Classification and clinical signs of motor neurone diseases

1. Big picture

Motor neurone diseases (MNDs) are diseases in which the main lesion is in the upper motor neurone, the lower motor neurone, or both.

The most important disease in this group is:

Amyotrophic lateral sclerosis (ALS) = combined upper and lower motor neurone degeneration.

The key exam idea:

MND causes progressive motor weakness without primary sensory loss. Sensation, eye movements, and bladder/bowel function are usually preserved, especially in ALS.

Motor neurone disease is not one disease only. It is a group that includes:

• ALS;
• primary lateral sclerosis;
• progressive muscular atrophy;
• progressive bulbar palsy;
• spinal muscular atrophy;
• spinobulbar muscular atrophy / Kennedy disease.

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2. Definition

Motor neurone diseases are progressive disorders affecting motor neurones:

• upper motor neurones: motor cortex and corticospinal/corticobulbar tracts;
• lower motor neurones: anterior horn cells of the spinal cord and motor cranial nerve nuclei.

Because motor neurones are affected, the dominant clinical problem is:

progressive weakness, wasting, fasciculation, spasticity, dysarthria, dysphagia, and respiratory failure depending on the subtype.

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3. Classification of motor neurone diseases