central nervous system represents a third form of compensation which occurs more gradually. Levels of glucose within the central nervous system are normally lower than the blood, regulated by an incompletely understood transfer process. Chronic hypoglycemia or hyperglycemia seems to result in an increase or decrease in efficiency of transfer to maintain CNS levels of glucose within an optimal range. In both young and old individuals, the brain may habituate to low glucose levels with a reduction of noticeable symptoms, sometimes despite neuroglycopenic impairment. In insulin-dependent diabetic patients this phenomenon is termed hypoglycemia unawareness and is a significant clinical problem when improved glycemic control is attempted. Another aspect of this phenomenon occurs in type I glycogenosis, when chronic hypoglycemia before diagnosis may be better tolerated than acute hypoglycemia after treatment is underway. Neuroglycopenia without hypoglycemia A rare metabolic disease of the blood-brain glucose transport system has been described in which severe neuroglycopenic effects occurred despite normal blood glucose levels. Low levels of glucose were discovered in the cerebrospinal fluid (CSF), a condition referred to as hypoglycorrhachia [or hypoglycorrhacia]. Hypoglycorrhachia is associated with Glucose transporter type 1 GLUT1 deficiency syndrome (De Vivo disease). Perhaps a much more common example of the same phenomenon occurs in the people with poorly controlled type 1 diabetes who develop symptoms of hypoglycemia at levels of blood glucose which are normal for most people. References Neurological disorders Medical emergencies