Glaucoma is a collection of eye conditions that cause progressive damage to the optic nerve, which transmits visual information from the eye to the brain. This damage frequently results from elevated intraocular pressure (IOP), which occurs when the eye’s natural drainage system malfunctions. While many assume glaucoma always affects both eyes simultaneously, it is possible to develop the condition in only one eye, known as unilateral glaucoma. This asymmetrical occurrence is usually linked to specific, acquired factors rather than the more common primary forms of the disease.
Glaucoma: The Potential for Unilateral Disease
The most prevalent type of the condition, Primary Open-Angle Glaucoma (POAG), is typically considered a bilateral disease. This means it affects both eyes due to systemic factors like shared genetic risk or widespread vascular issues. However, even in POAG, the disease progression is often asymmetrical, with one eye showing more advanced damage or higher pressure levels than the other. This difference occurs because subtle variations in the optic nerve’s structure or blood flow make one eye more vulnerable to pressure fluctuations.
True unilateral glaucoma, where only one eye is affected, is less common and usually points toward a secondary cause localized to that single eye. In a healthy eye, a clear fluid called aqueous humor is constantly produced and drained through a mesh-like structure at the angle where the iris and cornea meet, maintaining stable IOP. If this drainage system is compromised in only one eye—either by physical blockage, inflammation, or trauma—the resulting pressure spike damages the optic nerve in that specific eye.
The distinction between an asymmetrical bilateral condition and a truly unilateral one is important because it dictates the underlying cause and the long-term risk to the seemingly healthy eye. When the disease is confined to one eye, it is generally due to an isolated event or condition that physically alters the fluid dynamics of that eye alone. If the cause is systemic, the unaffected eye remains under close observation for signs of future damage.
Factors That Cause Glaucoma in Only One Eye
Unilateral glaucoma is most often the result of an acquired secondary condition that specifically targets the drainage structures within a single eye.
Trauma and Injury
One of the clearest examples is angle-recession glaucoma, which develops following blunt trauma to the eye, such as from an accident or sports injury. The force of the impact can cause a tear in the ciliary body, leading to a recession of the drainage angle. This damage eventually scars and impedes the outflow of aqueous humor, sometimes decades after the initial injury occurred.
Inflammation (Uveitis)
Ocular inflammation, or uveitis, is a common cause of unilateral secondary glaucoma, referred to as uveitic glaucoma. Inflammation introduces cells and debris into the eye’s internal fluid, which can physically clog the trabecular meshwork, the eye’s drain. Furthermore, the inflammatory process itself can damage the meshwork, reducing its ability to filter fluid and causing a rapid or slow rise in IOP confined to the inflamed eye.
Vascular Issues
Neovascular glaucoma is a severe form that is secondary to conditions like Central Retinal Vein Occlusion (CRVO) or advanced, asymmetrical diabetic retinopathy. These underlying issues cause an extensive lack of oxygen in the retina, which in turn triggers the growth of abnormal new blood vessels across the iris and into the drainage angle. This vascular growth creates a physical barrier to fluid outflow, leading to extremely high and rapidly damaging intraocular pressure in the affected eye.
Other Causes
Other factors that can cause unilateral glaucoma include:
- Prolonged use of corticosteroid eye drops administered topically to only one eye, which can cause a significant pressure response by altering the structure and function of the drainage meshwork.
- Conditions that create a mechanical obstruction, such as exfoliation syndrome or pigment dispersion syndrome. In these cases, material like pigment or protein deposits may accumulate far more heavily in one eye’s drainage system, causing pronounced asymmetry.
Managing and Monitoring Unilateral Glaucoma
The initial clinical approach to a unilateral diagnosis involves confirming the extent of the damage in the affected eye and rigorously assessing the condition of the seemingly healthy eye. Specialized diagnostic tools are employed to visualize the optic nerve and measure the thickness of its nerve fiber layer.
Diagnostic Tools
- Optical Coherence Tomography (OCT) is utilized to create cross-sectional images that detect subtle thinning of the retinal nerve fiber layer, which can occur before functional vision loss is noticeable.
- Visual field testing, known as perimetry, maps the peripheral and central vision to quantify any existing blind spots caused by nerve damage.
- Central corneal thickness (pachymetry) is measured. A thinner cornea can sometimes lead to an artificially low IOP reading, which influences the target pressure for treatment.
These tests help establish a baseline and confirm that the optic nerve in the second eye remains undamaged.
Treatment for the affected eye is focused on safely and consistently lowering the intraocular pressure to a level that prevents further optic nerve deterioration. This often begins with topical eye drops, which work either by decreasing the production of aqueous humor or by improving its drainage. Laser procedures, such as selective laser trabeculoplasty, may also be used to enhance the function of the drainage angle. In more advanced cases, surgery may be required to create a new drainage path for the fluid.
The unaffected eye requires particularly vigilant and frequent monitoring, especially because the initial diagnosis may represent an early, asymmetrical presentation of a systemic condition. For instance, after an acute angle-closure event in one eye, the anatomically similar second eye has a high probability of developing the same issue if left untreated. Therefore, the long-term management plan includes regular, often biannual, check-ups with repeat imaging and visual field testing to detect the absolute earliest signs of pressure elevation or nerve changes in the fellow eye.