Lumbar pneumoencephalograms, performed before craniotomy in the stereotactic procedure, from patients in the upright and supine positions demonstrate shifts (up to 2.5 mm) of deep-seated intracerebral structures, influenced by gravity. After trephining the burr hole with opening of a free communication of air into the subdural space, dislocation of the brain (up to 2 mm) in the direction of gravity may also be encountered. Thus, for exact localization of the stereotactic target point al-lowances for extracerebral air collections and consequent shifts of deep-seated target structures must be made.

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