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It can be difficult to get a diagnosis initially, partially due to the disease rarity. Patients need to be referred to a neurologist as soon as possible, but this does not always happen (in the UK).
Symptoms can be aligned with other conditions such as cauda equina or Guillanne Barre syndrome, but further investigation will eliminate these. An MRI scan is usually need to give a definitive diagnosis, but the inflammatory changes can be quite subtle, so an expert in reading the scans is needed.
Other tests include blood tests and lumbar puncture. Lumbar puncture also can provide definitive diagnosis, however I declined to have this test.