Case Example

• Asymmetry of the optic nerves
  • The right optic disc is tilted with oblique entry of the retinal vessels. The neuroretinal rim is irregular with thinning/notching at the inferior and temporal regions as well as marked adjacent beta-zone parapapillary atrophy. The right optic disc is also cupped with a vertical cup-disc ratio of 0.80. There is a pigment crescent at the nasal border.
  • The left optic disc appears relatively normal with healthy, thick regular neuroretinal rim, a vertical cup-disc ratio of 0.40, and non-specific parapapillary pigment crescents.
• Visual field changes
  • Both visual fields are reliable
  • There is right a superior arcuate visual field defect
• Decreased RNFL thickness
  • There is thinning of the right RNFL (average thickness 74.76 microns) compared to the left RNFL (average thickness 89.81 microns). The inferior quadrant is particularly thinned, in keeping with the appearance of the optic nerves and the appearance of the visual fields.

• Normal tension glaucoma
• Vascular insult to the optic nerve such as NAION, GCA, shock optic neuropathy
  • These tend to present with an acute change in vision, and late stage cupping is often associated with pallor
• Compressive optic neuropathy
  • A disproportionately large decrease in colour vision, age less than 50, neuroretinal rim pallor, vertically aligned visual field defects and a visual acuity less than 6/12 should raise the suspicion for compressive optic neuropathy and patients should be considered for neuroimaging
• Congenital optic nerve defects ¹

Normal tension glaucoma is defined as a patient with glaucomatous optic neuropathy where the pre-treatment IOP never exceeds 21mmHg. ²

Worldwide the proportion of POAG which is NTG varies. Most western studies report that NTG represents 25-50% of all cases of POAG, however in one Japanese study NTG represented 92% of all POAG cases. ¹

Our patient is at high risk of progression due to the following factors described by the Collaborative Normal Tension Glaucoma Study (CNTGS)

• Female sex
• History of migraine
• Family history of glaucoma ²

The other risk factor for progression of NTG identified by this study was disc haemorrhage at presentation

The CNTGS found that the rate of visual field progression in patients is variable between patients. Although some cases showed progression within months, around half of the patients receiving no treatment had not worsened at the 5 year follow up. Hence careful clinical decisions should be made based on the severity of disease and the presence of risk factors. ³

Progression of glaucoma is often based on worsening of HVF defects. However in patients with NTG receiving frequent HVF testing looking for subtle changes, there is a chance of falsely judging a worsening of visual fields. Hence the results of the CNTGS suggested that at least two VF showing progression were required for confirmation. ² However progression should also be determined based on clinical correlation, IOP and optic disc changes.

Again, the CNTGS found that lowering IOP by 30% slowed progression of visual field loss compared to those not treated. This was achieved by medical, laser, surgical or a combination of treatments. This 30% reduction could be achieved with medical therapy and laser trabeculoplasty in about half the patients. Surgical treatments produced the most cataracts. ³

As a vascular aetiology involving hypoperfusion of the optic nerve head is suspected in NTG. Theoretically, beta-blockers and alpha agonists could exacerbate this however there is no strong evidence to support this. In the CNTGS these two medical therapies were not included due to theoretical systemic effects. The low pressure glaucoma treatment study found that systemic beta-blockers were a risk factor for disc haemorrhage.