1.3. Musculoskeletal Growth Injury and Repair - Peripheral Nerve Injuries Flashcards
What is a Motor Unit?
A Motor Axon, leaving from the Anterior Horn Cell (Gray Matter) of the Spinal Cord, to supply Muscle Fibres
What is a Sensory Unit?
Cell Bodies in the Posterior Root Ganglia (lie outside of the Spinal Cord)
What do Nerve Fibers join to form?
- Ventral (Anterior) Motor Roots
2. Dorsal (Posterior) Sensory Roots
What do the Ventral and Dorsal Nerve Roots combine to form?
A Spinal Nerve
Where do the Spinal Nerves exit the Vertebral Column?
Via an Intervertebral Foramen
What is a Peripheral Nerve?
A bundle of Nerve Fibres which are the part of the Spinal Nerve, Distal to the Nerve Roots
What is the size range for a Peripheral Nerve?
0.3-22 μm
What surrounds Peripheral Nerves?
- Schwann Cells form a thin Cytoplasmic Tube
- Myelin Sheath - Large Fibres are in a Multi-layered insulating Membrane
- Multiple layers of Connective Tissue surround the Axons
What are Axons?
Long processes of Neurons
What are Axons coated with?
Endoneurium
What are Axons grouped into?
Fascicles
What are Fascicles covered in?
Perineurium
What is formed when Fascicles are grouped?
A Nerve
What is a Nerve covered in?
Epineurium
What is a Neurone?
A Nerve Cell
What surrounds Neurons?
Schwann Cells
What are the different Nerve Fiber Categories? And what are their relative sizes?
- A-Alpha (Group 1A and 1B afferent’s) - 15 microns
- A-Beta (Group 2 afferent’s) - 12-14 microns
- A-Gamma - 8-10 microns
- A-Delta (Group 3 afferent’s) - 6-8 microns
- B - 2-5 microns
- C (Group 4 afferent’s) - <1 micron
- What is the function of A-Alpha Neurons?
2. What speed does nerve signal travel through them? (in meters / second)
- a) Large Motor Axons
- b) Muscle Stretch Sensory Axons
- c) Muscle Tension Sensory Axons
- 60-100m/s
- What is the function of A-Beta Neurons?
2. What speed does nerve signal travel through them? (in meters / second)
- a) Touch Sensory Axons
- b) Pressure Sensory Axons
- c) Vibration Sensory Axons
- d) Joint Position Sensory Axons
- 30-60m/s
- What is the function of A-Gamma Neurons?
2. What speed does nerve signal travel through them? (in meters / second)
- Gamma Efferent Motor Axons
2. 15-30m/s
- What is the function of A-Delta Neurons?
2. What speed does nerve signal travel through them? (in meters / second)
- a) Sharp Pain Sensation
- b) Very Light Touch Sensation
- c) Temperature Sensation
- 10-15m/s
- What is the function of B Neurons?
2. What speed does nerve signal travel through them? (in meters / second)
- Sympathetic Pre-Ganglionic Motor Axons
2. 3-10m/s
- What is the function of C Neurons?
2. What speed does nerve signal travel through them? (in meters / second)
- a) Dull, Aching Pain Sensation
- b) Burning Pain Sensation
- c) Temperature Sensation
- < 1.5m/s
What are the 2 broad categories of injury which can occur to a nerve?
- Compression / Entrapment (at different possible levels)
2. Trauma
What are some classical Compressive Nerve Injury Conditions?
- Carpal Tunnel Syndrome (Median Nerve at the Wrist)
- Sciatica (Spinal Root by the Intervertebral Disk)
- Morton’s Neuroma (Digital Nerve in 2nd / 3rd Web Space of Forefoot)
What are the 2 Sub-Categories of Trauma Nerve Injury?
- Direct (Blow, Laceration)
2. Indirect (Avulsion, Traction)
What are the 3 types of Trauma Nerve Injury?
Note - This is not the Direct vs Indirect question
- Neurapraxia
- Axonotmesis
- Neuromesis
What is Neurapraxia?
Reversible conduction block of a Nerve caused by:
1. Stretching (8% will damage microcirculation)
2. Bruising
Leading to local Ischaemia and Demyelination
What is the prognosis of Neurapraxia?
Good (Weeks to Months)
Is Axonotmesis more severe than Neruapraxia?
Yes
What is Axonotmesis?
Disruption of Axons, with the Endoneurium intact, caused by:
- Stretching ++ (15% elongation disrupts axons)
- Crushed Nerve
- Direct Blow to the Nerve
What is the prognosis of Axonotmesis?
- Fair. Sensory recover better than motor, but often, not normal sensation returns but enough to recognize pain, hot/cold, sharp and blunt
- Wallerian degeneration follows
Is Neurotmesis more severe than Axonotmesis?
Yes
What is Neurotmesis?
Complete Nerve Division via either:
- Laceration
- Avulsion
Is there any recovery from Neurotmesis?
Not unless it is repaired via suturing / grafting
What is the prognosis of Meurotmesis?
Poor, Endoneural tubes are disrupted so there is a high chance of “Mis-wiring” during regeneration
What is a Closed Nerve Injury associated with?
This is associated with Nerve Injuries in Continuity (Neuropraxia / Axonotmesis)
How are Closed Nerve injuries treated?
- They aren’t, spontaneous recovery is possible (Axonal Growth rate is 1-3 mm/day)
- Surgery is indicated after 3 months
- If no recovery is identified then Electromyography is required
What is an Open Nerve Injury associated with?
Neurotmeic injuries (knives / glass) Note - The distal portion of the nerve undergoes Wallerian Degeneration (occurs up to 2 weeks after the injury)
How are Open Nerve Injuries treated?
Early Surgery
What are the clinical features of Nerve Injury?
Sensory: Dysaethesiae (Disordered sensation)
Motor:
1. Paresis (Weakness) or Paralysis +/- Wasting
2. Dry Skin
Reflexes: Diminished / Absent
How does the Nerve Heal after an Open Nerve Injury?
- Wallerian Degeneration (initial death of axons distal to the site of injury)
- Degradation of the Myelin Sheath
- Proximal Axonal Budding occurs (after 4 days)
- Regeneration proceeds at 1mm/day
Note - Pain is the first modality to return
What is the prognosis for recovery of an Open Nerve Injury dependent on?
- Whether the Nerve is:
- a) Pure - only sensory or motor
- b) Mixed - both sensory and motor
- How distal the lesion is (proximal is worse)
How can:
- Injury be assessed?
- Recovery be monitored?
- Electrophyiological Nerve Conduction Studies
- a) Tinel’s Sign (tap along the nerve to see how distally they can feel)
- b) Electrophyiological Nerve Conduction Studies
How can the Nerve be repaired?
- Direct Repair (For Laceration with no loss of nerve tissue)
- Nerve Grafting (Nerve loss)
What is the “Rule of Three” when it comes to 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
