Some early signs of Glaucoma are appearance of rainbow-colored circles around bright lights, Hazy or Blurred Vision, severe head and eye pain, vomiting, nausea which accompanying eye pain that is severe and even sudden sight loss.
Glaucoma occurs when the fluid inside the eye does not drain properly and the fluid increases pressure in the eye which is called the intraocular pressure which if not drained can damage the eyesight and even lead to complete loss of eyesight.
So it's important to get Glaucoma treated as soon as possible.