A 72-year-old man is brought to the emergency department by his family after nearly causing a motor vehicle accident. He reports that he did not see a car merging from his right side. His medical history is significant for hypertension, hyperlipidemia, and atrial fibrillation for which he is non-adherent with his medications. On physical examination, visual acuity is 20/20 in both eyes. Visual field testing by confrontation reveals a loss of vision in the right half of the visual field in both the right and left eyes. However, the central 5 to 10 degrees of vision remain intact. The pupils are equal, round, and reactive to light and accommodation. The remainder of the neurological examination is unremarkable.
Which of the following is the most likely location of this patient’s lesion?
The correct answer is:
C) Left posterior cerebral artery
This patient is presenting with a right homonymous hemianopia with macular sparing. A homonymous hemianopia (loss of the same side of the visual field in both eyes) indicates a lesion distal to the optic chiasm. Macular sparing is the hallmark of a lesion in the primary visual cortex (occipital lobe), most commonly due to an occlusion of the posterior cerebral artery (PCA). The macula is spared in PCA strokes because the macular projection area of the occipital cortex has a dual blood supply from both the PCA and the middle cerebral artery (MCA). Since this patient has a right-sided visual field defect, the lesion is located in the left occipital cortex, corresponding to a left PCA territory infarct.
Answer choice A: Left Meyer's loop, is incorrect. A lesion in Meyer's loop, which is located in the temporal lobe, results in a contralateral superior quadrantanopia, often described as a "pie-in-the-sky" defect. In this case, a left-sided lesion would cause a right superior quadrantanopia, not a complete hemianopia.
Answer choice B: Left optic tract, is incorrect. Lesions of the optic tract do cause a contralateral homonymous hemianopia (in this case, right-sided). However, optic tract lesions typically involve the entire half of the visual field, including the macula. Macular sparing is specifically associated with cortical lesions.
Answer choice D: Optic chiasm, is incorrect. A lesion at the optic chiasm typically compresses the decussating nasal fibers from both retinas, resulting in a bitemporal hemianopia and loss of the temporal half of the visual field in both eyes. This is classically seen with pituitary adenomas or craniopharyngiomas.
Answer choice E: Right posterior cerebral artery, is incorrect. A stroke involving the right PCA would result in a left homonymous hemianopia with macular sparing. Because the patient’s visual field defect is on the right side, the lesion must be on the left side of the brain.
Key Learning Point
A contralateral homonymous hemianopia with macular sparing is highly suggestive of a lesion in the occipital lobe, typically caused by a posterior cerebral artery (PCA) stroke. The macula is spared because the occipital pole receives collateral blood supply from the middle cerebral artery (MCA).
