A 58-year-old man with a 10-year history of type 2 diabetes presents to the ophthalmology clinic for routine screening. Gonioscopy reveals a wide, open angle with normal trabecular meshwork appearance bilaterally. Intraocular pressure (IOP) is 24 mmHg in both eyes. Visual fields show a superior arcuate defect in the right eye. Optic disc photography demonstrates a vertical cup-to-disc ratio of 0.8 with inferior neuroretinal rim thinning. Which of the following findings would be MOST helpful in differentiating this presentation from angle-closure glaucoma?