ISSN 2398-2950      

Glaucoma

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Introduction

  • Cause: glaucoma represents a group of heterogeneous diseases characterized by pathologically elevated intraocular pressure resulting in retinal gangllion cell death. In cats, the most commn cause is intraocular inflammation.
  • Signs: discomfort, blindness, episcleral congestion, conjunctival hyperemia, corneal edema, mydriasis, buphthalmos, lens luxation.
  • Diagnosis: ophthalmic examination including tonometry.
  • Treatment: ocular hypotensive and inflammatory tonometry.
  • Prognosis: guarded.

Pathogenesis

Etiology

Predisposing factors

General

  • Uveitis.
  • Systemic hypertension Hypertension.
  • Lymphoma.
  • Diffuse uveal melanoma.

Pathophysiology

  • Normal intraocular pressure is a balance between aqueous humor production and outflow:
    • Aqueous humor is produced by the ciliary body processes by active secretion (mainly) and ultrafiltration.
    • Aqueous humor then traverses the posterior chamber via the pupil into the anterior chamber.
    • From the anterior chamber, the majority of aqueous drains via the iridocornal angle.
    • The main components of this angle are the pectinate ligament and ciliary cleft which contains the sieve-like trabecular meshwork.
    • The vast majority of aqueous (~97%) filters through these structures and enters the angular aqueous plexus before entering the cleral venous plexus ('conventional' outflow).
    • Normal intraocular pressure in the cat is 10-25 mm Hg.
  • In cats, the only consistent risk factor for glaucoma amongst the group of heterogenous diseases is pathologically elevated intraocular pressure.
  • Overproduction of aqueous humor is not a recognized phenomenon in cats and, thus, all known forms of glaucoma relate to obstruction to aqueous humor outflow.
  • Aqueous humor obstruction can occur at any point from its release into the posterior chamber to its ultimate drainage into the angular aqueous plexus and include the following:
    • Pupil block:
      • Iris bombe. Peripheral synechiae (adhesions between posterior iris and anterior lens capsule) for 360° in chronic uveitis.
      • Lens luxation and subluxation.
      • Vitreous prolapse (following lens removal or lens luxation).
    • Obstruction of entrance to iridocorneal angle:
      • Obstruction by red blood cells (trauma), white blood cells and inflammatory debris (acute and chronic uveitis).
      • Anterior synechiae (chronic uveitis).
    • Obstruction within ciliary cleft:
      • Collapse of ciliary cleft in all forms of glaucoma.
      • Neoplastic extension (in particular with diffuse uveal melanoma).
      • Obstruction by red blood cells (trauma), white blood cells and inflammatory debris (acute and chronic uveitis).
  • Elevation of intraocular pressure causes interruption of vascular supply to optic nerve head and axoplasmic flow within optic nerve axons.
  • Increase in glutamate levels may cause excitotoxicity-mediated retinal ganglion cell death.

Timecourse

  • Usually chronic in cats as a result of chronic uveitis.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Dietrich U, Chandler M, Cooper T et al (2007) Effects of topical 2% dorzolamide hydrochloride alone and in combination with 0.5% timolol maleate on intraocular pressure in normal feline eyes. Vet Ophthalmol 10 (Suppl 1), 95-100 PubMed.
  • Rainbow M & Dziezyc J (2003) Effects of twice daily application of 2% dorzolamide on intraocular pressure in normal cats. Vet Ophthalmol 6 (2), 147-150 PubMed.
  • Hampson E C, Smith R I & Bernays M E (2002) Primary glaucoma in Burmese cats. Aust Vet J 80 (11), 672-680 PubMed.
  • Blocker T & Van Der Woerdt T (2001) The feline glaucomas: 82 cases (1995-1999). Vet Ophthalmol (2), 81-85 PubMed.
  • Olivero D K, Riis R, Dutton A G et al (1991) Feline lens displacement a retrospective analysis of 345 cases. Progress in Vet Comp Ophthalmol (4), 239-244 VetMedResource.

Other sources of information

  • Gould D & McLellan G (2014) BSAVA Manual of Small Animal Ophthalmology. 3rd edn. British Small Animal Veterinary Association. pp 293-294.
  • Gelatt K, Gilger B, Kern T (2013) Veterinary Ophthalmology. 5th edn. pp 1514-1518. 

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