A 55-year-old woman has a central scotoma in the right eye and a superotemporal visual field defect confined to the left eye. Neuroimaging shows a lesion at the anterior angle of the optic chiasm on the right side. This pattern, known as a junctional scotoma, results from involvement of which anatomical structure?

• A Meyer's loop in the left temporal lobe
• B Wilbrand's knee, the loop of crossing inferonasal fibres into the contralateral optic nerve ✓
• C The uncrossed temporal retinal fibres in the right optic tract
• D The macular fibres decussating in the posterior chiasm

Explanation

At the anteroinferior angle of the chiasm, fibres from the inferonasal retina of one eye briefly loop forward into the base of the contralateral optic nerve as Wilbrand's knee before crossing. B lesion here therefore damages the ipsilateral optic nerve, producing an ipsilateral central scotoma, together with these looping contralateral fibres, producing a contralateral superotemporal defect. This combination distinguishes a true anterior chiasmal lesion from simple optic neuropathy and demands imaging of the sellar region.

Reference: Miller's Walsh and Hoyt Clinical Neuro-Ophthalmology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.