A most useless doctor visit

For the past few years I've noticed that my wrists have been getting sore after extended periods of computer use. Knowing what this could mean, I attempted to cut down my computer use, and started using programs that encouraged me to take regular breaks¹. The soreness became more frequent and came on with shorter periods of use (which is, I might point out, one of many reasons why my posting frequency has dropped precipitously), and then recently I started noticing tingling in my fingers and my mouse hand feeling itchy. So, I decided that enough was enough: it was time to head to the doctor.

The nurse who first saw me had carpal tunnel syndrome herself, and she shared her experiences. Unfortunately, they boiled down to “don't do what I did - I had to have surgery, and still can't use my left hand all that well.” Practical advice on how not to do what she did was rather lacking from her monologue.

So, in came the doctor. He did the usual asking what was wrong, and after my explanation he said, “Well, you've got carpal tunnel syndrome.” He made the diagnosis without ever touching me, or without asking any clarifying questions. Given that after reading online I'd found that there are a few possible conditions that have similar symptoms, I was surprised that he didn't. He then proceeded to explain what causes carpal tunnel syndrome (pressure on a specific nerve; it's always interesting to listen to how doctors dumb down explanations) and that if it got bad enough I might have to have surgery.

Did he bother to say what I needed to do to avoid said surgery? No.

He then asked me whether I used a computer at work, and after I replied with a yes (and that I had a suboptimal work arrangement), he said that that was almost certainly the cause.

Did he bother to ask me if I did other activities (like, say, playing the guitar, or blogging) that might be related? No.

He did, however, go on to talk about how employers are very worried about carpal tunnel syndrome due to the fact that it's the #1 worker's comp injury, and that I should talk to my HR department about getting something ergonomic for my setup (his primary suggestion was, I kid you not, “a gel wrist-pad”). Did he give me any real suggestions on what to do to make my workspace (at home or work) more ergonomic? No. Did he bother talking about any of the range of possible treatments, and discuss with me which would be most appropriate? Other than suggesting that I take off my watch and get a gel wrist-pad, no.

He did ask if my hands tingled when I woke up (which is a symptom of sleeping with bent wrists, an indication that wrist braces may help if worn while sleeping), but then quickly went back to talking about worker's comp.

After a nearly useless conversation, he briskly asked me if I had any other questions, and then shooed me out the door. I learned more in a half hour of web-browsing the night before the appointment than I learned from him, and I'm now left to search the web for more information.

¹ Initially I used Ubuntu's built-in typing break program (found in the “Keyboard” menu, at least for Ubuntu 7.04). While this was an excellent start, it was suboptimal (it didn't have an option for micro-breaks, and it would lock the keyboard without much warning, which was frustrating if I was in the middle of a sentence). I also tried a Google Desktop add-in, but that was even less useful (as it just brought up a popup window, which was easy to ignore if I was busy). Most recently I've started using Workrave, a free Linux and Windows program that appears to be excellent: it has both micro- and macro-breaks, gives plenty of warning when breaks are about to appear (I can finish my sentences now!), and is very customizable.

Even limited exercise helps

One of the exercise mantras commonly floated about is that adults should get 30 minutes of moderate activity exercise at least five days a week. While this is a good goal, the majority of Americans don't get this amount of exercise¹. And, for a lot of beginning exercisers, exercising for 30 minutes a day probably seems like a huge commitment, if not an insurmountable challenge.

My SO and I have long viewed exercise in the "some is better than none" category, and thus try to do even little amounts whenever we can (e.g., walking to the store instead of driving, always taking the stairs at work, doing our own gardening). A recent paper (Church et al., 2007) has shown, via a randomized, controlled trial, that even small amounts of exercise are better than no exercise at all (at least for the subset of people they tested).

Church et al. divided overweight, post-menopausal women into four groups:

• Control: Did no exercise a week (other than normal walking)
  
• 4kcal/kg: Exercised about 70 minutes a week
  
• 8kcal/kg: Exercised about 135 minutes a week
• 12 kcal/kg: Exercised about 190 minutes a week

Participants were randomly assigned to a condition, and all exercise was performed in a lab². The participants exercised at their given level for six months, at which point physiological data were compared to data taken at the start of the study.

