Causes and effects of denervation Flashcards
Define denervation?
Damage to peripheral nerves
Give 4 effects of sensory denervation due to peripheral neuropathy?
Allodynia
Hyperalgesia
Paraesthesia
Numbness
What 2 aspects of the somatosensory system are altered by sensory denervation due to peripheral neuropathy?
Pain and temperature pathways
Discriminative/fine touch
Define discriminative/fine touch?
Perception of vibrations, shapes, sizes, textures
Does lower or upper motor denervation cause paralysis?
Lower
Does lower or upper motor denervation cause paresis, and what is paresis?
Lower
Paresis is muscle weakness
Does lower or upper motor denervation commonly cause muscle atrophy?
Lower
What are the 2 resulting effects of muscle atrophy caused by lower motor denervation?
Increases susceptibility to cramps
Leads to generalised sensitivity to acetylcholine along whole sarcolemma and causes fibrillation (spontaneous contraction of individual muscle fibres)
What is fibrillation?
Spontaneous contraction of individual muscle fibres
What is fasciculation, and what kind of denervation is it a common clinical feature of?
Visible quick twitch of muscle fibres
Common sign of lower motor denervation
If a patient has lower motor denervation, what is the result of their plantar reflex test?
Negative babinksi sign (hallux flexion)
Describe the reflexes of a patient with lower motor denervation compared to upper motor denervation?
Lower motor denervation causes hyporeflexia (underactive reflex)
Upper motor denervation causes hyperreflexia (overactive reflex)
If a patient has upper motor denervation, what is the result of their plantar reflex test?
Positive babinski sign (hallux extension)
What is flaccidity, and what kind of denervation is it a characteristic clinical feature of?
Muscle paralysis that makes them soft and yielding to passive stretching due to loss of peripheral nerves
Clinical feature of lower motor denervation
What is spasticity, and what kind of denervation is it a characteristic clinical feature of?
Muscle-control disorder characterised by stiffness and involuntary contractions
Clinical feature of upper motor denervation
How long are muscles viable for without a nerve supply, and what 2 processes occur in these muscles in this time period?
Chronic peripheral neuropathy causes fat build up and muscle fibrosis
Muscles are viable without nerve supply for up to 2 years
In chronic peripheral neuropathy, what happens to muscles if they have no nerve supply for over 2 years?
Severe muscle fibrosis and fat build up causes irreversible loss of normal function
Define upper motor neuron lesion and lower motor neuron lesion?
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)
Give 2 general ways in which UMN and LMN lesions can be distinguished?
Present with different clinical features
UMN lesions only affect central nerves and LMN only affect peripheral nerves
Define severance, and does it cause mild or severe peripheral nerve injury which leads to denervation?
Nerve is physically divided by excessive stretch or laceration
Severe PNI due to complete interruption of axon and connective tissue
What are the 2 characteristic features of severance of a peripheral nerve?
Immediate loss of sensation
and/or muscle paralysis in areas supplied by affected nerve
Give 2 common causes of mononeuropathy and 4 causes of polyneuropathy?
Mononeuropathy caused by trauma and ischaemia
Polyneuropathy caused by hereditary, autoimmune, metabolic disease and toxins
Is diabetes more likely to cause polyneuropathy or mononeuropathy?
Polyneuropathy
Are drug toxins more likely to cause polyneuropathy or mononeuropathy?
Polyneuropathy
Is HIV more likely to cause polyneuropathy or mononeuropathy?
Polyneuropathy
Are infections more likely to cause polyneuropathy or mononeuropathy?
Polyneuropathy
Are nutrition disorders more likely to cause polyneuropathy or mononeuropathy?
Polyneuropathy
Are fractures more likely to cause polyneuropathy or mononeuropathy?
Mononeuropathy
What is traumatic axonopathy?
Damage primarily occurs to axon of peripheral nerve which causes denervation
Define axonotmesis, and is it a mild or severe peripheral nerve injury?
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
Describe recovery of a peripheral nerve after axonotmesis has occured?
Spontaneous recovery often occurs and is slow (axon regrowth 1mm/day)
Often incomplete recovery due to myelin scarring
Does a patient regain full functional recovery after axonotmesis?
Degree of functional recovery depends on communication of regenerated axon with end-organ
Define neurotmesis, and is it a mild or severe peripheral nerve injury?
Complete transection (cut) of peripheral nerve
Severe PNI, and is more severe than axonotmesis and neuropaxia
Does spontaneous recovery occur after neurotmesis?
No, recovery requires surgical intervention to use nerve grafts to reconnect transected ends
Does a patient regain full functional recovery after neurotmesis?
Likely to never regain full function of affected nerve
What process occurs in peripheral nerves after axonotmesis and neurotmesis, and how long after does this occur?
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
Where in the damaged peripheral nerve does wallerian degeneration occur?
Distal to peripheral nerve lesion
What is traumatic myelinopathy?
Damage to myelin limited to site of injury in a peripheral nerve, which causes denervation
Define neuropaxia, and is it a mild or severe peripheral nerve injury?
Focal segmental (patchy) demyelination at site of peripheral nerve injury without disrupting axon or endoneurium
Mild PNI
How does neuropaxia affect somatosensation and movement?
Conduction block or slowed conduction causes impaired somatosensation and movement
Does a patient regain full functional recovery after neuropaxia?
Often full functional recovery that takes 8-12 weeks
What is the characteristic presentation of distal symmetrical length-dependent neuropathy?
Affects extremities first (feet then hands) as these are supplied by the longest axons
