Why Injections Not Drops

Here’s why eye drops don’t work for treating diseases of the retina.

Featured Image | Blood Brain Barrier

The blood brain barrier blocks most medications from reaching the retina. You may wonder why there isn’t a pill, or an eye drop that treats these retinal diseases and why the only effective treatment is a shot in the eye, an intraocular injection.

Intraocular injections are common and effective ways to treat wet macular degeneration, diabetic retinopathy and retinal vascular occlusions.

Pills, eye drops, and even IV solutions cannot penetrate to the retina because it is protected by the blood-brain barrier. Direct injection circumvents this barrier.

The eye is the only part of the brain that can be viewed directly. And that is done when an eye doctor uses an ophthalmoscope and shines a bright light into your eye and can view the innermost layers of the eye—the retina, and the optic nerve.

Blood Brain Barrier

The blood-brain barrier (BBB) is a protective layer of endothelial cells that protects the brain from any pathogens or toxins that may be circulating in the blood supply, but it also restricts entry of large-molecule medications and 98% of all small-molecule drugs. So, the only way to get therapeutics to the brain or to the deep structures of the eye is to go through or behind the BBB.

The BBB allows water, some gasses, and fat-soluble molecules to dissolve in the cell membranes and cross its boundary. Alcohol, caffeine, and nicotine easily cross the BBB because they are lipophilic (attracted to fat) and mix easily with the fat in the BBB to gain entry. Oxygen and anesthesia are gasses that can pass through the BBB.

As part of the brain, the BBB prevents therapeutics that have been ingested and are circulating in the blood to gain entry to the eyes. The medication will travel through the body, but almost none of it will reach the eyes.

Leaky Blood Vessels

Wet macular degeneration is caused when blood vessels behind the retina grow abnormally and leak blood and fluid that distorts vision. If left untreated the leaking blood vessels will cause irreversible damage to the photoreceptors that create central vision.

A Shot in the Eye

The only way to get enough medication directly to the retina to stop the growth of the leaky blood vessels is to use an intravitreal injection. An intravitreal injection means that the medication is injected into the vitreous (jelly-like fluid) of the eye where it will diffuse to the retina.

In addition, injecting anti-vascular endothelial growth factor (anti-VEGF) directly into the eye where the leaky vessels are growing, prevents the anti-VEGF from circulating in your blood supply and possibly adversely affecting systemic blood vessels as it moves along to the eye.

The intravitreal injections are painless because the eye is numbed, and the injections can be done in your doctor’s office. They are a safe, and effective way to prevent leaky blood vessel damage to your retina.

Coming Advances in Treatment

An implanted reservoir that delivers sustained-release anti-VEGF directly to the retina is in the experimental phase. And the field of nanotechnology is in the preliminary research stages of developing nanoparticles as a potential way to transfer medication in eye drops across the BBB.

If you would like to schedule an appointment, please call us (877) 245.2020.

Nader Moinfar, M.D., M.P.H.
Retina Specialist
Orlando, FL

Leave a Reply

Your email address will not be published.

Jon Doe