[Inferior oblique underaction].

We investigated the etiology and incidence of 100 cases of inferior oblique underaction. The difference between congenital and acquired underaction was discussed using Awaya's New Cyclo Tests (NCT) to examine cyclodeviation qualitatively and quantitatively. Inferior oblique underaction had been considered more frequently congenital than acquired. However, 63.0% of our cases were proved to be acquired. The congenital cases were Brown's syndrome and cases of unknown causes and the acquired cases were mainly of myasthenic or traumatic origin. The results of the NCT examination were as follows: incyclodeviation or no cyclodeviation was encountered in all cases of congenital underaction except for one postoperative case, but excyclodeviation rather than incyclodeviation occurred in cases of acquired underaction, which showed ocular torticollis to the side of the paretic eye. This result suggested that some other muscles were involved in acquired cases of seemingly isolated inferior oblique underaction, especially in myasthenia gravis. A diagnosis of myasthenia gravis should be considered first when an acquired inferior oblique underaction occurs with association of excyclodeviation demonstrated by the NCT as well as Cogan's lid twitch sign.