Plateau iris syndrome is a specific anatomical condition of the eye where the iris is positioned in a way that can narrow the eye’s drainage angle, increasing the risk for a type of glaucoma. This condition is characterized by a persistently narrow angle, even after a laser procedure designed to open the angle has been performed. Understanding this unique ocular structure is important for managing the potential for vision loss.
The Unique Anatomy of Plateau Iris
Plateau iris is distinguished by particular anatomical features within the eye’s anterior chamber. The condition often involves an anteriorly displaced or abnormally large ciliary body, which is the structure behind the iris that produces aqueous humor, the fluid filling the eye. This forward positioning of the ciliary body pushes the peripheral part of the iris forward, causing it to crowd the drainage angle. Imagine a sink drain partially covered by a flat plate; the water struggles to drain effectively because the opening is physically obstructed.
The drainage angle is located where the iris meets the cornea, and it contains the trabecular meshwork, a spongy tissue responsible for filtering aqueous humor out of the eye. In plateau iris, the peripheral iris, instead of curving gently backward, takes a steep, almost flat approach, then drops sharply just before the trabecular meshwork. This arrangement physically obstructs the meshwork, impeding the outflow of fluid and potentially leading to a buildup of pressure within the eye.
Symptoms and Diagnostic Process
Individuals with plateau iris syndrome may not experience any symptoms, with the condition often discovered during a routine eye examination. However, it can lead to acute angle-closure attacks, which manifest with noticeable symptoms. These may include sudden eye pain, blurred vision, seeing halos around lights, and headaches. The elevated intraocular pressure during such an attack can also induce nausea and vomiting.
Diagnosing plateau iris involves specific ophthalmological tests to visualize the eye’s internal structures. Gonioscopy is a primary diagnostic tool, allowing the ophthalmologist to directly view the drainage angle using a special lens. With this technique, the angle typically appears narrow or closed, and the iris often presents a flat plane. Indentation gonioscopy can reveal a “double-hump sign,” where the peripheral hump corresponds to the iris draping over the ciliary body, and the more central hump is formed by the iris curving over the lens.
Another diagnostic method is ultrasound biomicroscopy (UBM), which provides high-resolution, cross-sectional images of the anterior segment of the eye. UBM is useful for visualizing the ciliary body and the iris root’s position, confirming the anatomical abnormalities characteristic of plateau iris. It can show an anteriorly rotated ciliary body, a shortened iris root, and the contact between the iris and the trabecular meshwork.
Treatment Procedures
Initial management of plateau iris often involves addressing any co-existing pupillary block. Laser Peripheral Iridotomy (LPI) is typically the first procedure performed for narrow angles, creating a small opening in the iris to allow fluid to bypass the pupil and relieve pressure buildup behind the iris. However, LPI alone often does not fully resolve the angle narrowing in plateau iris syndrome.
If angle closure remains after LPI, Argon Laser Iridoplasty (ALI), also known as gonioplasty, is a common next step. This procedure uses low-energy laser burns applied to the peripheral iris. These burns cause the iris tissue to contract and shrink, pulling the peripheral iris away from the trabecular meshwork and physically widening the drainage angle. The laser energy causes immediate contraction of stromal collagen, with longer-term effects potentially involving fibroblast proliferation and membrane formation.
Other treatment options may include miotic eye drops, such as pilocarpine, which constrict the pupil and can pull the iris away from the angle, thereby opening the drainage pathway. However, these drops can have side effects like brow ache or vision changes due to pupil constriction. In some cases, cataract surgery, which involves removing the eye’s natural lens, can also deepen the anterior chamber and definitively open the angle, serving as a treatment for plateau iris.
Distinguishing Plateau Iris from Other Angle-Closure Causes
Understanding plateau iris syndrome involves recognizing how it differs from other, more common causes of angle closure, particularly pupillary block. In pupillary block, the most frequent mechanism of angle closure, fluid (aqueous humor) produced in the posterior chamber of the eye is unable to flow freely through the pupil into the anterior chamber. This trapped fluid builds up pressure behind the iris, causing it to bow forward and block the drainage angle. After a laser peripheral iridotomy, the iris in pupillary block typically flattens, and the angle widens significantly.
In contrast, plateau iris syndrome is characterized by the physical configuration of the iris and ciliary body. The ciliary body is positioned anteriorly or is abnormally large, directly pushing the peripheral iris forward against the trabecular meshwork. This anatomical arrangement means that even after an LPI has eliminated any pupillary block, the angle remains narrow or occludable because the underlying structural issue persists. This highlights why LPI, while a standard initial treatment for angle closure, has limited effectiveness in fully resolving plateau iris.