 The image
to the right is a rough
representation of how individual axons combine within several layers of
connective tissue sheaths to form larger nerve bundles. These in
turn combine to form nerve trunks. Note that even within the nerve trunks,
each individual axon retains its separate identity, thus traveling all the
way from the tooth to its cell body in the ganglion on that side of the
head.
The nerves in your body never exist separate from a support
system composed of several types of connective tissue, and blood vessels of
various sizes. In fact, wherever you find a nerve, you will find at
least one blood vessel accompanying it. Conversely, wherever you find
a blood vessel, you will find at least one nerve accompanying it. Any
complex of nerves and blood vessels is called a neurovascular bundle.
The blood vessels and nerves in a human body are like the roots of two
intertwining trees. The source of the blood vessels is the heart, and
the source of the nervous system is the brain and accompanying spinal cord.
The intertwined "twigs" in the outer areas of the body combine to form
larger and larger branches and "trunks" as they progress toward their
respective sources. The "nerve" inside a tooth
is really a neurovascular bundle, since it is composed of connective tissue
and blood vessels as well as nerve endings. Inflammation in the nerve of a tooth will
often affect the chemistry and physiology of the neurovascular tissues all along the
course of the nerve trunk making it difficult for anesthesia to penetrate
into nerve bundles even at points relatively far removed from the actual
site of the toothache. It is easy to see why producing good
anesthesia may be quite tricky if the patient puts off coming in for
treatment until the inflammation has gotten out of hand.
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The
detail on the right is a representation of the fine anatomy of a nerve bundle as
it enters into the apex (the tip) of the root of a lower front tooth. Note
the extent to which it branches again and again inside the tooth. As the
nerve bundle enters into the root, it is composed of several individual
axons with accompanying blood vessels, each axon representing a separate neuron (nerve cell). Remember that
these neurons actually live well outside of the tooth itself in the Gasserian
ganglion on one side of the head. Each branch contains fewer and fewer
axons until the branches become so fine that each one represents a separate axon
from an individual neuron.
Upon occasion, it is very difficult to produce numbness in a
particular tooth. The best way to avoid the (rare) horror show is to be treated
when you first feel the pain. Normal toothaches with moderate pain numb out just
as easily as any healthy tissue. If your dentist notes something on an x-ray and
recommends that you have a filling or a root canal, don't put it off until it
hurts. If you wait until you have acute pain or an abscess, even touching
the tooth is painful and getting you numb may be difficult, especially if you
are swollen. (Note that if the pulp is entirely dead in a tooth, and there
is no pain, the tooth is simply an empty tube that happens to be filled with
dead tissue, and the root canal can be done from beginning to end without any
shots at all.)
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How decay progresses inside
a tooth
 The
image on the left is a more detailed diagram of a cross section of a
tooth. The white covering on the tooth is called enamel.
It really is white in color, but somewhat translucent and allows the
color of the underlying structures to shine through. It
is also very hard and quite resistant to acid attack. The
brownish yellow material underlying the enamel is called dentin, and
it too is hard, but it is much less hard than enamel. The
dentin has a density like that of hard bone. It is much less
resistant to acid attack than enamel. Underlying the dentin is
the nerve of the tooth. The nerve is actually a complex organ.
In a healthy state, it is pink and soft, like the lining of your
mouth, and is composed of blood vessels, connective tissue and, of
course, nerve fibers. The dentin overlying the nerve is
permeated with thousands of tiny tubules which run perpendicularly
from the nerve to the enamel/dentin interface, and also to the outer
surface of the root in areas which are not covered by enamel. These tubules are filled with fluid.
The fluid is actually contained within tiny projections from cells
that line the inside of the nerve space. These cells are part
of the nerve complex and are called odontoblasts. Touching the living dentin (or even a
stream of air blown across exposed dentin) produces
movement of the fluid in the tubules which transmits impulses back to the nerve
making the dentin sensitive to any type of direct stimulus. Because of the
presence of these tubules, the dentin is actually quite permeable to fluids. |
 The image on the right is a picture of an actual tooth which has been
attacked by decay. It has been stained to
better show the structures within the tooth. Originally, the tooth was adjacent to two other teeth
which made contact with this one at the positions shown by the yellow arrows.
