Subjective visual vertical in Pisa syndrome

Parkinson's Disease (PD) alters perception and somatosensory information integration, including visual dependency and judgment of body position in space. PD may be associated with Pisa syndrome (PS), a lateral deviation of the longitudinal body axis (LBA) of unknown origin. We tested whether this inclination is associated with an altered perception of the subjective visual vertical (SVV) and if these alterations are secondary effects of the LBA deviation or of a primary perceptual dysfunction. Furthermore, we investigated the contribution of different sensory modalities and dopaminergic medication.

Seventeen PD patients (8 with PS, 9 without PS) and 18 healthy controls were tested. The SVV was assessed in a seated, in a lateral horizontal and - in PS patients - in a seated manually rectified position. Frame and moving-stimulus-patterns were used to test visual dependency. In PD and PS patients all trials were conducted in dopaminergic "on" and "off".

When seated, SVV values on PD in "on" and PS in "on" and "off" differed significantly from controls. This difference remained in PS patients after manual rectification in "off". The SVV in a lateral horizontal position was not significantly different between the three groups. When inclined, visual dependency was higher in PD "off" than in controls.

Both PS and PD patients showed SVV deviations compared to healthy controls. These cannot be explained by their intrinsic lateral deviation in PS patients. They must be secondary to either a primary perceptual dysfunction or alterations of internal models of verticality due to re-weighting of perceptual afferences.