What is the cause of decreased outflow of aqueous in a case of primary open angle glaucoma in diabetic?

In primary open angle glaucoma, the angle of anterior chamber is open and appears normal but there is increased resistance to the outflow of aqueous humor in the vicinity of canal of Schlemm. Primary open angle glaucoma is defined as glaucomatous optic neuropathy in the presence of an open angle and no other ocular abnormality to account for a secondary mechanism. It is a painless condition, and patients presents with loss of peripheral visual field but central visual acuity remains unaffected even in patients with severe visual field loss. Ref: Essentials of Rubin's Pathology By Howard Reisner page 803. Tsai L.M., Kamenetzky S.A. (2010). Chapter 37. The Eye & Ocular Adnexa. In G.M. Dohey (Ed), CURRENT Diagnosis & Treatment: Surgery, 13e.