migraine headaches

just came across this on another list i haunt:
From
http://www.imigraine.net/other/meta.html

10.5: Dialysis
Headache occurs with varying degrees of severity during dialysis in about 70 percent of dialyzed patients (Raskin, 1988). The most common form of "dialysis headache" is the precipitation of migraine headaches in those with preexisting migraine. Other patients without previous headache disorders experience headache only in association with dialysis, usually during the third or fourth hour. It is usually reported as bilateral and throbbing without focal neurologic symptoms (Bana et al, 1972). Headaches may disappear after nephrectomy or after a successful transplant. Later, if the transplant is subsequently rejected, headaches may reappear as a symptom of rejection continuing until nephrectomy is carried out (Graham, 1976). There is some evidence that arterial renin and 18-hydroxy-11-deoxycorticosterone levels, obtained during hemodialysis, are lower in patients who are subject to headache (Graham, 1976; Bana et al, 1972; Bana and Graham, 1978). However, the preponderance of evidence points to shifts of water into the brain as the major cause of "disequilibrium," the term denoting a conglomeration of symptoms, including headache, that occur during or after dialysis (Arieff et al, 1978).
Quoting from Raskin (1988), "The mechanism of these symptoms was originally believed to be the lag in the reduction of brain urea compared to blood urea because of the influence of the blood-brain barrier. This effect was thought to produce an osmotic gradient between blood and brain, resulting in movement of water into brain, leading to cerebral edema, increased intracranial pressure, and symptoms of encephalopathy. This hypothesis became known as the reverse urea effect; the unequal effects of dialysis upon tissue and blood solutes, resulting in an osmotic gradient, underlies the naming of this disorder the disequilibrium syndrome. Experimental models of disequilibrium support the hypothesis that osmotically active substances are ("idiogenic osmoles") are present in brain in the dialyzed uremic animal (and not in the dialyzed nonuremic animal), creating an osmotic gradient between brain and blood that results in shifts of water into brain (Raskin and Fishman, 1976). The nature of the idiogenic osmoles is not clear, but they must play a role because the amount of urea transiently retained in the brain is not sufficient to account for cerebral edema, nor are alterations in brain or plasma sodium likely to underlie disequilibrium. The marked lowering in cerebrospinal fluid (CSF) pH that occurs in dialyzed uremic animals may reflect the cerebral accumulation of organic acids, and such osmotically active solutes may also be important to the genesis of the neurologic disturbances that occur associated with hemodialysis."