10: MSK Growth, Injury and Repair - Peripheral Nerve Injuries Flashcards
what is a motor unit (efferent) composed of?
- anterior horn cell, (located in the grey matter of spinal cord)
- motor axon
- muscle fibres (neuromuscular junctions)
what is a sensory unit composed of?
- cell bodies in posterior root ganglia
- i.e. lie outside the spinal cord
what is a peripheral nerve?
- the part of a spinal nerve distal to the nerve roots
- a bundle of nerve fibres
what is the function of A-alpha, group IA and IB afferent fibre types?
large motor axons
muscle stretch and tension
sensory axons
what is the function of A-beta group II afferent fibres?
touch, pressure, vibration and joint position sensory axons
what is the function of A-gamma fibres?
gamma efferent motor axons
what is the function of A-delta group III afferent fibres?
- sharp pain
- very light touch
- temperature sensation
what is the function of B fibres?
sympathetic preganglionic motor axons
what is the function of C Group IV afferent fibres?
- dull, aching, burning pain
- temperature sensation
compression/entrapment of peripheral nerves: list some classical conditions
- carpal tunnel syndrome > median nerve at wrist
- sciatica > spinal root by intervertebral disc
- Morton’s neuroma > digital nerve in 2nd or 3rd web space of forefoot
describe neurapraxia
- Neurapraxia is the mildest form of peripheral nerve injury commonly induced by focal demyelination or ischemia.
- In neurapraxia, the conduction of nerve impulses is blocked (reversible).
- nerve in continuity
describe axonotmesis
- more severe than neurapraxia
- peripheral nerves become damaged due to stretch or crushed or indirect blow
- the endoneurium remains intacts (tube in continuity), but there is a disruption of axons.
- prognosis if fair
describe neurotmesis
- complete division of a peripheral nerve, caused by laceration or avulsion
- no recovery unless repaired by direct suturing or grafting
- endoneural tubes disrupted so high change of ‘miswiring’ suring regeneration
- prognosis is poor
what Sunderland grade is a neurapraxia?
grade 1
what sunderland grade is a axonotmesis?
grade 2
what sunderland grade is a neurotmesis?
grade 3-5 depending on severity
closed nerve injuries are associated with
- nerve injuries in continuity such as neuropraxis or axonotmesis
- typically stretching of nerve: brachial plexus injuries and radial nerve humeral fracture
when is surgery indicated for a closed nerve injury?
after 3 months if no recovery is identified clinically or by electromyography
what is the axonal growth rate?
1-3 mm/day
open nerve injuries are typically associated with?
nerve division > neurotmesis
e.g. knives/glass
how are open nerve injuries treated?
early surgery
In an open nerve injury, the distal portion of the nerve undergoes which type of degeneration?
Wallerian
- occurs up to 2-3 weeks post injury
what are some clinical features of a nerve injury?
sensory:
- dysaethesiae (disordered sensation): anaesthetic (numbness), hypo & hyper-aesthetic, paraesthetic (pins and needles)
motor:
- paresis (weakness) or paralysis +/- wasting
- dry skin
- loss of tactile adherence since sudomotor nerve fibres not stimulating
reflexes: diminished or absent
describe the healing process of a nerve injury
Starts with initial death of axons distal to site of injury:
- Wallerian degeneration
- then degredation of myelin sheath
- proximal axonal budding occurs after about 4 days.
- regeneration proceeds at a rate of about 1mm/day (can be 3-5mm in children).
- pain is first modality to return.
the prognosis for nerve injury recovery depends on…
whether the nerve is:
- ‘pure’ (only sensory or only motor)
- ‘mixed’ (sensory and motor within same nerve)
- how distal the lesion is (proximal worse)
which sign can be used to monitor nerve injury recovery?
Tinel’s sign
- (tap over site of nerve and paraesthesia will be felt as far distally as regeneration has progressed.
how can a nerve injury be assessed, and recovery monitored?
by electrophysiological nerve conduction studies
when is a direct repair of a nerve injury indicated?
- laceration
- no loss of nerve tissue
when is nerve grafting indicated?
nerve loss
- late repair (retraction, sural nerve)
describe the ‘rule of three’ surgical timing in a traumatic peripheral nerve injury
- immediate surgery within 3 days for clean and sharp injuries
- early surgery within 3 weeks for blunt/contusion injuries
- delayed surgery, performed 3 months after injury, for closed injuries
how do you tell the difference between a peripheral or central nerve lession (UMN vs LMN)?
strength, tone, deep tendon reflexes, clonus, Babinski’s sign, atrophy
