Disorders of the Motor Unit

Question 1

In diseases of the motor unit name the 4 possible areas affected and their associated pathology

Answer 1

m.n. cell body - motor neuronopathies
m.n axon - peripheral neuropathies
n.m.j - disorders of transmission
muscle fibres themselves - myopathies/myositis

Question 2

Name some symptoms and signs of motor unit disease

Answer 2

Symptoms: weakness, wasting, twitching, myalgia, myoglobinuria
Signs: wasting, weakness, fasciculations, reduced reflexes

Question 3

Can signs and symptoms help us distinguish between myopathy and neuropathy?

Answer 3

Difficult as wasting and weakness are key in both
Fasciculations tend to imply disease of the motor neurone itself
No sensory abnormality in myopathy
Brisk reflexes are a sign of upper motor neuropathy

Question 4

What are the 3 laboratory criteria to distinguish neuropathy from myopathy?

Answer 4

muscle enzymes
electrophysiological examination
muscle biopsy

Question 5

Which muscle enzymes are high in myopathy but not in neuropathy?

Answer 5

creatine kinase

lactate dehydrogenase

Question 6

What is electromyography?

Answer 6

Inserting a small needle into muscle, record motor units.
Records: spontaneous activity, no. of units under voluntary control (interference pattern), duration and amplitude of AP’s

Question 7

Neurogenic disease vs Myopathic disease in electromyography

Answer 7

Neurogenic 
- spontaneous activity at rest
- giant motor unit potentials
- reduced interference pattern
Myopathic
- no resting activity
- shorter and smaller motor unit potentials

Question 8

Is normal muscle electrically silent at rest?

Answer 8

yes

Question 9

What are nerve conduction studies?

How do they allow us to distinguish between neuropathy and myopathy?

Answer 9

measure motor and sensory conduction velocities of axons as in demyelinating neuropathies velocity is reduced
In muscle disease = conduction velocity is normal

Question 10

Describe the appearance of normal muscle, denervated muscle and muscular distrophy

Answer 10

Normal muscle - checkerboard staining pattern with myosin ATPase
Denervated muscle - fibre type grouping, axons sprout from neighbouring motor units to innervate the denervated fibres
Muscular dystrophy - huge variation in fibre size, increased fat and connective tissue, may see inflammation

Question 11

Give examples of neurogenic diseases of the motor neurone cell body and the peripheral nerve

Answer 11

amyotrophic lateral sclerosis (MND)

Guillain-Barre syndrome (acute)

Question 12

Give examples of myopathic diseases
2 x inherited
2 x aquired

Answer 12

duchenne muscular dystrophy, limb girdle muscular dystrophy
Dermatomyositis
Polymyositis

Question 13

What is babinski sign and what pathology does it indicate?

Answer 13

Scrap sole of foot, big toe curls up whilst other toes curl down
Normal in newborns as upper motor neurones are not yet myelinated
Indicates upper motor neurone dysfunction

Question 14

What is the pseudobulbar effect and what pathology does it indicate?

Answer 14

Laugh/cry more easily (inappropriate)

Indicates upper motor neurone dysfunction

Question 15

What is the technical term for motor neurone disease?

Which motor neurones does it affect?

Answer 15

amyotrophic lateral sclerosis

affects upper and lower motor neurones

Question 16

What is the aetiology of MND?

Answer 16

Unknown but one theory is due to protein disaggregation and abnormal precipitation and glutamate excitotoxicity –> leads to motor neurone death

Question 17

5 A’s and one P of treating MND

Answer 17

antispasticity agents
anticramping agents
antidepressants
agents to receive a dry mouth
agents to aid respiration

PEG feeding

Question 18

What is Guillain-Barre Syndrome?
Cause
Symptoms
Treatment

Answer 18

Syndrome occurring days or weeks after an infection
Due to cross-reacting antibodies that attack the peripheral nerves
Motor and sensory nerves involved
Symptoms can range from mild to profound weakness
Treatment - plasma exchange or donor immunoglobulin

Question 19

What are some of the main causes of peripheral neuropathy?

Answer 19

ABCD

Alcohol, B12 deficiency, carcinoma, diabetes

Question 20

What is Charcot-Marie-Tooth disease?
What are the symptoms?
What are the signs|

Answer 20

Hereditary motor and sensory neuropathy affecting both motor and sensory neurones
Symptoms: muscle weakness and wasting, hyporeflexia, anaesthesia, very slowly progressive
Signs: inverted champagne bottle legs, pes cavus (arched foot)

Question 21

Give the genotype phenotype of Duchenne Muscular Dystrophy

Answer 21

X linked recessive - boys mostly affected 
affects boys
onset in first 3 years of life
proximal muscle wasting/weakness
calf pseudo-hypertrophy 
cardiac muscle involvement
progressive disease

Question 22

What is gowers manouvre?

Which pathology is it a sign of?

Answer 22

Relies heavily on limbs when standing up due to proximal weakness of lower limbs
Indication of Duchenne Muscular Dystrophy

Question 23

What does dystrophin do?

Fun fact about the dystrophin gene!

Answer 23

Holds actin to muscle membrane

Its the largest gene yet identified in humans

Question 24

DMD investigations and diagnosis

Answer 24

Serum muscle enzymes - CREATINE KINASE high
Muscle biopsy - DYSTROPHIN ANTIBODIES
Molecular genetic analysis - deletions/mutation in DYSTROPHIN GENE

Question 25

Abnormalities in cytoskeltal proteins of the sarcolemma causes hereditary myopathies
What is the cause of limb girdle muscular dystrophy

Answer 25

Problems with the sarcoglycans in sarcolemma

Question 26

What is dermatomyositis?

Answer 26

Acquired condition
Characterised by a violaceous rash (face, chest, hands) and a myopathy of the proximal muscles.
Develops over weeks - mild/life threatening
10% of adult cases have an underlying malignant tumour
Very high levels of muscle enzymes