Asymmetric horizontal tropias, DVD, and manifest latent nystagmus: an explanation of dissociated horizontal deviation.

Four patients with dissociated horizontal deviation (DHD) are described and an explanation of their horizontal comitant deviations is offered. The patients had asymmetric comitant horizontal deviations dependent on the fixing eye. All of them had unequal visual acuity (VA) and asymmetric manifest latent nystagmus (MLN) documented with eye movement recordings. The subjects appear to use convergence to reduce their nystagmus when viewing with the eye having more severe nystagmus and hence poorer vision. When patients viewed with the eye having less nystagmus, little or no convergence was exhibited. We suggest that DHD may be the manifestation of an asymmetric nystagmus blockage syndrome (NBS). The dampening effect of convergence on nystagmus has been well documented in the past and seems to apply here. The patients demonstrate that a true, but asymmetric NBS, can be found with MLN.