The Accessory Drainage System: Its Role in Humans and Other Mammals

As mentioned in my November 5 article,¹ humans use two different routes to drain the brain: 1) through the transverse/sigmoid sinus system and into the internal jugular veins, which is primarily used to drain the brain in the recumbent position; 2) through the occipital marginal sinus system and into the vertebral venous plexus (VVP), which is preferentially used to drain the brain in the upright position. Some authors refer to this system as the accessory drainage system.

In the previous article I also described the role of the craniosacral primary respiratory system (CSPR) in moving cerebrospinal fluid (CSF) through the brain and the magnitude of its impact on cranial hydrodynamics as reflected in the shape of the sutures. We also discussed how the force of the CSPR comes from the combined effects of the motility of the nervous and musculoskeletal systems, arterial pulsations and respiratory pressure waves. In humans, upright posture further increases blood and CSF flow.

CSF is an ultra filtrate of the blood that is produced by both active and passive processes. The blood for CSF comes from vascular plexuses in the ventricles called the telea choroidea. The telea are covered with a layer of ependymal cells that are used for the active production of CSF. These cells secrete salts that draw blood out of the telea choroidea through the blood-brain barrier and into the ventricles. The passive production of CSF is caused by an increase in the CSF pressure gradient as a result of upright posture.

After leaving the ventricles, CSF flows into the cisterns and subarachnoid space. It then flows upward and into the superior sagittal sinus, where it is absorbed by arachnoid granulations to exit the brain through the dural sinuses. Because it is located at the top of the head, pressure in the superior sagittal sinus drops and becomes negative in the upright position. This increases both blood and CSF perfusion pressure gradients and flow. The accessory drainage system offsets the increase in blood and CSF flow; without it, their volumes would increase, causing hydrocephalus and edema.

The accessory drainage system in humans is an extensive system of valveless veins that includes the occipital marginal sinus system and the VVP, located inside the neural canal. Because it has no valves, blood flow in the accessory drainage system is determined by momentary respiratory and postural constraints. While it is currently believed that the accessory drainage system is unique to upright mammals such as apes, chimpanzees, humans and hominids, other mammals may use similar systems to control cranial hydrodynamics.

What makes upright posture in humans unique compared to other semi-upright mammals is the exposure to prolonged vertical upright posture, which caused significant changes in anatomical design. Some scientists believe that the changes in the cranium and venous drainage system, and the addition of the accessory drainage system, may have contributed to the large size of the human brain by increasing blood and CSF flow, and enhancing brain cooling. But while these changes in design may have contributed to the large size of the human brain, they also created some potential problems that may predispose humans to chronic normal pressure hydrocephalus (NPH) and subsequent degenerative diseases of the brain as a result of aging.

In most mammals the floor of the basicranium is flat. In humans it is bent. This forms upper and lower compartments. The lower compartment is called the posterior fossa. Each compartment is further divided into left and right halves by the falx cerebri and cerebelli. The pituitary also has its own compartment. The upper compartments are joined to the posterior fossa by the clivus, formed by the union of the body of the sphenoid and basilar process of the occipital bone, which is approximately forty-five degrees to horizontal when the head is held upright.

In most mammals, the foramen magnum is on the rear wall of the posterior fossa. In humans it is on the floor of the posterior fossa at the base of the clivus. The posterior fossa, which contains the cerebellum, is separated from the upper compartments by the tentorium cerebelli, which contains an opening. The thalamus and hypothalamus of the brain stem are above the tentorium. The rest of the brain stem passes down and through the opening in the tentorium and down through the foramen magnum. The angle of the clivus and the location of the foramen magnum places the weight of the brain stem more directly over the tentorium cerebelli and foramen magnum, which is more susceptible to prolapse and herniation into these structures.

A pressure conus occurs when the brain stem sinks into the foramen magnum, as a result of removing too much CSF during a spinal tap. The only thing that prevents a pressure conus from occurring during upright posture is the CSF water "jacket." This makes the correct volume of CSF critical to humans. On the one hand, too much CSF causes hydrocephalus. On the other hand, an insufficient amount of CSF causes a pressure conus. Ultimately a full pressure conus depresses all vital functions and consciousness. But there may be intermediary stages as well. That is, less extreme forms of pressure conus may occur in the aging brain as a result of weakness of the dura mater and failure of cranial hydrodynamics, and thus may contribute to chronic NPH by obstructing the outlets to the accessory drainage system.

