Aqueous Flare (Tyndall Effect): Grading and Meaning

Aqueous flare is the visible scatter of a slit-lamp beam by protein in the anterior chamber.

Normal aqueous is nearly optically empty, so the beam crosses it unseen.

When the blood-aqueous barrier fails, plasma proteins enter and the beam becomes visible.

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What Aqueous Flare Is

Dissolved protein causes this optical event.

The examiner sees a faint gray haze inside the beam.

Healthy aqueous contains very little protein and scatters almost no light.

As a result, any visible haze implies a raised protein level.

The Tyndall Effect at the Slit Lamp

The Tyndall effect describes light scattering by small particles in a clear medium.

A sunbeam crossing a dusty room is a familiar example.

In the eye, albumin and globulins act as the scattering particles.

Larger molecules scatter more light, so fibrin-rich aqueous looks especially dense.

Why Aqueous Flare Appears

The ciliary epithelium and iris vessels normally exclude most plasma proteins.

Inflammation, surgery, trauma and ischemia can all open this barrier.

New iris vessels are particularly leaky.

Therefore, flare signals barrier breakdown of any cause.

Flare Versus Cells

Cells are discrete particles, usually leukocytes, drifting in the convection current.

Flare, in contrast, is a uniform haze without individual points.

Cells also move, while flare stays still in the beam.

Pigment and degenerated red cells can mimic leukocytes (see ghost cell glaucoma).

Because cells reflect current leukocyte traffic, they track activity more closely.

Slit-Lamp Technique for Aqueous Flare

First, darken the room and use high magnification with maximum light.

Next, set a short beam, about 1 mm high, at 45 to 60 degrees.

Then focus inside the chamber with the dark pupil behind the beam.

Compare the haze with the crisp beam edges on the cornea and lens.

Finally, grade both eyes with identical settings.

SUN Grading of Aqueous Flare

The Standardization of Uveitis Nomenclature (SUN) group adopted a descriptive scale.

• None (0)
• Faint (1+)
• Moderate (2+): iris and lens details clear
• Marked (3+): iris and lens details hazy
• Intense (4+): fibrin or plastic aqueous

However, the scale is subjective, and corneal edema can exaggerate it.

Laser Flare Photometry

Laser flare photometry measures backscattered laser light objectively.

A photomultiplier counts photons from a small window in the chamber.

The device reports flare in photon counts per millisecond.

Sawa and colleagues described the method in 1988.

Commercial Kowa instruments later made it practical.

What Photometry Adds

Photometry detects flare below the slit-lamp threshold and gives reproducible numbers.

For example, Davis and colleagues linked higher readings with complications in children with chronic uveitis.

However, the devices are costly, and values differ between instrument models.

Persistent Aqueous Flare

Chronic uveitis can leave flare long after cells have cleared.

Damaged iris and ciliary vessels may stay leaky in a quiet eye.

A stable 1+ or 2+ flare without cells usually reflects this chronic leak.

For this reason, SUN criteria define inactive disease by the cell grade.

Flare and Treatment Decisions

Escalating steroids to clear stable residual flare adds cataract and glaucoma risk without clear benefit.

Therefore, titrate topical therapy to the cell grade.

Rising flare with returning cells, in contrast, needs closer review.

New flare in a quiet eye, however, should prompt a search for neovascularization.

Aqueous Flare After Surgery

Cataract surgery produces transient flare in most eyes, and it usually settles over weeks.

Laser iridotomy can also cause brief flare (see laser peripheral iridotomy).

Prolonged flare, however, raises concern for retained lens material or infection.

Conditions With Chronic Flare

• Chronic anterior uveitis, including juvenile idiopathic arthritis
• Diabetes and iris neovascularization
• Pseudoexfoliation syndrome
• Fuchs uveitis syndrome
• Previous intraocular surgery or trauma

Fibrin and Plastic Aqueous

At grade 4+, protein levels rise enough to form fibrin.

A gel-like aqueous or a pupillary membrane may then appear.

HLA-B27 uveitis and endophthalmitis typically produce this picture.

Therefore, check the red reflex and consider infection after surgery.

Counseling the Patient With Aqueous Flare

Patients often worry when reports still mention flare.

Explain that it can persist as a sign of past vessel damage.

Also explain that treatment follows cells, symptoms and complications.

Regular follow-up still matters, because chronic flare has been associated with complications.

Pitfalls to Avoid

• Do not grade flare through an edematous cornea
• Never escalate steroids for stable, cell-free flare
• Do not call pigment or red cells inflammatory cells
• Avoid long, wide beams when grading
• Do not compare readings across photometer models

For the latest studies, see the PubMed results for aqueous flare.

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References

1. Jabs DA, Nussenblatt RB, Rosenbaum JT; Standardization of Uveitis Nomenclature (SUN) Working Group. Standardization of uveitis nomenclature for reporting clinical data. Results of the First International Workshop. Am J Ophthalmol. 2005;140:509-516.
2. Hogan MJ, Kimura SJ, Thygeson P. Signs and symptoms of uveitis. I. Anterior uveitis. Am J Ophthalmol. 1959;47:155-170.
3. Sawa M, Tsurimaki Y, Tsuru T, Shimizu H. New quantitative method to determine protein concentration and cell number in aqueous in vivo. Jpn J Ophthalmol. 1988;32:132-142.
4. Tugal-Tutkun I, Herbort CP. Laser flare photometry: a noninvasive, objective, and quantitative method to measure intraocular inflammation. Int Ophthalmol. 2010;30:453-464.
5. Davis JL, Dacanay LM, Holland GN, et al. Laser flare photometry and complications of chronic uveitis in children. Am J Ophthalmol. 2003;135:763-771.