RICHARD BELLI,
D.C.
For years
clinicians have observed marvelous results from cranial bone manipulation,
but along with these observations have come a great deal of conjecture and
debate as to the mechanism. This discussion covers some of the plausible neurological
mechanisms for these observations. These mechanisms include tonic labyrinthine
reflexes, dural innervation, postural modulation by vestibulospinal projections,
and vestibular projections to the reticular formation, thalamus, and hypothalamus.
The most
striking observation of cranial manipulation is the diversity of physiological
and therapeutic response. For decades clinicians have observed resolution
of everything from low back pain to tachycardia. These observations, because
they are so diverse have given cranial manipulation an almost mystical connotation,
how else could such diverse responses be explained?
The debate
has raged amongst anatomist and clinicians as to whether cranial bones do
in fact move or if the sutures are fused making the cranium an immovable vault.
It has been well documented by Upledger that there is a cranial respiratory
mechanism. Many anatomist and clinicians claim that the cranium in vivo is
wet and flexible, thus allowing for movement. This all leaves us with a preponerence
of evidence that the cranium in vivo is an alive and movable mechanism.
The aforementioned
leads us to the discussion as to what mechanically happens when the cranium
is manipulated. Keeping it simple, and without getting into specific techniques,
we assume that cranial techniques generally restore normal juxtapositional
relationships and movement to the cranial respiratory mechanism. By restoring
normal motion and function, one would assume that there is a change in the
dura, or better said in dural tension, as well as a normalization of the bilateral
temporal bone relationship. As discussed by Walther, Upledger, and others,
the cranium is believed to function as a closed kinematic chain, in other
words you can not affect a bone as a single entity, manipulation of one bone
will affect all of them.
The dura
is supratentorially innervated by the trigeminal nerve and infratentorially
by the vagus nerve. This makes it enticing for cranial practitioners believe
that cranial manipulation may have it therapeutic responses via the afferents
of these nerves. Another school of practitioners finds it irresistible to
believe that changes in cranial motion and normalizing of the flow of cerebrospinal
fluids leads to therapeutic changes. And finally, some practitioners believe
that normalization of the temporal bone relationship to each other accounts
for the therapeutic effect.
Lets now
look at these suspected mechanisms and determine which can explain the spectrum
of clinical change. Logic would tell us that the modality that encompasses
the neurological pathways that explain the broad spectrum of clinical change
would be our prime candidate. Looking at the dura we have to take into consideration
the sensory innervation and such aspects as dural tension. As previously mentioned
the dura is innervated supratentorially by the trigeminal nerve and infratentorially
by the vagus nerve. When the vagus and trigeminal nerves are brought to threshold
by some sort of mechanical pressure the axons project to the sensory nuclei
of each one of these nerves These include the sensory nucleus for the trigeminal
nerve and the solitary nucleus for the vagus nerve. In theory the vagal and
trigeminal afferent input will facilitate via collateral's the motor nuclei
for the reticular formation and other bulbar nuclei facilitating visceral
and somatic efferents. This in theory can account for visceral changes seen
with cranial manipulation. The question at hand is whether the amount of movement
that the cranial manipulation imparts is enough to bring the sensory aspects
of the dura to threshold? Also we must consider that the majority of these
afferents are nociceptive in nature. As Upledger describes there is a great
deal of movement in the cranial sacral respiratory mechanism. If this movement
is not enough to bring these afferents to threshold it is not likely that
cranial manipulation will. However, this constant motion of the cranial sacral
mechanism may be enough to maintain a base line level of mechanoreceptor barrage
from the dura. If this is the case, then in theory, if there is a change in
cranial sacral motion then the afferent barrage from the dura to the sensory
nuclei of the trigeminal nerve and the solitary nucleus of the vagus nerve
would change. In other words, a normal amount of cranial sacral motion will
maintain a normal amount of afferent input to vital centers.
Looking at
cerebral spinal fluid flow as a candidate one must question the actual changes
that take place in pressure from normal abdominal and thoracic cavity pressure
changes. The CSF pressure ranges from 50 to 150 mmHg. Keeping this in mind
it is not likely that the subtle changes that are made with cranial manipulation
are going to have a significant effect on the overall pressures.
The vestibular
system is of primary importance in maintaining upright posture, muscle tone
and eye movement. The two vestibular apparati do not function properly unless
they are in normal juxtaposition with each other. If their positional relationship
is lost, the vestibular output is of two different messages, which makes accurate
central processing impossible. The central pathways of the vestibular system
are principally motor reflex connections to nuclei innervating extraocular
muscles, the motor reticular formation, the spinal motor neurons, and the
cerebellum. With this in mind one can see there is an enormous amount of potential
for affecting physiological function. The vestibular projections to the motor
reticular formation are multipurpose purpose in nature. When you consider
that blood pressure needs to change with position and muscle tone needs to
change with position it is easy to see the necessity of these projections.
Additionally, the autonomic effect that the reticular formation has are not
restricted to blood pressure changes. The reticulospinal pathways drive the
intermediolateral cell column and subsequently the autonomic system. The reticular
formation involvement alone can account for visceral as well as somatic changes.
There are
abundant vestibular projections to the cerebellum. The cerebellum has projections
to the reticular formation, and directly and indirectly to the thalamus. These
projections can account for autonomic changes via the thalamohypothalamo pathways,
and cortical changes from projections from the thalamus to the cortex. Also,
the cerebellum modulates movement via cerebellobasalganglionic and cerebellothalamocortical
projections. And finally, the reticulospinal and vestibulospinal pathways
modulate trunk flexion and extension.
Additionally,
the vestibular mechanism projects directly to the thalamus and hypothalamus.
The thalamic projections are thought to account for cortical appreciation
of position changes. And the hypothalamic projections are thought to be necessary
for autonomic response to movement.
After examining
the three possible mechanisms for the effect of cranial manipulation it is
easy to see that the vestibular apparatus has the largest potential for affecting
change on the central neuraxis and consequently the overall physiology. However
I also find it difficult to completely ignore the possibilities of dural feedback
from the mechanoreceptors located there. So the conclusion that I would draw,
with the available information, is that the vestibular mechanism is the primary
mechanism with the dural mechanoreceptor mechanism secondary.
If one takes
into account the vast neuronal network affected by the vestibular mechanism
it is easy to account for the far reaching effects that cranial manipulation
can have on the health of the patient. If the practitioner has these pathways
to memory and available for explanation it will be effortless for him to explain
to other practitioners why he is getting such fabulous results from his treating
techniques. This should be another example as to why we need to have a greater
understanding of what we are accomplishing when we are using manipulative
therapies.