Oculovestibular Reflex Testing During Brain Death Examination, by Dr. David K. Urion.
We would next try to stimulate this through caloric testing, or oculovestibular reflexes.
And for that, you need to have some preparation.
The head of the bed needs to be at roughly 30 degrees so we can have the semicircular
canals orthogonal to gravity.
It's always a good idea, since we're about to put water into the bed itself, to put a
towel under here to catch what might flow out of my basin.
We then fill this basin with ice water, and we would take into a syringe 50 CCs of ice
cold water, a very profound stimulus to the oculovestibular network.
Before we go ahead any further, we would take an otoscope and examine the ear to make certain,
first of all, that there is no wax blocking the pathway, so that we, in fact, really are
going to stimulate the tympanic membrane, and also make certain that there's no other
pathology there that would make us not want to do this test.
So we'll investigate on this side, and then, I have to climb around and ask Dr. Tasker
to look in the other side to make certain that the tympanic membranes are intact and
visible.
That being the case, we would then go ahead, and I'll ask Dr. Tasker to hold back his eyelids
so we can watch.
We'll position the basin just behind the ear like this, and then we would insert the tubing--
you can make this from any kind of an IV tubing, just removing the needle-- and we would then,
as rapidly as we can, instill 50 CCs of ice water into this space.
Now, what we're looking for is half of what would be usual caloric nystagmus.
Remember that nystagmus is named after its fast phase, cold-opposite, warm-same, since
the fast phase is the cortical reset.
And we've already demonstrated that there are no cortical responses.
We'd be looking for the brain stem half of this response, which, in this instance, would
be the eyes turning toward the cold water stimulus.
And we would put in these 50 CCs and wait roughly 90 seconds to 120 seconds and see
that there's no response.
We then come around and do the same thing on the other ear, again, making certain that
the eyes begin at midline.
And we would rapidly install 50 CCs of ice cold water into the ear
and look for any response, waiting for a period of time.
And again, we see none.
Oculovestibular Reflex.
With a normal and intact oculovestibular reflex, when you instill cold water into the otic
canal, you will observe that the eyes turn quickly away from the cold water stimulus,
and then turn slowly toward the cold water stimulus in a repetitive manner.
Therefore, if you instill cold water into the right ear, the fast phase of eye movement
will be toward the left side, and the slow phase of movement will be toward the right
side.
This is a normal physiological response known as nystagmus.
After instilling cold water into the ear, you may need to wait 90 to 120 seconds to
observe for any response.
For patients in which there is a lack of cortical response based on earlier tests, the brainstem
may still be intact.
When you instill cold water into the otic canal, if the brain stem is still intact,
you will observe the eyes turn slowly toward the cold water stimulus.
Therefore, if you instill cold water into the right ear, the eyes will turn slowly toward
the right side.
After instilling cold water into the ear, you may need to wait 90 to 120 seconds to
observe for any response.
If the brain stem of the patient is not intact, when you instill cold water into the otic
canal, you will observe that the eyes will not turn toward the cold water stimulus.
Therefore, if you instill cold water into the right ear, the eyes will stay mid-position.
After instilling cold water into the ear, you may need to wait 90 to 120 seconds to
observe for any response before concluding that there is no response.
Remember that cerumen in the otic canal will prevent the cold water from reaching the tympanic
membrane, and thus, you will not be able to assess this reflex.