Fitness improvement correlated linearly with the amount of exercise:



Figure 3 from Church et al. (2007)."Percent Change in Fitness Data for Each Study Group. The data represent the least-squares means adjusted for age, ethnicity/race, weight, and peak heart rate. The P values for pairwise comparisons of control with 4-kcal/kg, 8-kcal/kg, and 12-kcal/kg per week groups are P .001 for each variable. P for linear trend across groups .001 for each outcome. Error bars indicate 95% confidence intervals."
So, what this means is that even if you work out for only 20 minutes a day three days a week, you'll almost certainly see fitness benefits. And, once you start working out and getting used to doing it regularly, if you increase your workout durations (say, to 40 minutes a day three days a week), you'll see even more improvements. And, to help scare you into exercising, note that the people who didn't do any exercise actually had a small decrease in fitness³.

So, if you're one of the many sedentary Americans out there, take a look at those graphs and start exercising, even if it's only for a few minutes a day. It'll do ya good!

¹ As in "More than 60 percent of U.S. adults do not engage in the recommended amount of activity (30 minutes a day, 5 days a week)" and "Approximately 25 percent of U.S. adults are not active at all." (data from the 1996 Surgeon General's Report on Physical Activity and Health, specifically from here)
² "Women in the exercise groups alternated training sessions on semi-recumbent cycle ergometers and treadmills." The control (non-exercising) women tracked their daily steps with a pedometer and were "asked to maintain their level of activity during the 6-month study period."
³ In fact, it was the realization that our fitness was slowly declining (and would continue declining until we would no longer be able to walk when we were 70) that finally motivated my SO and me to start regularly exercising (and tracking our exercise goals and progress here) a few years ago.

Church, TS., DP Earnest, JS Skinner, and SN Blair. 2007. Effects of Different Doses of Physical Activity on Cardiorespiratory Fitness Among Sedentary, Overweight or Obese Postmenopausal Women With Elevated Blood Pressure. JAMA. 297:2081-2091. Abstract.

In the news

Looks like I was right; CNN has finally decided that 100% fruit juice won't make your kids fat after all.

Carbon dating of chicken remains in South America has provided evidence that Polynesians sailed to the Americas at least a hundred years before the Spanish did. Chickens are not native to the Americas, yet the chicken remains date to AD 1321 to 1407.

And, don't expect widespread Mad Cow testing to occur anytime soon in the US; the Bush administration is fighting to prevent a company from testing all their cows for the disease:

The Bush administration said Tuesday it will fight to keep meatpackers from testing all their animals for mad cow disease .

The Agriculture Department tests less than 1 percent of slaughtered cows for the disease, which can be fatal to humans who eat tainted beef. But Kansas-based Creekstone Farms Premium Beef wants to test all of its cows.

Larger meat companies feared that move because, if Creekstone tested its meat and advertised it as safe, they might have to perform the expensive test, too.

The Agriculture Department regulates the test and argued that widespread testing could lead to a false positive that would harm the meat industry.

A federal judge ruled in March that such tests must be allowed. U.S. District Judge James Robertson noted that Creekstone sought to use the same test the government relies on and said the government didn't have the authority to restrict it.

The ruling was to take effect June 1, but the Agriculture Department said Tuesday it would appeal -- effectively delaying the testing until the court challenge plays out.

Clear!

My mom just got the news a little while ago: where there was a lot of atypical ductal (and lobal) hyperplasia in the excised tissue, there was no cancer. This is about as good as we could have hoped for; there's still a high risk of cancer in the future, but it's a relief to know that there is none present now.

This afternoon we're going to get her an herb and flower garden going, and tonight we'll celebrate with slow-roasted pork ribs and collard greens, followed by an introduction to German-style board games.

So far so good

I'm pleased to report that all has gone well so far here in Colorado. My mom's surgery went about as smoothly as it could, and she was walking around and eating (and relatively pain free) within a few hours of the operation¹. We won't know the tissue test results until Friday at the earliest, but at least so far everything's gone about as well as we could have hoped for.

