Peripheral nerve injuries Flashcards
What makes up the motor unit/nerve fibre?
- Alpha motor neuron
- Motor axon
- Muscle fibres (NMJ)
Where do the sensory unit cell bodies lie?
The dorsal root ganglia which lie outside of the spinal cord. Their axons lead down to a variety of sensory endings.
What join together to form the spinal nerve which exits the vertebral column?
The anterior and posterior roots (motor and sensory)
Describe the structure of peripheral nerves
Highly organised structure comprised of nerve fibres, blood vessels and connective tissue
What covers axons?
Endoneurium
Fascicles are nerve bundles. What covers them?
Perineurium
What layer of connective tissue covers the nerve itself?
Epineurium
As nerve fibres increase in size what happens to the speed of transmission?
It also increases
Common causes of nerve trauma
- Direct - blow or laceration
- Indirect - avulsion or traction
Nerve trauma can be divided into which 3 categories?
- Neurapraxia
- Axonotmesis
- Neurotmesis
These are increasing in severity
What is neurapraxia?
- A nerve that has been damaged but is still in continuity
- Damage may be caused by compression, stretching or bruising from direct trauma
- What happens is you get reversible conduction block due to local ischaemia and demyelination - so in a small part of the nerve you cannot get conduction
- It settles after weeks or months but if the underlying cause was compression it will not heal if this is still present
What is axonotmesis?
- Endoneurium (the tube) is in continuity but you get disruption of the axon lying within it - more severe injury
- Causes: stretching or crushed or direct blow
- Wallerian degeneration follows - nerve fibre disappears but the tube remains.
- Prognosis is okay. Because the endoneurium tube is intact the nerve can regrow within it. Often the sensory recovery is not completely normal.
What is neurotmesis?
- Complete division of the nerve - loss of endoneurium tube.
- May even lose perineurium too.
- Occurs by laceration or avulsion.
- There is no recovery for a nerve if it is completely divided unless repaired by direct suturing or grafting. This is because there’s disordered growth of the nerve.
Open or closed nerve injuries
Another way of describing nerve injuries
- Closed
- Associated with nerve injuries in continuity i.e neuropraxis or axonotmesis.
- Spontaneous recovery is possible
- Surgery indicated after 3 months
- Typically stretching of nerve i.e brachial plexus injuries or radial nerve humeral fracture
- Open
- Frequently related to nerve division - knives/glass injuries
- Treated with early surgery
- Distal portion of the nerve undergoes Wallerian degeneration (2-3 weeks after injury)
Clinical features of nerve injury
Sensory
- Dysaethesia - disordered sensation this may be numbness, reduced sensation or pins and needles
Motor
- Paresis (weakness) or paralysis +/- wasting
- Dry skin - peripheral nerves carry parasympathetic and sympathetic nerves in addition to generalised sensory and motor nerves. There will be loss of tactile adherence since sudomotor nerve fibres are not stimulating sweat glands in skin
Diminished or absent reflexes as without a peripheral nerves the reflex has nothing to travel up.
Healing of nerve injuries
Is very slow!!
- Starts with the initial death of axons distal to the site of injury - Wallerian degeneration
- Then degradation of the myelin sheath
- And the nerve dies back to the site of the next node of ranvier
- The proximal axonal budding (very start of new growth) starts after about 4 days
- Regeneration proceeds at a rate of about 1mm/day
- Pain is the first modality to return
What does it mean by miswiring?
The damaged nerve regrows but misses its own tube and grows down another
Prognosis of nerve injury healing
Depends on whether the nerve is pure (only sensory or only motor) or mixed (both sensory and motor)
or how distal the lesion is (the more proximal the worse the recovery)
What is Tinel’s sign?
A way of detecting irritated nerves.
- If you tap over the end of nerve that is regrowing then a shooting discomfort or tingling feeling can be felt (parasesthesia) running into the site of where the nerve will end up
- This can help to monitor recovery
Nerve repair
Direct repair
- Laceration - the 2 nerve endings are close together and there’s no loss of nerve tissue then it can be repaired
- Surgeons try to repair the outer layer and also the individual bundles themselves
Nerve grafting
- Used when part of the nerve is lost
- Late repair
- Sural nerve is used to provide the tube - passage for axons to regrow down
How to determine between peripheral or central nerve injuries?
LMN lesions - peripheral nerve lesions - the whole nerve is transected
UMN lesions - central nerve lesions - the peripheral nerves are still in tact
- Strength - in both muscle power is decreased so strength is decreased
- Deep tendon reflexes - in the LMN lesions with the nerve transected there is no loop for the tendon reflexes to work along and therefore they are diminished or absent
- Atrophy - where there is no muscle stimulation or limited muscle stimulation the muscle will atrophy therefore in UMN where there is generalised increase in tone and often an increase in activity atrophy will mostly be absent. Where as LMN lesions will show significant atrophy
