It is perhaps significant that many of the instances in which gross enlargement of cerebral ventricles is compatible with normal life are cases where the condition develops slowly. Gross surgical lesions in rat brains are known to inflict severe functional disruption, but if the same damage is done bit by bit over a long period of time, the dysfunction can be minimal. Just as the rat brains appear to cope with a stepwise reduction of available hardware, so too do the human brains in some cases of hydrocephalus.
...
A group of researchers based at the New York University Medical Center has assembled a picture of the histological changes associated with hydrocephalus through experimental induction of the condition in cats. The group also observed the changes in tissue structure following the implantation of a shunt, the experimental equivalent to the normal treatment of hydrocephalus in humans. Speaking for the group, Fred Epstein says the following: "Hydrocephalus is principally a disease of the white matter. As the ventricles enlarge the layers of fibres above them begin to be stretched and very quickly they are disrupted, with the axons and the myelin sheaths surrounding them breaking down. Even in severe and extended hydrocephalus, however, the nerve cells in the gray matter were remarkably spared, though eventually there began to be a loss here too." The sparing of the gray matter even in severe hydrocephalus could go some way to explaining the remarkable retention of many normal functions in severely affected individuals.
Crucial to the approach to treatment of hydrocephalus is the brain's ability to recuperate following the release of fluid pressure when a shunt is implanted. One of the canons of neurobiology is that, once damaged, cells in the central nervous system are unable to repair themselves. Does Lorber's work dent this hallowed concept too? "When you implant a shunt in a young hydrocephalic child you often see complete restoration of overall brain structure, even in cases where initially there is no detectable mantle,"claims Lorber. "There must be true regeneration of brain substance in some sense, but I'm not necessarily saying that nerve cells regenerate,"he says cautiously; "I don't think anyone knows fully about that."
What, then, is happening when a hydrocephalic brain rebounds from being a thin layer lining a fluid-filled cranium to become an apparently normal structure when released from hydrostatic pressure? According to Epstein and on the basis of his colleagues' observations on experimental cats, the term rebound aptly describes the reconstitution process, with stretched fibres shortening, thus diminishing the previously expanded ventricular space. Within a short time scar tissue forms, constructed from the glial cells that pack between the nerve cells. "The reconstitution of the mantle,"report Epstein and his colleagues, "does not result in the reformation of lost elements, but rather in the formation of aglial scar and possibly a return to function of the remaining elements."