Unfortunately, however, things are not going quite so well back in California. My SO came home tonight (after a very long day of work) to find that our house was full of hundreds of buzzing, potentially stinging, uninvited arthropod guests. After some investigation (and careful shooing), my SO determined that we have a swarm (or other congregation) of bees in our chimney. As a friend in Oregon said when I told him about this: "So THAT'S where they've all gone!"

Anyone need some bees?

¹ In fact, my mom was feeling good enough today to head around town with me to help buy supplies for all the projects she wants me to do while I'm here. I should emphasize that I'm not complaining: many of those projects entail cooking, which I'm more than happy to do (and which I benefit from as much as she).

An unexpected trip

About two and a half weeks ago my mom had a breast needle biopsy to check an area that had changed suspiciously in two subsequent mammograms. The biopsy came back with a diagnosis of an "atypical ductal hyperplasia."

After talking with our resident pathophysiologist (it's convenient working at a college with a good human anatomy and physiology group), I've learned that atypical ductal hyperplasias fall into a category known as fibrocystic changes, meaning changes in the breast that produce lumps, yet are not full-blown cancerous growths. Some fibrocystic changes are associated with an increased risk of cancer (they're pre-cancerous, if you will), while others aren't. Unfortunately, it looks like atypical hyperplasias are associated with an increased risk of breast cancer in the future, and, based on what my mom has been told, sometimes atypical hyperplasias are even found around cancerous tissue.

My mom had a consultation with a surgeon this past Thursday, and he scheduled her for a full biopsy (essentially a lumpectomy) Tuesday. The quick timeline took us both by surprise, though it seems like a good idea.

My mom lives alone near Denver, and thus I'm going to be flying there on Monday to be with her during and after the procedure. While the operation sounds like it should be routine, I'm planning on staying the entire week just in case (and so I can be there when the tissue test results come in). Being that we've only got two weeks left in the semester, this is not an ideal time to be away from my classes (in fact, it's a crazy time to take a week off). However, I suspect my students can somehow manage without me¹.

From stats that my mom was given, no evidence of cancer is found about 85% of the time atypical hyperplasias are excised. Funny how statistics are not nearly as comforting as one wants them to be.

¹ Though I will be pre-recording my lecture and distributing it online, so my students can still get their daily dose of Prof. Radagast, if they so desire.

Testing for mad cow

I've written about bovine spongiform encephalopathy (BSE; mad cow disease) testing in cattle before. As the disease is extremely hard to detect in cattle (see here), and we don't know how easily it is transmitted to humans, it was thus somewhat surprising to learn that a few months ago the USDA decided to drastically reduce the number of cattle being tested for the disease.

Given that people are worried about the disease, one might expect that private beef companies would step in and start voluntarily testing their cattle before selling it (labeling the resulting beef as "mad-cow free" or some such). You've probably noticed that no such products are on the market.

Why? The USDA has threatened to sue any company that tests for mad cow separately from the government program (most notably Creekstone Farms). Just a few days ago this ridiculous prohibition was stopped by a federal judge, and, assuming that the government doesn't appeal the decision, private companies will now be free to test their own cattle for the disease.

Here's an excerpt from the New York Times article that has a bit more background on the specific case:

Creekstone Farms Premium Beef, a meatpacker based in Arkansas City, Kan., wants to test all of its cows for [mad cow] disease, which can be fatal to humans who eat tainted beef. Larger meat companies feared that move because if Creekstone tested its meat and advertised it as safe, they could be forced to do the expensive test, too.

The Agriculture Department currently regulates the test and administers it to less than 1 percent of slaughtered cows. The department threatened Creekstone with prosecution if it tested all its animals.

U.S. District Judge James Robertson ruled that the government does not have the authority to regulate the test. Robertson put his order on hold until the government can appeal. If the government does not appeal by June 1, he said the ruling would take effect.

...

The Agriculture Department argued that widespread testing could lead to a false positive that would harm the meat industry. Robertson said he was concerned by that possibility but noted that Creekstone sought to use the same test the government relies on.

Not the best of flights

I’m now in New Orleans for Innovations, but didn’t arrive at quite at the time I wanted to. Let me explain why.