Since teeth can move slightly when pressure is applied to them (such as when a
person chews or clenches his teeth) the teeth can rub together at the
contact points. The combination of acid attack from sugar soaked
plaque,
plus the friction of the constant rubbing of the teeth at the contact points
produces tiny holes in the enamel. The contact on the left side of the
tooth shows how acid plus friction can produce a hole in the enamel. This
one has not yet penetrated through to the dentin. The contact on the right
shows what happens when the enamel has been breached allowing the plaque
organisms to
penetrate into the dentin. Note that while the hole in the enamel is
relatively small, the decay has rapidly progressed within the dentin to a much
larger extent due to the relative softness and permeability of the dentin as
compared with the enamel. The decay has a tendency to spread along
the dentinal tubules from the enamel surface toward the nerve from which the
tubules arise.
Surprisingly, when a tooth is attacked by decay in this way, it only hurts
when the decay first penetrates through the enamel into the dentin.
After a few days, the pain stops because the odontoblasts inside the affected
tubules tend to die off fairly quickly. When this happens, the affected
tubules are called "dead tracts". Once the tracts die off, there is
no more pain until the decay actually approaches the nerve itself.
Referred pain
Tooth related pain can be rather hard to understand.
Pain emanating from one tooth may be felt in another tooth far removed from the
actual culprit. This is why it is sometimes difficult for a dentist to
make an accurate decision of which tooth to treat, especially if the tooth that
the patient believes is the one that needs treatment shows no actual signs of
disease. In my office, it is not unusual for a patient to present with
vague pain in an area of the mouth with no way to decide which tooth is actually
at fault. It becomes a real diagnostic problem when multiple teeth have
deep restorations or cavities, but only one of them is the actual culprit.
But which one?? When this happens, I generally send the patient home
with an analgesic and ask him or her to return when the pain localizes in one
area.
In other cases, one tooth may be obviously at fault, but the
patient is feeling pain in his ear, eye, temple, or in teeth in the opposite
arch as well as in the obviously affected tooth. Finally, you get
the really weird cases in which pain actually coming from, say, a top front
tooth is felt in a back bottom tooth. This actually happened to me when a
patient appeared with pain in a lone standing lower back tooth (a molar).
The molar had no cavities or fillings, and did not react badly to cold air or
tapping. I sent the patient home, and she returned the next day with an
abscess in a top central front tooth!
There
are essentially two reasons that pain in one area may refer to another.
 The diagram to the right is a schematic of
the Trigeminal nerve. This nerve is responsible for all the feeling on
one side of your face. (The other side of your face has its own
Trigeminal nerve.) There are three branches, all of which originate in
the semilunar (Gasserian) ganglion. The Ophthalmic branch gives feeling to
the face around the eye, bridge of the nose and the forehead. The Maxillary
branch is responsible for the feeling in your upper teeth and gums as
well as the facial area below the eye and above and including the top lip.
The Mandibular branch is responsible for conveying feeling from your
bottom teeth, gums and tongue as well as the skin below and including the
lower lip.The actual mechanism of pain referral from one area of the
head or neck to another is not well understood. One theory of
referred pain involves the way inflammation affects the functioning of
the nerves. Pain in a tooth understandably causes
inflammation in the nerve bundle that leaves the tooth and, if it is intense
enough, it may cause inflammation along the entire length of the affected
neurovascular bundle all the way up to the nerve cell bodies (a nerve cell is called a
neuron) located in the semilunar ganglion. Each neuron has hundreds
"dendrites" which make contact with other neurons and can, in
turn, excite
them by excreting chemicals called neurotransmitters. There are
several different kinds of neurotransmitters, some causing excitation of the
communicating neurons, and some causing inhibition. Under normal
circumstances each neuron is programmed to excrete "normal"
amounts of the various types of neurotransmitter molecules. When a
neuron is inflamed, it is "sick" and may excrete not only unusual
amounts of neurotransmitter, but perhaps even different ratios of different
types of neurotransmitters. This imbalance in neurotransmitters can
have unpredictable effects on communicating neurons, and may be responsible
for exciting neurons that lead to the brain in ways that create the
perception of pain in far flung areas of the head and neck. Thus pain in a back lower tooth may be felt in the ear, eye, or in an
entirely different tooth. Note that the pain never refers to any
structure on the opposite side of the face. This is because the
"short circuit" effects only one of the two Trigeminal nerves.
