Cataracts and glaucoma are two prevalent causes of vision impairment, especially among older adults, and they frequently occur together. When a patient has both a cataract and co-existing glaucoma, treatment often involves the surgical removal of the cataract, known as phacoemulsification. This procedure replaces the eye’s cloudy natural lens with a clear artificial intraocular lens (IOL). Cataract surgery often results in the beneficial side effect of lowering intraocular pressure (IOP), which is the primary treatment target for glaucoma. The degree of IOP reduction is highly variable, depending on the specific type of glaucoma and the patient’s pre-operative eye pressure.
Understanding Cataracts, Glaucoma, and Intraocular Pressure
A cataract is the clouding of the eye’s natural lens, causing blurred vision and difficulty seeing at night. Glaucoma is a group of diseases that cause progressive damage to the optic nerve, which transmits visual information to the brain. This damage is most often linked to elevated intraocular pressure (IOP).
The pressure within the eye is maintained by a continuous cycle of fluid production and drainage. A clear fluid called aqueous humor is constantly produced and flows out through the trabecular meshwork, located in the angle between the iris and the cornea. If this outflow system is compromised, the aqueous humor builds up, causing the IOP to rise. Elevated IOP is the most important modifiable risk factor in managing glaucoma.
The Physical Mechanism of Pressure Relief
The natural lens of the eye grows and thickens throughout life, a process known as phacomorphic change. As the lens thickens, it occupies more space within the anterior segment of the eye. This increased volume pushes the iris forward, which physically narrows the drainage angle where the trabecular meshwork resides.
In cataract surgery, the surgeon removes this large, thick natural lens using phacoemulsification. The lens is then replaced with a much thinner, synthetic intraocular lens (IOL). This replacement moves the lens-iris diaphragm backward, creating more space in the anterior chamber.
This spatial change deepens the anterior chamber and significantly widens the drainage angle. This mechanical widening alleviates crowding of the trabecular meshwork, allowing the aqueous humor to drain more freely. The resulting improvement in outflow directly leads to a reduction in intraocular pressure. This pressure-lowering effect is sustained because the new artificial lens will not thicken over time.
Isolated Outcomes for Different Glaucoma Types
The benefit of cataract surgery on eye pressure is not uniform; it depends on the underlying mechanism of the patient’s glaucoma. Cataract surgery provides the greatest therapeutic impact for patients with primary angle-closure glaucoma (PACG). In PACG, pressure elevation is caused by the physical obstruction of the drainage angle by the peripheral iris, often exacerbated by the bulky natural lens.
Removing the thick lens physically eliminates this obstruction, often leading to a substantial and sustained IOP drop, typically ranging from 2 to 12 mmHg. For many PACG patients, the procedure acts as a primary treatment, significantly reducing or eliminating the need for glaucoma eye drops. This physical re-engineering of the anterior chamber anatomy can be curative for the angle-closure component of the disease.
In contrast, patients with primary open-angle glaucoma (POAG) experience a more modest pressure reduction when cataract surgery is performed alone. In POAG, the drainage angle is anatomically open, but the trabecular meshwork itself is functionally impaired. Although the procedure widens the angle, the primary issue lies within the meshwork tissue, not a physical blockage.
For POAG patients, the typical IOP reduction is smaller and may not be sufficient to halt the progression of optic nerve damage. Studies suggest a mean pressure drop of approximately 1 to 4 mmHg, with the greatest reductions seen in patients who have a higher pre-operative IOP. Therefore, for most POAG patients, cataract surgery is considered an accessory benefit rather than a standalone treatment, and they often still require medication or further intervention.
Combining Cataract Removal with Glaucoma Procedures
When cataract surgery alone is unlikely to achieve the necessary IOP reduction, the procedure can be combined with a simultaneous glaucoma operation. This combined approach is common in patients with moderate to severe glaucoma or those unable to tolerate multiple medications. The most frequently utilized option involves Minimally Invasive Glaucoma Surgery (MIGS). MIGS procedures enhance the eye’s natural drainage system using microscopic devices or techniques that require small incisions. For instance, tiny micro-stents can be implanted directly into the trabecular meshwork during the cataract procedure, bypassing diseased tissue and creating a clear channel for aqueous humor outflow.
Combining cataract removal with a MIGS device offers a synergistic effect, providing the pressure-lowering benefit of lens removal alongside the targeted intervention. This dual procedure does not significantly prolong surgery or recovery time. For patients with advanced glaucoma, the surgeon may opt for a traditional filtering surgery, such as a trabeculectomy, performed concurrently (phaco-trabeculectomy). This more invasive combined surgery creates a new drainage pathway, offering a more profound pressure reduction for severe disease.