Question
I would appreciate your insight regarding when a true caloric perversion exists. I have seen several instances where a vertical component is present while performing air calorics using videonystagmography. I understand this to be quite rare and am perplexed by how many instances I have been finding recently. In one instance, the vertical nystagmus was present and persistent where there was no horizontal component, which seemed a clear case of perversion. In other cases I've seen a vertical component which matches the degree of the horizontal component in the tracings and changes from upbeat to downbeat nystagmus when the temperature changes. The eyes appeared to beat in an oblique fashion on the video. In other cases the vertical component is present in the caloric tracings and looks like rotary nystagmus on video. Eye blinks have been ruled out for all of these cases. Are there any test administration errors that might be leading to these results?
Answer
You are correct that a true caloric perversion is quite rare. When this phenomenon does occur, it is thought to suggest an abnormality in the vestibular nuclei or commissural pathway connecting the two vestibular nuclei. Previous studies on the subject have conflicting conclusions. Toupet and Pialoux (1981) found the majority of patients with vertical caloric nystagmus to have "pure central disease", while Elidan, Gay and Lev (1985) found no correlation between the two (vertical caloric nystagmus and central disease).
Historically, calibration error, or pre-existing vertical eye movement, have been suspected as the primary culprits for explaining vertical responses to horizontal canal stimulation (i.e. routine caloric testing), but since you visualized the vertical component, calibration error and pre-existing vertical movement can be ruled out.
There are only two administration errors I can think of that might cause this: 1. If the patient is not at a 30-degree angle, you might be stimulating more than just the horizontal canal. The direction of nystagmus will change if head position is changed significantly after irrigation. 2. If your stimulus is too strong (either temperature or duration) you might be stimulating the anterior and/or posterior canal as well as the horizontal canal, and the nystagmus pattern will vary accordingly.
I think a quote from one of the vestibular pioneers, Dr. Robert Baloh, may be helpful here, "One must be very cautious not to over-interpret so called "central ENG signs" particularly if they are not associated with clinical neurological symptoms or signs."
Alan Desmond, Au.D is the director of Blue Ridge Hearing and Balance Clinic in Bluefield and Princeton, WV. Thieme Medical Publishers released his book Vestibular Function: Evaluation and Treatment in April 2004. He can be reached at BRHBC@comcast.net