Box 1 The waterlogged human brain

Water intoxication (WI). WI is an older term referring to the disorientation and loss of consciousness (often with generalized seizure) that develops in response to excessive water drinking. WI develops when plasma osmolality acutely drops by 20 mOsm or more from a normal level of ~ 288 mOsm [13, 18, 15]. In each of the situations described below, undiagnosed SIADH in the individual is often a major contributor to the inadequate diuresis that results in dilute plasma and CSF. While hyponatremia invariably results, excess water is the more immediate concern because of brain swelling and associated hyperexcitability (“An Acute Drop in Osmolality Increases Excitability of the Live Brain Slice” section). The following are three causes of WI:

Forced water drinking. Each year, clinical case studies are published of individuals who imbibe excess water. The perception by the lay public is that such behavior is innocuous. The motivation includes carrying out a stunt or entering a contest for some reward [13]. A more common victim is the misbehaving child who is forced to drink excessive water as a punishment, often with lethal results [64]

Compulsive water drinking (polydipsia). Numerous studies have described compulsive water drinking by schizophrenic individuals. The institutionalized patient behaves normally in the morning, but with free access to a water fountain, becomes disoriented and may suffer a generalized seizure by the afternoon [13]

Overhydration in endurance sports. While it may seem conspiratory, hospitalizations of marathoners and triathletes were rare prior to the establishment of the sports drink industry in the 1970s [14]. Their promotion of imbibing fluids both before and during endurance races (supposedly to replace sodium lost from sweating) has resulted in death for numerous athletes. These victims were not hyponatremic. Rather, their symptoms were those of overhydration specifically: headache (uncommon with severe dehydration), bloating and swollen extremities, nausea and vomiting (from increased intracerebral pressure), generalized seizure [14]. Note that seizure is not the result of increased intracerebral pressure or brain “irritation” caused by expansion, but rather by elevated neuronal excitability as described in “An Acute Drop in Osmolality Increases Excitability of the Live Brain Slice” section [13]. For the past 15 years, the simple advice to endurance athletes has been the following: do not water load, drink when you are thirsty