["Paradoxical" herniation after decompressive trephining].

The intracranial space is divided into two large compartments by the tentorium. The hydrostatic pressure of spinal fluid is responsible for buoyancy of the brain within these compartments. In patients with craniectomy this equilibrium is exposed to atmospheric pressure. We report on four cases of reversible herniation after either bilateral or unilateral decompressive craniectomy performed for increased intracranial pressure (ICP) and failure of conservative ICP treatment. All four patients had survived a severe neurological disease (encephalitis, subdural haematoma, stroke) which required craniectomy to control raised ICP. All were successfully weaned from the ventilator and awake and CT scans showed no space-occupying lesion anymore. The patients showed a typical "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 either diagnostic or presumed therapeutic lumbar puncture. One patient developed herniation a second time while in the typical 30 degrees upright position. After craniectomy, transtentorial herniation is possible even in the absence of increased ICP. It is related to a negative gradient between atmospheric and intracranial pressure, which is enhanced by changes in the CSF compartment following lumbar puncture. Lumbar puncture should be avoided if possible and, when necessary, only be performed in the head-down position. Acute therapy in these cases is quite simple; it requires flat or even head-down positioning and early cranioplasty.