Musculoskeletal Growth/Injury and Repair - Nerve Flashcards
Describe how axons are grouped together to form peripheral nerves
- Axons are coated with endoneurium and grouped into fascicles (nerve bundles)
- Fascicles covered with perineurium and grouped to form nerves, which are coated with epineurium
How does nerve width influence AP conduction speed? How wide are the nerves that innervate muscle?
- Wider the nerve the faster the conduction
- Nerves innervating muscles tend to be wide, allowing for rapid responses
What are the three types of nerve injury caused by trauma?
Neurapraxia
Axonotmesis
Neurotmesis
What are some common peripheral nerve conditions caused by compression of peripheral nerves?
Carpal tunnel syndrome (median nerve at wrist)
Sciatica (spinal root by IV disc)
Morton’s Neuroma
Describe the characteristics of neurapraxia
- Nerve remains in continuity, damaged due to stretching / bruising etc.
- Causes a reversible conduction block, usually as a result of demyelination or ischaemia due to trauma
- Prognosis is good, usually recover in weeks / months
Describe the characteristics of axonotmesis
- Endoneurium remains intact but axons damaged
- Wallerian degeneration, but since endoneurium remains intact there can be nerve axonal growth to recover
- Prognosis is alright, sensory recovery more common but diminished in capability
What is Wallerian degeneration?
- Disappearance of nerve axons distal to the site of injury, the nerve fibres shrink back towards the last viable nerve of ranvier
Describe the characteristics of neurotmesis
- Nerve is completely divided in injury, no remaining endoneurium etc
- No recovery unless repaired (sutures / grafts) - high chance of nerve attempting to regenerate but mis-wiring occurring due to absence of endoneurium to guide growth
- Prognosis is poor
What is a closed nerve injury? Prognosis? Treatment?
Nerve injury where the nerve remains in continuity
- Spontaneous recovery usually possible
- If there is no sign of recovery after 3 months surgery to check integrity of nerve recommended
What is an open nerve injury? Treatment?
Nerve injury where the nerve is divided
- Treat with early surgery: suturing / graft (try repair endoneurium)
If an open nerve injury is sutured together early on, is there immediate recovery of function?
No
- There is still Wallerian degeneration of the nerve distal to the injury, surgery just gives the opportunity to regrow
What are some signs / symptoms of peripheral nerve injury?
- Dysaethesiae (disordered sensation: paraesthesia / anaesthesia)
- Paresis (weakness: progresses to paralysis +/- wasting)
- Dry skin (peripheral nerves supply sweat glands in skin)
- Diminished / absent reflexes
How long does it take for first nerve regeneration (proximal axonal budding) to occur after injury? At what rate does the nerve regrow? Which sensation is first to return?
Takes about 4 days to begin (have to wait for Wallerian degeneration and myelin sheath degradation)
- Regeneration occurs at a rate of about 1mm / day (up to 3/5mm / day in children)
- Pain usually first modality to return
What factors affect the prognosis of recovery from nerve injury?
- Type of nerve lesion (continuous / divided)
- How distal the injury is (more distal = better prognosis bc less chance end plate degenerates before nerve grows back)
- Whether the nerve is pure (only sensory or only motor) or mixed (prognosis better for pure)
What is Tinel’s sign? What is it used for?
- Paraesthesia (or other sensation) can be felt by tapping the distal end of the regrowing nerve
- Used to tell how far the nerve has regrown as sensation is felt at distal end of nerve
