Motor neuron disease is a neurological condition of unknown cause which can present with both upper and lower motor neuron signs. It rarely presents before 40 years and various patterns of disease are recognised including amyotrophic lateral sclerosis (ALS), progressive muscular atrophy and bulbar palsy.

There are a number of clues which point towards a diagnosis of motor neuron disease:

• asymmetric limb weakness is the most common presentation of ALS
• the mixture of lower motor neuron and upper motor neuron signs
• wasting of the small hand muscles/tibialis anterior is common
• fasciculations
• the absence of sensory signs/symptoms
  • vague sensory symptoms may occur early in the disease (e.g. limb pain) but 'never' sensory signs

Other features

• doesn't affect external ocular muscles
• no cerebellar signs
• abdominal reflexes are usually preserved and sphincter dysfunction if present is a late feature

The diagnosis of motor neuron disease is clinical, but nerve conduction studies will show normal motor conduction and can help exclude a neuropathy. Electromyography shows a reduced number of action potentials with increased amplitude. MRI is usually performed to exclude the differential diagnosis of cervical cord compression and myelopathy