Lesions of the optic tract and lateral geniculate nucleus

Summary

In this article we describe lesions of the optic tract and the lateral geniculate nucleus

Aetiology

The optic tracts connect the optic chiasm with the lateral geniculate nucleus (LGN). Each optic tract conveys signals from the contralateral nasal retina and the ipsilateral temporal retina.

Pathology behind the chiasma results in field defects on the contralateral side to the lesion. The same side of the field is affected in each eye. This is known as homonymous, and is in contrast to the bitemporal (and, heteronymous) defects seen in chiasmal lesions where different sides of the visual field are affected in each eye.

Congruity

A complete homonymous hemianopia affects the entire hemifield of both eyes. This can occur with a lesion anywhere posterior to chiasm, and based on visual field appearance alone cannot be localised. An incomplete homonymous hemianopia spares at least part of the vision on the affected side, and can be classified as either congruous or incongruous. A congruous visual field defect is identical between the two eyes, whereas an incongruous defect differs in appearance between the eyes.

Lesions of the optic tract

Homonymous hemianopia

Each optic tract, which arises from the posterior aspect of the chiasm, contains fibres from the contralateral nasal hemiretina (which crossed at the chiasm), and uncrossed fibres from the ipsilateral temporal hemiretina. The tracts leave the chiasm, diverge, and extend posteriorly around the cerebral peduncles before finally terminating at the lateral geniculate bodies.

The nerve fibres in the optic tracts are not closely aligned to where they originated from in the retina. Therefore, we see an incongruous homonymous hemianopia.

Wernicke hemianopic pupil

The optic tracts contain visual and pupillomotor fibres. Whilst the visual fibres end in the lateral geniculate body, the pupillary fibres exit the optic tract before reaching the LGN and enter the midbrain to synapse on the ipsilateral pre-tectal nucleus. An optic tract lesion before this point therefore may give rise to an afferent pupillary conduction defect.

The pupillary reflex will be intact upon stimulation of the unaffected hemiretina, but will be absent when the affected hemiretina is stimulated.

Optic atrophy

As the optic tract fibres are the axons of the retina ganglion cells, optic atrophy may occur if the optic tract is damaged. The pattern we classically see is atrophy of the superior and inferior aspects of the neuroretinal rim (represented by the temporal retina) whilst the contralateral eye shows atrophy of the nasal and temporal aspects of the neuroretinal rim (represented by the nasal retina)

Contralateral pyramidal signs

If the ipsilateral cerebral peduncle is affected contralateral pyramidal signs may occur - the tracts on each side lie on the lateral surface of the cerebral peduncle.

Lesions of the lateral geniculate nucleus

The lateral geniculate nucleus (LGN) is the nucleus in the thalamus that receives visual information from the retina and sends it to the visual cortex via optic radiations.

• Incongruous homonymous hemianopia

• Normal pupillary responses

• Optic atrophy (see above)

💡 Pupillary reflexes are normal, as fibres for pupillary reflexes from the optic tract are diverted to pre-tectal nucleus and do not reach the LGN.

References

1. Salmon, John F., and Jack J. Kanski. Kanski’s Clinical Ophthalmology: A Systematic Approach. Ninth Edition, Elsevier, 2020

2. Prasad, Sashank. “Retrochiasmal Disorders.” In: Liu, Volpe, and Galetta's Neuro-Ophthalmology: Diagnosis and Management, 3rd ed., Elsevier Inc., Edinburgh, 2019, pp. 293–339

3. Miller, Neil R., Newman, Nancy J.eds. Walsh & Hoyt's Clinical Neuro-Ophthalmology. 6th Edition. 530 Walnut Street, Philadelphia, Pennsylvania 19106 USA; 351 West Camden Street, Baltimore, Maryland 21201-2436 USA:Lippincott Williams & Wilkins; 2005.