What is Glaucoma and Its Mechanism?

Ben R. Wilkinson, FRCSC

Glaucoma is characterized by damage to the optic nerve head, often caused by a period of reduced blood supply. The condition referred to as elevated Intraocular Pressure (IOP) without evidence of optic nerve damage is accurately termed 'Ocular Hypertension.' The extent of the damage is assessed through examination of visual fields and the presence of notches in the nerve fiber layer at the optic disc, indicative of dead nerve fibers.

Causes of Elevated IOP: Common and Rare

The most frequent causes of elevated IOP include 'white coat syndrome,' an artificially high measurement of IOP similar to high blood pressure caused by 'white coat syndrome,' chronic or open-angle glaucoma, and acute or angle-closure glaucoma. Pigmentary glaucoma is a less common but viable cause of the condition. For chronic glaucoma, treatment typically involves medicated eye drops to reduce IOP, whereas angle-closure glaucoma is often treated with surgical procedures, possibly involving laser interventions.

Blood Pressure and Optic Nerve Dynamics

The pressure at the optic nerve is influenced by two primary factors: the condition of the 'plumbing' between the heart and the ophthalmic artery, such as arteriosclerosis, and the current state of systemic blood pressure. Any circumstance that leads to a drop in the arm-measured systemic blood pressure will similarly decrease the ophthalmic artery pressure. Conditions that can result in a drop in blood pressure include injury, severe illness, anesthesia, and surgery, a heart attack, or excessive blood pressure medication.

Risk Factors for Glaucoma: Low Tension Glaucoma

The commonest mechanism for glaucoma is a combination of elevated IOP accompanied by a decrease in blood pressure. Individuals can develop glaucoma even with normal IOP if there is a sufficiently significant drop in blood pressure, a condition known as low tension glaucoma. Each day a glaucoma patient remains symptom-free does not necessitate taking drops, yet these medications serve as a form of insurance. Much like fire insurance, they are necessary.

Symptoms and Early Detection

Unfortunately, there are no early symptoms of glaucoma that individuals can recognize, leading to a gradual loss of peripheral vision before the eye is almost blind. This is why it is crucial for professionals like optometrists and ophthalmologists to perform routine examinations to detect early signs of the condition. Alas, the vision lost due to glaucoma cannot be recovered, and the primary aim of treatment is to halt further vision loss.

Conclusion

Glaucoma's relentless, yet insidious, nature underscores the importance of regular eye examinations to catch and manage the condition before serious vision loss occurs. Early detection and appropriate treatment can significantly mitigate the progression of this often-silent thief in the night.