Open Angle Glaucoma



Glaucoma refers to the optic nerve damage that is caused by a significant rise in intraocular pressure. The raised intraocular pressure is caused by a blockage in aqueous humour trying to escape the eye. There are two types of glaucoma: open-angle and closed-angle.

Basic Anatomy and Physiology

The vitreous chamber of the eye is filled with vitreous humour.

The anterior chamber between the cornea and the iris and the posterior chamber between the lens and the iris are filled with aqueous humour that supplies nutrients to the cornea.

The aqueous humour is produced by the ciliary bodyThe aqueous humour flows from the ciliary body, around the lens and under the iris, through the anterior chamber, through the trabecular meshwork and into the canal of Schlemm. From the canal of Schlemm it eventually enters the general circulation.

The normal intraocular pressure is 10-21 mmHg. This pressure is created by the resistance to flow through the trabecular meshwork into the canal of Schlemm.

Pathophysiology

In open-angle glaucoma, there is a gradual increase in resistance through the trabecular meshwork. This makes it more difficult for aqueous humour to flow through the meshwork and exit the eye. Therefore the pressure slowly builds within the eye and this gives a slow and chronic onset of glaucoma.

In acute angle-closure glaucoma, the iris bulges forward and seals off the trabecular meshwork from the anterior chamber preventing aqueous humour from being able to drain away. This leads to a continual build-up of pressure. This is an ophthalmology emergency.

Increased pressure in the eye causes cupping of the optic disc. In the centre of a normal optic disc is the optic cup. This is a small indent in the optic disc. It is usually less than half the size of the optic disc. When there is raised intraocular pressure, this indent becomes larger as the pressure in the eye puts pressure on that indent making it wider and deeper. This is called “cupping”. An optic cup greater than 0.5 the size of the optic disc is abnormal.

Risk Factors

  • Increasing age
  • Family history
  • Black ethnic origin
  • Nearsightedness (myopia)

 

Presentation of Open-Angle Glaucoma

Often the rise in intraocular pressure is asymptomatic for a long period of time. It is diagnosed by routine screening when attending optometry for an eye check.

Glaucoma affects peripheral vision first. Gradually the peripheral vision closes in until they experience tunnel vision.

It can present with gradual onset of fluctuating pain, headaches, blurred vision and halos appearing around lights, particularly at night time.

 

Measuring Intraocular Pressure

Non-contact tonometry is the commonly used machine for estimating intraocular pressure by opticians. It involves shooting a “puff of air” at the cornea and measuring the corneal response to that air. It is less accurate but gives a helpful estimate for general screening purposes.

Goldmann applanation tonometry is the gold standard way to measure intraocular pressure. This involves a special device mounted on a slip lamp that makes contact with the cornea and applies different pressures to the front of the cornea to get an accurate measurement of what the intraocular pressure is.

 

Diagnosis

Goldmann applanation tonometry can be used to check the intraocular pressure.

Fundoscopy assessment to check for optic disc cupping and optic nerve health.

Visual field assessment to check for peripheral vision loss.

 

Management of Open-Angle Glaucoma

Management of glaucoma aims to reduce the intraocular pressure. Treatment is usually started at an intraocular pressure of 24 mmHg or above. Patients are followed up closely to assess the response to treatment.

Prostaglandin analogue eye drops (e.g. latanoprost) are first line. These increase uveoscleral outflow. Notable side effects are eyelash growth, eyelid pigmentation and iris pigmentation (browning).

Other options:

  • Beta-blockers (e.g. timolol) reduce the production of aqueous humour
  • Carbonic anhydrase inhibitors (e.g. dorzolamide) reduce the production of aqueous humour
  • Sympathomimetics (e.g. brimonidine) reduce the production of aqueous fluid and increase uveoscleral outflow

 

Trabeculectomy surgery may be required where eye drops are ineffective. This involves creating a new channel from the anterior chamber, through the sclera to a location under the conjunctiva. It causes a “bleb” under the conjunctiva where the aqueous humour drains. It is then reabsorbed from this bleb into the general circulation.

Last updated April 2019
WordPress Theme built by Shufflehound. Copyright 2016-2021 - Zero to Finals - All Rights Reserved