Imaging supraspinal locomotor control in balance disorders.

Patients with neurological gait disorders often present to their doctor with the key symptoms of dizziness and gait unsteadiness (e.g. cerebellar ataxia, progressive supranuclear palsy). In vestibular syndromes, on the other hand, the gait disturbance is a leading sign and many aspects of the syndrome can be recognized from the analysis of posture and gait (e.g. direction of falls). For therapy in particular it is important to better understand the physiological control of posture and gait to adapt rehabilitation programs. We recently succeeded in visualizing the hierarchic network for postural control in humans by means of functional imaging techniques. Growing evidence suggests that so-called "locomotor regions", groups of neurons able to initiate or modulate spinal stepping in the cat in response to electrical or chemical stimulation, also exist in humans. The most important locomotor regions are the mesencephalic, the subthalamic, and the cerebellar locomotor regions. Locomotor signals are transmitted from the midbrain to the spinal cord via the ponto-medullary reticular formation and integrate multisensory input at different levels. Functional imaging also demonstrated that the multisensory cortical areas are inhibited during locomotion, which is relevant for physical therapy of vestibular disorders which therefore should include exercises with different gait patterns and different speeds. The supraspinal network for locomotion is just beginning to be recognized as an important factor in the pathophysiology of common gait disorders. In Parkinson's disease, for example, low-frequency stimulation of the mesencephalic locomotor region (pedunculopontine nucleus) is already used to treat freezing and gait disturbance in selected patients. In this review we summarize different attempts to visualize human supraspinal locomotor control using functional neuroimaging techniques, both in healthy subjects and in patients suffering from balance disorders.