Glaucoma: Looking Beyond Eye Pressure

Eye Pressure Basics

Intraocular pressure (IOP) is the fluid pressure inside the eye. Elevated IOP, called ocular hypertension, can damage the optic nerve and lead to glaucoma – and eventually, vision loss.

Not everyone with glaucoma has high IOP, however. Normal-tension glaucoma (or low-tension glaucoma) is a condition in which optic nerve damage and vision loss occur despite normal pressure.

Beyond IOP

Based on findings of two large trials (1-3), experts agree that lowering IOP reduces risk of glaucoma progression both for those with elevated IOP and with normal-tension. But research also confirms that IOP is not the only risk factor, since the disease progresses despite good IOP reductions in about 20% of cases.

This finding has led to over a decade of research to identify risk factors other than IOP that might be addressed to protect the optic nerve. Though there’s a long way to go, progress has been made in the area of optic nerve protection (neuroprotection).

Retinal ganglion cells are a type of nerve cell (neuron) that transmits visual information from the retina to the brain to form images. The long “tails” of these nerve cells, called axons, make up the optic nerve. In glaucoma, ganglion cells in the retina die or become dysfunctional. So the goal of neuroprotection is to favorably alter factors that affect nerve cell survival.

Neuroprotective Strategies: Nutrients & Botanicals

Unstable ocular blood flow is thought to be an important contributor to ganglion cell death. Studies have reported that Gingko biloba can increase ocular blood flow. In a recent study, normal tension glaucoma patients taking ginkgo were reported to lose peripheral vision (known as visual field) at a slower rate (4).

A combination of flavonoids from bilberry and pine bark has also been tested in patients with ocular hypertension (but no signs of glaucoma), with results suggesting an improvement in ocular blood flow (5).

Disturbed blood flow can produce oxidative stress, where damaging free radicals outnumber the anti-oxidants needed to quench them. There are a number of antioxidants – CoQ10, vitamin E, and others – found to be protective of retinal ganglion cells in lab studies.

Neuroprotective Strategies: Drugs & Treatments

A number of promising treatment options are being tested. One topical drug, brimonidine, slowed visual field loss in people with normal tension glaucoma better than the IOP-lowering drug timolol, even though both similarly reduced IOP (6). Brimonidine, in addition to lowering IOP, may offer neuroprotection by enhancing or activating factors that protect neurons, or by blocking release of harmful compounds.

A strategy in early testing stages is to salvage poorly functioning retinal ganglion cells. One idea is to implant encapsulated cells in the eye that produce neuro-protective substances. Another approach could be to improve ganglion function by electrical stimulation (since these cells become dysfunctional with loss of electrical stimulation and ability to pass electrical signals).

Though more research on nutritional and other treatments is needed, progress to date is encouraging. Neuroprotective strategies, combined with standard IOP-lowering treatments, may well improve the outlook for glaucoma patients in the years to come.

References

1. Collaborative Normal-Tension Glaucoma Study Group: Comparison of glaucomatous progression between untreated patients with normal-tension glaucoma and patients with therapeutically reduced intraocular pressures. Am J Ophthalmol 126:487-97, 1998.
2. Collaborative Normal-Tension Glaucoma Study Group: The effectiveness of intraocular pressure reduction in the treatment of normal -tension glaucoma. Am J Ophthalmol 126:495-505,1998.
3. Leske MC, et al. Factors for glaucoma progression and the effect of treatment: The early manifest glaucoma trial. Arch Ophthalmol 121:48-56, 2003. 
4. Lee J, et al. Effect of ginkgo biloba extract on visual field progression in normal tension glaucoma. J Glaucoma [Epub ahead of print May, 16, 2012].
5. Steigerwalt RD Jr., et al.  Mirtogenol® potentiates latanoprost in lowering intraocular pressure and improves ocular blood flow in asymptomatic subjects. Clin Ophthalmol  4:471-76, 2010. 
6. Krupin, et al. A randomized trial of brimonidine versus timolol in preserving visual function: results from the Low-pressure Glaucoma Treatment Study. Am J Ophthalmol 151:671- 81, 2011.