My first flight of the day was to Dallas, and while I have a tendency to get motion sick, I’m usually fine on flights until the landing approach. This flight got me motion sick on the takeoff. In fact, the first hour of the flight was moderately miserable thanks to the near constant turbulence. The flight finally smoothed out for the middle half, and I was able to get a bit of work done (for maybe 30 minutes), but then it got turbulent again and was rocky for the entire descent into Dallas.

Normally I just slowly count in my head, clench my abdominal muscles, breathe deeply, and stare out the window during landing; I’m often unhappy, but usually fine. This time, however, that failed. And it failed when we were something like 50 miles up and a few decades away from the airport. So, I looked for an airsickness bag. There wasn’t one in my seat pocket.

A few minutes later we were still a few dozen miles above the ground in gale force winds (or something like that), so I asked my seat neighbor if she had an air sickness bag (note: that’s probably the question you least want to hear from the person sitting next to you on an airplane). She didn’t have one. Her neighbor didn’t have one. Finally they got a flight attendant to come up, and she found one a few rows up. It’s a great feeling knowing that you're about to throw up and that everyone around you knows that you’re about to throw up.

Before this gets too unpleasant, I should make it clear that I never needed the airsickness bag.

However, that didn’t mean all ended well. By the time we had landed I was probably hyperventilating (or something like it), my mouth was bone dry, my skin was clammy, my arms and legs were all extremely tingly, and I had lost virtually all muscular control over my hands, arms, and legs. In fact, as I sat there slowly figuring out that the muscles that control my fingers weren’t responding anymore, I realized that if I did actually throw up there was no way I could actually hold the bag open: my fingers were locked in a pinching position holding the bag by a corner. That was a problem I hadn’t considered before today.

After the plane landed I just sat there curled up with my eyes closed. I hardly even noticed the people getting off the plane. Even after everyone had left I was pretty sure I couldn’t walk, as I was just getting control of my hands back and I felt light headed anytime I even sat up. And my hands were shaking like mad anytime I tried to do anything with them. It was at that point that the flight attendants noticed me just sitting there; they ended up calling the paramedics.

I then got to meet some of the nicest folks in Dallas; the police officer, paramedics, and American Airlines staff were all exceptionally patient and caring. The paramedics helped me walk off the plane, then politely suggested that I not try walking up the sloped ramp to the terminal until they had checked me out. They didn’t find anything wrong (they initially suspected blood-sugar regulation problems, but they ruled that out with a blood sugar test result of 94 (units unknown)). They ended up saying that I should have eaten something for breakfast (and should have had something other than a carbohydrate-laden snack bar and can of soda for lunch), but didn’t posit a physiological mechanism for the symptoms.

The paramedics suggested that I have a “good meal of real food, not junk food” and see how I feel (recommending, of course, that I head to a hospital if I didn’t feel better; they also offered to take me to one right then and there). I agreed that a good meal was likely all I needed, and so I set about figuring out how I could make that happen since my connecting flight was scheduled to leave soon. The paramedics walked me up the ramp, and the American Airlines supervisor who had been hovering around for some time now went to a computer and started checking flights; it turned out that there was only one later flight to New Orleans, and it was already overbooked. My flight was scheduled to leave in less than half an hour, but I really didn’t feel like flying right then, and I think the supervisor got that idea. After a number of calls she finally was able to get me a guaranteed seat on the next flight out; I have no idea how she did it, but I’ll be eternally grateful, as the last thing I wanted to do at that moment was get on another plane.

A good meal and a Dramamine pill prepared me for the next flight, which thankfully was nausea free. I don’t know if it was the meal, the smooth(er) air, the Dramamine, or the Dramamine-induced drowsiness that made the flight fine; I’m just happy to be here on solid, non-moving ground.

So, the question I’m left with is this: what physiologically happened to me during the flight to remove my conscious muscular control? It has happened once before (on a SCUBA boat trip, actually), and it’s quite an odd phenomenon. I’m thinking that lack of blood flow to the extremities seems like a possible mechanism (maybe combined with breathing too rapidly), but once I get back I’ll have to try to dig up some references.

