If you are nearsighted, myopic, the eye has stretched a bit and the eye is more at risk of macular degeneration. The eye stretches at back of the retina, this is called a staphyloma. The stretching of the retina makes it thinner and thus weaker. Once it is too thin and too aged macular degeneration may occur. If this condition occurs the central vision, where the macula is, will be reduced. This reduction is likely due to a break in one of the layers of the retina. The break leads to fluild building up at the back of the retina. This fluid build up leads to a decrease in vision.
IF you notice a decrease in vision in the eye and this is not related to a change in prescription you will need an OCT scan of your retina. The OCT scan below shows the different layers at the back of the eye by the macula in a nearsighted eye with macular change: