Trick of the Trade: OKN drum to test psychogenic coma
Posted Jul 14 2010 12:00am
Occasionally, emergency physicians see patients who present because they are unresponsive despite normal vital signs and an otherwise normal exam. You detect no drugs or alcohol on board. You suspect a psychiatric or malingering etiology, but aren't sure. They seem non-responsive to voice and minimally responsive to very painful stimuli. Is this a case of psychogenic coma or true coma (with bilateral hemispheric dysfunction)?
What test can you do to reassure yourself that this may indeed be psychogenic coma?
Trick of the Trade:
Test for optokinetic nystagmus (OKN) using an OKN drum.
The optokinetic reflex allows us to follow objects in motion when the head remains stationary. Imagine as if you are a passenger in a car traveling on a highway, watching periodically-spaced telephone poles pass by. This can be reproduced by spinning an OKN "drum" directly in front of a patient. Spin the drum to create an alternating pattern of vertically-oriented black and white bars.
Normal OKN response
The eyes slowly pursue the moving bars in the same direction as the bars. Then quick saccade movement of the eyes in the opposite direction attempt to fixate on the next moving bar. A normal response requires a relatively normal visual acuity and intact fronto-parietal-occipital brain function. This nystagmus eye movement is called the optokinetic reflex. Check out this movie of a normal OKN response from the University of Utah's teaching files.
OKN Drum on the "Eye Handbook" iPhone app
Within the free iPhone app Eye Handbook, there's an OKN drum animation. You can speed up and slow down the pace at which the bars sweep through the screen.
For one particular "coma" patient, placing this screen in front of her revealed a normal optokinetic nystagmus response after "spinning the drum" leftward and rightward. I talked to her as if she were totally awake. I explained that we'd be starting an IV, getting labs, ordering a head CT if she weren't more awake soon. I told her to tell me when she was thirsty so that I could give her some water or juice. I asked her if she wanted to see a psychiatrist. No response. Thirty minutes later, she started talking and requested to talk to her psychiatrist.
Alternatively, a cold caloric test could have been performed to assess brain function.