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Normal tension glaucoma (NTG) remains a difficult diagnosis for the ophthalmologist who favours the argument that raised intraocular pressure (IOP) is essential for the diagnosis of primary open angle glaucoma (POAG). The concept of NTG challenges us to distinguish between pressure dependent and pressure independent causal factors. The purpose of this review is to present some of the current information on the epidemiology, aetiology, and management of NTG, based upon a review of recent literature and experience with more than 400 NTG patients attending the glaucoma service at Moorfields Eye Hospital.
Definition of normal tension glaucoma
It is important to define NTG in order to distinguish it from other forms of glaucoma and to be able to manage it in the most appropriate way. The inclusion of a typical glaucomatous optic neuropathy with characteristic field defects in the defining criteria poses few problems. However, the level at which intraocular pressure (IOP) becomes abnormal has been a source of conjecture in the ophthalmic literature and the pros and cons of setting an arbitrary limit for use in the definition of NTG have been discussed elsewhere.1 To try and simplify the issue we have included the following criteria for the definition of NTG as used in our clinic at Moorfields:
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A mean IOP off treatment consistently equal to or less than 21 mm Hg on diurnal testing, with no single measurement greater than 24 mm Hg
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Open drainage angles on gonioscopy
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Absence of any secondary cause for a glaucomatous optic neuropathy—for example, a previously raised IOP following trauma, a period of steroid administration, or an episode of uveitis
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Typical optic disc damage with glaucomatous cupping and loss of neuroretinal rim
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Visual field defect compatible with the glaucomatous cupping (disc/field correlation)
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Progression of glaucomatous damage.
The requirement that the disease be progressive is controversial.2 3 NTG …