26th June 2023

A 22-year-old man presents with a 3-month history of ascending numbness and tingling of his hands and feet

A 22-year-old man presents with a 3-month history of ascending numbness and tingling of his hands and feet, difficulty getting his balance and complains of regularly dropping things. On neurological examination, there is preservation of sensation to pain and temperature but loss of joint position sense and proprioception in all limbs. Power is globally reduced in all 4 limbs, with clonus present bilaterally. His cranial nerve examination was normal. CT head was normal and lumbar puncture cell count was normal. He discloses that he uses cannabis very regularly, and often also uses cocaine and nitrous oxide recreationally.

Which investigation is most likely to reveal the underlying diagnosis?