Ocular migraines and migraines and TIAs can be caused by high platelets and/or not taking aspirin. The condition is called microvascular ischemia. Platelets are sticky and cause the blood to clot which is good thing if you get cut or have surgery but in ET we have too many of these platelets so we can have unwanted clotting in vital organs. So if there are small clumping or clotting in the brain or eye or kidney or other vessels we can have trouble such as pain or visual disturbances. In more complicated scenarios loss of vision, heart attack or stroke could occur. That is why the protocol for ET usually involves baby aspirin to prevent clumping and clotting of platelets and some agent such as hydrea to lower platelets. The timing of when to start the platelet lowering agent varies but most often is started either by age (over 60), symptoms such as migraine or visual disturbances or by the number of platelets (this number varies depending on many factors), or by the documented genetic mutations such as JAK2 or CALR. This is a fairly simple explanation that does not go into great detail but is a general overview.
I have had ET since 2002 (yes over 20 years). I was started immediately on baby aspirin but did not start hydrea until age 60. I am now 71yo and doing well. Ask me anything and although I am not an expert, I have a lived with this a long time and have a medical background as a retired nurse practitioner. Stay healthy everyone! And yes, we can stay healthy even if we have been diagnosed with a cancer.