We present a documented observation suggesting the existence of a biased representation of verticality in PD, tilted backward dramatically, which resulted in a severe retropulsion and recurrent falls. A rehabilitation program aimed to modulate verticality perception improved the postural perception of the vertical, trunk posture and balance abilities and reduced retropulsion as well as lastingly fall frequency.

A 68 year-old patient with Parkinson's disease fall backward 3 times a day despite a severe camptocormia. No amyotrophy of posterior trunk muscles neither spinal canal stenosis could explain camptocormia (CT scan). The postural vertical (PV) was tilted backward at −9° (normally for this age −1.2±1.4°). This strongly suggested that retropulsion was the consequence of a backward PV tilt, the patient aligning implicitly his erect posture onto a biased internal model of verticality. This induced a backward disequilibrium and led to recurrent falls. We assume that camptocormia was mainly compensatory. An intensive rehabilitation program was set including during 2 weeks: erectus spinae muscles strengthening, postural exercises in front of a mirror, postural and general exercises. More specifically, on the basis of theoretical arguments (synthesis of graviceptive vestibular and somatosensory information) and at the light of published experimental studies, we also implemented 3 techniques supposed to recalibrate the internal model of verticality, undergone daily: 30° forward tilted posture on a tilt table, walking on a treadmill suspended by a cable (bodyweight support), vibration of tibialis anterior's tendons during dynamic postural tasks. Drugs were kept unchanged during this rehabilitation program. After 2 weeks of intensive training, the results were spectacular: reduction of 35mm in the C7 sagittal arrow, gain of 1 point on the Backward Disequilibrium Scale (BDS) and overall normalization of PV=0.1° (gain 9°). At discharge, the patient was instructed to regularly perform exercises taught. He was interviewed 2 and 6 months later and was very satisfied with a feeling to stand better and a net reduction of fall frequency.

This observation brings a new insight about the nature of some postural disorders in PD, in relation with retropulsion and backward falls, partially compensated by a camptocormia. It suggests the interest of a novel rehabilitation approach in PD showing these particular postural disorders.