What does blood in the back of the eye signify, anyway?

It could be a retinal vein occlusion, an ocular disorder that can occur in older people where the blood vessels to the retina are blocked.

The retina is the back part of the eye where light focuses and transmits images to the brain. Blockage of the veins in the retina can cause sudden vision loss. The severity of vision loss depends on where the blockage is located.

Blockage at smaller branches in the retinal vein is referred to as branch retinal vein occlusion (BRVO).  Vision loss in BRVO is usually less severe, and sometimes just parts of the vision is blurry.  Blockage at the main retinal vein of the eye is referred to as central retinal vein occlusion (CRVO) and results in more serious vision loss. 

Sometimes blockage of the retinal veins can lead to abnormal new blood vessels developing on the surface of the iris (the colored part of your eye) or the retina. This is a late complication of retinal vein blockage...

There are several treatable eye diseases that at their earliest onset have few or no visual symptoms. In fact, the three leading causes of legal blindness in the United States all start with almost no visual symptoms detectable by the person suffering from the disease. The three diseases are macular degeneration, glaucoma, and diabetic retinopathy. Each of these diseases gets more prevalent as people age. That is why the American Academy of Ophthalmology recommends regular eye exams at more frequent intervals as adults get older.

Macular Degeneration: The leading cause of legal blindness in the United States is a treatable but not curable disease. Early detection and treatment can significantly improve the long-term outcome. At the earliest stages, often when people are unaware that they have a problem, treating the disease with a very specific vitamin regimen called AREDS 2 can help. These vitamins have been shown to slow the progression of the disease and improve long-term...