Abnormalities of the spinal meninges in anencephalic fetuses.

The spinal cord and meninges were examined, at all levels, in seven anencephalic fetuses of between 19 and 24 weeks of gestation. Four of the seven anencephalics showed dense collections of cells resembling immature neural tissue throughout the spinal subarachnoid space. The leptomeninges were excessively vascular and contained many active macrophages and small round cells in all the anencephalics and the cord was more slender than normal. Control fetuses did not show these features, although inactive macrophages were seen occasionally. It is concluded that the abnormal collections are made up of heterotopic neural tissue which may have been dislodged downwards from the skull during earlier development and that the spinal leptomeninges show an inflammatory response. The significance of both the heterotopia and the phagocytic activity in these anencephalic fetuses is discussed. In particular, the findings are compared with those in patients with the "respirator brain" syndrome, in whom displacement of brain tissue is associated with raised intracranial pressure.