Neurodevelopmental disorders: Sufficient and necessary causes

- So let's take a look at a few of the more common events that can cause neurodevelopmental disorders. And maybe let's do that by sort of categorizing them here. So let's draw a line down here and pop "nature" down on this end, and "nurture" on the other end here. So now we've got a little spectrum that goes from nature all the way to nurture. And you've probably heard these terms before, nature and nurture. And what we mean when we say nature, what we are talking about are things that are innate, things that we are born with. So for example, our DNA, our genetic instructions that we have inside of ourselves that tells our body what to do and what to make. This is part of our nature. And let's actually put DNA down here on the nature side of our spectrum of causes of neurodevelopmental disorders. So we'll just say "genetics" here because changes in our DNA, like maybe if our mom or our dad, or maybe both of them, if they pass on mutated genes, well this is one of the possible events that can lead to a neurodevelopmental disorder. Right, and I should actually point out here that we're looking at sort of broad causes of neurodevelopmental disorders kind of in general. We'll look at some specific examples shortly, but right now let's just think broadly just so that we can get a sense of what sorts of overarching causes in general can lead to neurodevelopmental disorders. So on the other end of our spectrum here, when we say nurture, what we're talking about is our environment. All of our personal experiences that happen after we're born. So for example, I recently had an infection called meningitis which is when our meninges, these protective membranes that cover our brain and our spinal cord, they become infected. So, I developed this infection from my environment rather than something that I was born with and we can actually put infections, including meningitis, down here on the nurture end of our spectrum as a possible cause of a neurodevelopmental disorder. So you might be thinking, "Right, now hold on. "So if she had meningitis and if infections like meningitis "can cause neurodevelopmental disorders, does that "mean that she has a neurodevelopmental disorder?" And that's actually a really good question and it brings us to a really, really important point. And that's that all of these different events that we're talking about, in order for them to result in a neurodevelopmental disorder, they need to occur during the critical periods of development. So while the fetus is developing during pregnancy or during the actual birthing process or maybe shortly after birth, within the first few months to years of life. And that's why we call them neurodevelopmental disorders because when these events occur during these critical periods of development, they disrupt development in some way. So for me, I had meningitis around my mid-20's, so I passed these critical periods of development and my meningitis didn't result in a neurodevelopmental disorder. And let's actually keep this question in the back of our minds because we're going to look at it again shortly when we talk a bit more about this word, "cause." So let's jump back here to our spectrum and while we're here looking at nurture, let's pop down something else. Let's pop down trauma. And by trauma, what I'm talking about is something like head trauma, maybe from a car accident or a fall. And we can also put down deprivation. And when I say deprivation, what I mean is something like temporary hearing loss during the first few years of life. Maybe from recurrent ear infections in infancy or toddlerhood. And let's kind of move to the middle of our line here, at the middle of our spectrum, to talk about things that kind of fall in between nature and nurture. So these are things that might happen before a baby is born, so during pregnancy, but are due to the environment that the fetus is in. So if we're exposed to certain substances or toxins, maybe if mom drinks alcohol or smokes cigarettes while she's pregnant, this can cause certain neurodevelopmental disorders. Or maybe if mom doesn't get all of her important vitamins and minerals. So for example, maybe she doesn't get enough folate, one of our B vitamins, while she's pregnant. So these sorts of nutritional deficiencies can also result in a neurodevelopmental disorder because a fetus relies on getting its nutrients, the fuel that it needs to grow properly, from mom. So these are some of the main types of events or conditions that can cause neurodevelopmental disorders. And I promised that we would go over some specific examples and talk more about that term, "cause." So let's do that, let's put some examples down of some specific neurodevelopmental disorders. So how about Down syndrome and cerebral palsy and attention deficit hyperactivity disorder? We'll just put ADHD for that one. We'll use these three disorders in particular because each of them is going to help me demonstrate a particular point that I want to make. And let's actually draw a circle for each of these, and we'll put the causes of these different disorders in these circles here. We'll kind of make what we call a Causal Pie Chart. So if we look at Down syndrome, Down syndrome is caused by extra genetic material from chromosome 21. So let's put that in our Down syndrome circle here. Extra genetic material from chromosome 21. And sorry, why would this even happen? Well, this usually happens because the person has three copies of chromosome 