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A 28-year-old right hand-dominant mechanical engineer attended a number of neurologists because of several months of twitching in his thigh and calf muscles. His concern was that he may have developed motor neurone disease as he also had wasting of the muscles in his right thenar eminence. This had progressed over 3 years, with worsening grip strength, weakness and loss of dexterity in the right hand. There was no tenderness and no weakness elsewhere and no sensory, autonomic or systemic symptoms. His history included psoriasis, treated with oral methotrexate and corticosteroids.
On examination, there was significant wasting of the right thenar eminence with weakness of opposition and thumb abduction (figure 1). The remaining intrinsic muscles, all of the extrinsic muscles and sensory function of the hand were normal. There were brief fasciculations in the thighs and calves bilaterally, but none in the arms or tongue. Muscle bulk, strength and sensation were normal in the legs, and his reflexes were intact including the jaw jerk. There was no dysarthria, dysphagia or other cranial nerve deficits.
What is the differential diagnosis?
There are two dimensions to the presentation, which may or may not be related. First, the progressive right hand weakness with wasting and second, the fasciculations in the thighs and calves. Each of these is considered in turn.
Right hand weakness
Three years of weakness and wasting without spread beyond the thenar eminence favours a focal cause for right hand the symptoms, reflecting isolated distal median nerve involvement.
The most common site of compression of the median nerve is in the carpal tunnel, which normally manifests as weakness of the thenar muscles and dysaesthesia of the thumb, index, middle and radial half of the ring finger. The severity of the focal wasting and the lack of sensory features here indicate median nerve …
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