You are seeing a patient with suspected vestibular neuritis. The resident is having trouble with the physical examination. Will localization help her comprehend the bedside findings?
We understand neurolgical disease by localizing it. We can often localize vertigo, though it is more difficult for 3 reasons:
– the signal is tonic (there is always a basal tone which can then go up or down)
– it is bilateral
– Its sensory representation is space, which is not just within the body but includes the external world.
To localize vertigo, one only needs to know 2 physiological principles:
1) A head turn activates the ipsilateral labyrinth and suppresses the contralateral labyrinth.
2) The vestibulo-ocular reflex (VOR) makes the eyes move opposite the head turn.
Now let’s localize this patient’s vertigo.
Attending: “What direction is the spinning?”
Patient: “It is spinning to the left”
Attending (to the resident): “Now turn your head to figure out what induces a perception of ‘spinning to the left.’ (resident turns head to the right). Correct. A right head turn does this. So the patient has a more active right ear than left ear. Thus, the left ear is the suppressed ear. Let’s confirm that at the bedside with physical findings.”
Attending: “If the left ear is pathological, where will we see nystagmus, on right or left gaze?”
Resident: “Well the mnemonic of eyes to the ice would say left? Wait, no, to the right?”
Attending: “Forget about the mnemonic right now and use the head turn again. The VOR makes the eyes move opposite the head turn. So the eyes want to go to the pathologic side. When they look away from that you get nystagmus as the eyes try to overcome the pull, and then fatigue. So where do you see nystagmus?”
Resident: “When the eyes look away from the pull. So you get right beating nystagmus on right gaze.”
Attending: “Exactly. Nystagmus is the conscious mind overcoming the pull. We use nystagmus in cold calorics to confirm consciousness.”
Resident: (after testing the patient) “But there is no nystagmus either side.”
Attending: Ah, but you just got another localizing finding.
Resident: The hard blinking?
Attending: Yes, that is gaze aversion. It tells you the same thing nystagmus tells you. The eye doesn’t want to look that way. You can even ask the patient which side is harder to look at. That picks up subtler deficits.
Resident: So the lesson here is that the VOR makes the ear want to go to the pathologic side.
Attending: Exactly. The fast phase of nystagmus and gaze aversion on the right mean that the eye wanted to go to the left. Now we can confirm left pathology through one more test.
Resident: Head impulse test?
Attending: Exactly (this is performed and is equivocal to the left, normal to the right)
Resident: So do we need to get an MRI?
Attending: No. The studies you are thinking of were done on a select population with severe deficits. The patients were so sick they needed to consult neuro. This patient has a very mild presentation and we would not expect a definitive catch-up saccade. Future studies will catch up with this concept.
Take home points:
-You will forget these rules, but do not forget that turning your head to one side activates that ear and inactivates the opposite ear
-The VOR pulls the ear to the less active (pathologic) side, so the eye has trouble looking opposite that (fast phase nystagmus, gaze aversion on looking away from pathologic side)
The world spins toward the pathologic side
Head impulse test is positive to the pathologic side