Nerves from one side of the face do not anatomically contact any of the
nerves on the other. Thus, contralateral (opposite sided) "short
circuits" are impossible.
- Your brain is a learning machine. Not only does it
learn math and reading in school, it must also learn to localize pain as
well. Whenever someone touches your arm, hand or face, your brain
knows immediately where that stimulus came from because it has had lots of
experience being touched in those areas. Stimulation along any of the
pathways from any area of your skin is immediately localized in the brain so
you know exactly where the stimulus came from.
On the other hand areas inside your body are not so
public and do not receive outside stimulation very often. When the
brain receives stimulation from a deep area, it is not always aware of the
exact location of the stimulus. The brain has an idea of the general
area of the distribution of a particular deep nerve trunk, but it has no
experience of the specific neural pathways along that trunk. Thus, a
heart attack may be felt as indigestion, or pain radiating down the left
arm. Liver pain is frequently felt as a burning sensation on the skin
on the right torso.
Mouth pain is similar to deep body pain. Pain
experienced deep inside a tooth, or deeply inflamed tissue such as
that found in
pericorinotis
(infection around an unerupted tooth) or
aphthous
ulcers (canker sores--those painful sores with white centers and red
borders that occur on the underside of the tongue, or on the mucous membrane
inside the cheeks) can feel very painful in wide expanses of the mouth or
head.
Neuritis and Neuralgia
Neuritis means inflammation of a nerve (the suffix "itis" tacked onto
any biological entity means inflammation). The four classic signs of
inflammation are redness, swelling, pain and heat. This is easily
visualized when you think of what happens if you hit your finger with a hammer.
In the case of nerves, these processes are often hidden inside normal tissues,
but if one could actually see the neurovascular bundle where the neuritis is
taking place, one would see them in action. Inflammatory processes are
usually acute, which means that they are generally of short duration and resolve
over time. Neuritises generally manifest as pain in the distribution of
the affected nerve, however, in the case of inflamed motor neurons (nerves which
control muscles), they may manifest as paralysis of the affected muscles as in
the case of Bell's palsy, which is paralysis of the muscles of the
face--generally on one side. The treatment for neuritis is treatment of
the underlying condition causing it; antibiotics for infections, pain medication
to allow for healing, etc.
However, a small number of acute neuritises settle down to become chronic
situations, and these can present a serious long term problem, presenting as
phantom pain or serious sensitivity in the distribution of the affected nerve.
This condition is known as a neuralgia (The suffix "algia" means pain).
Neuralgias can be quite difficult on a patient, because the chronic pain may
require long term pain management. Sometimes non steroidal anti
inflammatory drugs are sufficient to control the symptoms, however, some cases
require more potent narcotics. This is a dangerous solution since
narcotics are habit forming and doctors are reluctant to write too many class II
narcotic prescriptions. No doctor wants to turn his or her patient into an
addict, and the DEA (Federal Drug Enforcement Administration) will eventually
arrest the doctor. When this type of pain occurs, it is always
advisable to try using one of the anticonvulsant drugs such as Neurontin (gabapentin),
Tegretol (carbamazepine) or Lyrica (pregabalin).
Trigeminal Neuralgia
(Tic doloureux) The most painful disease in all of medicine!
Trigeminal neuralgia is a seizure-like condition which causes
episodes of intense, stabbing, electric shock-like pain in certain areas of the face.
This pain is usually unilateral (on one side of the face only), but in 5% to 10%
of patients it may occur bilaterally (on both sides of the face), although
attacks do not generally occur on both sides of the face at the same time.
