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I enjoyed Pieh and Gottlob's article1 pointing out the association of a Chiari malformation with a “unique” form of nystagmus that they call “the nystagmus of skew.” The distinctive feature of this nystagmus is a disjunctive vertical oscillation in which the fast phase of one eye moves upward while, at the same time, the other eye moves downward.
The authors1 state that their second patient had a “rotatory component” by which I assume they mean torsional; this pattern of nystagmus is already established in the literature and is known as “jerk-waveform see-saw nystagmus”.2 3 In their first patient they point out that the amplitude of the vertical nystagmus was so small that they were unable to confidently exclude a torsional component. It would have been most interesting to obtain recordings looking for a torsional component using the modified scleral search coil technique4; I suspect that it would have shown a torsional component and that this patient also had jerk-waveform see-saw nystagmus.
Jerk-waveform see-saw nystagmus occurs with unilateral mesodiencephalic lesions, presumed due to selective unilateral inactivation of the torsional eye velocity integrator in the interstitial nucleus of Cajal2; during the fast (jerk) phases the upper poles of both eyes rotate toward the side of the lesion. With lateral medullary injury the fast phases of the torsional component jerk away from the side of the lesion.5 In both situations the torsional component is always conjugate. With mesodiencephalic lesions the vertical component is always disjunctive, but with medullary lesions it may be either conjugate (usually upward) or disjunctive.2
The authors reply:
We thank Lavin for his interesting comments.
We stated in our article that the possibility of a fine see-saw nystagmus could not be excluded. We did re-evaluate our patients with a torsional coil and did not record a torsional component. However, because of the fast improvement in both patients, all the eye movement abnormalities on re-evaluation were minimal. Clinically, even in the stage of maximal abnormalities, in one patient we did not detect any torsional component, which suggests that if there was an element of see-saw nystagmus, it was subclinical.
We did not state that the type of nystagmus associated with the Arnold-Chiari malformation was unique, precisely because we could not rule out with total certainty a see-saw nystagmus, which has been reported in one patient with the malformation. We did, however, point out that this association is unusual.
Because of the lack of strong evidence of a torsional component to the dissociated vertical nystagmus, we preferred the term, kindly suggested by a reviewer, “nystagmus of skew”. This would represent a more inclusive, descriptive term, of which both the pendular and the jerk see-saw nystagmus forms and the dissociated vertical nystagmus without demonstrable torsional component would represent subvariants.
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