Death by water intake - a look at osmosis and water balance

Many of you have probably already heard about Jennifer Strange, the woman who recently died while attempting to win a free Wii in a water-drinking contest. If you haven't, Orac has two posts that provide lots of background. Death from excessive water intake is not unheard of; for example, long-distance runners can die if they drink too much water in an attempt to stay hydrated (see here). Given water's relatively innocuous reputation, I thought it would be useful to look at a bit of the physiology behind what happens when a person drinks too much water.

Since much of the physiology of water balance depends on osmosis, I'm going to start by reviewing osmosis before getting into the physiology. If you're familiar with osmosis, feel free to skip the osmosis section (quiz to see if you can skip the osmosis section: which direction will water flow if a cell containing a 300 mOsm solution is placed in a beaker with a 150 mOsm solution?¹)

An introduction to osmosis

A key concept to understand when looking at water intake (and water balance) in organisms is osmosis. Osmosis is the tendency of water to move so that it ends up in equal concentrations on both sides of a semipermeable membrane. As an example, if you have a cell full of saltwater that's surrounded with pure water on the outside, the concentration of water inside the cell will be lower than that of the water outside (the salt inside the cell can be thought of as diluting the water), and thus water will move by osmosis from outside the cell to inside the cell. If you were to flip the concentrations (put pure water inside a cell that's surrounded with salty water), water would move the opposite direction (water would leave the cell). While osmosis may sound complicated, it's nothing more than the diffusion of water across a semipermeable membrane.

We can measure the likelihood that water will move into a solution via osmosis; we call this a solution's osmolarity (or osmotic pressure; units are mOsm). The osmolarity of a pure water solution is defined as having an osmolarity of 0 mOsm; the osmolarity of a solution is increased by adding solutes (salt, amino acids, glucose, or whatever else you want) to it. The higher the osmolarity of a solution, the more likely water is to osmose into it. So, to return to the salt and pure water example above, the pure water solution outside the cell has an osmolarity of 0 mOsm, while the salt solution inside the cell has a higher osmolarity (let's call it 100 mOsm, but at the least we know it's greater than 0), and thus water will move from the lower to the higher osmolarity solution (water will move into the cell).

We use osmosis in our bodies for many things. Osmotic gradients lead to water flowing into our bodies through tiny gaps between the cells of our small intestines (our guts are regulated so that the osmolarity of the gut contents is lower than that of the surrounding cells and interstitial spaces); this bulk flow of water brings along with it digested nutrients, and this is one of the mechanisms of nutrient absorption in our gut. Our kidneys use osmotic gradients to concentrate (or dilute) urine as it's being produced; to make concentrated urine the kidneys pass "pre urine" through an area of high osmotic pressure (high salt and urea concentration), thus causing water to osmose out of the urine (making the urine more concentrated).

Most regions of our body have the same osmolarity (~290 mOsm), and thus osmosis doesn't normally cause a net movement of water into or out of them. However, if we change the osmolarity of one component of the body (say, the blood plasma), then water will start moving from one region of the body to another. So, for instance, if you drank a lot of water, that water will be absorbed into your blood, and your blood plasma's osmolarity will be lowered (as the extra water dilutes the solutes). Once your blood osmolarity drops (say from 290 mOsm to 280 mOsm), water would start moving by osmosis from your blood into the other tissues of your body (as water would move from the lower osmolarity region in the blood plasma to the higher osmolarity region in your body tissues).

The physiology of drinking too much water

Now that we've gotten the general idea of osmosis and water movement, let's take a look at what happens when a person drinks too much water. Before I get into too much detail, however, I want to mention that my chemistry and physiology here will be filled with simplifying assumptions², and that I'm not a doctor and thus nothing I say should be taken as medical advice.

Jennifer Strange is reported to have consumed approximately 2 gallons (~7.5 liters) of water during the contest (data from Orac's post and this article). To put that volume into context, we need to look at how much water is in the human body.

