Normal Tension Glaucoma

Normal tension glaucoma (NTG) is a glaucomatous optic neuropathy with open angles, typical cupping and field loss, but intraocular pressure that never exceeds 21 mmHg on repeated measurements.

• Accounts for about 30 to 40 percent of open angle glaucoma in Western populations and up to 70 to 90 percent in East Asian populations.
• Associations: low blood pressure (nocturnal arterial hypotension, over treatment of hypertension), vascular dysregulation, migraine, Raynaud phenomenon, obstructive sleep apnoea, female sex, older age, disc haemorrhages, myopia, optineurin and TBK1 gene variants.
• Pressure dependent: lowering pressure by 30 percent slows progression in about 80 percent of patients.

• Asymptomatic until field loss, with paracentral defects close to fixation and more frequent threat to central vision.
• Optic disc: marked cupping with focal rim notching, frequent disc haemorrhage, peripapillary atrophy, and pallor not out of proportion to cupping.
• Normal pressure on office measurement, which can mask nocturnal peaks.
• Features suggesting an alternative diagnosis (non-glaucomatous optic neuropathy): pallor greater than cupping, visual loss out of proportion to disc appearance, young age, vertical meridian respecting or temporal field defects, colour vision loss, a relative afferent defect disproportionate to the field loss, or rapid progression.

• Serial tonometry over the day (diurnal curve) and consider an overnight or supine measurement; central corneal thickness (thin corneas underestimate pressure).
• Gonioscopy to exclude intermittent angle closure and secondary glaucoma (pigment, past uveitis, steroid use, previous trauma).
• Optic disc imaging, OCT, and repeated automated perimetry.
• Exclude non-glaucomatous causes in appropriate patients: MRI of brain and orbits with contrast (compressive lesion), blood tests (full blood count, ESR, glucose, thyroid function, B12 and folate, syphilis serology), and assessment for past shock, blood loss or anaemia.
• Assess vascular status: blood pressure (24-hour ambulatory monitoring when nocturnal hypotension is suspected), carotid examination, sleep apnoea screen, migraine and Raynaud history.

• Reduce intraocular pressure by at least 30 percent from baseline, using first line agents: prostaglandin analogue (latanoprost 0.005 percent nightly), brimonidine 0.2 percent, dorzolamide 2 percent. Avoid beta blockers if nocturnal hypotension is suspected (they lower night time perfusion).
• Selective laser trabeculoplasty is effective; trabeculectomy with mitomycin C for progression despite maximal therapy and low target pressure (below 12 mmHg).
• Systemic vascular optimisation: review antihypertensive regimen with the physician (avoid overtreatment and nocturnal dips; give antihypertensives in the morning rather than at night), treat sleep apnoea, correct anaemia, avoid hypotensive episodes.
• Calcium channel blockers (nifedipine 10 to 30 mg daily) or nimodipine for vasospastic features have been used without proven benefit.
• Monitoring: fields every 4 to 6 months initially, with OCT every 6 months, to establish the rate of progression, then individualise.
• Smoking cessation, regular exercise, and counselling.