A filtering bleb is a specialized structure formed during glaucoma surgery, primarily trabeculectomy, to manage intraocular pressure. Its main purpose is to create an alternative drainage pathway for the eye’s internal fluid, known as aqueous humor. This surgical modification helps control eye pressure, a significant factor in preventing optic nerve damage and vision loss associated with glaucoma. The bleb is a direct result of the surgery and plays a central role in the procedure’s long-term effectiveness.
Understanding a Filtering Bleb
A filtering bleb appears as a small, blister-like elevation on the sclera, the white part of the eye, typically hidden beneath the upper eyelid. This structure represents a new drainage system created to bypass the eye’s natural drainage pathways, which may be blocked or not functioning efficiently in glaucoma. The eye continuously produces aqueous humor; in a healthy eye, an equal amount drains out to maintain stable intraocular pressure. When this drainage system is impaired, fluid accumulates, leading to increased pressure that can harm the optic nerve. The bleb provides an outlet for this excess fluid, thereby lowering eye pressure and preventing further damage.
How Filtering Blebs Are Created and Function
The creation of a filtering bleb usually involves a surgical procedure called trabeculectomy, often referred to as “filtration surgery.” During this outpatient procedure, a tiny flap is made in the sclera, the white outer wall of the eye. A small opening is then created under this flap to connect the inside of the eye to the subconjunctival space, the area beneath the conjunctiva, the clear membrane covering the eye. This allows the aqueous humor to drain from the eye through the opening and collect in the newly formed bleb.
The fluid collected in the bleb is then gradually absorbed into the surrounding tissues and bloodstream. This continuous drainage and absorption helps maintain lower intraocular pressure. To prevent the surgical site from scarring and closing, surgeons sometimes apply anti-scarring medications like mitomycin C or 5-fluorouracil during the procedure. The scleral flap, which is loosely sutured back into place, acts as a valve to regulate fluid flow, ensuring balanced drainage.
Assessing Bleb Appearance and Health
Ophthalmologists routinely examine filtering blebs to assess their health and function, often using a slit lamp microscope. A well-functioning bleb typically appears diffuse, low-lying, thin-walled, and can be transparent or translucent. The presence of tiny microcysts within the bleb’s surface can indicate good fluid filtration.
Variations in these characteristics can signal different functional states or potential issues. For instance, a bleb that is very high and localized might suggest over-filtration, where too much fluid is draining, potentially leading to very low eye pressure. Conversely, a flat, opaque, or highly vascularized bleb can indicate excessive scarring, which may impede fluid drainage and lead to bleb failure.
Common Concerns with Filtering Blebs
While filtering blebs are effective in managing glaucoma, several concerns can arise. Hypotony, or excessively low intraocular pressure, is a potential complication, often resulting from over-filtration. This can lead to issues such as a shallow anterior chamber, choroidal effusions, or hypotony maculopathy, which can affect vision. Another concern is a bleb leak, where fluid escapes from the bleb onto the surface of the eye. Leaks can occur due to thinning of the bleb wall or incomplete closure of the surgical incision and increase the risk of infection.
Bleb infections, known as blebitis or, if more severe, endophthalmitis, are serious complications that can develop days to years after surgery. These infections can lead to significant vision loss and require prompt treatment. Signs of infection may include redness, irritation, discharge, pain, or blurred vision. Bleb encapsulation is another issue, where the bleb wall thickens due to excessive scarring, often forming a firm, localized cyst that impairs fluid drainage and can lead to bleb failure. Excessive scarring around the surgical site can also lead to bleb failure by obstructing aqueous humor flow.
Long-Term Care and Monitoring
Maintaining the health and function of a filtering bleb requires consistent, lifelong follow-up care with an ophthalmologist. These regular appointments allow the doctor to monitor intraocular pressure and assess the bleb’s appearance for any changes. Patients are typically advised to avoid rubbing their eyes, especially the area of the bleb, to prevent trauma.
Adherence to prescribed eye drops, including steroids for inflammation and antibiotics for infection, is important post-operatively. Patients should be aware of signs that warrant immediate medical attention, such as sudden changes in vision, severe eye pain, increased redness, or any discharge from the eye, as these could signal complications like infection or bleb failure. The health of a bleb can change over time, and interventions, such as laser procedures to loosen sutures or injections to prevent scarring, may be necessary to maintain its pressure-lowering function and ensure long-term success.