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Neuro II > Peripheral Nerve Degeneration > Flashcards

Peripheral Nerve Degeneration Flashcards

1
Q

Identify & define the indicated components of the provided image:

A
  • Enodneurium: surrounds individual axons
  • Perineurium: bundles groups of axons into fascicles
  • Epineurium: surrounds groups of fascicles & also forms the protective sheath on the nerve
    • find blood supply
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2
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3
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4
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5
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6
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7
Q

Which Sunderland degree classifications of nerve injury do not require physician intervention?

A

1, 2, & 3

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8
Q

What tests can be performed in order to classify the degree of nerve injury?

A

sensory & motor testing

nerve conduction studies

surgery

MRI - Diffusion Tensor Imaging

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9
Q

What defines the proximal end of a nerve injury? Distal end?

A

Proximal: where all the axons are still attached to the soma

Distal: only has axons (that will degrade)

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10
Q

What is the human response to nerve injury? How does it work?

A

Wallerian Degeneration (Controlled inflammatory response)

getting rid of distal end → clearing everything up, then regrow the axon to innervate the target tissue

  1. Degeneration (distal axon)
  2. Regeneration (of proximal axon)
  3. Re-innervation (target tissue)
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11
Q

Where in the body does Wallerian degeration occur? Why?

A

ONLY in PNS

degeneration in CNS is too slow (b/c oligodendrocytes do not help the same way the schwan cells do),

regeneration is not supported b/c it is not a permissive environment (d/t development of glial scars- that helps make sure structure is still intact, but inhibits growth)

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12
Q

Describe what happens within the first 24 hrs of Wallerian Degeneration

A
  • Calcium influx from extracellular space (both in proximal & distal end) → induces a sealing of the ends
    • proximal part usually degenerates up to first node of ranvier
  • Degeneration
    • proximal part usually degenerates up to first node of ranvier
    • all of distal nerve degenerates
      • Myelin breakdown begins
      • resident macrophages begin clean up (induced by intact schwann cells)
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13
Q

Describe what happens by 48hrs of Wallerian Degeneration

A
  • Axon & myelin breakdown continues distally
    • myelin debris inhibits outgrowth from proximal stump
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14
Q

What variable is important in determining timeline of Wallerian Degeneration?

A

Where the injury occurred & the length of distal nerve that needs to be degenerated & how much needs to be regenerated

(ie. if injury in proximal arm, a lot of nerve needs to be degenerated, much less if injury is in finger)

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15
Q

What substance inhibits the outgrowth of the axon from the proximal stump?

A

myelin debris

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16
Q

Describe what happens by 72hrs of Wallerian Degeneration

A
  • more macrophages called in from circulation that build up along whole distal nerve
  • reactive Schwann cells proliferate & line up → begin to form tube : Bands of Bungner
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17
Q

What is the purpose of the Bands of Bungner?

A

the regrowing axon will use a guidance to find the target tissue

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18
Q

Describe what is happening about 1 week into Wallerian Degeneration?

A
  • macrophages subside (when debris of myelin is mostly cleared up)
    • substances are gone that inhibit the growth cone
  • anti-inflammatory agents are present
  • reactive Schwann cells continue to form Bands of Bungner → basically ready to be used as a cue for regrowth
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19
Q

Describe what is happening weeks-years into Wallerian Degeneration?

A
  • once the regrown axon has found the target tissue (to increase conduction speed)
    • Schwann cells are regenerating myeline
    • axon fiber matures & thickens
  • hopefully → regain of function
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20
Q

24-36 hrs post injury, Schwann cells release what substances? This has what effect?

A
  • Proinflammatory agents → signal to resident macrophages to start clearing debris
    • TNFa
    • IL-1B
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21
Q

36-48 hrs post injury, what is happening at the molecular level in Wallerian Degeneration? This has what effect?

A
  • Proinflammatory agents → stimulate fibroblasts
    • TNFa
    • IL-1B → recruits monocytes from the circulation (differentiate into macrophages & help with clean up)
      • MCP-1
  • Fibroblast → tells schwann cells to proliferate & phagocytosis
    • GM-CSF
22
Q

48-72 hrs post injury, what is happening at the molecular level in Wallerian Degeneration? This has what effect?

A
  • Schwann Cell
    • MCP-1 → peak macrophage concentration helping to clear debris
  • Macrophages (after most of debris has been cleared)
    • IL-10 (anti inflammatory)
      • stops the controlled inflammatory response “Walllerian Degeneration” by preventing Schwann cells from releasing TNFa & IL-1B
  • Schwann cells start forming Bands of Bungner
23
Q

What is occurring at the molecular level 3-7 days post injury in Wallerian Degeneration?

A
  • Schwann cells →tell proximal stump that it is time to regrow the axon
    • NGF
    • BDNF
  • Fibroblasts → potentiate axon growth
    • NGF
  • Macrophage → numbers will decrease
    • IL10
24
Q

At what time point in Wallerian Deneration does the new axon start to grow?

