Disorders of the Motor Unit Flashcards
In diseases of the motor unit name the 4 possible areas affected and their associated pathology
m.n. cell body - motor neuronopathies
m.n axon - peripheral neuropathies
n.m.j - disorders of transmission
muscle fibres themselves - myopathies/myositis
Name some symptoms and signs of motor unit disease
Symptoms: weakness, wasting, twitching, myalgia, myoglobinuria
Signs: wasting, weakness, fasciculations, reduced reflexes
Can signs and symptoms help us distinguish between myopathy and neuropathy?
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
What are the 3 laboratory criteria to distinguish neuropathy from myopathy?
muscle enzymes
electrophysiological examination
muscle biopsy
Which muscle enzymes are high in myopathy but not in neuropathy?
creatine kinase
lactate dehydrogenase
What is electromyography?
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
Neurogenic disease vs Myopathic disease in electromyography
Neurogenic - spontaneous activity at rest - giant motor unit potentials - reduced interference pattern Myopathic - no resting activity - shorter and smaller motor unit potentials
Is normal muscle electrically silent at rest?
yes
What are nerve conduction studies?
How do they allow us to distinguish between neuropathy and myopathy?
measure motor and sensory conduction velocities of axons as in demyelinating neuropathies velocity is reduced
In muscle disease = conduction velocity is normal
Describe the appearance of normal muscle, denervated muscle and muscular distrophy
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
Give examples of neurogenic diseases of the motor neurone cell body and the peripheral nerve
amyotrophic lateral sclerosis (MND)
Guillain-Barre syndrome (acute)
Give examples of myopathic diseases
2 x inherited
2 x aquired
duchenne muscular dystrophy, limb girdle muscular dystrophy
Dermatomyositis
Polymyositis
What is babinski sign and what pathology does it indicate?
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
What is the pseudobulbar effect and what pathology does it indicate?
Laugh/cry more easily (inappropriate)
Indicates upper motor neurone dysfunction
What is the technical term for motor neurone disease?
Which motor neurones does it affect?
amyotrophic lateral sclerosis
affects upper and lower motor neurones
What is the aetiology of MND?
Unknown but one theory is due to protein disaggregation and abnormal precipitation and glutamate excitotoxicity –> leads to motor neurone death
5 A’s and one P of treating MND
antispasticity agents anticramping agents antidepressants agents to receive a dry mouth agents to aid respiration
PEG feeding
What is Guillain-Barre Syndrome?
Cause
Symptoms
Treatment
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
What are some of the main causes of peripheral neuropathy?
ABCD
Alcohol, B12 deficiency, carcinoma, diabetes
What is Charcot-Marie-Tooth disease?
What are the symptoms?
What are the signs|
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)
Give the genotype phenotype of Duchenne Muscular Dystrophy
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
What is gowers manouvre?
Which pathology is it a sign of?
Relies heavily on limbs when standing up due to proximal weakness of lower limbs
Indication of Duchenne Muscular Dystrophy
What does dystrophin do?
Fun fact about the dystrophin gene!
Holds actin to muscle membrane
Its the largest gene yet identified in humans
DMD investigations and diagnosis
Serum muscle enzymes - CREATINE KINASE high
Muscle biopsy - DYSTROPHIN ANTIBODIES
Molecular genetic analysis - deletions/mutation in DYSTROPHIN GENE
Abnormalities in cytoskeltal proteins of the sarcolemma causes hereditary myopathies
What is the cause of limb girdle muscular dystrophy
Problems with the sarcoglycans in sarcolemma
What is dermatomyositis?
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