There is still another problem with upright posture. Since it increases both blood and CSF flow and drainage, it likewise increases CSF outflow at a time when it is most needed in order to prevent a pressure conus. The increase in outflow, however, is made up for by an increase in the passive production of CSF. The passive production of CSF, however, relies on pressure gradients formed between the ventricles, where it is produced, and the superior sagittal sinus, where it is absorbed. This pressure gradient is extremely small and is easily overcome by momentary postural and respiratory pressure gradients, such as inversion and Valsalva maneuvers. In addition, it is also possible that craniocervical syndromes may obstruct the accessory drainage system, and thus affect the CSF pressure gradient and therefore, the passive production of CSF.

Hominids, humans, apes and chimpanzees have used a variety of different layouts for accessory drainage. There are even racial variations in designs among humans. But the hydrodynamic changes caused by upright posture naturally favored those with better drainage systems, and despite some slight variations all humans basically use an occipital/marginal sinus system that drains into the VVP inside the spinal canal. In contrast to the transverse/sigmoid sinus system, the occipital/marginal sinus system has numerous connections with the VVP through a network of emissary veins. These veins pass through various openings in the skull, including the posterior condyloid, mastoid, occipital, and parietal emissary foramina, and the hypoglossal canal. There are also other veins connected to the accessory drainage system, such as the basilar plexus, that don't pass through emissary foramina, and there may be others we don't know about yet.

From my studies of other mammals, I believe that the accessory drainage system also uses the diploic and facial veins, and possibly the central canal of the spinal cord. The diploe are the spaces between the cortices of the cranium that contain the diploic veins. They vary in size in different individuals, and are typically larger in males. The largest diploic spaces are typically found in the parietal bone and occipital bones, which lie over the dural sinuses. Emissary veins connect the diploic veins to the dural sinuses on the inside, and the scalp and facial veins on the outside of the cranium. The facial veins are likewise valveless so that blood can flow in either direction.

As far as the central canal is concerned, it is currently maintained that CSF in the central canal flows uphill and into the fourth ventricle to exit the brain through the superior sagittal sinus. The central canal however lacks a vascular plexus similar to the choroid plexus in the ventricles. Since it has no blood vessels to draw from, it seems unlikely then that the central canal produces much CSF. On the other hand, the central canal is lined with ependymal cells similar to those that cover the telea choroidea in the ventricles. It seems likely that these cells secrete salts similar to the ependymal cells that line the ventricles. This would create an osmotic pressure gradient that would draw CSF out of the fourth ventricle and either into the cisterns, where it is needed to support the brain in the upright position, or down the central canal, if the cisterns are full. Given its small pressure gradient it seems unlikely that CSF flows uphill during upright posture. Instead, it seems more likely that drainage of the central canal occurs in the recumbent positio,n and that its movement is assisted by rhythmical pressure fluctuations that emanate from the CSPR, which drive CSF back up the canal and into the fourth ventricle.

To look for evidence of accessory drainage systems in other mammals, I chose to investigate bats, giraffes and whales. Each of these must contend with extreme changes in cranial hydrodynamics. For example, in contrast to upright posture, bats must contend with prolonged periods of inversion. Next, I chose giraffes, because they have to contend with up to nine-foot columns of blood stacked against their brains when they lower their heads to drink. Lastly, I chose to study whales, because in contrast to inversion, whales have to contend with compression of the abdomen and thorax while holding their breaths during deep dives. This would be equivalent to an extreme Valsalva maneuver in humans. Both inversion and Valsalva maneuvers are known to increase intracranial pressure (ICP) in humans. It seemed likely that these mammals must have a way of controlling cranial hydrodynamics and ICP. While I could only scratch the surface, what I found suggested that they probably use the same system of valveless veins that humans use, but they develop and use different parts according to their particular needs.

In contrast to prolonged upright posture in humans, bats must contend with cranial hydrodynamics associated with prolonged inversion. In humans, the crista galli of the ethmoid bone forms a well that is horizontal to the ground when the head is held upright and the foramen cecum lies just in front of it. The foramen cecum contains an emissary vein that connects the dural sinuses to the facial veins. In contrast to humans, the ethmoid canal in bats forms a much deeper well, which points straight to the ground when the head is inverted. It occurred to me that bats probably use these and other routes as accessory drainage systems for prolonged inversion to help maintain brain blood flow and intracranial pressure. Primates that hang upside down probably also use the facial, diploic and emissary veins as alternative drainage routes during inversion.

