A 64-year-old man presents with a eight-month history of generalised weakness...

A 64-year-old man presents with a eight-month history of generalised weakness. On examination he has fasciculation and weakness in both arms with absent reflexes. Examination of the lower limbs reveal increased tone and exaggerated reflexes. Sensation was normal and there were no cerebellar signs. What is the most likely diagnosis?

A. Lead poisoning
B. Motor neuron disease
C. Vitamin B12 deficiency
D. Syringomyelia
E. Multiple sclerosis

Answer:
B. Motor neuron disease

Discussion: 'Fasciculations' - think motor neuron disease
These symptoms are typical of amyotrophic lateral sclerosis, the most common type of motor neuron disease.

Motor neuron disease: features
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, 
• progressive muscular atrophy and 
• bulbar palsy 

There are a number of clues which point towards a diagnosis of motor neuron disease:
 fasciculation
 absence of sensory signs/symptoms
 lower motor neuron signs in arms and upper motor neuron signs in legs
 wasting of the small hand muscles/tibialis anterior is common

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 an increased amplitude. MRI is usually performed to exclude the differential diagnosis of cervical cord compression and myelopathy