Probably my second-to-least favorite part of going to the optometrist (after the glaucoma test, which always skeeves me out), is the eye dilation. The blurriness, the light sensitivity, the eye strain — it leaves you blinder leaving the eye doctor than when you went in.
But last week, when I went in for a checkup, I got curious while I was in the chair having bright lights shined on my retina: What’s in those drops?
My optometrist told me the drug she uses is called tropicamide. Imagining sandy beaches and drinks with umbrellas (I had to distract myself somehow), I decided to learn a little more.
Tropicamide is a muscarine antagonist, which is to say, it’s a drug that blocks something on the surface of muscle cells called a muscarinic acetylcholine receptor. Your cells are coated with receptors for all kinds of proteins and molecules; they help cells pick up messages from other cells and sense what’s going on around them.
The muscarinic acetylcholine receptor is one of two kinds of receptors that lets muscle cells pick up messages sent from nearby nerve cells using the neurotransmitter acetylcholine. The muscarinic one is so called because it also responds to muscarine, an alkaloid compound found in certain species of mushrooms. You find muscarinic receptors in the central nervous system, smooth muscles, and the heart.
The other kind of acetylcholine receptor, the nicotinic receptor, responds to — you guessed it — nicotine. You find nicotinic receptors where nerves meet muscles and throughout your autonomic nervous system (the part of the nervous system that regulates involuntary actions, like heart beat and breathing).
When your optometrist tells you to tilt your head back and drops the drops in your eye, the tropicamide seeps through your cornea and floats into your iris sphincter muscle, the muscle that causes your iris to close. By binding to those muscarinic acetylcholine receptors, the drug then paralyzes the muscle, taking away your iris’s ability to close and triggering mydriasis — the fancy word for pupil dilation.
With your pupils wide open, your optometrist can then pull over the slit lamp — a microscope that shines a thin beam of light into your eye — and examine your lens, retina, and vitreous body (the gelatin inside your eye).
She can also take pictures like this:
Those are my retinas, as photographed with a laser scanning camera three years ago.
Tropicamide isn’t the only drug optometrists use to dilate patients’ eyes. There are three other iris sphincter paralyzers in common use: cyclopentolate, homatropine, and atropine (another alkaloid that, incidentally, also blocks the effects of nerve gas and can help reboot your heart).
Optometrists also have drugs that, instead of paralyzing the sphincter muscle, will stimulate the iris dilator muscle (which opens the iris up more). The most common one here in the States is phenylephrine (yes, the decongestant).
The eyes aren’t just windows to the soul — they’re the cameras through which we see the world. Drugs like tropicamide help make sure that those cameras are in good working order. Which I guess makes an occasional dilation a good thing.