A 38-year-old man reports blurred vision in the left eye for 3 months. Visual fields show a dense central scotoma in the left eye and a superotemporal field defect in the right eye. MRI shows a mass at the left optic nerve just anterior to its entry into the chiasm. This pattern, called a junctional scotoma, localizes the lesion to:
- A The anterior chiasmal junction where the optic nerve meets the chiasm, involving fibres of Wilbrand's knee ✓
- B The body of the chiasm
- C The posterior chiasm
- D The left optic radiations
Correct answer: A. The anterior chiasmal junction where the optic nerve meets the chiasm, involving fibres of Wilbrand's knee
Explanation
B junctional scotoma arises when a lesion damages one optic nerve at its junction with the chiasm. Ipsilateral optic nerve fibres are destroyed, giving a central scotoma, while looping fibres from the contralateral inferonasal retina (Wilbrand's knee) are caught, producing a contralateral superotemporal defect. B pure chiasmal body lesion gives bitemporal defects without a monocular central scotoma, which excludes option B.
Reference: Kanski's Clinical Ophthalmology, 9th ed.
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Written and medically reviewed by the StethoPrep medical team.