Giraffes have a similar but different challenge to cranial hydrodynamics than bats. They both have to contend with challenges to cranial hydrodynamics from head inversion. But while bats must contend with prolonged inversion giraffes must contend with up to a nine-foot column of blood inverted against the brain when they lower their heads to drink. What is amazing is that they don't pass out when they suddenly raise their heads back up.

Giraffes have several mechanisms to deal with the challenges to cranial hydrodynamics caused by head inversion. For one, giraffes have a rete mirabile at the base of the cranium. The rete mirabile is a vascular plexus that fills up when the head is inverted. It acts like a G-suit to offset the increase in arterial and venous pressures. Nonetheless, their cerebral perfusion pressure is reversed, which could result in an increase in blood and CSF volume, and potentially injure the brain. It seems likely, therefore, that giraffes use an alternative drainage route during head inversion.

The first thing that caught my eye when I studied the giraffe skull was the exceptionally large size of their diploe. They also have accessory horns located on top of the diploic spaces. These horns vary in size, number and location. They typically increase in size and number with the size and gender of the animal. Their location on the cranium makes them unsuitable for sparring, so their purpose has always puzzled scientists. Typically, they are located along the base of the cranium and over the bones of the nasicranium. It is my opinion that giraffes use the diploic spaces and veins along with the accessory horns as an accessory drainage system when they invert their heads. In humans, the diploic spaces in the parietal, occipital and mastoid bones tend to be the largest. The relative size of these spaces in both giraffes and humans most likely reflects their underlying hydrodynamic stresses caused by postural constraints.

In contrast to bats and giraffes, whales face an entirely different challenge to cranial hydrodynamics, and both the ethmoid canal used by bats and the diploic spaces used by giraffes are nearly obliterated. Instead of inversion, whales must contend with extreme compression. Among other things, this would cause strangulation to the carotid arteries; so, instead of being located on the outside of the neck, they are located inside the spinal canal along with the VVP. This not only protects them from strangulation, but may provide additional benefit.

Since whales have no sternum, their thoraxes and abdominal cavities tend to flatten out when they get compressed during deep dives. This squeezes blood out of both cavities, which is subsequently translocated into the VVP. This should increase the pressure in the VVP and may, therefore, serve as a G-suit similar to the rete mirabile in the giraffe, thus offsetting the increase in systemic arterial pressure and protecting the brain from an increase in ICP. I also believe that whales may use their air sacs as accessory drainage routes the same way that giraffes use the diploic spaces and accessory horns. The air sacs are connected to the eustachian tube. This may cause them to inflate during dives and possibly open up accessory drainage routes. Unfortunately, I was unable to continue my investigation.

In brief, the accessory drainage system is an extensive system of valveless veins connected to the brain that appears to be used by mammals to offset cranial hydrodyamic stresses. In humans, it has improved steadily over time, and will probably continue to improve, as long as we remain upright terrestrial beings. However, this may soon change as we explore space.

The accessory drainage system is gravitationally based, and will probably be useless in space. In fact, prolonged exposure to gravity-free conditions may predispose today's humans to chronic NPH. And if humans are born and live in space, they will most likely develop hydrocephalic skulls. The benefit of a gravity-free environment is that the brain doesn't need CSF to keep it from sinking into the foramen magnum. The problem is that without gravity, the brain loses the boost in CSF and brain blood flow that typically comes from upright posture. Furthermore, the daily active production of CSF will most likely continue at its normal rate, but without the usual relief in volume that comes from upright posture. Sluggish blood and CSF flow, combined with continued production of CSF, could therefore lead to chronic NPH and brain edema during prolonged exposure to gravity free environments. Over time, this could lead to chronic NPH and eventually brain degeneration, the same way that chronic glaucoma leads to blindness.

In summary, blockage of the gravitationally based accessory drainage system by any means may result in chronic NPH. The possible causes of blockage of the accessory drainage system, and why they have never been found before, will be discussed in the next article. I will also discuss how chronic NPH may occur as a result of failed hydrodynamics, rather than actual obstruction of the accessory drainage system. This should be of special interest to chiropractors, because the accessory drainage system passes through and into the craniocervical spine. Craniocervical syndrome, including those caused by subluxations of the upper cervical spine, may lead to obstruction of the accessory drainage system. Lifetime chiropractic care, therefore, may have a small, but nonetheless significant impact on limiting the incidence of certain degenerative diseases of the brain.

Reference

1. Craniocervical syndrome: its potential role in Alzheimer's, Parkinson's and other diseases of the brain - Part I: Cranial hydrodynamics and the sutures. Dynamic Chiropractic Nov. 5, 2001 (www.chiroweb.com/archives/19/23/09.html).