21 rather than two. And you may have heard this being referred to as trisomy 21. "Tri" meaning three and "somy" referring to chromosome and "21" because we're talking about the 21st chromosome. So when someone has this extra genetic material from chromosome 21, they will develop Down syndrome. So we call this type of cause, this extra genetic material a sufficient cause, where sufficient means that every time this happens, so in this case the extra genetic information from chromosome 21, the outcome, Down syndrome, will always follow. So a totally different example of a sufficient cause is if I throw water on you, right? You will definitely get wet. Each time I throw water on you, you'll get wet. So me throwing water on you is a sufficient cause of you getting wet whereas an insufficient cause of you getting wet would be maybe like walking past a sprinkler. It's there, but you may or may not get wet. Now if we come back to Down syndrome here, in order for someone to develop Down syndrome, they must have extra genetic material from chromosome 21. In other words, every person with Down Syndrome has this extra material from chromosome 21. So because of this, because this is a requirement for the development of Down syndrome, we also call this a necessary cause. Necessary meaning that we need this to happen in order for our event, Down syndrome to occur. So extra genetic material from chromosome 21 is both a sufficient and a necessary cause of Down syndrome. Sufficient because it alone will always cause Down syndrome, and necessary because you can't have Down syndrome without extra genetic information from chromosome 21. So with these definitions in mind, let's check out cerebral palsy next. Well, there are actually quite a few different events that can each result in cerebral palsy. Let's draw another pie chart here and let's say that each of these represents a different person with cerebral palsy. And we could potentially draw 100 more circles here to write down all of the causes of cerebral palsy. But I'm just going to use two examples for now. So in this first one here, one cause of cerebral palsy is trauma. So maybe this person was in a car accident when they were just a few months old. And let's make our other example here an infection. So maybe this person had an infection like meningitis a few days after they were born. Now this brings us back to that thought that I wanted you to hold on to when we were wondering why I don't have a neurodevelopmental disorder even though I've had meningitis. Because that question leads us really nicely into how we can use the word "cause" in different ways. So while both of these people here have cerebral palsy because of these events like trauma or infection, these events don't always cause cerebral palsy. So because they don't always cause cerebral palsy, these are insufficient causes, right? They are the sprinkler. And let's see if any of these events are necessary causes. In other words, does cerebral palsy require one of these events? Well this person here, they developed cerebral palsy but they never had an infection, like meningitis so meningitis isn't a necessary cause, right? We don't need meningitis in order for cerebral palsy to occur. And this person, they developed cerebral palsy but they haven't sustained any head trauma so head trauma can't be a necessary cause either. So none of the causes of cerebral palsy are necessary causes, right? Remember our example where I caused you to get wet because I threw water at you? Well there are other ways I could get you wet, right? I could spray you with a hose or push you into a pool. Both of these will leave you pretty soaked as well. So me throwing water at you isn't a necessary cause of you getting wet because I can accomplish my goal of soaking you without it. And it's the same kind of idea for these different causes of cerebral palsy here. For both of these people, these events for them have caused cerebral palsy, but remember that there are lots of potential causes of cerebral palsy. So infection or trauma aren't necessary causes of cerebral palsy. So let's look at ADHD. And with ADHD, well we haven't quite figured out all of the details behind the causes of ADHD but we have discovered some events that increase a person's chance of developing ADHD. So we think that these events sort of come together and form a sufficient cause of ADHD. So let's make up an example of what this might look like. So maybe for this person, ADHD runs in the family. Maybe mom or dad have ADHD themselves. So let's put genetic predisposition down here as one wedge in our pie chart. And maybe mom smoked while she was pregnant with our kid here with ADHD. So let's put maternal cigarette exposure here as another wedge. And let's make our last wedge exposure to toxins, maybe something like exposure to lead from old school lead paint while the person was a young kid. Maybe he used to eat paint chips from his bedroom wall when he was little. I know at least one person who's done this. So we know that these events increase the chance of ADHD developing, but alone might not be sufficient enough to cause ADHD. But if we combine them together like we have here, we think that the combination might be enough, making them together a sufficient cause of ADHD. And none of these individual events is a necessary cause of ADHD. Because not everyone with ADHD has a genetic predisposition or was exposed to lead or cigarette toxins. So we don't need any one of these individual events in order for ADHD to develop.