Rough physiological formulas indicate that about 60% of our body mass is water (Berne et al. 1998). About 2/3 of that water is contained inside our cells, and is called intracellular fluid (Berne et al. 1998). The remaining 1/3 is contained outside our cells, and is called extracellular fluid (Berne et al. 1998). About 1/4 of the water contained outside of our cells is blood plasma; the rest is contained in the interstitial spaces around our cells (Berne et al. 1998). So, if we assume that Jennifer Strange was 165 pounds (the average weight of adult females in the US³; I have no idea what her weight was), we get the following amounts of water:

• Body mass: 165 lb (~75 kg)
• Total body water content: 45 L (75 kg * 0.6 * 1 L/kg)
• Total intracellular fluid: 30 L (45 L * 2/3)
• Total extracellular fluid (including blood plasma): 15 L (45 L * 1/3)
• Total blood plasma: 3.75 L (15 L * 1/4)

While those numbers are clearly just approximations, we can now see how large a volume of water 2 gallons (7.5 L) is: it's double the volume of water in the average female's blood plasma, and half the total water content of the average female's extracellular fluid. Adding that much water to a solution can clearly alter its osmolarity (by diluting the ions in solution), and thus will have a strong effect on osmotic movement of water in the body.

Of course we have kidneys, and one of their functions is to excrete excess water. So, a person could safely drink 2 gallons of water, as long as they could excrete those 2 gallons as quickly as they absorbed them. Unfortunately, the kidneys are limited in how fast they can excrete water; Orac cites data that the kidneys of a healthy adult can excrete a maximum of about 1 L of water per hour.

So, to fully understand what happens when a person drinks a large volume of water, we must look at this as a dynamic process. Water is being ingested at a specific rate, is then absorbed into the body (first stop: the blood plasma), is circulated around the body (where it is exposed to the various tissues of the body), and is then excreted from the body by the kidneys.

Based on the newspaper reports, contestants were given 0.25 L of water every 12 minutes, which is about 1.25 L per hour⁴. However, at some point in the contest the ingestion rate was increased, as contestants are reported as being given a "larger bottle." I don't have specific information on the duration of the contest, but to simplify things lets assume that contestants drank 1.25 L of water each hour for the first two hours, and then drank the remainder of the water in the next 2 hours (4 hours total seems like a good estimate for the duration of a radio show contest). Here's the net balance:

• Intake:
• First 2 hours: 2.5 L
  
• Last 2 hours: 5 L
• Total intake: 7.5 L
  
• Kidney filtration:
• First 2 hours: 2 L
• Last 2 hours: 2 L
• Total excretion: 4 L
  
• Net balance:
• Gain of 3.5 L of water

Note that this assumes both that all the water ingested was absorbed immediately (an incorrect assumption, as osmosis of water from the gut into the body will take time), and that all of the kidney filtration occurred immediately (again an incorrect assumption, especially as the contestants were prevented from urinating during the contest). However, lets hope that the delayed absorption of water in the gut is canceled out by the delayed (and possibly inhibited) filtration of water by the kidneys, and keep things simple by ignoring both.

The osmolarity of blood plasma is determined largely by the concentration of sodium in the plasma (typically 145 mmEq/L, which leads to a net blood plasma osmolarity of ~290mOsm; Berne et al. 1998). Given a starting blood plasma volume of 3.75 L (with 145 mmEq/L Na), and a final volume of 7.25 L (3.75 L starting + 3.5 L gain), the sodium concentration of the blood at the end of the contest (assuming no input of sodium from other body stores) would drop to 75 mmEq/L (3.75 L * 145 mmEq/L Na * 1/(7.25 L)). Orac specifies that a sudden drop in blood plasma sodium concentration from normal levels to below 120 mmEq is often fatal, so this drop in sodium concentration would be fatal.

As the sodium concentration of the blood plasma drops, the blood plasma's osmolarity will also drop (75 mmEq/L Na would lead to a blood plasma osmolarity of ~150 mOsm). This drop in plasma osmolarity would create an osmotic gradient between the blood plasma and extracellular fluid (the extracellular fluid would be 290 mOsm initially, as it is generally isotonic to the blood plasma under normal conditions). Thus, once some water is absorbed into the plasma, it will osmose from the blood plasma to the extracellular fluid.

