Active blockage and rest position nystagmus: electromyographic demonstration of two types of ocular induced head-turn.

A differentiation of two types of head-turns due to nystagmus, by means of electromyography (EMG) is demonstrated in this paper. The first type is represented by patients who actively block the nystagmus, by means of an increase of discharge of the extraocular muscles who are synergistic and responsible for the head-turn. This block has the same features of the block of nystagmus in convergence and usually exceeds 10-15 degrees from the primary position. The second type is made out of patients whose head-turn is explained with the null-position of Kestenbaum. Here the nystagmus simply disappears in the position of head-turn, which usually is of no more than 10-15 degrees. Both types of patients show the same electronystagmographic features in the position of head-turn. This differentiation is useful from the clinical standpoint. In fact, only the first type of head-turn may require, besides a classical Anderson or Kestenbaum procedure, also a posterior fixation suture according to Cüppers. This operation would be useless in the second type of head-turn. Clinical signs useful for differentiating these two types of head-turn are presented as well.