Final answer:
The treatment for a whitish membrane behind the IOL, known as posterior capsule opacification, after ECCE with IOL implantation is a YAG Laser Capsulotomy, a simple laser procedure that restores vision.
Step-by-step explanation:
When a patient experiences a gradual progressive loss of vision years after undergoing extracapsular cataract extraction (ECCE) with intraocular lens (IOL) implantation, and a whitish membrane is observed behind the IOL, this typically indicates the development of a posterior capsule opacification (PCO). The treatment for PCO is a relatively simple and effective procedure called YAG Laser Capsulotomy. In this outpatient laser procedure, a laser is used to create a small opening in the cloudy capsule behind the IOL, allowing light to pass through and restoring clear vision.