Keratic Precipitates (KPs)

Keratic precipitates - usually abbreviated as KPs - are clumps of inflammatory cells that settle on the inner surface of the cornea (the corneal endothelium) in patients with uveitis. The doctor sees them with a slit lamp as small white or pigmented dots on the back of the cornea. KPs are a sign that inflammation is or has been present inside the eye, and the pattern of KPs - their size, color, and distribution - is one of the most useful clues for identifying the type of uveitis.

What KPs Look Like

A normal corneal endothelium is glassy-smooth. In active uveitis, white blood cells circulating in the aqueous humor settle out of suspension and adhere to the endothelium, where the slit-lamp beam reveals them as small dots.

The doctor describes KPs by:

• Size - fine, medium, or large
• Shape - round, stellate (star-shaped), mutton-fat (greasy and lumpy)
• Color - fresh white versus pigmented (older, partially absorbed)
• Distribution - diffuse versus the classic inferior triangular pattern (Arlt triangle)
• Number - sparse versus dense

Patterns of KPs and What They Suggest

Fine White (Non-Granulomatous) KPs

Small, dust-like, evenly spread or in the inferior triangle. Typical of:

• HLA-B27-associated anterior uveitis - acute, often unilateral, classically in young adults with low back pain
• Idiopathic anterior uveitis
• Reactive arthritis-associated uveitis
• Inflammatory bowel disease-associated uveitis

Granulomatous (Mutton-Fat) KPs

Large, lumpy, greasy-appearing, often described as resembling globules of melted fat. They form when inflammatory cells aggregate into larger clumps containing macrophages. Suggest:

• Sarcoidosis - bilateral granulomatous uveitis is one of the most common ocular manifestations
• Tuberculosis-associated uveitis
• Syphilis - can mimic almost any uveitis pattern
• Vogt-Koyanagi-Harada (VKH) disease
• Sympathetic ophthalmia
• Herpetic uveitis - can occasionally show granulomatous features (especially zoster)
• Less commonly: Lyme uveitis (more typically posterior or intermediate)

Stellate KPs

Star-shaped, distributed diffusely across the entire endothelial surface rather than just inferiorly. This pattern strongly suggests:

• Fuchs heterochromic iridocyclitis - chronic low-grade uveitis with iris heterochromia
• Cytomegalovirus (CMV) anterior uveitis
• Herpetic anterior uveitis (HSV, VZV) - stellate KPs can occur, often with sectoral iris atrophy and elevated IOP

Pigmented KPs

Brown rather than white, reflecting older inflammation in which iris pigment cells have been incorporated. Indicate:

• Resolved or chronic uveitis - the active inflammation may already be settling
• Past anterior uveitis episodes - useful for history-taking when the patient cannot recall

Old "Ghost" KPs

Faint, dust-like residues that persist for months to years after the active episode. They document past inflammation but do not require treatment by themselves.

Where the Doctor Looks: The Arlt Triangle

KPs typically settle in a triangular pattern on the inferior cornea, with the apex at the center and the base along the lower endothelium. This Arlt triangle distribution reflects the convection of aqueous humor inside the anterior chamber:

• Aqueous humor is warmer near the iris (heated by iris blood flow) and cooler near the cornea (where heat is lost externally)
• Warm aqueous rises along the iris surface, cools at the cornea, then descends along the corneal endothelium
• Inflammatory cells suspended in the descending current settle out where the flow is slowest - the inferior third of the cornea - producing the characteristic Arlt triangle

A non-Arlt distribution - diffuse stellate KPs across the entire cornea - is itself a useful clue (especially for Fuchs heterochromic iridocyclitis and CMV anterior uveitis).

How KPs Are Detected

KPs are seen during a routine slit lamp examination. The clinician uses an oblique slit beam to look at the back of the cornea while the patient looks straight ahead, often using indirect retro-illumination from the iris to highlight subtle KPs.

KPs are usually graded informally - "1+ fine KPs in inferior triangle" - rather than counted exactly. Photography (slit-lamp imaging) is sometimes used for documentation in chronic disease.

What KPs Mean for the Patient

KPs do not directly impair vision. The visual symptoms a patient experiences come from the underlying inflammation rather than the precipitates themselves:

• Eye pain and photophobia from anterior chamber inflammation
• Blurred vision, often described as a "haze" or "smoke"
• Red eye with ciliary flush (perilimbal redness)
• Floaters if vitreous involvement is also present

The presence of KPs tells the doctor that the inflammation is in the front of the eye and gives clues about its nature, which then directs treatment.

Workup Triggered by KPs

The pattern of KPs guides the differential, which then guides which laboratory tests the doctor orders. A typical first-time uveitis workup may include:

• Blood tests - CBC, basic chemistry, ESR, CRP
• HLA-B27 typing if young adult with acute non-granulomatous anterior uveitis
• Syphilis serology (RPR/VDRL with treponemal-specific confirmation)
• Tuberculosis testing (interferon-gamma release assay or PPD), often with chest imaging
• ACE level and chest CT if sarcoidosis suspected
• Lyme serology in endemic areas
• Toxoplasma serology if posterior involvement
• HIV testing in selected patients
• Specific viral PCR of aqueous humor in granulomatous or stellate KP cases

The exam may be supplemented by fundus photography and OCT of the macula to look for cystoid macular edema, a common complication of uveitis.

Treatment Implications

KPs themselves resolve as the underlying inflammation is controlled. Standard treatment of anterior uveitis includes:

• Topical corticosteroids (for example prednisolone acetate, difluprednate, or loteprednol) tapered over weeks
• Cycloplegic drops to relieve ciliary spasm and prevent posterior synechiae
• Treatment of any identified specific cause (for example, antiviral therapy for CMV or herpetic uveitis, antibiotics for syphilis)
• Systemic immunosuppression for severe, chronic, or bilateral disease - see methotrexate, azathioprine, mycophenolate

KPs may persist as faded ghost KPs even after the eye is white, comfortable, and inflammation-free.

Frequently Asked Questions

Are keratic precipitates dangerous?

KPs themselves do not damage the cornea or vision. Their importance is what they signal - active inflammation in the front of the eye that needs to be evaluated and treated to prevent complications such as glaucoma, cataract, and macular edema.

Why are mine called "mutton-fat" KPs?

The term describes large, greasy-appearing KPs in granulomatous uveitis. The doctor's note may say "granulomatous KPs" or "mutton-fat KPs" - the two terms are interchangeable. They suggest a particular set of underlying causes (sarcoidosis, tuberculosis, syphilis, herpetic uveitis among others) and warrant a specific workup.

Will my keratic precipitates go away with treatment?

Active KPs typically resolve as the inflammation is controlled. Some pigmented or "ghost" KPs may persist permanently as a footprint of past inflammation, but they do not affect vision and do not require ongoing treatment.

Can I see KPs in a mirror?

No. KPs are tiny - typically a few hundred microns or smaller - and on the inner surface of the cornea. They are visible only with the magnification and slit-beam illumination of a slit lamp.

Do KPs always mean uveitis?

Usually, but the slit-lamp diagnosis matters. True KPs point to uveitis of one type or another, including infectious, autoimmune, idiopathic, and post-traumatic causes. Some endothelial dystrophies and corneal pathologies can mimic KPs, so the finding is interpreted with the rest of the exam.

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