Abstract
Purpose: :
Convergence dampening (CD) is often observed in patients with Infantile Nystagmus Syndrome (INS). However, it is not clear why it occurs only in some of the patients and not in others. To better understand the mechanism of the CD, we investigated the relationship between the CD and heterophoria in two groups of patients with and without CD.
Methods: :
Six INS patients were recruited. Eye movements were measured using a video-based eye tracking system (EyeLink 1000 desktop). First, the CD was identified in a near/far viewing paradigm where patients alternatively looked at two targets, one at 30 cm and the other 150 cm from the eyes. The alternate duration was 5 seconds. Then heterophoria for each patient was measured using an alternate cover test in a fixation paradigm where patients fixated on a 0.5 degree target at 150 cm for one minute. The relationship between heterophoria and CD was analyzed with relation to visual acuity, binocular function and degree of clinically observed heterophoria.
Results: :
Of the six patients recruited, five showed horizontal heterophorias and are listed in group A and one patient who did not show any heterophoria was put into group B. In group A, 100% of patients demonstrated an observable change in eye movements, as evidenced by eye movement recordings, with either convergence or divergence. Of these five patients, 3 showed convergence dampening and 2 showed divergence dampening. In contrast, CD was not obvious in group B.
Conclusions: :
The presence of a heterophoria seems to contribute to the CD although other central neural mechanism may be involved.
Keywords: nystagmus • vergence • strabismus