Life without a cerebellum.

ischaemic attacks or carotid disease was all very well, but did not answer the more relevant question: do patients with severe carotid disease and limb shaking transient ischaemic attacks have more impaired cerebrovascular reactivity than similar patients with 'con-ventional' transient ischaemic attacks (i.e. loss of function such as paralysis contralateral to severe carotid disease)? In this issue of Brain we now have that empirical evidence, from the well-known cerebrovascular group in Utrecht, for what we had anticipated from the clinical characteristics of the attacks (Persoon et al., 2010). First, the attacks are mostly short, lasting a few minutes, presumably because they can often be aborted by stopping the precipitating activity, even perhaps by just sitting down, or the systemic blood pressure rises spontaneously. Second, as far as the authors could tell, the attacks are usu-ally—but not always—precipitated by something that might lower the systemic blood pressure. Third, patients with limb shaking are more dependent on collateral blood supply distal to the severe carotid disease. And fourth, they have more impaired cere-brovascular reactivity. Of interest is that the patients with limb shaking were more often hypertensive, which does make one wonder whether sometimes their treatment might have been an unrecognized precipitant of the attacks. So in some patients at least, transient ischaemic attacks are due to focal low flow rather than emboli, and so maybe in the fullness of time we will know whether it is these 'low flow' patients—and not just any patient with carotid occlusion—who benefit from extra-to intracranial bypass surgery. In the meantime, I wish we could get our geographical analogy right. The watershed is the high area of land between two river basins; water drains 'from' the watershed, not towards it. It is not therefore an appropriate adjective to describe the potentially low cerebral perfusion and ischaemia between the areas of supply of adjacent cerebral arteries. Unfortunately, to confuse the situation, in North American usage the watershed is the river basin itself, but even that analogy is not right for the ischaemia that may exist where the area of supply of one artery meets the area of supply of an adjacent artery. The boundary zone, between the areas of supply from two cerebral arteries, describes better what we mean. Reference Persoon S, et al. Limb-shaking TIAs in patients with internal carotid artery occlusion: a case-control study. The human cerebellum is reported to contain 85 billion neurons, around …