PARADOXICAL HERNIATION AFTER DECOMPRESSIVE CRANIECTOMY

The intracranial space is divided into two large compartments by the t entorium. The hydrostatic pressure of spinal fluid is responsible for buoyancy of the brain within these compartments. In patients with cran iectomy this equilibrium is exposed to atmospheric pressure. We report on four cases of reversible herniation after either bilateral or unil ateral decompressive craniectomy performed for increased intracranial pressure (ICP) and failure of conservative ICP treatment. All four pat ients had survived a severe neurological disease (encephalitis, subdur al haematoma, stroke) which required craniectomy to control raised ICP . All were successfully weaned from the ventilator and awake and CT sc ans showed no space-occupying lesion anymore. The patients showed a ty pical,sunken pattern'' at the trepanation site. All patients developed clinical signs of transtentorial herniation (i.e. unilateral dilated pupils,deteriorated alertness,and extensor posturing) shortly after ei ther diagnostic or presumed therapeutic lumbar puncture. One patient d eveloped herniation a second time while in the typical 30 degrees upri ght position. After craniectomy,transtentorial herniation is possible even in the absence of increased ICP. It is related to a negative grad ient between atmospheric and intracranial pressure,which is enhanced b y changes in the CSF compartment following lumbar puncture. Lumbar pun cture should be avoided if possible and, when necessary,only be perfor med in the head-down position. Acute therapy in these cases is quite s imple; it requires flat or even head-down positioning and early cranio plasty.