A hypopyon is a visible pool of white blood cells that collects in the front of your eye, settling along the bottom edge of the iris as a whitish-yellow layer. It signals severe inflammation inside the eye and always requires prompt evaluation by an eye care specialist. While it can look alarming, the outcome depends heavily on what’s causing it: inflammatory cases tied to autoimmune conditions generally resolve well, while infections carry a higher risk of lasting vision loss.
How a Hypopyon Forms
Your eye has a small fluid-filled space called the anterior chamber, located between the clear cornea at the front and the colored iris behind it. When something triggers intense inflammation inside the eye, whether an infection, an autoimmune flare, or a reaction to surgery, your immune system floods the area with white blood cells. Normally these cells circulate through the eye’s tiny internal spaces. But when the immune response is severe enough, the sheer volume of white blood cells overwhelms those spaces, and the excess spills into the anterior chamber. Because white blood cells are heavier than the surrounding fluid, they sink and settle at the bottom, forming a visible crescent or horizontal line.
Despite its appearance, a hypopyon is not pus in the traditional sense. Pus contains dead white blood cells mixed with bacteria or other germs. The white blood cells in a hypopyon are typically sterile, meaning they don’t contain infectious organisms themselves. Eye specialists sometimes refer to a hypopyon as “sterile pus” for this reason, even when the underlying trigger is an infection elsewhere in the eye.
Common Causes
Eye Infections
Bacterial endophthalmitis, a serious infection inside the eye, is one of the most common infectious causes. It usually develops within days of an inciting event like cataract surgery or an eye injection. A hypopyon appears in roughly 80% of post-cataract surgery infection cases. The bacteria most frequently responsible are skin-dwelling staphylococci, which account for about 70% of post-surgical cases, followed by other staphylococci and streptococci.
Fungal infections follow a different timeline. Rather than appearing within days, symptoms tend to worsen gradually over days to weeks. Corneal infections caused by fungi like Aspergillus and Fusarium can progress to endophthalmitis, particularly in tropical climates or after eye injuries involving plant material. Candida is the leading cause when a fungal eye infection spreads through the bloodstream from elsewhere in the body.
Eye injuries that penetrate the outer wall of the eye carry their own infection risk. The organisms involved are more varied, including soil bacteria and fungi that enter through the wound.
Autoimmune and Inflammatory Conditions
About half of all hypopyon cases seen in emergency eye departments are caused by uveitis, which is inflammation of the eye’s middle layer without an active infection. Most of these are linked to a genetic marker called HLA-B27, which is associated with conditions like ankylosing spondylitis and reactive arthritis. These patients tend to be younger, more often male, and generally have better vision at the time they’re seen compared to those with infections.
Behçet’s disease, a chronic condition involving blood vessel inflammation throughout the body, is a classic but less common cause of sterile hypopyon. People with Behçet’s typically also have recurrent mouth sores, genital ulcers, and skin eruptions. Eye involvement is one of the most serious features of the disease and can lead to blindness if untreated. In some cases, Behçet’s disease first reveals itself through a hypopyon that appears weeks or months after cataract surgery, mimicking a delayed infection.
Medications
Certain drugs can trigger eye inflammation severe enough to produce a hypopyon. The antibiotic rifabutin, used to prevent infections in immunocompromised patients, causes anterior uveitis with hypopyon at doses above 300 mg per day. The antiviral cidofovir, given intravenously, is another known trigger. Drug-induced uveitis is rare overall, with a prevalence estimated at about 0.5%.
A related phenomenon called pseudohypopyon can occur when steroid crystals injected into the eye for other conditions migrate into the anterior chamber. This looks identical to a true hypopyon on casual inspection but is composed of medication particles rather than white blood cells.
What It Looks and Feels Like
From the outside, a hypopyon appears as a white or yellowish-white horizontal line or crescent at the bottom of the iris. It’s usually visible to the naked eye in moderate to severe cases. Because it’s driven by gravity, the layer shifts if you tilt your head.
The symptoms you notice depend on the cause. Infectious cases typically bring significant eye pain, redness, light sensitivity, and rapidly worsening vision, often within days of surgery or an injury. Inflammatory cases from autoimmune conditions may come on somewhat more gradually, with aching pain, blurred vision, and a red eye. In either scenario, a noticeable drop in vision is the symptom that most reliably signals something serious is happening.
How It’s Diagnosed
An ophthalmologist examines the eye with a slit lamp, a specialized microscope that illuminates the interior structures of the eye in cross-section. This allows the clinician to see individual inflammatory cells floating in the anterior chamber, measure how high the hypopyon layer has risen, and assess the degree of protein leakage (called “flare”) in the eye fluid. The height of the hypopyon gives a rough indication of severity.
Distinguishing between an infection and an autoimmune flare is the most critical diagnostic question because the treatments are very different. The clinical history matters enormously here: recent surgery or injection points toward infection, while a history of autoimmune disease, mouth sores, or joint pain shifts suspicion toward inflammation. When infection is suspected, the ophthalmologist may take a small fluid sample from inside the eye to identify the specific organism.
Pseudohypopyon and Masquerade Syndromes
Not every white layer in the anterior chamber is made of white blood cells. In rare cases, tumor cells can accumulate in the same location and create an identical appearance called a pseudohypopyon. Retinoblastoma, a childhood eye cancer, can present this way when tumor cells seed into the anterior chamber and vitreous, mimicking uveitis or endophthalmitis. Lymphoma involving the eye can do the same in adults. These “masquerade syndromes” are uncommon but important to recognize because treating them as simple inflammation delays the cancer diagnosis.
Treatment Based on the Cause
For infectious endophthalmitis, treatment is urgent. Broad-spectrum antibiotics are injected directly into the eye’s vitreous cavity because topical drops and oral medications don’t penetrate well enough on their own. If the infection doesn’t improve within 48 hours, culture results from the fluid sample guide a switch to more targeted antibiotics. The role of anti-inflammatory steroids alongside antibiotics remains debated. Steroids can reduce the immune-mediated damage to delicate eye tissues, but they also suppress the very immune response needed to fight the infection, which is especially concerning with fungal cases.
For autoimmune or inflammatory hypopyon, steroids are the mainstay. Steroid eye drops are the first-line approach, sometimes supplemented with injections around the eye or oral steroids for severe flares. In the emergency setting, all cases of non-infectious hypopyon uveitis in one large study resolved with steroid treatment. Serious complications and irreversible vision loss were uncommon in this group.
Visual Outcomes and Prognosis
Prognosis splits sharply depending on the underlying cause. Patients with HLA-B27-related uveitis, the most common inflammatory cause, generally do well. They tend to present with better initial vision and recover to good final visual acuity. Recurrences are common with autoimmune conditions, but individual flares typically respond to treatment.
Infectious cases carry more risk. Corneal infections can leave scars that permanently blur vision, and endophthalmitis can cause severe, irreversible damage even with prompt treatment. Herpes-related eye infections (both HSV and shingles virus) are particularly concerning: in one study, two-thirds of patients with herpes-associated hypopyon experienced moderate or severe vision loss, mostly from corneal scarring.
Fungal infections tend to have worse outcomes than bacterial ones, partly because their slower onset means they’re often diagnosed later, and partly because antifungal drugs are less effective at penetrating eye tissues. The speed of diagnosis and treatment initiation is the single biggest factor influencing how much vision can be preserved in infectious cases.