🎯 Pathology

• a group of chronic neurological disorders that causes progressive degeneration of motor neurons

  • can affect upper motor neurons (UMN), lower motor neurons (LMN), or both

  • UMNs: these are the corticospinal and corticobulbar (projections to the brainstem lower motor neurons such as the facial nerve) tracts

  • LMNs: these are the anterior horn cells that project from the brainstem and spinal cord to innervate muscles

 

⚡️ Corticospinal tract

• function

  • initiation of voluntary movement

• location of tract in the spinal cord

  • note that there are two descending corticospinal tracts:

    • laterally: lateral corticospinal tract

    • anteromedially: anterior corticospinal tract

• course

  1. upper motor neurons arise from the primary motor cortex (which corresponds to the precentral gyrus)

  2. axons descend to the cerebral peduncles in the midbrain via the internal capsule

  3. axons continue down the brainstem until they reach the medullary pyramids where they decussate

     • approximately 85% of the corticospinal fibres decussate, and these make up the lateral corticospinal tract

     • the rest of the corticospinal tract continues as the anterior corticospinal tract

  4. corticospinal tract fibres enter the spinal cord and synapse with lower motor neurons at the ventral horn

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🧩 UMN vs LMN lesions

• upper motor neuron signs

  • increased muscle tone

  • spasticity

  • hyperreflexia

  • positive Babinski sign (upgoing plantar reflex)

  • weakness

• lower motor neuron signs

  • muscle wasting

  • fasciculations (involuntary twitching of a muscle that is visible)

  • decreased muscle tone

  • hyporeflexia

  • weakness

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💡 Causes

• there is a genetic component to the disease

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🧐 Types

• amyotrophic lateral sclerosis (most common)

• progressive muscle atrophy (accounts for 5% of cases)

• primary lateral sclerosis

• progressive bulbar palsy

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💆‍♂️ Clinical features

 

General features

• asymmetrical involvement of nerves

• extraocular muscles of the eye are preserved

• no cerebellar signs

 

Amyotrophic lateral sclerosis

• affects both UMNs and LMNs and therefore characterised by a mixture of UMN and LMN signs

• asymmetric involvement of the distal muscle

  • e.g. difficulties with using keys to open doors, gait disturbances leading to frequent falls, and writing

• dysphagia

  • leads to hypersalivation because patients cannot swallow properly

  • increased risk of aspiration

  • leads to weight loss and nutritional deficiencies

• dysarthria

  • affects the ability to communicate effectively

• type II respiratory failure

  • due to impairment of respiratory muscles leading to hypoventilation, and therefore the development of hypercapnia and hypoxia

 

Progressive muscle atrophy

• affected LMNs and characterised by LMN signs

• LMNs are affected asymmetrically, and the distal limbs are usually affected

 

Primary lateral sclerosis

• affects UMNs and characterised by UMN signs

 

Progressive bulbar palsy

• affects the lower motor neurons that arise from the brainstem

• CN V —> impaired chewing

• CN VII —> facial paralysis

• CN IX and X —> dysphagia

• CN X —> dysarthria

• CN XI —> paralysis of the tongue

 

🕵️‍♂️ Investigations

• clinical

  • thorough history and neurological examination​

• nerve conduction studies

  • this looks at demyelination

  • in MND, the results are normal

  • it is a useful test to rule out demyelinating diseases

• electromyography​

  • measures the response of a muscle following the stimulation of nerve that innervates the muscle​

• lumbar puncture

  • elevated CSF neurofilament levels

• neuroimaging of the spine

  • MRI is the preferred investigation​

  • done to rule out neurological conditions that can affect the spine including multiple sclerosis and cervical cord compression

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💆‍♂️ Management

• multidisciplinary approach

  • physiotherapist and occupational therapists to aid with

    • mobility

    • independence

    • functional ability

    • exercise regimes

  • speech and language therapists to help with

    • speaking

    • swallowing

  • respiratory therapists can help with

    • monitoring respiratory function

    • evaluate the need for non-invasive ventilation (NIV) such as bilevel positive airway pressure (BiPAP)

  • nutritionist

    • patients may need to get nutrition via the enteral route due to swallowing issues

      • possible installation of a percutaneous gastrostomy tube (PEG)

  • neurologist

    • follow-up

    • consider the initiation of riluzole, an NMDA receptor antagonist

      • studies show it can improve survival by 3 months

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🧲 High-yield tips

• the extraocular muscles are preserved in motor neuron disease

• management is optimised via a multidisciplinary approach