A 33-year-old woman and a 26-year-old man, who developed horizontal, vertical and oblique nystagmus that changed its direction only under Frenzelglasses, were evaluated by neurological and neuro-otological examinations.
The results were as follows:
(1) The alternating nystagmus was detected only through the Frenzelglasses. It could not be detected under conditions of light, dark with eyes open or with eyes closed. It was elicited by photostimulation of the electric light of the Frenzelglasses.
(2) The nystagmus changed its direction irregularly. In the first patient the main nystagmus was upward vertical and oblique, and in the second it was horizontal.
(3) They suffered from sudden onset of rotatory vertigo for several days or two weeks without tinnitus or impaired hearing.
(4) The righting reflex, Fukuda's stepping test, eye tracking test and optokinetic nystagmus were all in normal range. Caloric nystagmus revealed normal and leftdirectional predominance, respectively.
A similar case report has not been yet reported in the literature and etiology of this nystagmus is unknown, but we concluded the following.
(1) The brain-stem may be responsible for this nystagmus, particularly the midbrain was suspected to be the region of disorder in the first patient and the pons was in the second.
(2) This may be a reversible disorder because it was a short-term illness and organic change or wide-spread lesion could be ruled out.
(3) The most important point of this peculiar nystagmus was that it was elicited by photostimulation under the condition in which the eye fixation was impossible.