A

7-10 days post injury

25
What is the rate of axon growth during regeneration?
1-4 mm/day
26
What variables impact the prognosis for a functional recovery?
* site of injury * proximal (long) vs distal (short) * axon diameter/length * small regenerates better (C \> A) * patient's age * better in younger patients * delay before intervention * observation for spontaneous recovery
27
Functional recovery is highly dependent on what variable?
TIME!
28
Chronically axotomized neurons leads to:
* schwann cell atrophy * target tissue atrophy * muscle & end-plate only good for 1 yr
29
What situations can lead to proximal degeneration?
direct trauma to cell body → necrosis axotomy → controlled apoptosis (not well understood)
30
Are spinal motor neurons or sensory neurons more likely to experience retrograde cell death?
* sensory (exogenous neutrophins help survival) * motor: * 20-30% apoptosis in proximal lesion * most peripheral lesion regenerate
31
What is the intervention if a nerve gap is present? If it is \<5mm? If it is \>5mm?
* \< 5mm → suture ends together * \>5 mm → another approach * just suturing will lead to excessive tension that impairs regeneration
32
What is the biggest problem in suturing the two end of a nerve gap?
scar formation if a scar develops between proximal & distal nerve ends, this physically blocks axonal sprouts from finding distal nerve
33
What is the most critical step in all nerve repairs?
nerve debridement - getting rid of the scars
34
What are the 3 different methods to suture the ends of nerves back together?
1. **Epineural neurrhaphy** 1. use topography of vasculature to align proximal & distal ends 2. faster & less traumatizing to the tissue 2. Groups fascicular neurorrhaphy 1. strip away epineurium & suture groups of fascicles together 3. individual fascicular neurorrhaphy
35
What are the benefits & risks of choosing epineural neurorraphy?
* benefit * good alignment using blood vessels * fast * risk * no guarantee of good fascicular alignment
36
What are the benefits & risks of choosing fascicular neurorraphy?
* benefit * better change good fascicular alignment * risk * slower * may not produce better clinical outcomes
37
What are the benefits & risks of choosing individual fascicular neurorraphy?
* benefit * best chance for good fascicular alignment * risk * very slow * increased number of sutures may cause too much scar formation
38
What structures are used to bridge the gap between nerve ends when the gap is \> 5 mm?
nerves blood vessels synthetic conduits
39
What is an “autograft” & what are the benefits and risks to this approach to bridging nerves?
nerve from the patient is sacrificed to repair the damaged nerve * benefit * “gold standard” * sural nerve is commonly used (don't need it really) * risk * additional surgical trauma is required * undergo normal Wallerian degeneration to provide natural pathways for outgrowth → double suture site increases scar formation to impair regeneration
40
What is an “allograft” & what are the benefits and risks to this approach to bridging nerves?
a nerve is taken from cadaver & is used to repair the damaged nerve * benefit * no additional patient surgical procedures required * risk * rejection is a large problem * not a permanent problem b/c axon regrows through graft & graft degenerates * patients on immunosuppressant drugs
41
What is a “conduit” & what are the benefits and risks to this approach to bridging nerves?
a way to trap important matrix proteins, growth factors, & schwann cells at the place where it is needed
42
What are the 3 types of conduits & important features of each?
* Biological conduit * graft taken from patient (ie. venous) * biodegradable * synthetic collagen * FDA approved * degeneration with time * maximum gap length 3mm * Nonbiological material * silicone * experimental * does not degenerate - additional surgery
43
What are the phases of regeneration through a conduit?
1. **fluid phase** 1. accumulation neutrophilic factors & ECM molecules 2. **Matrix phase** 1. fibrin cable formation 3. **Cellular phase** 1. Schwann cell migration, proliferation & alignment, and tissue cable formation 2. formation fo Bands of Bungner 4. **Axonal phase** 1. growth of daughter axons (proximal to distal) across de novo tissue cable 5. **Myelination phase** 1. myelination of regenerated immature axons forming mature axonal fibers
44
What is a major problem with conduit repair?
Axons are all over the place in the distal nerve b/c you cannot really create alignment
45
What is something that patient can do to enhance regeneration?
exercise
46
What are the problems that occur with poor regeneration?
* loss of sensory & motor function * neuroma - intense, chronic pain * axonal sprouts fail to locate distal nerve * axonal sprouts continue to grow but form un-inervated mass
47
What is the process behind neuroma formation?
scar formation blocks neurite access to distal nerve → but they continue to grow the can lead to the formation of a neuroma; particularly in areas that are exposed to repeated trauma
48
What can be done if a neuroma is formed?
* surgery is often required to remove the neuroma * nerve regeneration can be attempted (nerve graft / conduit) * the resected nerve can be buried in muscle to protect it & prevent regeneration of the neuroma
49
What drugs can be given to patients suffering from a neuroma? Mechanism?
Gabapentin & Pregabalin inhibit Cava2d & reduce pain
50
What ion channels are increased that lead to chronic neuropathic pain often seen with neuromas?
Nav1.3 Nav1.8 Nav1.9 Cava2d

Neuro II (30 decks)

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