What is the prevalence and prognosis of Normal Pressure Glaucoma (NPG) compared to Primary Open-Angle Glaucoma (POAG)?

Normal Pressure Glaucoma: Prevalence and Prognosis

Normal tension glaucoma (NTG) is a common form of open-angle glaucoma worldwide, characterized by glaucomatous optic neuropathy despite normal intraocular pressure (IOP), with studies suggesting it represents a significant subset of primary open-angle glaucoma (POAG) cases where IOP-independent factors play a more important role in disease progression. 1

Prevalence

• Population-based studies suggest that approximately half of all individuals with POAG have IOP levels below 22 mmHg, which is the usual cutoff for normal pressure 2
• NTG is considered either a subvariant of POAG or a separate entity, defined as chronic open-angle glaucoma with IOP between 10-21 mmHg 3
• NTG represents a significant proportion of all glaucoma cases, with higher prevalence in certain populations, particularly in Asian countries 4

Clinical Characteristics and Diagnosis

• NTG presents with all classic symptoms of glaucoma except elevated IOP, including glaucomatous optic disc excavation, nerve fiber bundle defects, and corresponding visual field defects 3
• Papillary rim hemorrhages are frequently found in NTG patients 3
• Visual field defects in NTG tend to be more central compared to POAG, making them more noticeable and disturbing to patients 3
• Diagnosis requires comprehensive eye evaluation including:
  • Fundoscopic evaluation of the optic nerve head
  • Visual field assessment using perimetry
  • Measurement of IOP (consistently below 22 mmHg)
  • Open anterior chamber angle on gonioscopy 5, 4

Pathophysiology and Risk Factors

• NTG and POAG represent a continuum of open-angle glaucomas, with IOP being the predominant factor in POAG while additional IOP-independent factors take increasing importance in NTG 1
• Circulatory disturbances, including arterial hypo- and hypertension, may play an essential role in NTG pathophysiology 3
• Lower intracranial pressure has been demonstrated in patients with NTG compared to those with POAG and ocular hypertension 2
• Recent evidence suggests that the cornea is more deformable in NTG patients than in POAG patients or controls, indicating potential biomechanical differences 6
• Low diastolic perfusion pressure (<50 mmHg) is associated with a higher prevalence of POAG and may be particularly relevant in NTG 2

Prognosis and Disease Progression

• NTG typically has a worse prognosis compared to POAG with elevated IOP because:
  • The disease may be diagnosed later due to normal pressure readings giving false reassurance 7
  • Visual field defects tend to be more central, affecting quality of life more significantly 3
  • The multifactorial nature of NTG makes it more challenging to treat effectively 1
• Studies have shown that patients with NTG have even lower levels of intracranial pressure compared to those with POAG, which may contribute to a higher translaminar pressure gradient and potentially faster progression 2

Treatment Approach

• Despite normal IOP, the primary goal of treatment remains IOP reduction, which can slow or stop disease progression 3
• Treatment of NTG requires:
  • Lowering IOP through medications (typically prostaglandin analogs as first-line) or surgery 5
  • Consideration of IOP-independent factors, especially those affecting optic nerve head perfusion 1
  • Interdisciplinary approach to address systemic factors that may contribute to disease progression 3
• Research continues to investigate IOP-independent neuroprotective agents that may be particularly beneficial for NTG patients 1

Monitoring and Follow-up

• Regular monitoring of visual fields and optic nerve appearance is essential to detect progression 5
• More frequent follow-up may be necessary for NTG patients due to the potential for progression despite seemingly "normal" IOP readings 1
• Careful assessment of risk factors for progression, including low ocular perfusion pressure and systemic vascular dysregulation 2