Week 7: Dental and Orofacial Pain (Sensory Innervation to the Oral Cavity and Chronic Pain)

Question 1

Describe the myelination, diameter, category type and thermal threshold of Aβ primary afferent axons

Answer 1

Aβ Fibers:
* Myelinated
* Large diameter
* Proprioception, light touch
* Thermal threshold: Nil

Question 2

Describe the myelination, diameter, category type and thermal threshold of Aδ primary afferent axons

Answer 2

Aδ Fibers:
* Lightly myelinated
* Medium diameter
* Nociception (mehcnaical, thermal, chemical)
* Thermal threshold: ~53°C Type I and ~43°C Type II

Question 3

Describe the myelination, diameter, category type and thermal threshold of C primary afferent axons

Answer 3

C Fibers:
* Unmyelinated
* Small diameter
* Innoccus temperature, itch
* Nociception (mechanical, thermal, chemical)
* Thermal threshold: ~43°C

Question 4

Describe the Trigeminal Sensory Pathway (Generally)

Answer 4

• Sensory inputs (e.g. TMJ, oral mucosa)
• Cell bodies in the the Trigeminal ganglion
• Central brain region (lateral medulla where the the Spinal Trigeminal Nucelus is located?)
• Processing of nociceptive signals
• Thalamus
• Cortex

*Three-neuron chain in the pathway

Question 5

In orofacial pain processing which fibers terminate in Subnucleus Caudalis?

Answer 5

C-fibers

*some Aδ fibers too

Question 6

Where do Aδ fibers terminate?

Answer 6

Aδ fibers terminate in subnucelus oralis, subnucleus interporalis, and (some) in subnucelus caudalis.

Question 7

Which fibers terminate in the Main Sensory nucleus?

*MAIN SENSORY NUCLEUS (OR PRINCIPAL/PONTINE SENSORY NUCELUS)

Answer 7

Aβ Fibers

Question 8

Which nucleus is involved in processing proprioceptive information associated with the jaw/teeth?

Answer 8

Mesencephalic Trigeminal Nucelus

Question 9

What are receptive fields?

Answer 9

Receptive fields:
An individual neuron innervates a defined area of skin, a group of neurons, with axons in a segmental nerve, innervate a dermatome

Question 10

What is somatotopy?

Answer 10

Somatotopy:
The spatial organization of this information is preserved at different levels of the neuro-axis

Question 11

The majority of nociceptive output is frm which nucelus?

Answer 11

Subnucelus Caudalis

Question 12

What can be used in the treatment of the Nociceptive Trigeminal Inhibitory (NTI) or Jaw Opening reflex?

Answer 12

Splints

Question 13

What is the definition of pain?

Answer 13

The International Association for the Study of Pain (IASP) defines pain as:

An unpleasant sensory and emotional expereince associated with actual or potential tissue damages, or described in terms of such damage

Question 14

How can pain be described?

Answer 14

Pain is commonly described as acute/nociceptive/physiological OR chronic when it lasts for longer periods

Question 15

What is the definition of chronic pain?

Answer 15

IASP defines chronic pain as pain without apparent biological value that has persisted beyond normal tissue healing time (usually taken to be 3 months)

Question 16

What percentage of the population expereince migraines?

Answer 16

~20% of the population

Question 17

Head and orofacial pain are common. Describe how cranial structures and orofacial pain are connected?

Answer 17

Cranial structures such as the dura mater, cerebral vasculature, cornea, dental pulp and TMJ aare innervated nociceptors. Actionvations of neurons usually results in pain.

Question 18

What are the key underlying mechanisms of migraines?

Answer 18

Peripheral vascular is a key underlying mechanism for migraines. The meninges are the deepest structures that contain nociceptors. The throbbing aspects of headaches are attributed to the pulsations in blood vessels.

Question 19

Which drug group is used in the treatment for migraines? Provide examples and describe the type and drug targets.

Answer 19

Drug group: Triptans
Examples: Sumatriptan, Rizatriptan
Type/target: Serotonin1B, Serotonin1D, Serotonin1F receptor agonists

Question 20

What is the definiton of neuropathic pain?

Answer 20

IASP defines neuropathic pain as “pain caused by a lesion or disease in the somatosensory nervous system”.

Lesion can be seen by imaging, biopsies etc.

