because of forward displacement of the final roll and root of the iris against the cornea, resulting in the inability of the aqueous fluid to flow from the posterior to the anterior chamber and then out of the trabecular network. This accumulation of aqueous humor causes an acute increase in pressure and pain. Degeneration of axons of the retinal ganglion cells (the optic nerve) is a hallmark of glaucoma. The inconsistent relationship of glaucomatous optic neuropathy with increased intraocular pressure has provoked hypotheses and studies on anatomic structure, eye development, nerve compression trauma, optic nerve blood flow, excitatory neurotransmitter, trophic factor, retinal ganglion cell/axon degeneration, glial support cell, immune system, aging mechanisms of neuron loss, and severing of the nerve fibers at the scleral edge. Diagnosis Screening for glaucoma is usually performed as part of a standard eye examination performed by optometrists and ophthalmologists. Testing for glaucoma includes measurements of the intraocular pressure using tonometry, anterior chamber angle examination or gonioscopy as well as examination of the optic nerve to discern visible damage, changes in the cup-to-disc ratio, rim appearance and vascular change. A formal visual field test is performed. The retinal nerve fiber layer can be assessed with imaging techniques such as optical coherence tomography, scanning laser polarimetry or scanning laser ophthalmoscopy (Heidelberg retinal tomogram). Visual field loss is the most specific sign of the condition, though it occurs later in the course of the disease. As all methods of tonometry are sensitive to corneal thickness, methods such as Goldmann tonometry may be augmented with pachymetry to measure the central corneal thickness (CCT). A thicker cornea can result in a pressure reading higher than the true pressure but a thinner cornea can produce a pressure reading lower than the true pressure. Because pressure-measurement error can be caused by more than just CCT (such as by corneal hydration or elastic properties), it is impossible to adjust pressure measurements based only on CCT measurements. The frequency-doubling illusion can also be used to detect glaucoma with the use of a frequency-doubling technology perimeter. Examination for glaucoma is assessed with attention given to gender, race, history of drug use, refraction, inheritance and family history. Glaucoma has been classified into specific types: Primary glaucoma and its variants Primary glaucoma (H40.1-H40.2) Primary open-angle glaucoma, also known as chronic open-angle glaucoma, chronic simple glaucoma, glaucoma simplex High-tension glaucoma Low-tension glaucoma Primary angle closure glaucoma, also known