Neurology - Disorders of peripheral nerves I Flashcards
3 types of connective tissue surrounding peripheral nerve
Axons present in peripheral nerves grouped into fascicles
1.) Epineurium - binds individual fascicles together; outer layer merges with the dura mater of spinal roots
- ) Perineurium - binds each fascicle with elastic fibres; acts as diffusion barrier
- ) Endoneurium - supports individual axons within each fascicle
Blood supply to peripheral nerves
Vasa nervorum - blood vessels supplying the fascicles located in the epineurium
Do all peripheral nerves have a Schwann cell?
Yes - but only in some cells (myelinated fibres) does the Scwann cell membrane spiral around the axon
What are the 3 types of fibre in peripheral nerves?
Type A (myelinated) - carries vibration and position sense; alpha motor neurones
Type B (myelinated) - autonomic preganglionic
Type C (unmyelinated) - pain and temperature afferents
3 types of pathology affecting peripheral nerves
1) Wallerian degeneration = degeneration of the axon distally following damage
- distal to the injury the axon disintegrates and the myelin breaks up into globules
- basement membrane of the Schwann cell survives and forms a skeleton along which the axon can grow
2) Segmented demyelination = scattered destruction of myelin sheath occurs without axonal damage
3) Distal axonal degeneration = damage to the cell body or the axon will cause the axon to “die back” from the periphery; myelin is lost as a secondary event
What is a mononeuropathy?
Neuropathy involving a single nerve - e.g. median nerve
If multiple single nerves are affected in an asymmetrical pattern this is called mono neuritis multiplex
Polyneuropathy
General dysfunction of peripheral nerves that is usually symmetrical
Usually starts distally with some proximal progression
Classified based on:
- ) Timing - acute or chronic (often points to the aetiology)
- ) Function - autonomic, sensory, motor or combo
- ) Pathology - axonal or demyelinating
E.g. GBS is an acute, predominantly motor, demyelinating polyneuropathy
Length dependance
Polyneuropathy affects the longest axons first and affected the worst
What are negative sensory symptoms of polyneuropathy?
Negative symptoms = loss of sensation
Large myelinated fibres - loss of touch and joint position
- Difficulty in discriminating textures
- Hands and feet feel like cotton wool
- Gait unsteady (especially in darkness where vision cannot compensate)
Small unmyelinated fibres - loss of pain and temp leading to painless trauma
- Neuropathic joints (Charcot’s joints) - painless traumatic deformity
Positive sensory symptoms of polyneuropathy
Large myelinated fibre disease can cause paraesthesia (“pins and needles”)
Small unmyelinated fibres produce painful positive phenomena:
- Analgesia
- Hyperalgesia (increased sensitivity to painful stimuli)
- Allodynia (pain provoked by non painful stimuli)
Glove and stocking distribution
= sensory loss in the feet and hands
Usually length related and therefore signs in the hand will not develop until there is sensory loss up to at least mid shin level
What are common motor symptoms in peripheral neuropathy?
Weakness is usually the main presenting feature
- distal - e.g. difficulty clearing the curb when walking
- proximal - e.g. difficulty climbing stairs or combing hair
Cramps
Fasciculations may be present (although normally associated with anterior horn cell disease)
Sensory signs in polyneuropathy
All modalities tested - large and small fibre
Identify area of total sensory loss
Sensory loss is usually symmetrical in polyneuropathy with glove and stocking distribution
Examine gait - sensory ataxia and positive Romberg’s test
Trophic changes - cold blue extremities, cutaneous hair loss
What is the “axon reflex” used for?
“Places” the lesion in a sensory pathway
Normally scratched skin leads to (1) local vasoconstriction (white reaction) then (2) local oedema (red reaction) and finally surrounding vasodilation or flare (dependent on antidromic impulses from the DRG)
Distal sensory lesion - absent flare response
Proximal root lesion (past DRG) will not impair the response
What motor signs indicate polyneuropathy?
Muscle wasting - present in axonal but absent in demyelinating neuropathies (look at 1st dorsal interosseous), fasciculations may also be present
Muscle weakness - proportional to the number of affected motor neurones
- usually starts distally and spreads proximally
- usually symmetrical
- respiratory muscle weakness can occur in severe cases
What causes loss of tendon reflexes? Are they lost in all forms of neuropathy?
Any interruption to the reflex arc leads to loss of tendon reflexes
Reflexes are lost (i) and (ii) proximally before distally
Neuropathies that affect predominantly small fibres may preserve tendon reflexes because spindle afferent fibres are large myelinated proprioceptive fibres
What polyneuropathies are acute in onset?
Acute = days up to 4 wks
A useful clue is that most have motor functional loss
1) Inflammatory (GBS) - predominantly motor + autonomic disturbance (demyelinative)
2) Diphtheria - cranial nerve onset + mixed sensory/ motor (demyelinative)
3) Porphyria - motor with minimal sensory loss + abdo pain + psychosis + seizures (may begin in arm) (axonal)
Some polyneuropathies are subacute and asymmetrical. Give some examples
Subacute = months-years
These are normally asymmetrical (cf. most are normally symmetrical) and multifocal
1) Infections:
- Leprosy - sensory neuropathy with pigmentation and thickened nerves
- HIV - range of presentations
2) Vasculitic disorders:
- PAN, Wegeners, Churg-Strauss - often painful, usually presents with mononeuritis multiplex or asymmetrical sensorimotor neuropathy (causes Wallerian degeneration + vasculitis)
There are lots of causes of subacute and chronic, symmetrical polyneuropathies. Name some metabolic or endocrine disorders
- Diabetes - distal sensorimotor most common but wide range of other forms
- Uraemia - distal sensorimotor
- Hypothyroidism - distal sensorimotor
- Acromegaly - distal sensorimotor
Most metabolic and endocrine disorders causes polyneuropathy via axonal degeneration
What nutritional deficiencies cause polyneuropathy?
1) Vit B1 (thiamine); includes alcoholic neuropathy
- Predominantly sensory with burning feet
- Weakness may develop
2) Vit B12 - predominantly sensory + SACD
What malignant diseases cause polyneuropathy?
Polyneuropathy caused by cancer = carcinomatous polyneuropathy
- Paraneoplastic - sensory or sensorimotor; may be associated with anti-Hu antibodies in serum
- Infiltrative - multifocal (often a polyradiculopthay); more common with lymphoma
What type of polyneuropathy does amyloid cause?
Amyloid causes subacute or chronic, symmetrical sensorimotor neuropathy often with autonomic involvement
Associated with thickened nerves with amyloid deposits
