We will recruit 30 patients with progressive visual function loss, due to Normal tension glaucoma (NTG) or chronic open angle glaucoma (COAG), despite optimal conventional or intraocular pressure reducing surgery treatment so far. We will proceed with an optic nerve sheath fenestration (ONSF) (according to the studies in the literature, e.g. Wax MB, Barrett DA, Hart WM Jr, Custer PL. Optic nerve sheath decompression for glaucomatous optic neuropathy with normal intraocular pressure. Arch Ophthalmol. 1993;111(9):1219-28.) and compare the cerebrospinal fluid (CSF) taken from the subarachnoid space (SAS) of the optic nerve (ON) with the CSF taken during a lumbar puncture, on the same day. Concentrations of neurobioactive substances will especially be studied. Visual acuity and visual field development before and after ONSF will be studied. Quality of life assessment will be done before and several times after ONSF.
