Angle-closure classification and mechanisms

Angle-closure glaucoma is a leading cause of bilateral irreversible blindness worldwide. While less prevalent than POAG in European populations, primary angle-closure glaucoma (PACG) carries a disproportionately high risk of severe visual impairment due to acute, severe pressure elevations and chronic asymptomatic synechial angle closure. Mastery of anterior segment gonioscopy, mechanistic classification, emergency medical rescue protocols, and definitive laser and lens-based interventions is essential for the European Board of Ophthalmology Diploma.

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Natural History & The EGS / Foster Staging Classification

Angle closure is an anatomical disorder characterized by apposition or adhesion between the peripheral iris and the trabecular meshwork, impeding aqueous outflow. The European Glaucoma Society (EGS) adopts the three-stage epidemiological classification established by Foster et al., which stratifies patients along an anatomical continuum:

1. Primary Angle-Closure Suspect (PACS):
   • Gonioscopy: Iridotrabecular contact (ITC) of $\ge 180^\circ$ (at least 2 quadrants) on static gonioscopy in the dark without indentation (the posterior pigmented trabecular meshwork is not visible over $\ge 180^\circ$).
   • Intraocular Pressure: Normal untreated IOP ($\le 21$ mmHg).
   • Anatomic Integrity: No peripheral anterior synechiae (PAS).
   • Optic Nerve & Field: Structurally normal optic nerve head, normal RNFL, and completely normal visual fields.
2. Primary Angle Closure (PAC):
   • Gonioscopy: ITC of $\ge 180^\circ$.
   • Pathological Signs: Associated with features indicating trabecular outflow damage, specifically elevated IOP ($>21$ mmHg) and/or the presence of peripheral anterior synechiae (PAS).
   • Optic Nerve & Field: Absence of glaucomatous optic neuropathy; visual fields remain completely normal.
3. Primary Angle-Closure Glaucoma (PACG):
   • Criteria: Fulfills all criteria for PAC (ITC $\ge 180^\circ$ with elevated IOP and/or PAS).
   • Definitive Neuropathy: Presence of established glaucomatous optic neuropathy (neuroretinal rim thinning, excavation, notching) with corresponding reproducible visual field defects.

Anatomic Predisposing Features

Eyes predisposed to primary angle closure share distinct biometric features: a shallow axial anterior chamber depth (ACD $<2.5$ mm), short axial length (AL $<22.0$ mm), increased crystalline lens thickness ($>4.5-5.0$ mm), anteriorly positioned lens, small corneal diameter, steep corneal curvature, and moderate-to-high hyperopia. Demographic risk factors include advanced age (lens thickness increases progressively throughout life), female sex (shallower anterior chambers), and East/Southeast Asian ancestry.

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Pathophysiological Mechanisms: Pupillary vs. Non-Pupillary Block

Accurate diagnosis requires identifying the exact level of anatomical obstruction responsible for iridotrabecular contact.

1. Relative Pupillary Block

Pupillary block is the primary mechanism in approximately 90% of angle-closure cases:

• Pathogenesis: Resistance to aqueous humor transit from the posterior chamber to the anterior chamber occurs at the interface between the posterior pupillary border of the iris and the anterior capsule of the crystalline lens.
• Biomechanics: This physiological resistance is maximal in the mid-dilated pupillary state (3.5 to 4.5 mm), where iris-lens contact remains substantial while the peripheral iris stroma is lax. The resulting pressure differential between the posterior chamber and the anterior chamber pushes the pliable peripheral iris forward (iris bombé), causing appositional closure of the iridocorneal angle.
• Definitive Resolution: Relieved completely by creating a full-thickness hole in the peripheral iris (laser peripheral iridotomy), which equalizes the trans-iris pressure gradient.

2. Non-Pupillary Block Mechanisms

When angle closure persists despite a widely patent iridotomy, non-pupillary block mechanisms must be diagnosed and managed:

Acute Angle-Closure Crisis: Clinical Presentation

An acute angle-closure crisis is an ophthalmic emergency caused by sudden, total $360^\circ$ appositional angle closure with severe, rapid IOP elevation (often 50 to 80 mmHg). The clinical presentation is dramatic:

• Symptoms: Severe, excruciating unilateral ocular, periorbital, and hemicranial pain radiating along the ophthalmic distribution of the trigeminal nerve ($V_1$); pronounced nausea and profuse vomiting (frequently misdiagnosed as an acute abdominal emergency, gastroenteritis, or subarachnoid hemorrhage); marked visual blur or drop in acuity; perception of colored rainbow halos around lights caused by diffraction through microcystic corneal epithelial edema.
• Signs on Examination:
  • Palpation: Rock-hard globe.
  • Cornea: Microcystic epithelial corneal edema with epithelial blebs and stromal haze.
  • Anterior Chamber: Markedly shallow anterior chamber with peripheral iridocorneal touch, accompanied by flare and inflammatory cells.
  • Pupil: Mid-dilated (4 to 6 mm), vertically oval, fixed and unreactive to direct light, secondary to ischemic paralysis of the iris sphincter muscle.
  • Conjunctiva: Intense ciliary and conjunctival vascular engorgement (ciliary flush).
  • Contralateral Eye: Gonioscopy of the unaffected fellow eye almost invariably demonstrates an occludable, anatomically narrow angle, confirming predisposing bilateral anatomy.
• Post-Attack Sequelae (Signs of Previous Crisis): Glaukomflecken (multiple, small, grey-white anterior subcapsular lens opacities representing necrotic lens epithelial cells from severe ischemia and mechanical pressure), sectoral iris stromal atrophy with pupil distortion, and persistent peripheral anterior synechiae (PAS).

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