Pathology of the visual pathways

Topography and visual field defects

Specific visual impairments develop depending on the localization of the pathological process along the visual pathway. This correlation allows clinicians to suspect the level of the lesion even before neuroimaging:

• Optic nerve (Nervus opticus): complete loss of vision (blindness) on the affected side (monocular).
• Optic chiasm (Chiasma opticum): loss of the outer (temporal) halves of the visual fields bilaterally — bitemporal hemianopia.
• Optic tract (Tractus opticus): homonymous hemianopia (loss of corresponding halves of the visual fields on the side opposite to the lesion).
• Optic radiation (Radiatio optica): involvement of fibers in the temporal lobe (Meyer's loop) results in superior quadrantanopia, whereas involvement in the parietal lobe results in inferior quadrantanopia.
• Primary visual cortex: homonymous hemianopia, often with preservation of macular vision (macular sparing). This is due to the dual blood supply of the macular zone from the posterior and middle cerebral arteries.

Congenital anomalies of the anterior pathway

Congenital pathologies include optic nerve hypoplasia and septo-optic dysplasia.

Optic nerve hypoplasia can result from chromosomal abnormalities, sporadic mutations, or intrauterine insults. MRI (especially T1 and STIR sequences) reveals a thinned optic nerve without pathological contrast enhancement and an anatomically narrowed optic canal.

Septo-optic dysplasia is a complex developmental malformation, the classic triad of which includes:

1. Hypoplasia of the optic nerves or chiasm.
2. Partial or complete absence of the septum pellucidum (septum pellucidum).
3. Hypothalamic-pituitary axis dysfunction.

Clinically, the disease manifests with visual impairments, endocrine disorders, and developmental delay. MRI typically shows the absence of the septum pellucidum, a low-lying fornix, and a characteristic pointed shape of the frontal horns of the lateral ventricles.

Tumors and vascular malformations

Intra-axial visual pathway masses include gliomas and cavernous malformations.

Visual pathway gliomas behave differently depending on the patient's age. In children, they are most often benign pilocytic astrocytomas involving the nerve or chiasm. If the hypothalamus is involved, mortality increases. In adults, however, visual pathway gliomas are highly malignant and lead to 100% mortality.

An optic pathway cavernoma is an angiographically occult venous malformation. It is often asymptomatic, but may occasionally become symptomatic due to slow growth or rare hemorrhages. On MRI, it has a classic "popcorn" appearance and demonstrates a blooming artifact on T2* (gradient-echo) sequences due to hemosiderin deposition, whereas on CT it can be mistaken for an aneurysm.

Inflammatory and demyelinating processes

The most common inflammatory pathology is retrobulbar optic neuritis. This is a typical (and in 15–20% of cases, the first) manifestation of multiple sclerosis. Half of patients presenting with such neuritis are subsequently diagnosed with the disease. Clinically, neuritis presents as a painful loss of vision developing over several days.

The primary diagnostic modality is contrast-enhanced brain MRI with fat-suppression sequences (STIR). Signs of neuritis include nerve swelling, high T2/STIR signal, and gadolinium enhancement.

If the neuritis is bilateral or affects extensive segments of the posterior visual pathways, neuromyelitis optica spectrum disorder (NMOSD) must be ruled out.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomy of the postchiasmal region: lateral geniculate body and Meyer's loop
• Structure and functions of the primary visual cortex (striate cortex)
• Blood supply to the visual cortex and consequences of calcarine artery occlusion
• Differential diagnosis of optic neuritis in multiple sclerosis vs. NMOSD
• MRI protocols (STIR, T2*) for evaluating visual pathway pathologies

Other topics in the Neuroimaging section

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