Glaucoma is progressive ocular neuropathy, which causes damage to the optic nerve thus impairing the vision. Generally, the pressure within the eyes is elevated in glaucoma which is responsible for the damage to the optic nerve which carries the impulse to the brain.
Glaucoma is of three types namely open-angle glaucoma, angle closure glaucoma and normotensive glaucoma. Open-angle glaucoma is insidious in onset and is painless whereas angle closure glaucoma is acute in onset and present with severe pain.
Out of various causes of glaucoma, ocular hypertension that is an increase in the intraocular pressure is the most important cause for the development of glaucoma.
There are various causes of glaucoma and various theories have been put forward to explain its etiopathogenesis. The main risk factor for the glaucoma is increased intraocular pressure and the main course of treatment is also focused on decreasing intraocular pressure. Intraocular pressure is the pressure exerted by the aqueous humor fluid present within the eye. Aqueous humour is produced by the ciliary process of the ciliary body and is drained into the trabecular network. The aqueous humor flows from ciliary body to posterior chamber and from posterior chamber to the anterior chamber through the pupil. And from the anterior chamber, it is drained into trabecular mesh work, which drains into sinuses.
How Does Glaucoma Affect Your Vision?
Glaucoma is a progressive ocular neuropathy which leads to defective vision. The visual field defect in the glaucoma is permanent and irreversible.
In open-angle glaucoma, reduction of the flow of Aqueous is due to degenerative and obstructive changes of the trabecular network lead to the impaired absorption and thus resulting into raised intraocular pressure.
In angle-closure glaucoma, there is a reduction of iridocorneal angle of the eye due to forward displacement of iris and/or subluxated lens. There occurs accumulation of Aqueous in the anterior chamber.
This consistently raised intraocular pressure causes degeneration of the optic nerve head, compression of the blood vessels, reduction of trophic factors, and degeneration of retinal ganglion cells.
Retinal Ganglion Cell Death
Retinal ganglion cell is the neurons present in the retina. The death of these retinal ganglion cells is crucial in the pathophysiology of glaucomatous vision loss. Retinal ganglion cell death is initiated when some pathologic even blocks the transport of growth factors. Neurotrophins (growth factors) are essential for normal functioning of the retinal ganglion cells. Blockage in the transmission of these neurotrophins from the brain to retina initiates the death cascade in these cells. These cells are unable to perform normal functioning in absences neurotrophins and thus undergo apoptosis.
The death of these retinal ganglion cells is associated with loss of the retinal nerve fibers. The characteristic optic disc changes and specific visual field defect are Apparent when there is a significant loss of these cells or nerve fibers.
Etiological Factors for RGC Death
Mechanical theory: raised intraocular pressure causes mechanical stretching of the lamina leading to axonal deformation and ischemia, this result in the impaired transportation of neurotrophins to the ganglion layer.
Vascular Insufficiency Theory
Retina and optic disc Have mechanism of autoregulation of blood flow. Raised intraocular pressure causes the failure of the autoregulatory mechanism.
Vasospasm affects the vascular perfusion of optic nerve head.
Systemic hypotension due to the nighttime administration of antihypertensive drugs causes a dip in the systemic blood pressure which causes decreased vascular perfusion of the optic nerve head in normotensive glaucoma.
In early glaucoma, there is a gradual and imperceptible failing of the peripheral vision.
As glaucoma progresses, central vision may remain clear, but the peripheral vision starts to fail. That is there is a loss of peripheral vision of the field.
In the advanced stages of glaucoma, there is an extensive loss of nerve fiber cells so that only a small Central area of vision remains. Eventually with the time damage to optic nerve will lead to total blindness.
The visual field defect in the glaucoma is Irreversible because the Degenerated optic nerve or nerve fibers cannot be regenerated, unlike the peripheral nerve fibers. Hence, the visual field defects in glaucoma are permanent.
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