You’ve seen it on head CT reports. “The patient has a collection of extra-axial blood…” Then it goes on to describe the location and size of a subdural hematoma. But why is it called “extra-axial?”

The answer lies in the embryology of the central nervous system. Yes, it’s been a long time since any of us have read anything about that. Early animals had a straight neural tube, which slowly evolved into a brain and spinal cord. This is known as the axis of the nervous system.

The brains of early vertebrates developed at the end of the neural tube, and were oriented in the same longitudinal axis as the rest of it. As brains got bigger, a 90 degree bend occurred at the cephalic flexure.

So in humans, there is a difference between the body axis and the brain axis. But the brain axis is what really counts. This means that any blood outside of the brain axis is defined as extra-axial.

Bottom line: Extra-axial blood is defined as any bleeding outside of the brain parenchyma. This includes subdural and epidural hematomas, and subarachnoid hemorrhage. It does not include any intraparenchymal bleeding like contusions or hematomas.