Ask the patient to stand with their feet together (touching each other). Then ask the patient to close their eyes. Remain close at hand in case the patient begins to sway or fall.
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With the eyes open, three sensory systems provide input to the cerebellum to maintain truncal stability. These are vision, proprioception, and vestibular sense. If there is a mild lesion in the vestibular or proprioception systems, the patient is usually able to compensate with the eyes open. When the patient closes their eyes, however, visual input is removed and instability can be brought out. If there is a more severe proprioceptive or vestibular lesion, or if there is a midline cerebellar lesion causing truncal instability, the patient will be unable to maintain this position even with their eyes open. Note that instability can also be seen with lesions in other parts of the nervous system such as the upper or lower motor neurons or the basal ganglia, so these should be tested for separately in other parts of the exam.