Patients with multiple sclerosis are more likely than other people to experience
trigeminal neuralgia, and when they do have TN, are more likely to experience it
bilaterally.
The
distribution of the pain varies from patient to patient depending upon which branches
are affected. The pain is debilitating. The "shocks last
seconds, but come in bursts that can last for several
hours. It is sometimes referred to as the "suicide disease"
because of its intractability and persistence, and because of the sheer misery
it causes the patient and those who must live with him or her. This
disease generally strikes after the age of fifty, but rare cases have been seen
in younger patients. Women are effected more frequently than men. The pain may be spontaneous, but most of the time it
is stimulated by light touching of certain "trigger points" located
virtually any place on the head, face or inside the mouth or nose. Men may
avoid shaving particular areas of their faces and women may not apply makeup to
their trigger points. In rare instances, patients have been known to
starve to death because the trigger point is located in the mouth.
Trigeminal neuralgia is not treated by dentists, but dentists
are often involved in its diagnosis, mostly by way of misdiagnosis of
toothaches. Trigeminal neuralgia is often mistaken for very serious
tooth pathology when it first appears, and almost invariably results in the loss
of one or more of the patient's natural teeth. This result is the tragic
consequence of the rarity of the disease and the similarity of the symptoms of
trigeminal neuralgia with the symptoms of an acute pulpitis (inflammation of the
nerve inside a tooth) which may refer serious pain
throughout large parts of the distribution of the trigeminal nerve. The
tipoff that this is not a normal toothache comes after the tooth (teeth) have
been extracted, but the pain still persists unabated. A dentist may
practice for decades before seeing a case of trigeminal neuralgia. His job
is to relieve pain, and the worse the pain, the more pressing is the need to
relieve it! There are no definitive methods for the differentiation of the
pain caused by trigeminal neuralgia from the pain that may be referred by a severe
pulpitis, so a misdiagnosis and the
consequent extraction of one or more teeth is quite understandable in light of
the severe pain the patient presents in the dental office. The tooth
may be a most likely culprit simply because trigeminal neuralgia may manifest
(in part) as a severe toothache.
There is hope. Trigeminal neuralgia can be treated in
one of two ways:
- Drug therapy--A number of anticonvulsant drugs are
available to reduce the frequency, intensity and duration of attacks:
- Surgery--Several forms of surgical correction of
trigeminal neuralgia are available. They are generally effective, but not
always permanent. Most are done under general (complete) anesthesia and
are
considered safe for all categories of patient. These surgeries fall
into three general categories.
- The most frequently performed types are
called rhizotomies. A rhizotomy is done either by
the chemical, electrical or mechanical ablation (destruction) of the
Semi Lunar ganglion (also called the gasserian ganglion).
Rhizotomies usually result in permanent numbness of at least part of the
effected side of the face. The numbness is considered by most
patients to be a small price to pay for relief from this painful
condition.
- The second category of treatment is called microvascular
decompression surgery and is done by the placement of Teflon implants between the nerve and
the offending blood vessels. This option rarely produces
numbness, but is a complicated and very expensive procedure.
- The third
and newest surgical treatment is not surgery at all. It is called
Gamma Knife Radiosurgery (GKRS), and it involves aiming 201 beams of
cobalt-60 radiation focused precisely on a specific region in the
brain--in this case the trigeminal nerve root. This precise
concentration of radiation radiates the structure in question without
damaging surrounding areas. The pain of TN usually subsides within
several weeks. Click
here to download an excellent brochure on
this subject. The brochure is in pdf format and requires
Adobe acrobat (free download). The
Gamma knife is used for numerous brain lesions including virtual cures for
some seizure disorders and Parkinson's disease.
 In my opinion, the very best graphic demonstration explaining
the mechanism and various treatments of trigeminal Neuralgia has been posted on
the web by the University of Manitoba in Canada and can be seen by clicking on
the icon to the right.
The tutorial requires the installation of the Macromedia Shockwave Flash
plug-in, which can be downloaded for free at the linked site.
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neuron
cell that continues into one of the nerve trunks that issue from the ganglion. 
