Optic Nerve Head and Glaucoma

Optic Nerve Head

• The optic nerve head is the intra-ocular segment of the optic nerve. It is composed of 3 segments:
  • Surface nerve fibre layer: Convergence of retinal ganglion cell axons onto the optic nerve head.
  • Pre-laminar region: Nerve fibres form into bundles to pass through the lamina cribrosa.
  • Lamina Cribrosa: Connective tissue with multiple openings. Provides nutrition to the nerve fibre bundles. The pores are larger in the superior and inferior lamina cribrosa due to larger nerve fibre bundles.

Optic Disc

• Optic disc: Optic cup + Neuro-retinal rim (NRR).
• Small disc size: 2.2mm.
• Large discs: More likely to be damaged, particularly in Normal Tension Glaucoma.
• ISNT Rule: Expected thickness: Inferior > Superior > Nasal > Temporal. High sensitivity in detecting early glaucomatous changes (81%). However, healthy optic nerves may not always follow this rule.

Glaucoma

• Group of conditions causing chronic progressive optic neuropathy.
• Often associated with raised intraocular pressure (IOP). This is a key modifiable factor.
• Aqueous humour: Fluid that circulates in the eye.
• Aqueous drainage pathways:
  • Trabecular pathway: 90% of aqueous exits through this route.
  • Uveoscleral pathway: 10% of aqueous exits through this route.

Retinal Nerve Fibre Layer Defects

• Types of defects:
  • Focal: Localized wedge-shaped defects.
  • Diffuse: Widespread defects.
• Appearance: Easier to see in red-free illumination.
• Significance: Subtle defects that precede optic nerve head changes and visual field loss.

Laminar Dot Sign

• Definition: Visible lamina cribrosa pores or increased lamina cribrosa pore size due to loss of retinal ganglion cell axons.
• Prevalence: Observed in 70% of patients with Primary Open Angle Glaucoma.

C:D Ratio Asymmetry

• Definition: Discrepancy in cup to disc ratio (C:D ratio) between the two eyes due to differences in disc size.
• Significance: Can be a sign of glaucoma.

Aqueous Drainage

• Aqueous humor exits the eye via two pathways: trabecular pathway (90%) and uveoscleral pathway (10%)
• Trabecular pathway: Aqueous flows through the trabecular meshwork, Schlemm's canal, collector channels, and then into the episcleral venous system.
• Uveoscleral pathway: Aqueous drains into the space between the ciliary body and sclera, exits through veins in the ciliary muscle and anterior choroid, and then drains into vortex veins.

Optic Nerve Recap

• The optic nerve head is the intraocular segment of the optic nerve.

Glaucoma Overview

• Glaucoma is a group of eye diseases that damage the optic nerve, which can lead to blindness.
• Primary Open-Angle Glaucoma (POAG) is the most common type and involves a gradual increase in intraocular pressure (IOP) due to blockage of the trabecular meshwork.
• Normal Tension Glaucoma (NTG) is characterized by glaucomatous optic neuropathy and visual field loss despite normal IOP.
• Ocular Hypertension refers to consistently elevated IOP (>21 mmHg) without optic nerve damage.

Ocular Hypertension

• 4.5-9.4% of individuals over 40 years old have ocular hypertension.
• 10% of patients with untreated ocular hypertension develop POAG within 5 years.
• 45.5% of individuals with ocular hypertension develop POAG within 20 years.
• Symptoms: Asymptomatic
• Signs: Elevated IOP (>21mmHg) measured by Goldmann Applanation Tonometry (GAT).

