Background
Glaucoma refers to a range of ocular diseases that ultimately result in increased intraocular pressure (IOP) and decreased visual acuity. Acute angle-closure glaucoma (AACG) is an ocular emergency and is differentiated by its acute presentation, need for immediate treatment, and well-established anatomic pathology. [1] Rapid diagnosis, immediate intervention, and referral can have profound effects on patient outcome and morbidity.
The acute angle closure literature has been plagued by the lack of a uniform definition and specific diagnostic criteria. Only in recent years has there been a strong push to standardize the definitions of the various forms of angle closure disease. Primary angle closure, primary angle-closure glaucoma, acute angle closure, and acute angle-closure glaucoma were previously used interchangeably. Acute angle closure is defined as at least 2 of the following symptoms: ocular pain, nausea/vomiting, and a history of intermittent blurring of vision with halos; and at least 3 of the following signs: IOP greater than 21 mm Hg, conjunctival injection, corneal epithelial edema, mid-dilated nonreactive pupil, and shallower chamber in the presence of occlusion.
Primary angle closure is defined as the presence of signs of trabecular obstruction on physical exam in the setting of an occludable drainage angle. These signs include peripheral anterior synechiae, increased IOP, distortion of iris fibers [iris whorling], lens opacities, and excessive trabecular pigmentation deposits. Further, the term primary angle closure suspect refers to an eye in which there is contact between the peripheral iris and the posterior trabecular meshwork, or in which contact is considered possible based on ocular anatomy, and there is no evidence of acute disease. The term glaucoma is added if glaucomatous optic neuropathy is present.
Pathophysiology
AACG occurs as a result of the impaired drainage of aqueous humor and subsequent pathologic increase in IOP. Aqueous humor is produced by the ciliary body in the posterior chamber of the eye. It diffuses from the posterior chamber, through the pupil, and into the anterior chamber. From the anterior chamber, the fluid is drained into the vascular system via the trabecular meshwork and canal of Schlemm.
There exist several anatomic abnormalities which can lead to anterior chamber crowding, predisposing individuals to AACG. These include a shallow anterior chamber, thin ciliary body, thin iris, anteriorly-situated lens, thickened lens, [2] and short axial eye length. Studies have suggested that increased iris thickness and cross-sectional area are also associated with increased risk. [3] Of the many predisposing anatomical variations, a narrow angle has the most devastating consequences.
In the traditional model of AACG, the eye's physiologic pupillary dilation (as in response to environmental or chemical stimuli) contributes to a pathologic iris-lens apposition. When there is increased apposition or contact between the lens and the iris caused by this dilatory action, there is a risk of pupillary block (resistance in the aqueous flow from the posterior to anterior chamber). The increasing pressure in the posterior chamber then pushes the iris forward in an action termed "iris bombé," further narrowing the angle and obstructing aqueous drainage. Each step in this process can result in further increasing IOP, leading to a vicious cycle of ever increasing pressure.
Research has suggested an alternative pathway for AACG. Cronemberger et al propose that an acute increase in sympathetic tone is the underlying pathophysiology of AACG and this is compounded by excessively active or large iris dilatory muscles. Emotional distress, sympathomimetic drugs, or conditions of low light precipitate increased ocular sympathetic tone causing pupillary dilation, and thickening of the middle-peripheral iris. This thickening can lead to angle closure, thereby obstructing the outflow of aqueous humor. [4]
Other proposed mechanisms of AACG include plateau iris, lens swelling, and ciliary block. Plateau iris, thought to be less common than pupillary block, is an anatomic configuration in which the iris is flattened with an anterior attachment in the setting of a normal anterior chamber depth. The pathologic consequences are more or less due to the anterior insertion of the iris. The superfluous and crowded iris tissue blocks the trabecular meshwork thereby leading an increased IOP.
The remaining causes that will be discussed in this article, lens swelling and ciliary block, are extremely rare. Lens swelling is typically the result of lens necrosis secondary to cataracts. As the lens swells, it crowds the anterior chamber and, through mechanisms previously discussed at length, leads to an increase in IOP. Finally, when there is increasing force posterior to the lens (as can be seen in post-surgical inflammation or uveitis), both the lens and iris are pushed forward, termed ciliary block.
Epidemiology
Prevalence
The prevalence of AACG worldwide is about 0.6% but varies with ethnicity. [5]
Mortality/Morbidity
Outcome after AACG is dependent on duration from onset to treatment, underlying ocular disease, and ethnicity. The degree of IOP elevation has been shown to have less impact on future visual acuity. Studies report that as many as two thirds of individuals with AACG had no visual field loss; however, Asians appear to be be less likely to respond to initial medical management, and are more likely to experience a progressive increase in IOP and deterioration in visual acuity even with definitive intervention. [6]
Race
AACG occurs in 1 of 1000 whites, about 1 in 100 Asians, and as many as 2-4 of 100 Eskimos. [7, 8]
Sex
Women are at increased risk for developing AACG and for developing subsequent blindness from AACG. [9]
Age
Elderly patients in their sixth and seventh decades of life are at greatest risk. [9]
Prognosis
Decrease time to presentation is associated with a more favorable prognosis, as is prophylactic iridectomy on all fellow eyes. Surprisingly, degree of IOP elevation is not associated with a worse prognosis. [10]
