How does a diagnosis of astigmatism in left eye with continual eye strain with or without glasses relate to pressure on top of the head?

There are nerves that surround the eye area. Because of astigmatism in your left eye your right eye is making up for the loss of proper sight and this is a strain which effects the nerves and it can cause a pressure on the top of your head. It's wise to see a specialist (Ophthalmologist.) One can never be too careful with their eyesight.