Air filling of the subarachnoid space over the cerebral convexities at repeat encephalography

In order to check the constancy of convexity block, the airencephalograms (AEG) of seventeen patients who had undergone two separate examinations were reexamined. A satisfactory filling by air of the basal cisterns or Sylvian fissures was a prerequisite for including the AEG in this series. Consistent results related to air filling of the parietal region were obtained at the two examinations except in one patient with clinical signs of a pontine neoplasm. When the AEG was repeated in a very short time and with unchanged clinical conditions, the degree of air filling over the convexity was practically identical. But, with progressive neurological signs, the parietal air blocks showed a slight increase and also extended to the frontal region. It was concluded, firstly, that upper convexity block is a reproducible finding; secondly, that it is related to organic brain disease, the extent of the convexity block being correlated to the degree of clinical deterioration. As judged from other studies using isotope cisternography, the subarachnoid space is usually not complelely obstructed. It is suggested that the organically changed brain and leptomeninges create special physical conditions for the appearance of convexity air block in the AEG.