Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine you and obtain a history and review your imaging, I can not tell you what the exact cause of your symptoms is nor what the best management for your microadenoma would be. However I will try to provide you with some useful information.
If a pituitary adenoma is not hormone secreting, in general, for small pituitary adenomas, that are around 5 mm in diamter or less that is discovered incidentally, further intervention is often not necessary, and all that needs to be done is a repeat MRI after a certain interval (once over twice over a 2 year period), to monitor for growth of the adenoma. An adenoma that small would be unlikely to cause headaches or visual disturbances, but in any one with a concern for vision problems such as in yourself, with a pituitary adenoma, evaluation by an opthalmologist, with formal visual field testing, would be appropriate.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.