Trabeculectomy is a surgical procedure in which a small hole is made in the sclera (eye wall) to lower the intraocular pressure (IOP) in the eye in patients with glaucoma. The aqueous humour, which is the fluid inside the eye, drains through this opening into a small reservoir under the eye surface that is hidden by the eyelid. Draining the aqueous humor prevents or slows further damage and further loss of vision in glaucoma. The procedure will not restore vision already lost from glaucoma. In certain complicated cases, the doctor may also place a glaucoma drainage device implant to keep the drainage opening from healing and closing down.