25th December 2023

Disorders of the visual pathway

Most intracranial disorders are associated with significant neurological morbidity; however, occipital pathology may be clinically silent. Patients often describe vague or unilateral problems with vision. A dense homonymous hemianopia may be missed unless visual fields are adequately assessed.

Anatomical considerations

The optic chiasm permits the hemi-decussation of the visual field information, such that the information from the left visual field (left temporal and right nasal field) and right visual field (right temporal and left nasal field) is processed in the contralateral visual cortex. Lesions at the optic chiasm produce bi-temporal hemianopia. Retro-chiasmal lesions produce homonymous hemianopia. Congruity describes the degree of similarity between the visual field defects in both eyes. The more posterior the lesion in retro-chiasmal visual pathway, the more congruous the visual field defect is.

Causes of neurological visual field defects

• Tumours
  • Pituitary adenoma
  • Craniopharyngioma
  • Meningioma
  • Lymphoma
  • metastases
• Infection
  • Intracerebral abscess
• Inflammation/infiltration
  • Sarcoidosis
  • Histiocytosis
  • Lymphocytic hypophysitis
• Vascular
  • Stroke
  • Pituitary apoplexy
  • Arteriovenous malformation
  • Intracranial aneurysm
• Trauma

Image Source: Wikimedia Commons (2017). Hemianopsio eo, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Hemianopsio_eo.png (Accessed on: 15  May 2022).

1 – right monocular vision loss 2 – bi-temporal homonymous hemianopia 3. Left complete homonymous hemianopia 4. Left superior homonymous quadrantopia 5 – Left inferior homonymous quadrantopia with macula sparing 6 – left superior quadrantopia with macula sparing.

Visual aspects of stroke

Visual impairment is a common consequence of stroke. There is significant overlap between the arterial circulations. The lateral geniculate nucleus may be affected by compromise to the internal carotid artery (anterior choroidal artery branch), middle or posterior cerebral artery. The optic radiations are served by both the anterior choroidal and middle cerebral arteries. The occipital cortex is supplied by the middle and posterior cerebral arteries. A posterior cerebral artery stroke may produce a complete homonymous hemianopia with macular sparing owing to an intact blood supply from the middle cerebral artery.

Exceptional visual field defects

Rarely, intracranial insults may give rise to unusual visual field defects that may mimic an intra-ocular or pre-chiasmal insult. There are two circumstances of note:

1. Bilateral occipital lobe lesions: will give rise to bilateral altitudinal visual field defect mimicking retinal or optic nerve head lesion. Full dilated fundus examination is recommended to exclude local causes. Neuroimaging is diagnostic. Patients are at risk of cortical blindness syndrome.
2. Unilateral temporal crescenteric visual field defect: The nasal visual field extends to 60 degrees from fixation, whereas the temporal field extends to 90%. This leaves an area of the temporal visual field that has no nasal correlate in the contralateral eye, represented within the parieto-occipital sulcus. Damage in this area of the visual cortex leads to a monocular, contralateral temporal crescent defect.

References

• Ali K The temporal crescent syndrome Practical Neurology 2015;15:53-55.
• File [modified]: Hemianopsio eo.png. Wikimedia Commons [Internet]. Commons.wikimedia.org. 2021 [cited 15 May 2022]. Available from: https://commons.wikimedia.org/wiki/File:Hemianopsio_eo.png
• Newman RP, Kinkel WR, Jacobs L. Altitudinal Hemianopia Caused by Occipital Infarctions: Clinical and Computerised Tomographic Correlations. Arch Neurol. 1984;41(4):413–418.