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A 43-year-old right-handed woman presented with a two month history of drooling and difficulty eating due to food falling out of her mouth, particularly on the right, initially diagnosed as a Bell’s palsy. She then noted progressive limb weakness, beginning with difficulty lifting heavy objects above her head, climbing stairs and running. Her power rapidly deteriorated over six weeks. There was no fatigability. She lost 1 kg in weight and complained of painful spontaneous muscle cramps of all limbs. There were no sensory, autonomic, ocular or cognitive symptoms.
Several weeks before her symptom onset, she had suffered from brief diarrhoea and vomiting (as had several other family members) but there was no other past medical history. She was on no medication, did not smoke and rarely drank alcohol. Her mother was said to have died of suspected motor neuron disease at the age of 53, but the details were unclear.
On initial examination she had bilateral lower motor neuron facial weakness, more marked on the right, with associated dysarthria. There were sparse tongue fasciculations but no wasting. Her jaw jerk was normal, as were her ocular movements, and she did not have ptosis. She was generally thin but had possible wasting of deltoids and quadriceps with occasional fasciculations in these muscles. She was weak (MRC grade 3–4/5) in all four limbs, particularly proximally, with normal tone, easily elicited reflexes and flexor plantar responses. Sensation and coordination were normal.
What is the differential diagnosis and what investigations would you do?
The signs suggested a lower motor neuron disorder, although she had retained her reflexes. There were no sensory features, so one must consider the following possible sites of pathology and conditions:
Motor neuron/nerve roots
Motor neuron disease (either the “lower motor neuron only” subtype progressive muscular atrophy, or classical amyotrophic lateral …