Isolated cases of alcohol poisoning almost never cause any long term brain damage if survived and certainly none if survived without hospital treatment. This is due to the fact that alcohol indirectly causes death (slower heart rate, respiratory depression, and possibly dehydration or hypoglycemia). Blood oxygenation levels throughout a case a alcohol poisoning play a critical role in determining whether a particular case will be fatal. It seems, however, that deprivation of oxygen has a "critical point" or very small range where the chance and relative amount of brain damage that might occur increases exponentially.
Consider, for instance, a study done to determine the extent of brain damage at different blood oxygen saturation for different durations. It was found that significant brain damage would not occur for three hours at 35 percent of normal blood oxygen saturation. However, It was found that only 30 minutes at 30 percent blood oxygen saturation would cause lasting damage. Furthermore, damage at eight minutes at 25 percent BOS was observed and cardiac arrest would usually occur at levels lower than 20 percent. Now consider alcohol's effect on the respiratory system also seems to be nonlinear, that is, becoming exponentially pronounced with any increase in blood-alcohol content. A person with alcohol poisoning will usually either: 1) reach a level where the rate of alcohol elimination (and its effect on the respiratory system) far outpaces the time required (so to speak) for brain damage to occur at any particular BAC level (also realize that at high BAC levels, elimination is non-linear due to the action of enzymes not usually involved in the process with elimination rates around -.04/hour) or 2) reach a BAC level which depresses the respiratory system to a level where damage (and low oxygen hampers its ability to pump efficiently) to the cardiac muscle occurs, slowing blood flow which, in turn, further lowers blood oxygen levels until the heart stops. Now, of course, dehydration and hypoglycemia may come into play here, but I won't get into the specifics.
Overall, dehydration would only concern me as a possible cause of brain damage if alcohol were used to excess (non stop) for many days by an experienced drinker. Hypoglycemia-induced brain damage would only concern me in cases of alcohol poisoning where the person has not eaten anything for a day or longer, when dealing with someone under the age of 5 years, or when dealing with an alcoholic (or anyone who I might suspect to have a poor diet). The effects of dehydration and hypoglycemia otherwise would not warrant consideration because their contribution to neuronal death in the brain would only be significant at BAC levels much higher than those that would cause death from respiratory arrest. People who survive alcohol poisoning with brain damage are always the ones who only survive due to intensive medical care, and these patients usually remain in coma for a day or more.
Those who are anxious about a case of suspected alcohol poisoning where the victim was not taken to the hospital but survived, should rest assured that these victims never have any neurological impairment (because brain damage only starts to occur when the individual has surpassed a "critical point" where death is always the outcome in victims who do not get urgent medical care) In layman's terms: If you suspect you had alcohol poisoning and you did not go to the hospital, then you have no significant long term damage. So don't worry! If you went to the hospital, speak to your doctor (but damage is unlikely).
I wrote this because I was extremely anxious about a case of alcohol poisoning where I was the victim. This anxiety led me to imagine symptoms that would affect my studies. This continued until I did extensive research as part of an undergrad thesis.
If you suspect alcohol poisoning in a friend, call 911! Those who receive treatment are more likely to survive (usually unharmed if they do).
If the hard logic doesn't convince you, when is the last case that you have heard about (or researched) where a person was released from a hospital with an outcome other than: 1) treated and released the next day, 2) Dead on arrival, or 3) Dead after a week in a coma (or something to that effect). Any other outcome is quite rare.
Chronic alcoholism is another matter! But I will not get into that.