Loss of osmotic thirst in multiple system atrophy: Association with sinoaortic baroreceptor deafferentation

We evaluated plasma osmolality (pOsm), thirst, and vasopressin response to hypertonic saline infusion in 14 patients with multiple system atrophy (MSA). This disease is characterized by the degeneration of noradrenergic neurons in the central nervous system and severe orthostatic hypotension. Seven patients were also characterized by the lack of vasopressin response to hypotension (group B) and seven by a preserved response (group A). In group A pOsm rose from 290 ± 2 to 312 ± 6 mosmol/kgH₂O, vasopressin from 0.9 ± 0.3 to 5.7 ± 0.5 pmol/l, and thirst from 1.1 ± 0.1 to 8.7 ± 1.1 cm on the visual analog scale. After saline, patients drank 1,215 ± 150 ml of water (no different from healthy controls). In group B patients' pOsm rose from 296 ± 3 to 325 ± 6 mosmol/kgH₂O and vasopressin from 1.2 ± 0.1 to 19.6 ± 0.4 pmol/l (P <0.01 vs. group A and controls). Group B patients had no thirst during saline and drank little after the challenge (175 ± 50 ml; P <0.01 vs. group A and control). Forced drinking decreased vasopressin in patients before changes in pOsm, showing that inhibitory afferents from oropharyngeal mucosa were intact. In MSA patients with altered afferent control of vasopressin there is a dissociation between the osmotic control of thirst and the osmotic control of vasopressin.