Aqueous flare is a clinical sign detected during an eye examination, representing a breakdown of the eye’s internal barrier against inflammation. The aqueous humor is the clear, watery fluid that fills the anterior chamber (the space between the cornea and the lens). This fluid provides essential nutrients and maintains proper pressure. When inflammation occurs, the normally clear aqueous humor becomes hazy. The presence of this haziness, referred to as aqueous flare, indicates an active inflammatory process in the eye’s anterior segment.
Understanding Aqueous Flare
The physical appearance of aqueous flare results from the Tyndall effect, where light scattering is caused by tiny particles suspended in a fluid. In a healthy eye, the aqueous humor contains very little protein, allowing light to pass through without scattering. Clarity is maintained by the blood-aqueous barrier, which strictly controls what enters the fluid.
When inflammation occurs, the tight junctions forming the blood-aqueous barrier weaken. This disruption permits larger molecules, primarily blood proteins like albumin and fibrin, to leak from surrounding blood vessels into the aqueous humor, causing the fluid to become turbid.
Inflammatory cells, such as white blood cells, may also pass into the anterior chamber, further contributing to the visible flare. When a beam of light is passed through, these suspended proteins and cells scatter the light, making the beam visible to an examiner. The density of the particles correlates directly with the intensity of the observed flare.
Conditions Linked to Aqueous Flare
The most common cause of aqueous flare is anterior uveitis, an inflammation affecting the iris and the ciliary body. The severity of the flare is often proportional to the activity of the uveitis, making it important for monitoring the disease. This condition can be triggered by autoimmune disorders, infections, or trauma.
Aqueous flare is also a predictable, though temporary, occurrence following intraocular surgery, particularly cataract removal. Surgical manipulation causes a transient disruption of the blood-aqueous barrier, resulting in increased protein leakage and inflammation in the immediate postoperative period. High levels of flare after surgery are associated with an increased risk of complications, such as pseudophakic cystoid macular edema (swelling in the retina).
In conditions involving ocular ischemia, such as advanced diabetic retinopathy, aqueous flare levels are significantly elevated. In diabetic retinopathy, severe damage to the small blood vessels is often accompanied by a breakdown of the blood-aqueous barrier. Higher flare values correlate with more advanced stages of the disease and are related to the duration of the diabetes.
How Doctors Measure and Monitor Flare
Aqueous flare is typically assessed visually using a slit lamp biomicroscope, an instrument that projects a narrow, high-intensity beam of light into the eye. The doctor sets the beam to a specific width, usually one millimeter, and views the anterior chamber at a right angle. This technique makes the light scattering effect visible, appearing like a faint headlight beam through fog or smoke.
To standardize the observation and monitoring of inflammation, doctors use a qualitative grading system, such as the Standardization of Uveitis Nomenclature (SUN) Working Group scale. This scale assigns a grade from 0 to 4+ based on the visual appearance of the flare. Monitoring the change in this grade over time helps gauge the effectiveness of treatment.
SUN Grading Scale
- Grade 0 indicates no haze is present.
- Grade 1+ describes a faint haze that is barely discernible.
- Grade 2+ indicates a moderate haze where the details of the iris and lens are still visible.
- Grade 3+ is a marked haze that makes the iris and lens hazy to view.
- Grade 4+ is an intense haze often containing visible fibrin, causing the aqueous humor to appear almost solid.
Addressing the Underlying Inflammation
Management of aqueous flare requires treating the root cause of the barrier breakdown. The goal of therapy is to restore the integrity of the blood-aqueous barrier, stopping the leakage of proteins and cells into the anterior chamber. Corticosteroids are the primary medication used to achieve this anti-inflammatory effect.
Corticosteroids can be administered topically as eye drops, or through periocular or systemic routes for more severe cases. These medications reduce the inflammatory response, sealing compromised blood vessels and clearing the aqueous humor. Nonsteroidal anti-inflammatory drugs (NSAIDs) may also be used, often combined with corticosteroids, particularly to manage postoperative inflammation. The resolution of the aqueous flare is a reliable clinical sign that the underlying intraocular inflammation is receding.