(photo illustration with iStock image)

One late-summer day last year, my surroundings started playing tricks with me. The letters I typed on my computer screen looked fuzzy. Objects on my desk seemed to slip sideways, escaping their own outlines. My colleagues, viewed across the room, appeared to have shifted slightly so that now they stood or sat as ghostly silhouettes beside themselves.

I put it down to fatigue or lack of food.

The next morning, neither sleep nor sustenance had cured me. I squinted my way out of my apartment and reached for the handrail that runs alongside the front steps. As my left arm extended, my forearm divided somewhere between my elbow and my wrist, so that now I had two left hands and 10 fingers, groping for two railings that ran not parallel to one another but diverged into the distance. Below my four feet, the neat brick geometry of a Capitol Hill sidewalk had become a muddled mosaic. I looked up to see two identically dressed men, swinging their arms in unison as they marched, lockstep, toward me.

I closed one eye and then the other. Both worked well. In fact, each restored reassuring order to the world: One man. One left hand. One railing. But when I tried to walk with only my right eye open, I keeled over to the side. I lost my balance — and a little confidence.

"Not good," the ophthalmologist murmured later that morning as he tracked the movement of my eyes from left to right and back again. Diplopia was his diagnosis. Greek for double vision. How did I feel? he asked. Had I had a virus? I needn't go to the ER, he said, unless I developed a splitting headache or started vomiting. But I should see a neuro-ophthalmologist. Soon.

For many doctors, double vision is daunting because its causes run the gamut from benign to quickly fatal.

"I would have hated to have you as a patient," my brother, a general practitioner, later told me. "Not once I got to know you," he assured me, but he would have worried about what doctors refer to as looking for zebras — the danger of searching for exotic diseases instead of focusing on simpler diagnoses. The medical-school adage says, "When you hear hoofbeats, think of horses, not zebras." But when it comes to diplopia, zebras abound.

Double vision can be the result of something as mundane as a virus or of something extremely rare. It might demand dramatic intervention such as emergency brain surgery or be the first sign of chronic illness requiring long-term care. In many cases, left untreated, it eventually disappears.

About 500 neuro-ophthalmologists — experts in both the visual and nervous systems — work in the United States, many at major academic medical centers. Eight are headquartered at Johns Hopkins, where assistant professor Amanda Dean Henderson began an immediate and meticulous evaluation of my sight and general health. She and her team at the Wilmer Eye Institute tested my visual field and my depth perception, and they placed prisms in front of my eyes to measure the degree my vision deviated from normal. Henderson took my blood pressure, ordered a handful of blood tests and booked an MRI scan for me.

The problem, she explained, was not with my eyes but with one of the 12 pairs of cranial nerves, which control such diverse things as tongue movement, taste, swallowing and vision. Damage to the first — or olfactory — nerve affects your sense of smell. A poorly functioning seventh nerve creates the characteristic facial droop of Bell's palsy. My horizontal double vision, in which two images of the same object appeared side by side, suggested that one of my sixth nerves was damaged.

The sixth, or abducens, nerve, is among the longest of the cranial nerves, making it prone to injury in many places as it travels upward from the brain stem, then turns abruptly forward toward the eye. When it's working properly, the nerve operates the tiny lateral rectus muscle, which pulls your eye away from your nose toward your ear. When it isn't, the eye cannot turn outward. That's what was happening to my right eye. Anyone who looked at me could see it drift, disconcertingly, toward my nose.

Looking straight ahead, I saw two images of each object, one produced by each eye. As I turned my gaze to the right, my left eye moved but my right eye didn't, so the duplicate images appeared farther and farther apart. When I looked to the far left, the images came close to reuniting.

Sixth nerve palsy, or paralysis, can be caused by trauma, but the reasons for a case like mine include aneurysm, brain tumor, stroke, viral infection, Lyme disease, diabetes and illnesses I had never heard of. Commonly, the cause remains a mystery.

A 2004 article in the journal Ophthalmology estimates that sixth nerve palsy occurs in 11 people per 100,000 each year. Henderson, who sees about one case every week, couldn't be sure what caused my double vision, but her tests showed that I was healthy and that my case was probably benign — a horse, not a zebra. She expected the palsy to resolve over the next three months, and she didn't think it would recur. While I waited, I should cover my right eye with a patch.

Life with one eye proved nothing short of, well, eye-opening. The glare of lights above an office computer screen made it hard for me to read. I found myself holding my head at an angle, straining to use my left eye to make up for the peripheral vision I lacked on my right, until my neck and shoulders ached and ached. When I took the patch off, I could feel my brain wrestling to realign the images.

And, oh, how clumsy I proved to be. I could see what lay in front of me, but I couldn't tell how far away things were. One evening, when having dinner with my mother-in-law, I offered her a glass of wine, picked up the bottle and — whoosh! — poured red wine onto her tablecloth.

I developed some compensatory strategies. (Grip the glass, and pour only when the bottle rests on its lip.) And I found a company called the Magic Thimble, which makes eye patches for every occasion: baseballs prints, seasonal orange pumpkins, even gold swirls on black for evenings out. I resisted the holly-berry pattern, determined to be better before Christmas, and I sometimes stuck with drugstore black.

"Mom, is that a real pirate?" one child asked his parent. I wasn't, I assured him. But I made myself a cardboard cutlass and dressed as one for Halloween.

And I honed my diplopia jokes. "You getting any better?" people wanted to know, as weeks went by. "Fingers crossed!" I'd quip, noting that I had twice as many fingers to cross as anybody else.

Sometimes I would take the patch off to marvel at the kaleidoscopic distortions of my sight.

"I wish I could show you what I'm seeing," I told my husband, watching dueling images dance toward and away from one another as I moved my head or eyes. Objects in the distance looked farther apart than those close to me. I remember lying on the ground looking up a flagpole. It split in two as I stared up to where twin flags were waving in the wind. If I tilted my head or turned my eyes, the flags shifted their relationship — nearer, farther, up and down they flew, as did the clouds behind them.

I soon learned never to remove the patch while riding in a car, where the shifting sights created nausea-inducing chaos. Think of being in bumper cars hurtling along the highway, where each parked car has a duplicate, often directly in your path, and a second road seems to veer ahead, sending traffic careening across your route.

And in the evenings, I sometimes searched the Internet for zebras, scanning Google's horizons for exotic landscapes where rare afflictions roamed. Could it be Wernicke's encephalopathy that my knowledgeable physician had somehow failed to spot? Maybe myasthenia gravis? Or even Gradenigo-Lannois?

I should have known better. I'm from a family of physicians, and I understand the risk of focusing on ill-informed diagnoses. But I allowed my online safari to take me to unlikely destinations even while — as Henderson had told me to expect — my symptoms gradually improved.

First, I found that I could read using both eyes when I held a book to my left.

In time, I could see just one image of objects that were a few feet in front of me.

Then, one November morning, I took a bath before putting on my eye patch. I lay in the tub glancing above my two feet at the chrome taps. I was astounded by what I saw. My eyes were beginning to work together again, fusing twin images and restoring the depth perception I had temporarily lost. And I was, for that moment, acutely conscious of the three-dimensional nature of every single thing around me: the bend in the base of the metal taps, the gentle curve of the roll-top tub, the contour of a toenail.

It was at once absolutely breathtaking and completely distracting — a sensory overload that was blessedly short-lived.

I am relieved, of course, that I can now assess how far away things are. I no longer have to hold a bottle against the lip of a glass before I pour. Thanks to myriad interactions between our two eyes and our brain, we make those kinds of complex calculations all the time. But for the most part, we do so unthinkingly.

Like so many other things we take for granted.