Duane's Retraction Syndrome (DRS) is a neurogenic brainstem ocular motor dysfunction so named because of the retraction of the globe (caused by synchronous contraction of the horizontal recti) and narrowing of the lid fissure that occur on attempted adduction. DRS may be bilateral (15 - 20% of cases).

By using EMG examination, this syndrome can be classified into three types (Huber, 1974, Brit. J. Ophthalmol., 58: 293-300): I) Palsy of abduction with retraction on adduction. EMG shows absence of electrical activity in the lateral rectus muscle on abduction but paradoxical electrical activity on adduction. II) Palsy of adduction with retraction, and intact abduction. EMG reveals electrical activity with contraction of the lateral rectus muscle on both abduction and adduction. III) Palsy of adduction and abduction, with retraction on attempted adduction. The EMG demonstrates co-contraction of the horizontal recti on both adduction and abduction.

Because the EMG is not easily applicable to paediatric patients, other instrumental examinations like eye movements recording have been proposed. As shown by Metz (1975, Am. J. Ophthalmol., 80: 901-906), in type I DRS the average abduction saccadic velocity is usually quite slow, while the average adduction velocity is only moderately reduced. In type II DRS average abduction saccades are generally normal in speed, but adduction saccadic velocity is markedly subnormal. The type III DRS has both slowed saccades medially and laterally. Gourdeau (1981, Arch. Ophthalmol., 99: 1809-1810) recorded slowed adduction saccades in the sound eyes in all the considered 5 cases of monolateral DRS, as well as asymmetric vestibulo-ocular reflex, optokinetic nystagmus, and after-nystagmus. Previously, Nemet and Ron (1978, Brit. J. Ophthalmol., 62: 528-532) found that the motility of the sound eye was affected in all their 10 patients. On the contrary, Metz (1982, Arch. Ophthalmol., 100: 843-844) found normal saccadic velocities in the sound eye of 28 DRS patients.

In our study we measured the electromyographic firing patterns of the horizontal recti of the normal (right) eye, in one case (16-year-old boy) of monolateral type II DRS with alterations of the saccadic velocities in both eyes.

Saccadic eye movement recordings of both eyes revealed an increased slope of the Duration-Amplitude relation in adducting saccades. The linear regression fit for the left eye was D = 10.9 * A + 35.6 ms for rightward saccades and D = -6.8 * A + 38.6 ms for leftward saccades while for the right eye it was D = 1.9 * A + 31.3 ms for rightward saccades and D = -4.4 * A + 28.8 ms for leftward saccades. Also the Peak Velocity-Amplitude relation showed significant differences between abducting and adducting saccades with remarkable slow down of the adducting movements. We used the best-fit with the non-linear relation Vp = 1/(a + b/A) to quantify these differences. For abducting saccades 1/a was 523 and 1045 and 1/b was 40 and 64 respectively in the left and in the right eye; for adducting saccades 1/a was 388 and 426 and 1/b was 77 and 84 respectively in the left and in the right eye. The EMG of the horizontal recti of the right eye showed, as present in the affected left eye, a paradoxical activity of the lateral rectus muscle during adduction.

Our results about the saccadic characteristics of the normal eye in one DRS patient show a bilateral saccadic velocity alteration. It is in accordance with Nemet and Ron (1978) and partially with Gourdeau (1981), who found slower saccades although still within the normal range. In our case, detecting the saccadic alterations in a boy where an extraocular muscles EMG seemed to be possible to execute, the EMG of both eyes showed a bilateral co-contraction of lateral rectus during adduction, being this one a case of II type DRS. We agree with Gourdeau (1981) that DRS may possibly relate to a disturbance not only of the sixth nerve and its nucleus but also to a disturbance of the horizontal-gaze system controlled through the internuclear projections of the abducens nucleus via the medial longitudinal fasciculus to the contralateral oculomotor complex and that the other alterations of eye movements he detected suggest again that premotor structures in the brainstem may be involved in DRS. Our case demonstrates that clinical evidence of monolateral DRS can be inadequate for a correct diagnosis, being indispensable the EMG of the normal eye at least in the cases with bilateral saccadic alterations, before speculations about the possible origin of the syndrome.

In conclusion, EMG revealed a bilateral type II DRS in a patient with an apparently normal right eye, but where dynamic alterations were present during the adducting saccades.

Acknowledgement: work partially supported by the University of Trieste, Italy (MURST 60%).