Why does lateral geniculate body (LGB) involvement cause sector anopia?

Lateral Geniculate Body Involvement and Sector Anopia

Lateral geniculate body (LGB) involvement causes sector anopia due to its unique retinotopic organization and vascular supply, where damage to specific portions of the LGB results in corresponding wedge-shaped visual field defects while preserving other portions of the visual field. 1

Anatomical Basis of Sector Anopia

• The lateral geniculate nucleus (LGN) is the site where ganglion cell axons of the optic tract synapse with neurons that form the optic radiations 2
• The LGB has a precise retinotopic organization, with axons from different parts of the retina terminating in specific regions of the LGB 3
• Axons originating from inferior, central, and superior retina are located laterally, centrally, and medially, respectively, in the LGB 3
• This specific organization explains why discrete lesions in different parts of the LGB produce distinctive visual field defects 3

Vascular Supply and Mechanism

• Sector anopia typically involves ischemia or infarction of parts of the lateral geniculate body supplied by the distal anterior choroidal artery 1
• The lateral choroidal artery, which supplies the LGB, has a specific distribution pattern that, when compromised, leads to the characteristic wedge-shaped visual field defects 4
• Vascular steal phenomena, as seen in arteriovenous malformations, can divert blood away from the LGB, causing ischemia and resulting in sector anopia 1
• When blood flow is restored to viable areas of the LGB (e.g., after treatment of an arteriovenous malformation), partial campimetric recovery can occur 1

Clinical Characteristics of Sector Anopia

• Sector anopia presents as homonymous, wedge-shaped visual field defects that extend from the central fixation point to the periphery 5
• These defects can be horizontal, affecting either upper or lower visual fields, or can involve multiple sectors (quadruple sectoranopia) 1
• Visual acuity may remain intact (20/20) despite the presence of sector anopia 1
• The defects are typically homonymous (affecting the same portion of the visual field in both eyes) due to the post-chiasmatic location of the LGB 2

Diagnostic Features

• Neuroimaging, particularly high-resolution MRI with heavily T2-weighted images, can demonstrate discrete lesions of the LGB 3
• The location of the lesion within the LGB correlates with the pattern of visual field defect:
  • Lateral LGB lesions produce homonymous upper quadrantanopia 3
  • Medial LGB lesions produce homonymous horizontal sectoranopia 3
• Partial retinal nerve fiber layer atrophy matching the campimetric deficit can confirm LGB involvement 1

Differential Diagnosis

• Sector anopia must be distinguished from visual neglect, which is a neurological disorder where a person ignores visual stimuli on one side despite having intact visual ability 6
• Unlike visual neglect, sector anopia represents an actual visual field defect rather than an attentional deficit 6
• Sector anopia should also be differentiated from hemianopia, although they can coexist 6
• Careful visual field testing and neuroimaging are essential for accurate diagnosis 2

Potential for Recovery

• Some cases of sector anopia may show partial reversibility if blood flow to the LGB is restored 1
• In cases of vascular steal due to arteriovenous malformations, treatment of the malformation can lead to improvement in the visual field defect 1
• Recovery typically begins several months after treatment and may be incomplete, with some sectors showing complete reversal while others remain affected 1