Test Your Knowledge - Anatomic Pathways - Case 1

Test your knowledge of anatomic pathways.

NOVEL: Test Your Knowledge Collection A lesion in the left optic tract would cause which of the following disturbances in the pupils? A) Left afferent pupil defect B) Right afferent pupil defect C) Left light-near dissociation D) Right light-near dissociation Courtesy: Jonathan D. Trobe Right afferent pupil defect Correct Answer: B) Right afferent pupil defect A lesion in the optic tract is apt to cause a contralateral afferent pupil defect because axons coming from that eye and crossing into the opposite optic tract outnumber non-crossing axons. Such an afferent pupil defect is more likely to occur if the lesion damages the optic tract severely enough to cause a complete or nearly complete homonymous hemianopia. Why is this phenomenon important clinically? Because the combination of a unilateral complete homonymous hemianopia and an ipsilateral afferent pupil defect localizes the lesion to the contralateral optic tract, provided there is no evidence of an optic neuropathy on the side of the afferent pupil defect. Without that afferent pupil defect, you might not be able to localize a complete homonymous hemianopia to a particular region in the retrochiasmal visual pathway. Light-near dissociation occurs with lesions of the dorsal midbrain, not from lesions in the optic tract.