Be certain when diagnosing normal tension glaucoma

December 1, 2009

Practitioners who diagnose patients with normal-tension glaucoma must be careful not to do so prematurely due to insufficient evaluation, one expert cautions.

Speaking at International Vision Expo West, Dr. Sowka said that the occurrence of glaucoma at statistically normal pressures is somewhat uncommon, although some population-based studies show that 20% to 39% of patients who have open-angle glaucoma have NTG.

Still, the diagnosis of NTG should be one of exclusion. With proper diligence, most NTG cases can be correctly ascribed to another condition.

NTG defined

NTG is a condition where there is progressive cupping of the optic nerve and visual field loss resembling that seen in other forms of chronic glaucoma. Also, an IOP greater than 21 mm Hg cannot be documented, and there is no other obvious or apparent cause for these changes.

Without proper evaluation, NTG-also known as pressure-independent glaucoma-can be overdiagnosed. Because elevated IOP is a strong risk factor for glaucoma development, practitioners should consider other possibilities when it is not present, Dr. Sowka said.

Before diagnosing NTG, a doctor should rule out other possible causes and perform a thorough evaluation including visual field analysis; multiple pressure readings; measurement of central corneal thickness; diagnostic imaging studies; and, possibly, a general medical exam.

Dr. Sowka said that the two most common conditions misdiagnosed as NTG are suspicious-appearing congenital anomalies, and the misdiagnosis of primary open-angle glaucoma (POAG). The POAG misdiagnosis occurs when too few IOP measurements are made, a high diurnal variation is not identified, or a thin cornea masks elevated IOP.

In rare cases, a tumor in the anterior visual pathway can mimic NTG. These patients present with decreased visual acuity, disc pallor, and visual field defects unlike those associated with POAG.

Patients who develop what looks like NTG may actually have a systemic disease with ocular manifestations. They should be evaluated for severe vascular disease, autoimmune disease, or very low blood pressure and subsequent poor ocular perfusion pressure.

Clinicians should not accept the findings of an initial examination and fall back on the label of NTG, Dr. Sowka said. In fact, he never uses the term NTG.

"We should probably strike this from our vernacular altogether," he said. "Glaucoma is glaucoma, no matter what the pressure is. When diagnosing glaucoma, remove pressure from the equation. If patients have characteristic glaucomatous neuropathy and corresponding visual deficits, then just make the diagnosis of glaucoma, irrespective of what the pressure is."

Studies from sleep labs have shown that the highest IOP spike occurs while patients are asleep in the supine position. "We cannot safely say that the pressure is normal in these situations," Dr. Sowka said. "Thus, the term NTG is really not accurate."

When to consider IOP

When treating patients with diagnosed NTG, IOP becomes a consideration, he said. If the patient has glaucoma and if it is progressive, the goal should be to reduce IOP by at least 30%.

There is considerable difference between managing a patient whose IOP is 19 mm Hg versus a patient whose pressure is 11 mm Hg. The person with the higher pressure could probably be treated relatively easily and successfully with a prostaglandin analogue. Patients whose IOP is 11 mm Hg or lower should initially be observed-if fixation is not threatened-and treated with medication, laser, or surgery if their condition worsens, Dr. Sowka concluded.