The optic nerve is the “power cable” of the eye, connecting the eye to the brain and is essential for vision (Figure 2).
Glaucoma is a condition whereby there is damage to the optic nerve resulting in reduced function and is the world’s leading cause of irreversible blindness. The majority of patients with glaucoma have little or no symptoms and hence the reason why a diagnosis can often be delayed or missed. If patients, do experience symptoms, these are often non-specific to glaucoma and include areas of blurred vision or “missing areas” in the vision. By the time symptoms develop, the disease is often in its advanced stages.
Glaucoma is broadly divided into open angle and closed angle glaucoma, with the angle referring to an area of the eye where the majority of fluid produced within the eye drains drain’s out of the eye. Figure 3 shows a cross section of the eye with fluid (aqueous humor) forming behind the iris and draining out of the eye through structures in the angle of the eye (red arrowed line). When this angle either malfunctions (as in open angle glaucoma) or is closed (as in closed angle glaucoma) patients can develop high intraocular pressure which can lead to glaucoma.
Risk factors for glaucoma include raised eye pressure (known as the intraocular pressure or IOP), increasing age, ethnicity (with Afro-Caribbean’s and Asians being more affected) and refractive error (power of your glasses prescription). The normal IOP is between eleven and twenty-four millimeters of mercury (11-24mmHg) for the majority of people. For the patient with the commonest form of glaucoma (Primary open angle glaucoma) the IOP is elevated above 24mmHg at the time of diagnosis. However, the Intraocular pressure (IOP) can be within normal limits at the time of diagnosis of glaucoma (known as Normal tension glaucoma) due to risk factor’s other than intraocular pressure alone – such as migraine, cold hands and feet (Raynaud’s), anaemia and obstructive sleep apnoea.
Figure 2 – Cross section of the eye showing the main structure of the eye and the optic nerve. Source: National eye institute media library
Figure 3 – Cross section of the eye showing the region where fluid (aqueous humour) is produced and where it starts to drain out of the eye. The red arrow illustrations the path of aqueous humor flow. Source: National eye institute media library