Causes and effects of denervation

Question 1

Define denervation?

Answer 1

Damage to peripheral nerves

Question 2

Give 4 effects of sensory denervation due to peripheral neuropathy?

Answer 2

Allodynia

Hyperalgesia

Paraesthesia

Numbness

Question 3

What 2 aspects of the somatosensory system are altered by sensory denervation due to peripheral neuropathy?

Answer 3

Pain and temperature pathways

Discriminative/fine touch

Question 4

Define discriminative/fine touch?

Answer 4

Perception of vibrations, shapes, sizes, textures

Question 5

Does lower or upper motor denervation cause paralysis?

Answer 5

Lower

Question 6

Does lower or upper motor denervation cause paresis, and what is paresis?

Answer 6

Lower

Paresis is muscle weakness

Question 7

Does lower or upper motor denervation commonly cause muscle atrophy?

Answer 7

Lower

Question 8

What are the 2 resulting effects of muscle atrophy caused by lower motor denervation?

Answer 8

Increases susceptibility to cramps

Leads to generalised sensitivity to acetylcholine along whole sarcolemma and causes fibrillation (spontaneous contraction of individual muscle fibres)

Question 9

What is fibrillation?

Answer 9

Spontaneous contraction of individual muscle fibres

Question 10

What is fasciculation, and what kind of denervation is it a common clinical feature of?

Answer 10

Visible quick twitch of muscle fibres

Common sign of lower motor denervation

Question 11

If a patient has lower motor denervation, what is the result of their plantar reflex test?

Answer 11

Negative babinksi sign (hallux flexion)

Question 12

Describe the reflexes of a patient with lower motor denervation compared to upper motor denervation?

Answer 12

Lower motor denervation causes hyporeflexia (underactive reflex)

Upper motor denervation causes hyperreflexia (overactive reflex)

Question 13

If a patient has upper motor denervation, what is the result of their plantar reflex test?

Answer 13

Positive babinski sign (hallux extension)

Question 14

What is flaccidity, and what kind of denervation is it a characteristic clinical feature of?

Answer 14

Muscle paralysis that makes them soft and yielding to passive stretching due to loss of peripheral nerves

Clinical feature of lower motor denervation

Question 15

What is spasticity, and what kind of denervation is it a characteristic clinical feature of?

Answer 15

Muscle-control disorder characterised by stiffness and involuntary contractions

Clinical feature of upper motor denervation

Question 16

How long are muscles viable for without a nerve supply, and what 2 processes occur in these muscles in this time period?

Answer 16

Chronic peripheral neuropathy causes fat build up and muscle fibrosis

Muscles are viable without nerve supply for up to 2 years

Question 17

In chronic peripheral neuropathy, what happens to muscles if they have no nerve supply for over 2 years?

Answer 17

Severe muscle fibrosis and fat build up causes irreversible loss of normal function

Question 18

Define upper motor neuron lesion and lower motor neuron lesion?

Answer 18

UMN lesions: Lesions that occur in motor neurons that originate in cerebral cortex and terminate in brainstem/spinal cord (CNS)

LMN lesions: Lesions that occur in motor neurons that originate in spinal cord and terminate in gland/muscle (PNS)

Question 19

Give 2 general ways in which UMN and LMN lesions can be distinguished?

Answer 19

Present with different clinical features

UMN lesions only affect central nerves and LMN only affect peripheral nerves

Question 20

Define severance, and does it cause mild or severe peripheral nerve injury which leads to denervation?

Answer 20

Nerve is physically divided by excessive stretch or laceration

Severe PNI due to complete interruption of axon and connective tissue

Question 21

What are the 2 characteristic features of severance of a peripheral nerve?

Answer 21

Immediate loss of sensation

and/or muscle paralysis in areas supplied by affected nerve

Question 22

Give 2 common causes of mononeuropathy and 4 causes of polyneuropathy?

Answer 22

Mononeuropathy caused by trauma and ischaemia

Polyneuropathy caused by hereditary, autoimmune, metabolic disease and toxins

Question 23

Is diabetes more likely to cause polyneuropathy or mononeuropathy?

Answer 23

Polyneuropathy

Question 24

Are drug toxins more likely to cause polyneuropathy or mononeuropathy?

Answer 24

Polyneuropathy

Question 25

Is HIV more likely to cause polyneuropathy or mononeuropathy?

Answer 25

Polyneuropathy

Question 26

Are infections more likely to cause polyneuropathy or mononeuropathy?

Answer 26

Polyneuropathy

Question 27

Are nutrition disorders more likely to cause polyneuropathy or mononeuropathy?

Answer 27

Polyneuropathy

Question 28

Are fractures more likely to cause polyneuropathy or mononeuropathy?

Answer 28

Mononeuropathy

Question 29

What is traumatic axonopathy?

Answer 29

Damage primarily occurs to axon of peripheral nerve which causes denervation

Question 30

Define axonotmesis, and is it a mild or severe peripheral nerve injury?

Answer 30

Injury and compression of axon and myelin but endoneurium and other connective tissues remain intact

Usually mild PNI as intact endoneurium provides supportive framework for axon to regenerate, but is more severe than neuropaxia

Question 31

Describe recovery of a peripheral nerve after axonotmesis has occured?

Answer 31

Spontaneous recovery often occurs and is slow (axon regrowth 1mm/day)

Often incomplete recovery due to myelin scarring

Question 32

Does a patient regain full functional recovery after axonotmesis?

Answer 32

Degree of functional recovery depends on communication of regenerated axon with end-organ

Question 33

Define neurotmesis, and is it a mild or severe peripheral nerve injury?

Answer 33

Complete transection (cut) of peripheral nerve

Severe PNI, and is more severe than axonotmesis and neuropaxia

Question 34

Does spontaneous recovery occur after neurotmesis?

Answer 34

No, recovery requires surgical intervention to use nerve grafts to reconnect transected ends

Question 35

Does a patient regain full functional recovery after neurotmesis?

Answer 35

Likely to never regain full function of affected nerve

Question 36

What process occurs in peripheral nerves after axonotmesis and neurotmesis, and how long after does this occur?

Answer 36

Wallerian degeneration: Active degenerative process of axon distal to peripheral nerve lesion that occurs due to being crushed/cut

Occurs distal to peripheral nerve injury 24-36 hours after

Question 37

Where in the damaged peripheral nerve does wallerian degeneration occur?

Answer 37

Distal to peripheral nerve lesion

Question 38

What is traumatic myelinopathy?

Answer 38

Damage to myelin limited to site of injury in a peripheral nerve, which causes denervation

Question 39

Define neuropaxia, and is it a mild or severe peripheral nerve injury?

Answer 39

Focal segmental (patchy) demyelination at site of peripheral nerve injury without disrupting axon or endoneurium

Mild PNI

Question 40

How does neuropaxia affect somatosensation and movement?

Answer 40

Conduction block or slowed conduction causes impaired somatosensation and movement

Question 41

Does a patient regain full functional recovery after neuropaxia?

Answer 41

Often full functional recovery that takes 8-12 weeks

Question 42

What is the characteristic presentation of distal symmetrical length-dependent neuropathy?

Answer 42

Affects extremities first (feet then hands) as these are supplied by the longest axons