Introduction There are many causes of progressive muscle wasting and weakness from which motor neuron disease (MND) is a common and dreaded differential diagnosis. Routine investigations such as neurophysiology study, imaging and laboratory tests are important to confirm or exclude the diagnosis. We report a young man who had a presentation mimicking motor neuron disease but eventually was given a diagnosis of myotonic dystrophy.

Case description An 18 year old right handed man presented with progressive muscle wasting and weakness of the left upper limb over a year. Initially he complained of knuckle cramps and reduced finger dexterity in winter. There was no family history of neurological problems. Clinical examination essentially showed wasting of forearm and hand, significant weakness in all fingers, thumb and lesser extent in wrist with preserved reflexes and normal sensory testing. There was no fasciculation. His cranial nerves were intact. The initial impression was progressive muscular atrophy, a form of motor neuron disease. He had a normal MRI cervical spine, CSF study and negative anti–ganglioside antibodies. His nerve conduction studies and EMG showed severe denervation in C7–8/T1 distribution in left arm and milder change in the right. Whilst many causes were excluded, a review of his presenting complaint and re–examination led to detection of myotonia in thenar eminences and tongue. He went on to have a genetic testing which showed 128 CTG repeats in DPMK gene and a final diagnosis of myotonic dystrophy type 1 was made.

Discussion Myotonia is typically worse in the cold environment and is most prominent in the early stage of myotonic dystrophy1. Early onset of myotonic dystrophy without other systemic involvement can mimic MND.