Motor Neurone Disease Flashcards
What is a motor neurone disease?
Cluster of major degenerative diseases characterised by selective loss of neurons in motor cortex, cranial nerve nuclei and anterior horn cells
What are the five steps involved in the organisation of a movement?
- Idea of movement - association areas of cortex
- Activation of UMN in precentral gyrus
- Impulses travel to LMN and their motor units via the corticospinal tract
- Modulating activity of the cerebellum and basal ganglia
- Further modification of movement depending on sensory feedback
What is a LMN?
“Final common pathway” by which the CNS controls voluntary movement.
Where are LMN’s located?
In the anterior horns of the spinal cord and in cranial nerve nuclei in the brainstem
What is a motor unit?
Basic functional unit of muscle activity
Its the LMN plus the axon plus the skeletal fibres that it innervates
How is muscle tone regulated?
- Stretch receptors in muscle (muscle spindles) innervated by gamma motor neurones
- Muscles stretched -> afferent impulses from muscle spindles -> reflex partial contraction of muscle
- Disease states eg spasticity and rigidity alter muscle tone by altering the sensitivity of this reflex
What are the 6 potential sites of damage along the motor pathway?
- Motor nuclei of cranial nerves
- Motor neurons in spinal cord
- Spinal ventral roots
- Peripheral nerves
- Neuromuscular junction
- Muscle
What can cause motor neurone disease?
Its usually sporadic and has no known cause
What are the main cells affected in motor neurone disease?
Anterior horn cells
What are the risk factors for motor neurone disease?
- Males > females
- 5-10% cases are familial
What is the most common motor neurone disease?
Amyotrophic Lateral Sclerosis (ALS)
What are the signs of Amyotrophic Lateral Sclerosis (ALS)?
Weakness + UMN signs
- Eg upping plantars + LMN wasting / fasciculations - usually in one limb
- Split hand sign - thumb side of the hand seems adrift due to excessive wasting around it - there is much less hypothenar (little finer) wasting
- Cramps are a common but non-specific symptom
- Examination reveals UMN signs such as brisk reflexes (brisk reflex in a waster muscle is a classic sign), extensor plantar response and spasticity
- Wrist and foot drop
What happens in progressive muscular atrophy (PMA)?
Lower motor neuron only
- Everything goes down (think LOWER = DOWN)
- Muscle tone reduced = flaccid
- Muscle wasting
- Fasciculation - visible spontaneous contraction of motor units - NOT ENOUGH TO DIAGNOSE LMN, NEED WEAKNESS TOO
- Reflexes depressed or absent
- Usually starts in one limb and gradually spreading to involve other adjacent spinal segments
- Affects distal muscle group BEFORE proximal
What happens in Primary Lateral Sclerosis (PLS)?
Upper motor neuron only
- Everything goes up!
- Spasticity (increased muscle tone)
- Brisk reflexes - tendon and jaw reflexes
- Plantars are upturned on stimulation (positive babinski sign)
- Characteristic pattern of limb muscle weakness (pyramidal pattern)
- Upper limbs extensor muscles are weaker than flexors
- Lower limb flexor muscles are weaker than extensors
- Finer more skilful movement are impaired
- Affects groups of muscles instead of individual muscles
- Velocity dependent spasticity (faster you move patient’s muscle = more resistance)
- Clasp knife
- Muscle wasting less prominent
- Can mimic LMN lesion in the first few hours before spasticity and hyperreflexia develops
- No cognitive decline
What investigations would you do in suspected motor neuron disease?
Diagnosed based on clinical findings
- Definite: LMN + UMN signs in 3 regions
- Probable: LMN + UMN signs in 2 regions
- Probably with lab support: LMN + UMN signs in 1 region or UMN sign in more than 1 region + electromyography (EMG) shows acute denervation in more than 2 limbs
- Possible: LMN + UMN signs in 1 region
- Suspected: LMN or UMN sign only in one or more regions
Brain / cord MRI
Lumbar puncture
Nerve conduction studies (NCS) + electromyography (EMG)
