There are a few theories about what exactly causes headache pain in people with spinal CSF leaks. The most obvious one is the loss of cerebrospinal fluid and the resulting lowered intracranial pressure: When a person with a spinal CSF leak stands up, that loss of brain buoyancy, and the effects of gravity while a person is upright and the brain shifts downward, can put pressure on pain-sensitive structures in the head and neck. Another theory is based on what’s called the Monro-Kellie hypothesis (named for the two eighteenth-century Scottish physicians who coined it), which posits that within the rigid compartment of the skull, there is a constant pressure-volume relationship between the brain, cerebrospinal fluid, and intracranial blood. A decrease in any one of those three things is compensated by an increase in the others. When the amount of cerebrospinal fluid is lower than normal, due to a leak, this volume has to be made up somehow. Brain tissue can’t really expand, at least as far as we know; and so what happens is the intracranial blood vessels start to dilate, to compensate for the lack of cerebrospinal fluid. This expansion of arteries and veins, which increases rapidly when a leaking person stands up, can be a painful sensation.
Another explanation of what causes pain in people with spinal CSF leaks involves the notion of compliance, or craniospinal elasticity. Just as some individual people can be more psychologically resilient than others, individual people’s physical anatomies can be more resilient than others (such as in Dr. Kranz’s example of the dural sacs of two different hypothetical people with the same level of spinal fluid pressure, one of whom has a dura that is flexible like a latex balloon, and one whose dura is more rigid, like a Mylar balloon). The headache pain that is such a hallmark of spinal cerebrospinal fluid leaks may come from changes in compliance. The amount of compliance or flexibility within the cranial space is limited, since it’s a closed space covered by an unyielding, impenetrable skull: The intracranial blood vessels can dilate, as in the Monro-Kellie hypothesis, but there’s only so much room for expansion. In the lumbar region, however, there’s more room to expand, as there’s no skull to contain it, only the self-limiting capacity of the tough (but more flexible than bone) dura. So there is a contradiction: the dura tight and noncompliant in the cranial space, with veins dilating and becoming stiffer, more engorged with fluid; and the dura looser and more floppy in the lumbar space, where there’s more room to expand, and where, due to gravity and a leak preventing the normal flow and circulation of spinal fluid, the fluid can collect, stretching and straining the dura mater. This contradiction, this interruption of the normally uniform distribution of craniospinal elasticity, can be a factor in causing headache pain.
The dura mater in general is not a stretchy thing, and although it accommodates changes in fluid pressure, it’s not elastic in the way we think of a rubber band or a bouncy ball being elastic. While I was leaking, the dura was explained to me in many different ways. I heard it described as a thin covering, like a cloth. I heard it described as a hollow plastic tube. I was told it was like packing tape, the kind with strings throughout it for added toughness and durability. I was told it was like duct tape, the super strong and sticky kind that’s hard to puncture. I was told it was a kind of connective tissue, which it is, and not a muscle, which it is not, and that it wasn’t something I could either strengthen or diminish through exercise or physical therapy. But what I didn’t realize was that this tube, as thin as cloth, as tough as packing tape, is, as yet another doctor explained, richly endowed with nociceptive fibers. Which means that this plastic tube encasing the brain and spinal cord is capable of sensing and transmitting pain.
Nociceptive is a word that comes from the Latin nocere, “to harm,” and what it describes is a kind of receptor in a sensory neuron that detects and responds to unpleasant stimuli in the body by sending electrical signals to the central nervous system. We have these kinds of nociceptors all over our bodies, and the electrical signals they send to the spinal cord and brain alert us to possible threats, and to harmful sensations. Think of your fingers touching a hot surface: In an instant, the nociceptive fibers in your skin send the message to your brain that there is danger, and before you can even form the thought, Ouch, this is hot, your hand has already jerked away to safety from that hot place. Those particular fast-acting fibers (called A-delta nociceptor fibers) are large in diameter and have a thick myelin sheath
