· Types of pain
· Pain pathways
· Stage 1 pain
· Stage 2&3 pain
· Analgesia
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Somatic pain |
· Mainly Aδ fibres · Fast, sharp, well localized · Triggers withdrawal |
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Visceral pain |
· Mainly C fibres · Slow, dull, poorly localized · Elicits guarding |
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Referred pain |
· Pain perceived at a location distal to the site of insult · Due to convergence of somatic and visceral 1° afferents on same WDR projection neurons |
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Somatic afferent |
· Nociceptive receptor in tissues e.g. perineum · 1° afferent: soma in dorsal root ganglion, synapse at dorsal horn (e.g. via sacral nerve roots) · 2° afferent: decussates in anterior commissure and ascends via spinothalamic tract · 3° afferent: to cortex (somatic) and subcortex (visceral, autonomic centres) |
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General visceral afferent |
· Nociceptive receptor in tissue e.g. cervix · 1° afferent: piggybacks onto autonomic efferents o With SNS: via white ramus communicans to spinal cord T1-L2 o With PSNS: via sacral nerves to S2-4 (also cranial) o Direct synapse with wide dynamic range 2° afferents in deeper layers o Convergence onto somatic nociceptive-specific 2° afferents in superficial layers · 2° afferent: o Decussation via anterior commissure (incomplete for older pathways) o Ascent via multiple spinothalamic tracts · 3° afferent: o From brainstem or diencephalon to higher structures |
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Definition |
· Until cervix effaced and 10cm dilated |
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Somatic pain |
· Minimal |
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Visceral pain |
· Rhythmic myometrial contraction – distension of lower segment and cervix · GVA pathway: T10-12 via ovarian and inferior hypogastric plexuses · Location: lower abdomen, sacrum, back |
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Definitions: |
· 2: Until baby is delivered · 3: Until placenta is delivered |
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Somatic pain |
· Continuous stretching and tearing of perineum and outer 1/3 vagina · Pathway: pudendal nerve (S2-4), ilioinguinal and genitofemoral nerves (L1-2) · Location: perineum and vagina |
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Visceral pain |
· Continuous pressure on pelvic viscera · GVA pathway: T10-L1 via ovarian and inferior hypogastric plexuses; S2-4 via sacral plexus · Location: pelvis, inner thigh |
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Endogenous |
· Ascending: via non-pain modalities o Activation of inhibitory interneurons · Descending: by NAd > 5-HT from brainstem nuclei (e.g. periaqueductal grey) o Disinhibition of brainstem nuclei by endogenous opioids |
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Neuraxial |
· Local anaesthetic: o Spinal nerve roots o Dorsal horn o Spinal cord · Opioid: o Dorsal horn o Brain via CSF o Systemic absorption |
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