Keratoconus is a progressing eye disorder where the cornea of eye starts thinning. This progressive thinning leads to forward bulging of cornea and it appears cone shaped. Symptoms of keratoconus are blurred vision, astigmatism, double vision and poor sensitivity to light. Picture credit: https://clinicalophthalmology.wordpress.com/keratoconus/
Another symptom would be a non moving black or dark spot or line somewhere in your field of vision where the the lenses is thinnest.