(Reuters) - Canadian biotechnology company QLT Inc said the U.S. health regulator granted an orphan status to its key eye drug.

The drug, Visudyne, is already approved as a treatment for age-related macular degeneration, a cause of blindness in people over the age of 55, and the company is now seeking approval for chronic or recurrent central serous chorioretinopathy (CSC).

CSC symptoms include blind spots in the central visual field, reduced contrast sensitivity and color vision.

Orphan drug designation is granted by the U.S. Food and Drug Administration to drugs or biologics that treat a condition affecting less than 200,000 Americans. If approved, the drug could get a seven-year marketing exclusivity.

The eye drug developer has been grappling with slowing growth for Visudyne and last month forecast lower revenue for the full year. (Reporting by Bhaswati Mukhopadhyay in Bangalore; Editing by Sriraj Kalluvila)