Keratitis is an inflammation of the cornea, the clear tissue at the front of the eye that covers the pupil and iris. It can be caused by infection from bacteria, viruses, fungi, or parasites, or by non-infectious sources like an injury or wearing contact lenses too long. Since the cornea focuses light, this inflammation can cause pain, redness, blurred vision, and light sensitivity. Diagnosing keratitis requires a detailed look at the eye’s microscopic structures with a specialized instrument.
The Slit Lamp Examination
A slit lamp examination is the primary method for diagnosing conditions affecting the front of the eye. The slit lamp is a table-mounted biomicroscope that provides high magnification with a powerful, adjustable beam of light. This setup allows an eye doctor to view the cornea, iris, and lens in three dimensions, revealing details invisible to the naked eye.
During the painless procedure, you will rest your chin and forehead against a support bar to keep your head steady. The doctor sits opposite you and looks through the microscope eyepieces. They will manipulate the instrument to focus the light beam across the surface and into the layers of your eye.
To see different parts of the eye more clearly, the doctor can adjust the light beam’s width, height, and angle. A wide beam illuminates the overall surface, while a narrow slit creates a cross-sectional view through the cornea to examine its layers for abnormalities. In some cases, pupils are dilated with eye drops for a better view of deeper structures.
Identifying Keratitis Signs
Under the slit lamp’s magnification, an eye doctor can identify specific signs of keratitis. A common finding is the presence of corneal infiltrates, which appear as hazy, white, or yellowish opacities within the corneal tissue. These infiltrates are collections of inflammatory cells, and their size, location, and density provide information about the condition’s severity.
Another visible sign is corneal edema, or swelling, which causes the cornea to lose some transparency and can create folds in its inner layers. The doctor may also identify epithelial defects, which are breaks on the cornea’s outermost surface. These defects show where protective surface cells have been lost due to infection or inflammation.
To make these signs more visible, eye doctors use a diagnostic dye called fluorescein. A sterile paper strip containing the orange dye is touched to the eye’s surface, or it is administered as an eye drop. When the doctor switches the slit lamp’s light to a cobalt blue filter, any areas with missing epithelial cells absorb the dye and glow bright green. This staining highlights the size and shape of the damage.
Differentiating Keratitis Types
The appearance of keratitis on the slit lamp offers clues about its underlying cause, which helps guide treatment. Different infectious agents create characteristic patterns on the cornea. Observing these patterns helps an ophthalmologist distinguish between bacterial, viral, fungal, or parasitic infections, often before laboratory results are available.
For example, keratitis caused by the herpes simplex virus (HSV) often produces a dendritic ulcer. When stained with fluorescein and viewed under a cobalt blue light, this ulcer appears as a branching, tree-like lesion on the corneal surface. This pattern is a hallmark of HSV keratitis and allows for the prompt initiation of antiviral therapy.
In contrast, bacterial keratitis often appears as a dense, well-defined, yellow-white stromal infiltrate, sometimes accompanied by significant discharge. Fungal keratitis may have fluffier margins and could present with smaller “satellite lesions” around a primary infiltrate. The combination of the patient’s history and these signs helps the doctor determine the cause.
Monitoring Treatment and Healing
The slit lamp examination is also used throughout the treatment process to monitor the eye’s response to therapy. During follow-up appointments, the ophthalmologist performs repeated exams to track the progress of healing. These examinations allow for a direct comparison with the initial findings, providing evidence of improvement.
Signs of healing are visible through the slit lamp. A shrinking infiltrate indicates that inflammation is subsiding and the infection is clearing. A reduction in corneal edema, observed as increased clarity and fewer folds in the cornea, also signals that the tissue is returning to its normal state.
The healing of an epithelial defect is also monitored using fluorescein dye. As the cornea’s surface regenerates, the area that stains green will get progressively smaller with each visit. A complete resolution is marked by the absence of any staining, indicating the protective epithelial barrier has been fully restored.