Monday, May 26, 2008

When the Body Lies

Sometimes the human body can be deceptive. That really scares me.

I saw a woman recently who had blurred vision that was getting worse over a few weeks. Looking inside the eye, it looked perfectly normal, but the vision was profoundly reduced. The vision in the other eye was normal. When the eye looks normal, and the vision is reduced, then there is something behind the eye causing the reduced vision. But where?

Well, the first step in finding the location of the lesion is to perform a visual field exam. The eye chart measures your straight ahead vision, but not your peripheral vision. You might be able to see 20/20, but you might be looking through a tiny tunnel of lost peripheral vision. A visual field test checks the peripheral vision. You look straight ahead while lights of various intensities pop up in different areas of the peripheral vision, and every time you see one of those lights, you press a button connected to a fancy computer. The computer maps out your visual field.

They eyes are wired in an interesting way. All the information from the retinae of each eye gathers into the optic nerve of each eye and takes off to the brain. But then, once just inside the brain, a funny thing happens. Some of the fibers from each eye cross over to join fibers from the other eye. They "decussate" in a place called the "optic chiasm." The result is that if there is a problem behind the optic chiasm -- after the fibers have crossed -- say, on the right side of the brain, both eyes (not just the right eye) will manifest a visual field defect since fibers from the left eye have crossed over to the right and are being affected by the lesion in the brain. The wiring is so specific that you can even tell how far back the lesion is by the type of visual field pattern loss in each eye. If the damage is in front of the chiasm, the fibers haven't yet crossed over, and the field defect will show up in only one eye.

I'm thinking this as I examine this woman, and I know that in order to find the location of the lesion, I need to know the visual fields of the unaffected eye. Because if the visual field is normal, the problem is in front of the chiasm in the optic nerve of the abnormal eye. If the visual field of the good eye has a defect, then the problem is in the brain. Got it?

I get the visual field. The bad eye is totally wiped out. The good eye is completely normal. That pretty much clinches the diagnosis. Her lesion is in the optic nerve, and in a woman her age, the likely cause is optic neuritis -- actually a specific variant called "retrobulbar" optic neuritis in which the inflammation is behind the eye, not in it, therefore when you look in the eye it looks normal. The maxim for retrobulbar neurtiits is "the patient sees nothing, and you see nothing."

So she has this variant of optic neuritis. What causes optic neuritis? The most likely cause for her is multiple sclerosis -- a lousy diagnosis, but one you want to know about. The next step is to get an MRI of the brain, because the brain in MS has characteristic plaques that light up on the MRI. The MRI won't change the outcome of the optic neuritis, but it helps the patient plan for their future if it is MS.

Problem: there is no MRI machine on Saipan. The closest one is on Guam. I could get a CT here, but it might not show anything, which might mean that the MS plaques may still be there, but CT couldn't see them. If the CT is positive, it helps, but if it's negative, you just spent $800 of her money unecessarily. She makes her way to Guam and gets the MRI.

And this is what can scare the snot out of you. The MRI results come in, and it doesn't show the characteristic MS plaques. It shows a tumor the size of a fist in her brain, back behind her optic chiasm where it should have shown an abnormality in the visual fields of both eyes. Everything that I thought was right, is just plain wrong. There is no way the visual field can be normal in her good eye with a tumor this size, but it is. It's likely that the tumor itself isn't actually causing any visual field problems. The MRI shows the tumor cells have infiltrated down one of her optic nerves and is choking it to death. Thus, only the vision and the visual field in one eye is abnormal. Her body isn't following the rules. But I am.

And the most scary part is that when everything pointed to optic neuritis, I was tempted to save her the trouble and expense of going to Guam for the MRI and just admit her to CHC and treat her according to the guidelines of the optic neuritis treatment trial. That's the sort of thing we often do on Saipan where resources are limited, and people can't afford to go off-island for further testing. If she hadn't been able to go to Guam, I would have done exactly that -- treated it as MS -- and thought nothing of it, confident in the information the human body was conveying to me. What a disaster that would have been.

Sometimes, you don't conquer disease. Sometimes you just accidentally trip over it and it dies. In the instant that you realize how close you were to making a deadly mistake, you find yourself totally humbled. The tumor was found, and she's going for neurosurgical treatment, but it was just dumb luck.

I keep replaying the events, the decisions, wondering what to do differently next time, and really, there isn't much, other than to realize that the body, sometimes will smile coolly at you and just lie with the deepest deception. And there's no way to know of its betrayal, other than with luck and the help of angels.

4 comments:

Jeff said...

Your job is heavy.

James said...

We put too much pressure on ourselves as doctors. The tumor would've grown and you would've discovered it eventually, right? Is the earlier diagnosis going to save her life? You can't really know, right? I think that much of what we consider to be "medical mistakes" are not really mistakes at all but they are simply built in to the system. When doctors practice as they are trained to practice they make wrong diagnoses sometimes. Sometimes the doctor's fault, sometimes not.
Sorry, the comment went somewhere I wasn't really intending. I enjoyed the post :)

The Welch Family said...

I love your humility. My daughter was diagnosed with a huge tumor in her chiasm...the doctor did an eye exam after and said her nerves were pink and vision perfect - he couldn't believe that the scan was from the same child as he tested.
Our daughter was diagnosed 1 day after sudden onset nystagmus (but still could see perfectly and grasp a cherrio first shot). Other children have gone to eye doctors for 2 years before diagnosis. Each day is a learning opportunity. It sounds like you are a wonderful doctor who is willing to learn.

E said...

Doctor, as a current student in lower division physiology, I found your post fascinating, I have a test on the cranial nerves tomorrow!
Thanks for the fun and insightful break.