"Oh, right." She nods, returning to her more recognizable, friendly-but-cold demeanor. "It's a condition that surfaced when she was a Filly. The eyes cross, affecting vision, balance, depth of field... Derpy specifically suffers from a severe case, a mix of Hypo and Hypertopia. Typically, only one of the eyes is affected, and turns to face one direction more than the others... in her case, one eye suffers Hypotopia, the other Hypertopia. One looks up, one looks down. Our usual treatment, which is specialized glasses... don't really work, because they don't... well, survive."