Overview of Glaucoma:
Glaucoma is an umbrella term for optic nerve pathologies (diseases) characterised by progressive degeneration of retinal ganglion cells in the nerve fibre layer of the retina.
The optic nerve is a bundle of over one million nerve fibres (extensions from the retinal ganglion cells) at the back of the eye that carry visual messages from the retina to the brain. These nerve fibres exit the eye through a hole in the sclera called the lamina cribrosa. The lamina cribrosa is the weakest point of the wall of the pressurised eye. Intraocular pressure-induced stress and strain can disrupt this area causing damage to the nerve fibres.
Glaucoma is the result of retinal ganglion cell death and damage which is characterised by thinning of the nerve fibre layer. With retinal ganglion cell death and optic nerve fibre loss in glaucoma, characteristic changes referred to as “cupping” in the appearance of the optic nerve head and retinal nerve fibre layer occur.
Figure 1 (a) shows a normal, healthy optic nerve head fundus image on the left, and the cupping of the optic nerve head in an eye with glaucoma on the right, the diameter of the optic cup is indicated by a double ended arrow, the optic disc is indicated by a single ended arrow. Figure 1 (b) provides an illustration of the changes in the optic cup and disc of a normal healthy eye on the left, compared to the glaucomatous cupping on the right. This image was adapted from Marais, Andre & Osuch, Elzbieta. (2017). The medical management of glaucoma. South African Family Practice. 59. 6-13.
Glaucoma is often, but not exclusively associated with high pressure in the eyes (intra-ocular-pressure, IOP). IOP can occur due alterations in the drainage angle of the eye. The drainage angle is the point in the eye where the coloured part of the eye (the iris) and the white part of the eye (the sclera) meet. Blockage of this angle can lead to increased pressure in the eye. The increased pressure can damage the optic nerve and cause glaucoma.
Specifically, IOP occurs as a result of aqueous humour inflow balanced against aqueous humour outflow and both are approximately 2.75 μl/min. Inflow is not very sensitive to pressure until very high pressures are achieved. IOP is regulated primarily by controlled adjustments of the outflow by apart of the eye called the trabecular meshwork (TM). When parts of the TM are altered and there is an increased resistance to outflow of aqueous humour, then IOP increases. The increased ocular pressure causes degenerative changes and progressive death of the retinal ganglion cells (RGCs) and damages their axons, which transfer visual information to the brain via the optic nerve.
Vision lost to glaucoma cannot be regained as retinal ganglion cells do not regenerate. At present there is no cure, however there are therapy and surgery options to delay the progression of the disease. People with diabetes are at risk for several types of glaucoma.