Mechanisms causing angle closure can be described according to the anatomical location of the obstruction

Mechanisms causing angle closure can be described according to the anatomical location of the obstruction:

• pupillary block mechanism
  • seen in around 75%
  • aqueous flow from the posterior chamber to the anterior chamber through the pupil is hampered resulting in an increase in posterior chamber pressure and anterior bowing of the iris
  • may be physiological (during reading in poor light) or pharmacological
  • narrow anterior segment and the age related increase of lens volume may act as predisposing factors
  • is associated with hypermetropia
    
    
• obstruction at the level of the iris and/or cilliary body
  • caused by variation in anatomy of the iris and cilliary body resulting in iridotrabecular contact
  • e.g. - thick iris, more anterior iris insertion and more anterior cilliary body position
    
    
• obstruction at the level of the lens
  • thicker (in diabetes or post traumatic cataract), more anteriorly positioned (retinal gas or oil tamponade) or subluxation (Marfan syndrome or trauma) of the lens may result in secondary angle closure
    
    
• obstruction posterior to the lens (aqueous misdirection syndrome)
  
  • may be seen after trabeculectomy, lens extraction, laser iridotomy
  • aqueous is misdirected in to the vitreous causing lens/iris diaphragm to push forward and obstruct the anterior chamber angle (1)

Reference:

• 1. European Glaucoma Society (EGS) 2008. Terminology and guidelines for glaucoma (3rd edition)