Staphylococcal marginal keratitis is an inflammatory condition that develops at the outer edge of the cornea, the clear, dome-shaped surface at the front of the eye. It is not a direct infection of the cornea itself, but rather an immune system reaction to bacteria that commonly live on the skin and eyelids. This response occurs when the body’s defenses react to the presence of these bacteria, causing inflammation on the peripheral cornea.
The Role of Eyelid Bacteria
This form of keratitis is closely linked to bacteria called Staphylococcus aureus, a common bacterium that resides on human skin. An overgrowth of these bacteria on the eyelids can lead to a condition known as blepharitis, which is an inflammation of the eyelid margins. The glands in the eyelids can become blocked, causing irritation and allowing the bacteria to multiply, creating an excess of bacterial byproducts like toxins and cell wall fragments.
These bacterial substances then mix with the tear film that lubricates the eye’s surface. As the tears circulate, these antigens are carried to the edge of the cornea. The peripheral cornea is equipped with a network of immune cells. When these cells encounter the staphylococcal antigens, they can initiate an inflammatory cascade, viewing the bacterial byproducts as a threat.
This defensive reaction leads to the signs associated with marginal keratitis. The body sends inflammatory cells to the site, creating localized inflammation without the presence of a live, multiplying infection in the cornea.
Recognizing Key Symptoms
Common symptoms include eye discomfort, a feeling that something is in the eye, and general irritation. Redness of the eye, sensitivity to light, and excessive tearing are also frequently reported. The discomfort can range from mild to significant, depending on the severity of the inflammatory response.
An eye doctor will observe specific clinical signs. The most definitive sign is the appearance of one or more small white spots, known as infiltrates, in the periphery of the cornea. These infiltrates are accumulations of inflammatory cells. A characteristic feature is that these spots are separated from the absolute edge of the eye, or limbus, by a small, clear zone of uninvolved corneal tissue.
Medical Diagnosis and Treatment
Diagnosing marginal keratitis involves a detailed examination by an eye care professional using a slit lamp. This high-magnification microscope allows the doctor to closely inspect the cornea, eyelids, and eyelashes to identify the characteristic infiltrates and signs of underlying blepharitis.
Treatment focuses on two main goals: calming the immediate inflammation and managing the underlying cause to prevent recurrence. To address the inflammation, doctors often prescribe a combination eye drop or ointment containing both a steroid and an antibiotic. The steroid component works to suppress the immune system’s overreaction, reducing redness and discomfort, while the antibiotic helps to control the staphylococci on the eyelids.
For long-term management, addressing the source of the bacteria involves a consistent routine of eyelid hygiene. Warm compresses applied to the closed eyelids for several minutes help to melt oily secretions that can block glands. Following the compresses, gentle lid scrubs using a commercial eyelid cleanser or a diluted solution of baby shampoo can remove bacterial debris from the base of the eyelashes.
Distinguishing from an Infectious Ulcer
It is important to distinguish staph marginal keratitis from a microbial infectious ulcer. Marginal keratitis is a sterile condition, meaning the corneal infiltrates are composed of the body’s own immune cells and there is no live, replicating bacteria invading the corneal tissue. It is an inflammatory response to bacterial antigens, not an active infection within the cornea.
In contrast, a bacterial corneal ulcer is a direct and active infection of the cornea by microorganisms. In this scenario, bacteria are actively invading and destroying corneal tissue, which can pose a more immediate threat to vision. The treatment for an infectious ulcer is focused on aggressively eliminating the pathogens with targeted antibiotics.
This distinction is the reason why a topical steroid is a primary treatment for marginal keratitis. The steroid effectively calms the immune system’s response. Using a steroid on a true infectious ulcer could be detrimental because suppressing the immune response would hinder the body’s ability to fight off the invading bacteria, potentially worsening the infection.