A 52-year-old woman with a refractive error of +3.5 D presents with a 6-month history of intermittent blurred vision and mild ocular discomfort. Intraocular pressure (IOP) is 18 mmHg in both eyes. Gonioscopy reveals a narrow angle of 20 degrees with appositional contact between iris and trabecular meshwork in the superior quadrant. Anterior chamber depth is 2.1 mm. Optical coherence tomography (OCT) of the optic nerve head shows a cup-to-disc ratio of 0.3 with intact neuroretinal rim. Which of the following is the most appropriate next step in management?