][7]

• Vision impairment: reduced visual acuity, subacute vision loss, dyschromatopsia, visual field defects (e.g., central scotoma)
• Retrobulbar pain (increases with eye movements)
• Typically monocular

Optic neuritis typically manifests with unilateral, subacute vision loss and orbital pain that worsens with eye movements. ^[5]

Diagnostics ^[4][6]

Optic neuritis is a clinical diagnosis.

• Physical examination and examination of the eye
  • Swinging-flashlight test: relative afferent pupillary defect ^[8]
  • Ophthalmoscopy
    • Retrobulbar neuritis: may be normal
    • Papillitis: may show inflammatory changes (e.g., poorly defined papilla and hyperemia)
  • Visual acuity testing
  • Color vision testing, e.g., assessing for red desaturation
  • Neurological examination
• Confirmatory studies and evaluation for the underlying cause
  • MRI head and orbits with and without IV contrast (first-line studies) ^[4][9]
  • CSF analysis: Findings may indicate the underlying cause, e.g., oligoclonal bands in MS.
  • Serology (e.g., Lyme serology, antinuclear antibodies): obtained in consultation with specialists to assess for infectious and autoimmune etiologies
  • See also “Multiple sclerosis” and “Neuromyelitis optica spectrum disorders.”
• Specialist diagnostics
  • Optical coherence tomography: can show subtle changes in the retina and optic disc
  • Visual evoked potential (VEP)
    • Used to assess for conduction disorders of the optic nerve
    • VEP cannot be used to differentiate between etiologies of optic neuritis. ^[4]

Treatment ^[4][6][7]

Although many patients experience symptom improvement without treatment, research has shown that treatment reduces recovery times and may improve outcomes. ^[6]

• Consult neurology and consider inpatient admission.
• Begin high-dose glucocorticoids, e.g.:
  • IV methylprednisolone (off-label) for 3–7 days ^[4][6]
  • Followed by oral prednisone (off-label) for 11–14 days ^[4][6]
• Depending on the underlying cause, advanced treatment modalities (e.g., plasmapheresis and intravenous immunoglobulin) may be used.
• Manage the underlying condition in consultation with specialists, e.g., for treatment of MS or treatment of Lyme disease.

Prognosis ^[4]

• Prognosis depends on the underlying etiology.
• Visual acuity typically starts improving within weeks.
• Some degree of impairment often persists.

• 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

• 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.

• 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 ^[10]
  • 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. ^[10]
    • Direct TON: optic disc swelling and hemorrhage
  • CT scan of the head: optic canal fracture, optic nerve hematoma, and/or bony fragment
• Treatment ^[11]
  • Observation for mild symptoms
  • Corticosteroids and/or surgical decompression of the optic nerve

Definition ^[12][13]

Toxic-metabolic optic neuropathy is damage to the optic nerve caused by toxins and/or nutritional deficiencies.

Etiology ^{[7][12][13][14]}

• Methanol poisoning
• Nutritional deficiencies, e.g., vitamin B₁₂ deficiency
• Drug-induced, e.g., ethambutol, aminoglycosides, barbiturates, linezolid, infliximab

Clinical features ^[7][12][13]

• Reduced visual acuity, dyschromatopsia, and visual field defects (commonly central scotoma)
• Typically bilateral, progressive, and painless

Diagnostics ^[12][13]

• Focused toxicological history and physical examination
• Examination of the eye
  • Swinging-flashlight test: no relative afferent pupillary defect
  • Fundoscopy: often initially normal; swollen disc and/or hyperemia may be seen
  • Visual field examination: central scotoma
• MRI head and orbits to exclude compressive lesions
• Additional diagnostics depending on the suspected etiology (e.g., serum methanol levels in suspected methanol poisoning)
• Assessment of nutritional status, including vitamin B₁₂ and folate levels

Treatment ^[12][13]

• Management of the underlying cause, e.g., discontinuation of the causative drug, management of toxic alcohol poisoning
• For a comprehensive approach to poisoning, see “Approach to the poisoned patient.”

Definition ^[15][16]

Papilledema is swelling of the optic disc due to elevated intracranial pressure (ICP).

Etiology ^[15][16]

• Hypertensive encephalopathy due to hypertensive emergency
• Idiopathic intracranial hypertension (pseudotumor cerebri)
• Space-occupying lesions: intracranial tumors (e.g., medulloblastoma), cerebral abscess
• Hydrocephalus
• Drug-induced intracranial hypertension (e.g., due to hypervitaminosis A)
• Venous sinus thrombosis
• See also “Causes of elevated ICP.”

Pathophysiology

Increased cerebrospinal fluid pressure disrupts the axoplasmic flow in the optic nerve. ^[17]

Clinical features ^[15][16]

• Acute
  • The most common symptom is headache. ^[15]
  • Visual impairment is typically minimal; transient blurry vision or loss of vision may occur.
  • Other signs of elevated ICP may be present (e.g., nausea and vomiting).
• Chronic (rare): impaired vision, visual field defects, and blindness

Diagnostics ^[7][15][16]

• Visual field examination: arcuate scotoma, widened blind spot
• Fundoscopy
  • Acute
    • Edematous, poorly defined, prominent optic disc with blurry margins
    • Radial hemorrhage around the disc margin
  • Later stages of disease: Optic atrophy may be present.
• Diagnostics for elevated ICP: CT or MRI head
• Once other causes of elevated ICP have been ruled out: Obtain diagnostics for IIH (e.g., lumbar puncture).

Treatment ^[7][15][16]

• Initiate management of elevated ICP immediately.
• Further treatment focuses on the underlying cause, e.g., management of idiopathic intracranial hypertension, management of hypertensive emergencies.

• 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