External filtration and glaucoma

External filtration develops with a small hole in the wall of the filtration cushion, which leads to the outflow of intraocular fluid with direct communication between the outer surface and the inner cavity of the cushion. A risk factor for the development of external filtration is the intraoperative use of antimetabolite drugs.

The mechanism of development of external filtration: the ischemic filtration pad is stretched and surrounded by massive scar tissue, which limits the flow of aqueous humor beyond its limits. The filtration pad locally expands. When the tissue is stretched beyond the maximum possible threshold, a traction hole is formed.

External filtration is best determined by applying fluorescein to its surface and examining it under a slit lamp with a blue cobalt filter. A positive Seidel test is indicated by a change in the color of the dye to green-yellow when intraocular fluid flows out of the opening. Sometimes external filtration can only be detected by gently pressing on the eyeball.

External filtration increases the risk of infectious complications and endophthalmitis, so early detection and treatment of this condition is necessary. Careful surgical actions are very important to reduce the risk of external filtration during surgery. Particular attention should be paid to the technique of trabeculectomy, conjunctival suturing. time, the area of application and washing out of antimetabolites, and caution should be exercised during laser lysis of sutures.

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Treatment of external filtration

Conservative treatment

The advantage of methods that improve reparative processes is that they protect the patient from surgical intervention. Their disadvantages include the possibility of recurrence of filtration if they are ineffective. These treatment methods are not surgeries, but each of them has its own risk factors.

• Use of 18 mm soft contact lenses for 2 weeks.
• Using butyryl methacrylate glue and silicone disc.
• Introduction of autogenous blood into the filtration pad.
• Application of compression sutures.

Surgical treatment

The following options are possible.

• Conjunctival repositioning has been shown to be a highly effective technique. Patients with late-onset external filtration who were treated with conjunctival repositioning had better final results and fewer severe intraocular infections than patients who were treated more conservatively.
• Free conjunctival graft. Transplantation of a free conjunctival autologous graft is a safe and effective method for reducing the filtration pad and restoring its function.

Patients should be aware that after revision, medication or surgery may be required to control intraocular pressure in the postoperative period. Amniotic membrane. Amniotic membrane grafting is an alternative treatment option if the surgeon feels that the available conjunctival tissue is limited (e.g., due to thinning or scarring) or if there is already some ptosis. The technique described below differs slightly from that of Budenz et al. In this technique, the graft is folded, leaving the base layer on the outside and the stromal layer on the inside.

Technique of amniotic membrane suturing.

• The conjunctiva surrounding the ischemic filtration pad is separated.
• The old ischemic filtration pad is removed.
• They take the donor amniotic membrane and fold it.
• The anterior edges of the graft are sutured at the corners to the corneal portion of the limbus with 9-0 nylon.
• The posterior edge of the amniotic membrane is placed under the free, separated anterior part of the conjunctiva.
• The graft is securely sutured to the anterior edge of the patient's free conjunctiva with a running 8-0 Vicryl suture.
• In the limbal area, a 9-0 nylon compression suture is placed on the anterior edge of the graft.
• The entire area is checked for external filtration with fluorescein strips.
• The anterior compression suture can be removed after 1 month.

Variations of this technique can also be used for free conjunctiva transplantation, adding only the steps of removing tissue from the target area and not folding the free graft. Badens et al. in a study of amniotic membrane transplantation do not offer an effective alternative to conjunctival transplantation for the correction of glaucoma filtration pads. The accumulated data on the survival time of the amniotic membrane graft were 81% at 6 months, 74% at 1 year, and 46% at 2 years. Over the entire observation period, the overall survival rate of the transferred conjunctiva was 100%. Badens et al. in their study found that amniotic membrane transplantation was less effective than standard conjunctival transplantation. However, their results showed that the use of amniotic membrane can be successful in certain situations, suggesting the existence of an alternative method of treating external filtration in specific circumstances. In addition, if the amniotic membrane transplant fails, there is always the option of a conjunctival transplant. Even modifications of the surgical technique are possible, which affects the final results. The latter statement requires proof in a randomized clinical trial to compare with the data of Badens et al. and, of course, the test of time.