Charles Bonnet syndrome and periodic alternating nystagmus: Moving visual hallucinations.

ABSTRACT

OBJECTIVE: To describe and discuss potential mechanisms for modulation of visual hallucinations by nystagmus. METHODS: We present 2 patients with coexistent Charles Bonnet syndrome and periodic alternating nystagmus in the context of acquired visual loss. RESULTS: The combination has given rise to a rare phenomenon visual hallucinations that move in a manner governed by the nystagmus, specifically by the direction and velocity of the slow phase. The perceived modulation of movement is selective for a surface in one case and a landscape in the other but not present for hallucinated individual objects and people separate from the hallucinated background visual scene. CONCLUSIONS: The collision of Charles Bonnet syndrome and periodic alternating nystagmus in these 2 patients has demonstrated that some visual hallucinations can be modulated by, or collaterally with, ocular movements. We propose 2 potential mechanisms based on ocular proprioceptive input from extraocular muscles projecting to either extrastriate processing of visual scene, or to higher-order visual cortical areas involved in analysis of motion signals across the whole visual field.