Primary Open Angle Glaucoma (POAG)

• Prevalence: 4% in individuals over 40
• Increases with age: ~1% at 40 years old, ~3% at 60 years old, ~8% at 80 years old

POAG Risk Factors

• Raised IOP: Main modifiable risk factor.
• Increasing age
• Family history of POAG
• Ethnicity
• Corticosteroid use
• Type 2 Diabetes
• Myopia
• Vascular disease
• Ocular perfusion pressure (difference between arterial blood pressure and IOP)

POAG Aetiology & Pathogenesis

• Aetiology: Increased resistance to drainage through the trabecular meshwork, leading to progressive optic neuropathy.
• Pathogenesis:
  • Mechanical damage: Retinal nerve fibers are damaged at the optic nerve head (ONH) as they pass through the lamina cribrosa.
  • Ischaemic damage: Compression of blood vessels supplying the ONH.

POAG: Work Up

• Central visual field assessment: Using standard automated perimetry (full or suprathreshold).
• Optic nerve head assessment: Slit lamp and OCT.
• IOP measurement: Goldmann-type applanation (or Perkins).
  • IOP threshold value: ≥24mmHg.
  • Pachymetry.
• Peripheral anterior chamber depth and configuration: Van Herick, OCT, or Gonioscopy.

POAG: Visual Field Defects

• Paracentral scotoma: Central blind spot near the fovea.
• Nasal step scotoma: Step-like depression in the nasal field.
• Arcuate scotoma: Arc shaped defects in the visual field.
• Ring scotoma: Ring-like defects in the visual field.

POAG: Optic Disc Changes

• Notching: Focal or diffuse indentations in the optic nerve rim.
• Asymmetry in the C:D ratio: Differences in the cup-to-disc ratio between the two eyes.
• Neuroretinal rim (NRR) thinning: Thinning of the rim surrounding the optic cup.
• RNFL defects: Deficiencies in the retinal nerve fiber layer.
• Lamina dot sign: Grey dot-like fenestrae in the lamina cribrosa become visible.
• Peripapillary atrophy: Atrophy of the tissue surrounding the optic disc.
  • Alpha Zone: Superficial RPE changes, outer zone.
  • Beta Zone: Inner zone, chorioretinal atrophy, larger in glaucoma, signifies a risk of progression.
• Vascular changes:
  • Splinter hemorrhages: Near the optic disc, may also occur in systemic vascular conditions.
  • Baring of circumlinear blood vessels: Early sign, gap between NRR and superficial blood vessels.
  • Bayoneting: Double angulation of a blood vessel.
  • Collaterals: Vessels connecting two veins at the disc.

POAG Referral

• Refer if any of the following are present:
  • Optic nerve head damage.
  • Visual field defect consistent with glaucomatous damage.
  • IOP ≥24mmHg.

POAG Management (covered in more depth in next week's lecture)

• First-Line treatment:
  • Selective Laser Trabeculoplasty (SLT).
• If IOP > 24mmHg and SLT is unsuitable/declined:
  • Prostaglandin analogue eye drops (first choice)
  • Topical beta blocker (second choice)
  • Carbonic anhydrase inhibitors
  • Combination therapy.

Normal Tension Glaucoma (NTG)

• Characteristics:
  • Glaucomatous optic neuropathy.
  • Characteristic visual field loss.
  • IOP consistently ≤ 21mmHg.
  • Open anterior chamber angle.
  • No signs of secondary glaucoma.

NTG Risk Factors:

• Age: Patients tend to be older than those with POAG.
• Race
• Family history of NTG
• Thin central corneal thickness
• Systemic hypotension: Including dips in blood pressure at night.
• Abnormal vaso-regulation: In patients with migraine or Raynaud's syndrome.
• Low cerebrospinal fluid pressure

NTG Work Up:

• History and symptoms:
  • History of migraine, Raynaud's phenomenon, episodes of hypovolemic shock, headaches, neurological symptoms, intermittent steroid use, and systemic beta-blockers.

This quiz covers the anatomy of the optic nerve head, including its segments and the optic disc. It also explores the implications of optic disc size in glaucoma, particularly the ISNT rule and its sensitivity in detecting glaucomatous changes. Test your knowledge on these critical aspects of ophthalmology.