What is the pathophysiology of chronic glaucoma?
Chronic glaucoma, the most prevalent form of glaucoma, is characterized by a progressive degeneration of the optic nerve and loss of retinal ganglion cells (RGCs), leading to irreversible vision loss. The primary risk factor for chronic glaucoma is elevated intraocular pressure (IOP).
Pathophysiology:
1. Elevated Intraocular Pressure (IOP):
- IOP is the fluid pressure within the eye. In chronic glaucoma, the aqueous humor, the clear fluid that flows through the anterior chamber of the eye, fails to drain properly through the trabecular meshwork, leading to increased IOP.
- Elevated IOP causes mechanical stress and damage to the optic nerve head, the point where the optic nerve exits the eye.
2. Optic Nerve Damage:
- The increased IOP compresses the delicate optic nerve fibers, causing damage to their axons and loss of RGCs.
- The exact mechanisms of optic nerve damage are not fully understood but involve various factors:
- Ischemia: Increased IOP reduces blood flow to the optic nerve, leading to ischemic damage.
- Mechanical Damage: Direct mechanical compression of the optic nerve fibers disrupts their axonal transport and function.
- Excitotoxicity: IOP elevation can trigger the release of excitotoxic substances that damage RGCs.
- Oxidative Stress: Increased IOP induces oxidative stress, leading to the generation of free radicals that damage optic nerve tissues.
3. Progressive RGC Loss:
- The sustained compression and damage to the optic nerve fibers result in progressive RGC loss.
- RGCs are responsible for transmitting visual information from the retina to the brain. Their loss leads to the characteristic visual field defects associated with glaucoma, such as peripheral vision loss and blind spots.
4. Remodeling of the Optic Nerve Head:
- Chronic glaucoma also involves structural changes in the optic nerve head.
- The lamina cribrosa, a mesh-like structure that supports the optic nerve fibers, weakens and allows the optic nerve fibers to protrude posteriorly, creating a cupping appearance of the optic disc.
5. Neurodegeneration:
- In advanced stages of chronic glaucoma, the neurodegenerative process extends beyond the optic nerve head, affecting other retinal neurons and visual pathways, contributing to further visual impairment.
It is important to note that the progression of chronic glaucoma is typically slow and may span several years. Early detection and management of elevated IOP are crucial to preventing irreversible vision loss in chronic glaucoma.