Attenuated melanopsin-mediated activity in focal dystonia

Focal dystonia (FD) is an extrapyramidal movement disorder, but various disease aspects challenge the view of a condition only involving neuronal motor systems. Here, we were interested in potential changes of retinal functions, accessible by chromatic pupillometry.

Quantitative chromatic pupillometry was performed in 26 persons with FD and 20 age-/sex-matched healthy controls. Upon dark adaptation, standardized red (632 nm) and blue (462 nm) light pulses (100 cd/m²) were delivered to the pupils. Constrictions were recorded one and 6 s after the stimuli. The 1s-constriction is rod/cone-mediated, whereas the post illumination pupil response (PIPR) reflects the melanopsin-related activity of intrinsic photosensitive retinal ganglion cells (ipRGC) to blue light only.

Compared with controls, dystonia patients did not differ in baseline pupil diameter prior to red-light stimulation. Peak transient constriction, indexed by the minimal pupil size during the 1-s light pulse, was similar between groups for both red and blue light. The blue light-specific PIPR was significantly attenuated in patients.

The findings point to altered retinal light sensitivity in FD with a hypofunctional ipRGC system. This suggests an impact of the disease beyond movement-related neuronal systems.