Spontaneous Nystagmus Violating the Alexander's Law: Neural Substrates and Mechanisms.

Alexander's law states that spontaneous nystagmus increases when looking in the direction of fast-phase and decreases during gaze in slow-phase direction. Disobedience to Alexander's law is occasionally observed in central nystagmus, but the underlying neural circuit mechanisms are poorly understood. In a retrospective analysis of 2,652 patients with posterior circulations stroke, we found a violation of Alexander's law in one or both directions of lateral gaze in 17 patients with lesions of unilateral lateral medulla affecting the vestibular nucleus. Patients with vestibular neuritis served as a control. When Alexander's law is violated, the time constant (Tc) was larger than that in the controls (median [interquartile range, IQR]: 14.4s [6.4-38.9] vs 9.0s [IQR 5.5-12.6], p = 0.036) while the Tc did not differ between the groups when Alexander' law is obeyed (9.6s [3.6-16.1] vs 9.0s [5.5-12.6], p = 0.924). To test the study hypothesis that an unstable neural integrator may generate nystagmus violating Alexander's law, we utilized the gaze-holding neural integrator model incorporating brainstem leaky neural integrator and negative velocity feedback loop via the cerebellum. The lesion-induced changes included false rotational cue, primarily attributed to central vestibular imbalance, and unstable neural integrator, examined in two ways: hyperexcitable brainstem neural integrator and paradoxical excitatory effect of Purkinje cells. With normal integrator function, the false rotational cue generated nystagmus consistent with Alexander's law. However, both types of unstable neural integrators tested produced nystagmus that violated Alexander's law. We propose that when the neural integrator is unstable with lesions in the brainstem neural integrator itself or the neural synapse between Purkinje cells and the brainstem vestibular nucleus, nystagmus violates Alexander's law. The spontaneous nystagmus violating Alexander's law may be the useful clue for identifying central vestibular syndrome.