Cranial and Craniosacral; the central role of glymphatic circulation
SUMMARY - The human cranium is composed of bony plates connected by wavy sutures that give it the flexibility required to squeeze through the birth canal. Later, these sutures fill in with bone until almost all of them have "ossified" by adulthood, but that ossification of the sutures does not completely immobilize them. Afterwards, they participate in an almost imperceptible pumping system in which the cranium expands and contracts in synchrony with breathing in a process that circulates cerebrospinal fluid (CSF). This process is called glymphatic circulation, like lymphatic circulation for the glial cells of the brain. Cranial osteopaths described the movements of each cranial bone in this cranial respiration process half a century before they or anyone else understood its cause. Glymphatic circulation diminishes with age, and that decrease is believed to be responsible for the accumulation of waste products in the brain that causes Alzheimer’s disease, Parkinson's disease, glaucoma, age related cognitive decline, and dementia. Disruption of glymphatic circulation may also cause fluctuations in localized CSF pressures that produce chronic and exertional headaches.
BACKGROUND - The cranium was considered a rigid shell with a genetically determined shape until high precision ultrasound in the late 20th century showed that it is actually a dynamic system that changes shape in response to gravity and forces like coughing and sneezing. Now we understand that it also changes shape a little back and forth rhythmically in conjunction with respiration. Metabolic activity everywhere produces waste products that need drainage. In other parts of the body, waste products are flushed out by functional movements; because veins have one-way valves, and exercise operates them like a pump to help drain the capillary beds. The brain produces more than 1300 grams of macromolecular metabolites daily. However, the cranium is too rigid to employ movement to pump out its waste products, so it uses respiration.
GLYMPHATIC DRAINAGE – During sleep, especially slow wave sleep, the interstitial spaces (between cells) in our brains increase as much as 60% to become a mixing chamber, and the perivascular spaces (surrounding the vessels like sleeves) open up to house a CSF circulation process that employs the negative intrathoracic pressure produced by inhalation along with positive arterial pressure to pump fluids through the cranium while rhythmically moving the cranial bones back and forth in very small amounts to accommodate the pumping. The resupply relies on arterial pressure, which is relatively steady. The drainage portion relies on the negative intrathoracic pressure. Each inbreath sucks out 1-2 drops of interstitial fluid and neurologic waste products from the cranium into venous circulation by shortening the cranium antero-posteriorly, along with flexion of the spheno-occipital synchondrosis and external rotation of the temporal bones in a smooth steady process that moves relatively large volumes of CSF slowly, while peri-arterial flow slows. Then, during each exhalation, the cranium rebounds, along with extension of the spheno-occipital synchondrosis and internal rotation of the temporal bones, due to arterial pulsation in a stepwise process that moves smaller volumes of CSF more rapidly. The inhalation portion of this glymphatic circulation process serves to drain the by-products of neural metabolism into venous and lymphatic circulation much like functional forces serve to drain the by-products of metabolism from other parts of the body.
The cranium complies with this expansion and contraction during breathing by a folding process that resembles the way it folds to get through the birth canal, - expanding into extreme extension to become as long and narrow as possible while squeezing through; and then rebounding while contracting into extreme flexion by becoming shorter and wider during the first inhalation. This pattern of extension and flexion of the cranium slows down about 99.9% rapidly, and eventually probably about 99.9999999%, but then it continues slowly throughout life, functioning like a pump to keep the brain healthy.
CRANIAL OSTEOPATHY - The complex movements of the individual cranial bones during this expansion and contraction process were elegantly described long ago by cranial osteopaths, although they did not understand the source of the movements. Sutherland believed that cranial respiration was a primary process, - beginning in the brain and then moving through all the other tissues. Magoun understood that inhalation was associated with cranial flexion, but he still believed that cranial respiration had a timing different from lung respiration. Numerous cranial osteopaths described low force manual manipulations that could be used to restore restricted cranial movements.
It's certainly reasonable to hypothesize that restricted cranial movements could impair glymphatic circulation. Completely immobilizing a cranial suture in craniosynostosis impairs growth in all the surrounding cranial bones, alters head shape, and causes a number of symptoms. A partial or limited craniosynostosis could cause a less dramatic but still significant health impairment by impairing the CSF circulation process in portions of the cranium. The brain consistently turns out to be more sensitive than we can measure, and it could be affected in many ways that we do not yet understand by even minute changes in the pressure of its housing.
RESTRICTING BONE MOVEMENT IN THE MIDFACE - One area where dentists can inadvertenty produce a partial craniosynostosis is by limiting movement across the midpalatal suture by making thick upper oral appliances or nightguards. To prevent that problem, we make all our oral appliances with minimal acrylic crossing the midline, as shown below left. We can also make appliances that have no rigid materials crossing the midline but instead have the two sides connected only by loops of flexible braided stainless steel wire, as shown below right.
CHEWING - The pumping movements of the cranial bones during chewing can be an additional source of glymphatic drainage from the frontal cortex. In a study illustrated below, bite forces applied to a cranium coated with stress sensitive paint showed that they are distributed throughout the front of the cranium.