So, let's assume that the extra 3.5 L of water isn't all stored in the blood plasma, but is also moved to the extracellular fluid that's in the interstitial spaces. In this case we start with 15L of extracellular fluid with a sodium concentration of 145 mmEq/L (extracellular fluid has about the same sodium concentration as blood plasma), and end with 18.5 L of extracellular fluid (15 L + 3.5 L gain). This scenario leads to a final extracellular fluid sodium concentration of 118 mmEq/L (~236 mOsm), again low enough to lead to death.

But what is actually causing death? While I'm not an expert here (and haven't been able to find a good reference for this quickly), one of the problems that the body runs into in this situation is that those 3.5 liters of water have to go somewhere. As we've seen above, that somewhere will initially be the blood plasma and extracellular fluid, which will be followed by movement of water into the intracellular fluid pool (i.e., inside cells, which are ~290 mOsm to begin with, and will thus begin absorbing water as soon as the osmolarity of the extracellular fluid drops). All of this water movement means that tissues throughout the body are going to be gaining water, and when tissues gain water they swell. This swelling can be tolerated to some extent in many tissues (e.g., your leg muscles), but can lead to extremely negative effects when it occurs in tissues that have only a limited ability to expand, such as your brain (which is mostly surrounded by bone). Swelling in the brain increases pressure on the tissues of the brain, which can lead to many problems, including reduced transport of nutrients from the capillaries in the brain to the cells of the brain⁵. Many of the symptoms of hyperhydration/hyponatremia are neurologic (fatigue, headache, loss of alertness, cognitive impairment; see here and here), and thus it seems likely that swelling in the brain is at least a contributing factor to why people die after drinking too much water⁶.

To end with a little taxonomic diversity, plants can actually tolerate this type of osmotic situation very easily (you water your plants with tapwater, right?) The difference is that plant cells are surrounded by a rigid cell wall, whereas animal cells have just a wimpy little plasma membrane (plant cells have a plasma membrane too). This cell wall restricts the ability of plant cells to expand (it's like a little suit of armor), and thus once the plant cell is full of water (turgid), the cell wall exerts a force (a pressure) that counteracts osmosis and prevents water from entering the cell. Thus, plant cells can have a higher osmolarity than their environment (e.g., be immersed in gallons of pure water), but not risk death due to excessive water intake⁷.


Reference

Berne, R. M., M. N. Levy, B. M. Koeppen, and B. A. Stanton. 1998. Physiology: 4th edition. Mosby, St. Louis.


¹ Water will flow into the cell.
² In addition to the assumptions stated in the rest of the article, I'm assuming (among other things) that all consumed water was absorbed into the body (it could have been vomited out, remained in the gut, or excreted with fecal material), that the water ingested was pure water (it wasn't; fresh water typically has an osmolarity around 70 mOsm), and that non-excretory sources of water loss (breathing, sweating, crying, etc.) were minimal. I'm also entirely ignoring the lymph system, which transports excess interstitial fluid back to the blood.
³ Data from the Wikipedia and this article.
⁴ Based on this quote "participants were given two minutes to drink an 8-ounce [~0.25 L] bottle of water and then given another bottle to drink after a 10-minute break," and then this quote "Sherrod said she managed to drink eight, eight-ounce bottles but became nauseated after drinking half of a larger bottle." (both quotes from here)
⁵ Nutrient delivery from capillaries to the surrounding tissues requires a pressure difference between the inside of the capillaries and the interstitial fluid surrounding them (the fluid in the capillaries is under higher pressure thanks to the heart; this higher pressure forces nutrient-filled fluid out of the capillaries). Increased pressure in the brain would negate this pressure difference, and could thus reduce (or eliminate) nutrient delivery.
⁶ Note, however, that I am not certain of this. Other factors could also play a role; for example, it seems at least feasible that the changes in ion concentrations might alter nerve and muscle resting membrane potentials enough to cause problems.
⁷ So why do terrestrial plants die from overwatering, you ask? It's actually because plant roots need oxygen to survive (they're an oxidatively metabolizing tissue; they can't do photosynthesis to generate oxygen or carbohydrates because it's rather dark in the soil), and when soil is saturated with water the roots can't obtain enough oxygen. Thus, overwatered plants functionally die by drowning.

[Updated to correct the name to Jennifer Strange.]
[Update 2, July 2007: The Georgetown Medical Center has a detailed article on water requirements during exercise here.]