In one breath
A handful of words describe how pain sensitivity has gone wrong: hyperalgesia (lowered threshold), hypoalgesia (raised threshold), analgesia (none at all), hyperpathia (an exaggerated reaction once pain is felt) and allodynia (pain from a stimulus that should not hurt, like light touch). The named syndromes then follow: causalgia, a burning pain after nerve trauma that outlives the injury and is maintained by sympathetic fibers; inflammatory pain, the commonest of all; neuropathic pain from nerve injury, which responds poorly to analgesics; tic douloureux, or trigeminal neuralgia; thalamic syndrome after a posterior thalamic stroke; and toothache, from the richly innervated dental pulp.
Builds on: Pain: concepts, receptors and fibers · Endogenous pain control and gate control theory · Leads to: Trigeminal pathway and trigeminal neuralgia · Thalamus: nuclei, functions and thalamic syndrome · Sensory lesions and sensory function tests
Terminologies
| Term | What it means |
|---|---|
| Hyperalgesia | greater sensitivity, lowered threshold to painful stimuli |
| Hypoalgesia (hypalgesia) | less sensitivity, raised threshold |
| Analgesia | complete loss of pain sensation |
| Hyperpathia | pain perception is faulty, but the reaction is exaggerated once pain is perceived |
| Allodynia | excessive response to a mild stimulus |
- Hyperalgesia: inflamed skin is the everyday cause.
- Analgesia need not take the other sensations with it; pain alone can be lost (Sensory lesions and sensory function tests).
- Hyperpathia: the reaction to other stimuli is increased as well, not only to painful ones.
- Allodynia: the classic example is light touch, which is never painful in health, producing pain.
Hyperalgesia, hyperpathia, allodynia
Three words that get swapped in vivas. Pin them to the stimulus:
- Hyperalgesia — a painful stimulus hurts more than it should.
- Allodynia — a harmless stimulus hurts at all.
- Hyperpathia — the reaction to the stimulus is out of proportion, and other stimuli are over-reacted to as well.
Assessing a patient’s pain
Six things the physician notes:
- How it began.
- Its quality — burning, stabbing, aching.
- Its location, and any referral (Visceral and referred pain).
- Its intensity and duration.
- What provokes it and what relieves it.
- The physiological and behavioural responses that go with it.
Causalgia
- A burning pain that follows traumatic injury of a peripheral nerve.
- It is continuous, and usually carries hyperalgesia and allodynia with it.
- The striking thing: it persists even after the nerve damage has fully recovered.
- The sympathetic fibers are thought to keep it going, because it is relieved by sympathectomy or by adrenergic blockers.
- The mechanism proposed: noradrenergic sympathetic fibers grow into the dorsal root ganglia serving the injured area. After that, every sympathetic discharge produces pain there.
Applied: why a pain can be sympathetically maintained
Normally the sympathetic system has nothing to do with sensation. In causalgia the sprouted noradrenergic fibers hand it a line straight into the sensory neurons of the DRG, so ordinary sympathetic traffic now produces pain — which is why removing or blocking the sympathetic supply relieves it.
Inflammatory pain
- Inflammation is the commonest cause of pain, and inflammation follows injury.
- The mediators, including cytokines and substance P, raise sensitivity and lower the pain threshold in the injured area.
- So a minor painful stimulus now produces severe pain: these substances increase pain perception.
- It is one of the two kinds of pathologic pain (Pain: concepts, receptors and fibers).
Neuropathic pain
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Pain caused by injury to a nerve. Neuralgia is the general term for it.
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Trigeminal neuralgia from herpes zoster, causalgia included, is the book’s example.
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Other causes: diabetic neuropathy, nerve injury, the polyneuropathies, root irritation as in a prolapsed disc, and spinal cord injury.
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It usually comes with hyperesthesia (increased cutaneous sensitivity), hyperalgesia, hyperpathia and allodynia.
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It does not respond well to analgesics. Treatment climbs in steps:
- usually helpful: an epidural injection of analgesic together with a steroid;
- in severe cases, cutting the pathway —
- nerve section: cut the nerve carrying sensation from the injured site;
- dorsal rhizotomy: cut the pain fibers in the dorsal root;
- anterolateral cordotomy, also called spinothalamic tractotomy: cut the ascending spinothalamic fibers in the cord.
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Worth knowing, though not in your pages: the reason anterolateral cordotomy is done on the side opposite the pain is that the second-order fibers have already crossed in the cord (Pain pathways and perception).
Tic douloureux
- Another name for trigeminal neuralgia.
- Severe, stabbing, acute facial pain that lasts only briefly each time.
- Most patients get it after compression of the trigeminal nerve roots (Trigeminal pathway and trigeminal neuralgia).
Thalamic syndrome
- Chronic, severe and extremely unpleasant pain, arising spontaneously or from a trivial stimulus.
- The pain is felt in the brain, with no peripheral pathology at all.
- Cause: an ischemic or hemorrhagic lesion of the posterior thalamic nuclei, after obstruction of the thalamogeniculate branch of the posterior cerebral artery (Thalamus: nuclei, functions and thalamic syndrome).
- Treatment: the pain is cured by stereotaxic destruction of the posterior thalamic nucleus.
Central pain, not peripheral
Thalamic syndrome is the standard example of central pain: cut every nerve to the limb and the pain stays, because the pain is generated in the thalamus. Compare causalgia, where the trigger sits in the periphery.
Toothache
- Teeth are full of nociceptors.
- They are supplied by axons of the maxillary and mandibular divisions of the trigeminal nerve — C, Aδ and Aβ fibers.
- Route of the fibers: in through the root apex, branching within the pulp, ending in the pulp plexus.
- Enamel and cementum carry no nerve fibers. The pulp is the sensitive part, and enamel is its cover and shield.
- So exposure of the pulp, or inflammation of it, is what hurts.
- Four things set a toothache off: heat, cold, inflammation, and probing the pulp mechanically.
Exam-answer skeleton: "Define hyperalgesia, hypoalgesia, hyperpathia and allodynia, and give an example of each" (short note)
- One line on pain threshold, and on inflammation lowering it, so the definitions have something to hang on.
- Hyperalgesia: lowered threshold, greater sensitivity; example, inflamed skin.
- Hypoalgesia: raised threshold; example, a local anesthetic or a partial nerve lesion.
- Analgesia: complete loss of pain, which may spare the other sensations.
- Hyperpathia: faulty perception with an exaggerated reaction once pain registers, and over-reaction to other stimuli too.
- Allodynia: pain from a stimulus that should not hurt; example, light touch.
- Where they cluster in practice: inflammatory pain (hyperalgesia, allodynia), neuropathic pain and causalgia (all four, plus hyperesthesia).
- Close with the six points a physician records when assessing pain.
Asked in exams
- Allodynia and the other pain terms: Mar 2024, MCQ