What Causes Constant Headache?
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Detailed Answer:
I read your question carefully and I am sorry about the persistent headache you have been suffering from.
Looking at your description I must say that it is a typical description of tension type headache. As you must already have been told it is the most common primary headache by far, experienced by up to 80% of the population at some time in their lifes.
Whether it could be something else...usually when my patients have no history of headache in the past and the headache doesn't subside after a week I do consider the possibility of conditions like brain tumor, cerebral vein thrombosis, idiopathic intracranial hypertension, a Chiari malformation etc and have a MRI scheduled. Since in your case it has been already 9 months you shouldn't be alarmed as those conditions in the meanwhile would usually manifest other signs, so MRI would probably be normal, but still I think it should be scheduled.
Looking at causes not related to the brain itself, I might mention high blood pressure and sinusitis as the most common ones, but I suppose they've already been checked by your doctors.
I remain at your disposal for other questions.