A 58-year-old Indian man with a 5-year history of treated hypertension presents for routine eye check-up. Examination reveals IOP 24 mmHg (both eyes), healthy optic disc with normal cup-to-disc ratio, and normal visual fields. Gonioscopy shows open angles with mild trabecular meshwork pigmentation. Which finding best distinguishes this patient's ocular hypertension from established Primary Open-Angle Glaucoma?