Visual impairment in pterygium is due to -
Visual disturbances occur when it encroaches the pupillary area or due to corneal astigmatism induced due to fibrosis in the regressive stage. Occasionally diplopia may occur due to limitation of ocular movements. ref: A K KHURANA OPHTALMOLOGY,E4, Page-81
Astigmatism
Myopia
Hypermetropia
Hazy cornea
Conjunctiva