Let's start with some anatomy. In order for an infection to occur, an organism has to get
through the body's defenses and one of the most robust defense mechanisms is sheltering
of the brain and the nervous system from the outside world. What does that? Well, when we
move from the outside world into the brain, we got to pass through the scalp. And the scalp
stands for first the skin. Slightly deeper we get to the connective tissue, that's the C. The A
is called the aponeurosis, a thick fascial layer. It is just beneath the skin and connective
tissue and is a thick fascia. Aponeurosis are called the galea aponeurotica. Beneath the galea
is loose connective tissue, a very thin layer of loose, L, connective tissue. And then the
periosteum. And together those 5 layers make up what we call the scalp. Beneath the scalp
is the bone and we know that calvarial or cranial bone is composed of an outer cortical bone
and intercortical bone and in between that is a diploic space, soft tissue or soft area of bone.
And then beneath the bone is the outer covering of the brain, which we call the meninges.
First, we see the dura mater and there are reflections in the dura where the venous sinuses
occur, and you can see that in the middle part of this picture. Beneath the dura, we have the
arachnoid and the subarachnoid space, a potential space where blood vessels lie, and then
the pia mater. And together, the pia, arachnoid, and dura make up what we call the PAD or
the meninges. And deeper still, we get to the brain or the intraparenchymal compartment
of the brain tissue. So let's take a closer look at this and each compartment of the nervous
system and brain. Again, we have the dura, the arachnoid, the pia mater, and the brain and
we're going to look at infections that happen that are concentrated in each of those areas.
When we think about the dura and look at the dura, the infection that we're going to look into
and focus on will be meningitis. Meningitis is an infection that primarily affects the dura
mater. That's where we see it on imaging, that's where we evaluate it when we're looking at
patients and this is where many of the symptoms come from. In the level of the arachnoid
and pia, the infection that occurs primarily in that area is encephalitis. And when we think of
encephalitis, we're going to think of a problem that's in the arachnoid, the pia mater, and that
subarachnoid space, that potential space in between those 2 layers. The last layer is the
brain, the intraparenchymal tissue and then the infection that occurs in that area is
cerebritis. Cerebritis is an infection in the brain parenchymal compartment, then that will
account for both cerebritis and brain abscesses. So again, we're going to learn about 3 types
of infections; meningitis, encephalitis, and cerebritis. And when we think about those, we're
going to think about those infections affecting the dura mater, the arachnoid and
subarachnoid space, and pia, and the brain tissue.