Diagnosis: Acute angle closure glaucoma
Acute angle closure glaucoma is an ophthalmic emergency caused by sudden blockage of the outflow tract of the eye, or trabecular meshwork, causing an acute, significant, elevation in intraocular pressure. In addition to decreased vision, and pain, the elevated IOP can cause rapid, irreversible damage to the optic nerve, causing permanent vision loss. Therefore, prompt diagnosis and management of this condition is critical.
The most common mechanism for acute angle closure glaucoma is referred to as “pupillary block.” In a healthy eye, aqueous humor is produced by the ciliary body, travels over the lens, then through the pupil to enter the anterior chamber (see diagram below). Once in the anterior chamber, it exits the eye through the trabecular meshwork, and then is drained via Schlemm’s canal into the venous system. In acute angle closure glaucoma, the pupil becomes dilated, typically prompted by an environment with dim illumination, such as a movie theatre or room with lights off. The iris in its dilated state then mechanically obstructs the angle, or outflow tract of the eye. Simultaneously, fluid continues to be produced by the ciliary body. However, because it cannot exit the eye, a relative pressure differential is created between the posterior chamber, including the ciliary body where aqueous humor is being produced, and the anterior chamber, where the aqueous humor is trying to exit the eye. The increased pressure in the posterior chamber, causes shallowing of the anterior chamber, and because of this shallowing, the iris becomes pushed against the lens, thereby blocking any fluid from passing from the posterior to the anterior chamber. This mechanism is therefore referred to as a “pupillary block” mechanism. The iris essentially becomes “stuck” in this dilated position, because it is pressed against the lens. Cataract development and thickening of the crystalline lens can also lead to a relative narrowing of the angle and shallowing the anterior chamber, increasing risk of angle closure glaucoma (Above left image credit: Troy Teeples & Griffin Jardine, MD. Acute Angle Closure Glaucoma. MoranCORE, Moran Eye Center).
The treatment for pupillary block angle closure glaucoma, is essentially the creation of a “new pupil” or passageway for fluid to pass from the posterior to the anterior chamber. This is achieved through performing a laser peripheral iridotomy. Using laser, a small hole is a created in the peripheral iris to restore flow of fluid from the posterior to anterior chamber. With flow restored, the relative pressure differential between the posterior and anterior chamber is resolved, allowing the anterior chamber and angle to deepen, and re-establishing space between the lens and the pupillary margin. Treatment with aqueous suppressant drops is also indicated to lower the IOP. Lowering the IOP often reduces the corneal edema, improving the examiner’s view in preparation for a laser iridotomy (above left image credit: CC BY-NC-ND 3.0).
If a significant cataract is present, and is thought to have been a contributing factor to angle closure, cataract surgery can be also be considered in the management of this condition. The EAGLE study sought to evaluate outcomes of primary treatment of angle closure with laser iridotomy versus cataract extraction.
Traditionally, a preventative laser iridotomy is recommended in the fellow eye as well, after stabilization of the affected eye. This established practice versus observation was recently investigated by the ZAP Trial.
References and Additional Resources:
1. “Primary vs. Secondary Angle Closure.” Available at: https://eyewiki.aao.org/Primary_vs._Secondary_Angle_Closure_Glaucoma.
2. Nongpiur ME, Ku JY, Aung T. Angle Closure Glaucoma: A Mechanistic Review. Curr Opin Ophthalmol. 2011;22(2):96-101.
3. He M, Jiang Y, Huang S, Chang DS, Munoz B, Aung T, Foster PJ, Friedman DS. Laser peripheral iridotomy for the prevention of angle closure: A single-centre randomized controlled trial. Lancet. 2019;393(10181):1609-1618.
4. Azuara-Blanco A, et al. Effectiveness of early lens extraction for the treatment of primary angle-closure glaucoma (EAGLE): a randomized controlled trial. Lancet. 2016;388(10052):1389-1397.
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