Diagnosis & Testing: Why Gonioscopy is Mandatory

When it comes to diagnosing primary angle-closure glaucoma (PACG), a standard eye pressure check is not enough. Because the “closure” in your eye can be intermittent or happen when you are asleep, relying solely on eye pressure (IOP) readings taken during office hours can lead to a dangerous misdiagnosis.

The Mandatory “Gold Standard”: Gonioscopy

Gonioscopy is the single most important test for diagnosing PACG. During this exam, your doctor places a special mirrored lens directly on the surface of your numbed eye. This is the only way for a clinician to physically see into the “corner” of your eye to determine if the drainage angle is open, narrow, or completely blocked ^[1][2].

Without this test, it is impossible to definitively diagnose the type of glaucoma you have. Research shows that other common screening methods, like the “Van Herick” lamp test, can miss a significant number of cases where the angle is actually closed ^[3]. If your doctor has not performed a gonioscopy, you are encouraged to ask for one—it is the baseline requirement for safe and accurate glaucoma care ^[4].

The Trap of “Normal Tension Glaucoma”

A major risk for patients with PACG is being misdiagnosed with Normal Tension Glaucoma (NTG). This happens when a doctor sees damage to your optic nerve but finds that your eye pressure is in the “normal” range during your appointment ^[5].

However, in PACG, eye pressure is often a “moving target.” Patients frequently experience diurnal fluctuations—massive pressure spikes that occur outside of office hours. Research has found that:

• Pressure peaks in PACG patients often occur around 4:00 AM while they are asleep ^[6].
• These spikes can be more than 5 mmHg higher than the readings taken during a 2:00 PM doctor’s visit ^[6].
• Because the pressure is normal when you are in the clinic, the physical closure of the angle may be the “hidden” cause of your nerve damage ^[7].

If you suspect this is happening to you, discuss with your doctor whether a 24-hour pressure check or a change in the timing of your medication is appropriate.

High-Tech Imaging Adjuncts

While gonioscopy is the clinical gold standard, your doctor may also use advanced imaging to get a “cross-section” view of your eye’s internal structures. These tests provide objective data that help tailor your treatment plan:

• AS-OCT (Anterior Segment Optical Coherence Tomography): A non-contact, high-resolution scan that uses light waves to create a detailed map of your drainage angle ^[8]. It is excellent for monitoring how your angle changes over time ^[9].
• UBM (Ultrasound Biomicroscopy): This uses high-frequency sound waves to see behind the iris. It is particularly useful for identifying plateau iris, a condition where the structure holding the lens is pushing the iris forward into the drain ^[10][11].

These tools do not replace gonioscopy, but they offer “under-the-hood” insights that help your doctor decide if you need a laser procedure, medication, or surgery ^[12]. Don’t be afraid to ask your care team which of these tests are appropriate for your specific eye anatomy.