Evaluation of normal-tension glaucoma includes broad differentialdiagnosis

The diagnosis of normal-tension glaucoma (NTG) should be approached as one of exclusion, according to Andrew G. Iwach, MD.

There are a number of ophthalmic and non-ophthalmic possible causes for observed changes in function and/or structure. It is also important, however, to consider whether the patient might have primary open-angle glaucoma (POAG) with an underestimated IOP. Recent alcohol ingestion may lower IOP. The tonometric measurement also can be falsely low in patients with a thin cornea, including those who have a thin cornea as a result of refractive surgery.

Diurnal variation

Diurnal variation is another possible explanation for a normal IOP measurement in a patient who truly has POAG.

"IOP can easily vary by 2 to 6 mm Hg, and considerably more in some instances, over the course of the day. So it can be worthwhile to obtain a diurnal IOP. When undertaking that study, patients should be instructed not to vary from their usual daily routines," Dr. Iwach said.

The evaluation of patients with suspected NTG should also consider whether the optic nerve damage and accompanying visual field defect is the result of a prior IOP-related injury. For example, patients may have had intermittent angle-closure glaucoma, pigmentary glaucoma, glaucomatocyclitic glaucoma, uveitic or steroid-induced disease, or trauma that was not diagnosed and findings may be difficult to detect.

In addition, various systemic factors or conditions can be associated with a visual field defect, such as anemia, collagen vascular disease, syphilis, carotid artery disease, or systemic hypotension.

Visual field loss can also result from a number of nonglaucomatous ophthalmic problems. In that regard, the differential diagnosis might also consider the presence of congenital defects (optic nerve pits and colobomas), arteritic and non-arteritic anterior ischemic optic neuropathy, hereditary optic neuropathies, infection, optic nerve head drusen, optic neuritis, vascular occlusion, or retinal or chiasmal compressive lesions.

Role of neuroimaging

The role of neuroimaging in patients with suspected NTG was discussed by Christopher A. Girkin, MD, MPH, associate professor of ophthalmology and director, glaucoma service, Callahan Eye Foundation Hospital, University of Alabama at Birmingham. He said that in patients with visual field loss and a lack of structural damage, there are seven signs that signal a need for further work-up with magnetic resonance imaging (MRI) to investigate the presence of a compressive tumor. Those are:

For patients with typical NTG, debate continues over whether to perform neuroimaging routinely. Dr. Girkin noted that results of several studies could not establish a cost benefit for routine imaging. However, in a trial reported by Ahmed et al. in 2002, cranial MRI revealed significant lesions compressing the anterior visual pathway in four of 62 patients with NTG versus none of 70 patients with POAG.