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. Author manuscript; available in PMC: 2012 Sep 1.
Published in final edited form as: Vet Ophthalmol. 2011 Sep;14(Suppl 1):15–29. doi: 10.1111/j.1463-5224.2011.00912.x

Table 2.

Potential locations of obstruction to aqueous humor uutflow, with examples of pathologic mechanisms.

Block at the level of the ciliary Body/ posterior chamber/ Vitreous
“Crowding” of posterior chamber by:

  • Intraocular neoplasia

  • Cilio-vitreo-lenticular block / condensed anterior vitreous (FAHMS)

  • Iridociliary cysts

Vitreous expanded or pushed forward (so-called “malignant” glaucoma):

  • Posterior misdirection of aqueous

  • Choroidal effusion

Pupil block
Obstruction to flow of aqueous through pupil by:

  • Posterior synechiae / iris bombe (absolute block)

  • Lens within pupil / lens and iris anatomically apposed (relative block) : posterior misdirection of aqueous, luxation/ subluxation, intumescent lens (phacomorphic), spherophakia

  • Vitreous within pupil

Block at the level of the irido-corneal angle
Primary angle-closure

  • narrow-angle glaucoma

  • Pectinate ligament dysplasia

Secondary angle closure

  • Due to anterior “pull” by peripheral anterior synechiae or pre-iridal fibrovascular membrane

  • Due to posterior “push” e.g. by posteriorly misdirected aqueous humor or mass effect in the posterior chamber

Trabecular Meshwork obstructions
Primary malformation of the ciliary cleft

  • Primary congenital glaucoma

Secondary to obstruction of a conformationally “open angle” by

  • Inflammatory cells

  • Neoplastic cells

  • Pre-iridal fibrovascular membrane

  • Erythrocytes (both intact and “ghost cells”)

  • Vitreous

  • Proteins including fibrin (uveitis) or globulin (hyperviscosity syndrome)

  • Epithelial downgrowth

Post-trabecular block / Increased episcleral and/or scleral venous pressure
Innapropriate restraint

  • Including effects of tight neck collars or a stress response

Feline primary open-angle glaucoma

  • Myxomatous change around episcleral, intra-scleral and vortex veins

Feline primary congenital glaucoma

  • Paucity of intrascleral blood vessels

Orbital space occupying lesions

  • Increasing episcleral venous pressure

Infiltration of the episclera and / or sclera

  • Inflammatory

  • Neoplastic

Note that aqueous humor outflow is frequently obstructed at multiple locations within individual patients and that the proposed mechanisms should not be considered mutually exclusive. (Adapted from : Miller PE. “Feline Glaucoma”. In: Bonagura JD and KirkRW (eds). Kirk’s Current Veterinary Therapy XIV: SmallAnimal Practice. Philadelphia, Pennsylvania, WB Saunders Co. 2009.)