Disease is used when the underlying cause is known (e.g. diabetes)

Question 21

True or false: Neuropathic pain is a form of physiological pain

Answer 21

FALSE
NEUROPATHIC PAIN IS FORM OF CHRONIC PAIN

Question 22

Discuss the characteristics, pain, triggers, treatments, the differential diagnoses of Trigeminal Neuralgia, and the relation to dentistry.

Answer 22

Trigeminal Neuralgia:
* Characterized by sudden, brief, paroxysms of pain on one side of the face in the distribution of one of the branches of V
* Pain lasts only for short periods of time and is often described as “stabbing” or like an electrical shock
* Pain can be triggered by trivial (normally innocuous) sensory stimuli (e.g tooth brushing, brushing hair, touching the face, talking)
* Sometimes a specifc trigger point is located in the gingivae
* No sensory loss in the specific area
* Pain may remit for weeks, then return
* Treatment usually with drugs: Carbamazepine is often effective
* Differential: post-herpatic neuralgia, Multiple Sclerosis in those >40
* Patients often present to the dentist with toothache
* The desscription of dental pain, especially pulpitis, is similar to that of Trigeminal Neuralgia.
* If findings: examination/radiographs etc. are inconsistent with symptoms of pulpitis then do not commence irreversible procedures
* Trigeminal Neuralgia does not usually wake patients from sleep whereas dental pain may

*Treatment of Trigeminal Nerualgia may also include surgical intervention

Question 23

Discuss Herpes Zoster

Answer 23

Herpes Zoster (Shingles) is caused by the chicken pox virus, Varicella Zoster virus. In Shingles, the is usually a rash and is often quite painful from bilsters. Once Shingles subsides the virus remains in the patient in the appropriate dermatome. The affects of the virus may reappear years later, especially when the immune system is compromised. The incidence of Shingles is age-related (up 3.4:1,000 person years in healthy ppulation increasing to 12:1,000 person years in those of 65).

Question 24

Discuss Post-Herpes Neuralgia and treatment

Answer 24

When nerves are damged by an outbreak of shingles , post-herpatic neuralgia can develop. This pain can last more than a month (some studies report pain lasting years). Post-Herpatic Neuralgia is a condition responsible for 10-15% of vists to pain clinics. The main symptom of this condition is pain (mild-severe) that may come and go. The pain may be described as deep aching, burning, stabbing and the area may be very sensitive to touch or change in temperature.

Treatment:

Anti-virals e.g. acyclovir
Anti-epileptic drugs e.g. carbamazepine, gabapentin (for pain)
Topical local anaesthetic patches (short-term use)
Other analgesia
Tri-cylic anti-depressants

Question 25

True or False: Opioids are ineffective in the treatment of chronic pain

Answer 25

TRUE
Opioids are effective in the treatment of acute pain but not (that) useful in the treatment of chronic pain

Question 26

Discuss opioid tolerance

Answer 26

• Opioid drugs target Gi/o coupled receptors
• Shift dose-response curve to the right
• Respiratory depression is the MAIN side effect of opioids (constipation etc.)
• Opioid tolerance- change in how the Gi/o receptors work and the drug concentrations highly increase to produce effects

Question 27

Describe the drug treatments in chronic pain

Answer 27

• Tri-cyclic anti-depressants (TCAs)
  -Example: Amitriptyline
  -used at lower than doses, onset of action is quicker and efficacy unlikely to be related to mood
  -act by blocking the reuptake of serotonin and noradrenaline = increasing activity in these descending inhibitory pathways
  -Treatment for Trigeminal neuralgia and post-herpatic neuralgia
• Anti-epileptic drugs: Example Carbamazepine
  -Blocks voltage-gated sodium ion channels = lowers neuronal hyperexcitability
  -Treatment for Trigeminal Neuralgia
  -Side effect (Dental): xerostomia and ulceration
• Gabapentin
  -Increase GABA release from neurons and acts on voltage-gated Calcium ion channels to decrease excitatory inputs to the spinal cord
• Tramadol
  -Used as an adjunct
  -Weak opioid agonist, which also inhibits re-uptake of noradrenaline and serotonin
  -Benefit = causing less dependence, respiratory depression and constipation than stronger opioids
• Ketamine
  -NMDA (glutamate) receptor antagonist (key to central sensitization)
  -Problems: can cause hallucinations, delusions, psychosis