In fact, chewing is such an important source of glymphatic drainage in animals that depriving older animals of chewing by softening their food or removing their molars leads to loss of spatial memory, reduced learning capacity, neuro-endocrinal changes, and hippocampal degeneration. Also in humans chewing is so important for glymphatic drainage that, in most of the world, the rate of cognitive decline is proportional to the number of teeth that have been lost. MRI shows that chewing stimulates cranial circulation and frontal cortex activity by increasing glymphatic drainage.
Chewing provides glymphatic drainage primarily by producing a wave of compressive force on the working side. Healthy power-crushing has an anterior component of force that produces a wave of compressive force in the frontal cortex anteriorly toward the cribiform plate, where 15-30% of glymphatic drainage occurs. However, most modern chewing is more mashing than power crushing.
YAWNING - Is also a significant source of glymphatic drainage. The dramatic increase in CSF flow seen during yawning on MRI is probably due to the deep inbreath producing maximal negative intrathoracic pressure combined with a maximal mouth opening that pulls down on the front half of the cranium to compress it.
EXERCISE - is another source of glymphatic drainage, because the lymphatic and glymphatic systems are connected. Hamsters who have access to a wheel for running undergo cognitive decline more slowly than those who get less exercise.
THE ROLE OF THE BITE IN POSITIONING THE FACIAL BONES - Because chewing forces are much stronger than the light manual forces used to make cranial osteopathic adjustments or even stronger forces used to make cervical adjustments, biting can probably relapse those adjustments as soon as the interdigitation of the steeply cusped teeth returns the facial bones to the exact positions (within microns) that they had before the adjustment. In an integrated biological system, a change anywhere requires at least a small change everywhere else, and a steeply interdigitated bite prevents even a minute change in the braced position and thereby also the posture of the mandible.
The direction, location, and strength of the bite forces depends on the contours of the bite table. People who wear dentures or are undergoing orthodontics with braces have low bite forces. People under extreme stress often have sustained increases in jaw muscle tonus. Some people have bites that are directed unilaterally (they chew on one side), some have bite forces that extend out widely bilaterally, and others have bite forces that are directed almost entirely vertically. People with steep unworn dentitions and tall interlocking cusps on their teeth are more likely to encounter transversely directed forces from chewing.
The direction, location, and strength of the bite forces can also be altered by changing the contours of the bite tableA front flat bite plate oral appliance moves the location of the bite forces anteriorly. A tall oral appliance can produce sustained bite forces from passive stretch of the jaw closing muscles. An oral appliance can be used to shift bite forces to either side or to increase the strength of bite forces used in nocturnal bruxism by stabilizing the bite.
A bite that is too steeply interdigitated to adapt to the different growth patterns between the upper and lower jawbones can produce progressive strains between all the surrounding bones, because the bite functions like a cranial suture connecting the upper and lower jawbones, and cranial sutures must provide adaptability to accommodate irregularities in growth of individual bones. Such adaptability is needed in the bite, because the upper and lower jawbones grow by very different mechanisms, at slightly different rates, and in slightly different directions. However, in many modern human bites, the unworn teeth are so steeply interdigitated that they leave the bite without sufficient adaptive capacity to accommodate the natural discrepancies between the normal growth patterns in the upper and lower jawbones.The elongation of the mandible cannot push its corpus (the area containing the lower teeth) further forward, because that area of the mandible is locked to an upper jaw that grows by expanding rather than translating; and the upper jaw cannot expand, because it is locked to a mandible which grows by translating rather than expanding. The result of this locking together of the upper and lower jawbones by the bite is best described as a partial craniosynostosis of the maxillo-mandibular suture. Like in other craniosynostoses, the resulting strains are likely to be transmitted throughout the cranium.
CRANIOSACRAL TREATMENT - is an extension of cranial work to include the spinal canal, which is enclosed in the same CSF reservoir as the cranium; but where CSF flow is slower, more turbulent, and probably less consequential. The early osteopaths described how the tail end of the spine moves up during flexion (inhalation) and down during extension (exhalation). The hydrodynamics of CSF in the spinal canal are better known than those in the cranium, because they are more easily imaged and monitored. Blockages to the normal CSF flow through the spinal canal due to Chiari malformation, spina bifida, a cyst, spinal cord tethering, space occupying lesions, trauma, or infection impair the flow immediately above and below the blockage; but distal to the blockage, CSF flow resumes with epidural and cord pulsation. In minor blockages, the flow reflects local turbulence and eddies. In major blockages, the flow may be stopped completely. Removing the blockage surgically restores the flow and eliminates the symptoms.
Much of the clinical success from craniosacral treatment may be produced by other benefits of manual manipulation of the spinal column. The cranium functions as the superior end of the spine, therefore holding the occiput in one hand and the sacrum in the other when a patient lies on a table enables a therapist to feel the mobility of the whole spinal column and manipulate it in various ways that can enhance its range of motion, improve intervertebral joint circulation, or provide other benefits that we don't yet understand.